Conformational Effects on the Lowest Excited States of Benzoyl-Pyrrolopyridazine: Insights from PCM Time-Dependent DFT

Conformational Effects on the Lowest Excited States of Benzoyl-Pyrrolopyridazine: Insights from PCM Time-Dependent DFTDan Maftei,† Gheorghita Zbancioc,‡ Ionel Humelnicu,† and Ionel Mangalagiu*,‡

†Department of Physical and Theoretical Chemistry and ‡Department of Organic Chemistry, Al. I. Cuza University, 11 Bd. Carol I,700506 Iasi, Romania

*S Supporting Information

ABSTRACT: Time-dependent density functional theory(TD-DFT) computations and steady-state electronic spectros-copy measurements are performed on two recently synthesizedpyrrolopyridazines to account for the detrimental effect ofbenzoyl substitution on the blue fluorescence emission. In thecase of the highly fluorescent ester derivative, planar in groundstate, we show that TD-DFT using the PBE0 and B3LYPhybrid functionals in the state-specific solvation approachprovides an accurate description of absorption and emissionproperties. In benzoyl-pyrrolopyridazine, the (pretwisted) orientations of the benzoyl group and the solvent polarity are bothfound to modulate the nature of the lowest excited states. The first excited state has nπ* character at ground-state geometry ofthe main conformer (carbonyl group facing the diazine ring) in nonpolar solvents and become nearly degenerate with a ππ* statein polar solvents. The latter, lower than the nπ* state at the ground state geometry of a minor conformer, relaxes into a twistedintramolecular charge transfer. Experimental absorption and excitation spectra are consistent with the conformational-dependentpicture of the lowest excited state (as derived from TD-DFT). A rather qualitative agreement in predicting the fluorescenceemission wavelength is achieved in computations employing the CAM-B3LYP and BH&HLYP functionals, whereas globalhybrids with low or moderate amounts of exact exchange exhibit the expected TD-DFT failure with up to 1 eV underestimatedtransition energies.

■ INTRODUCTION

Photophysics of indolizines and azaindolizines has receivedincreasing interest during the last years driven by a wide rangeof potential applications, from electroluminescent materials1 tomacrocyclic fluorescent sensors.2,3 In particular, the interest in5-azaindolizine derivatives arises from their highly efficient bluefluorescence emission,4−6 which makes them attractivematerials in optoelectronics for blue organic light-emittingdiodes.Indolizine is an aromatic 10 π-electron N-fused heterocycle

isomeric with indole, containing a bridgehead nitrogen atomshared by an electron-excessive pyrrole and an electron-deficient six-membered ring.7 In addition to the uneven π-electron distribution between the two fused rings, the planargeometry of the indolizine is an important feature that makesroom for electron delocalization within the entire heterocycleskeleton. The planarity is preserved in the presence of anadditional nitrogen atom in the six-membered ring, as inisoelectronic azaindolizines, or upon substitution with differentgroups.In the case of 5-azaindolizine, referred to in the following as

PP, few experimental1,8,9 and combined computational andexperimental insights4,10 exist, aiming at a rational descriptionof the relative effects induced by the substituents on theabsorption and fluorescence properties. In the former studies,the PPs were shown to exhibit efficient fluorescence emission

yet modest molar extinction coefficients for the first absorptionband located in the near-UV region and little variation ofmaximum absorption wavelength with the number or thenature of the substituents. Because both absorption andemission wavelengths show little differences from onederivative to another, the electronic spectra for several of PPswere satisfactory modeled by time-dependent density func-tional theory (TD-DFT) calculations performed on theunsubstituted PP heterocycle.4 Large Stokes shifts were alsopredicted to originate in a bond-equalization process within thepyridazine ring in the first excited state of PP. This suggests thatabsorption and fluorescence spectra, while showing smallvariations from one derivative to another, arise essentially fromthe same electronic transitions taking place within theheterocycle and leave the substituents with the key role intuning the electronic transitions by electron-withdrawing orelectron-donating effects.In a systematic approach to establish how functionalization of

PP is influencing the fluorescence emission, Zbancioc et al.9

show that the best results in terms of quantum efficiency areobtained in PPs bearing two or three ester groups at the pyrrolering. From a large series of compounds, while a certain

Received: November 19, 2012Revised: March 20, 2013Published: March 25, 2013

Article

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© 2013 American Chemical Society 3165 dx.doi.org/10.1021/jp311396m | J. Phys. Chem. A 2013, 117, 3165−3175

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influence of the nature and the position rather than the numberof substituents is found, the group in position 7 of the pyrrolering (Scheme 1) was considered to play a crucial role in

fluorescence emission of PP. When an ester or amide groupfrom position 7 of a highly fluorescent derivative 1 is replacedwith a benzoyl moiety as in 2 in an attempt to further increaseπ-electron conjugation in either ground or excited state, theresulting compound is nearly nonfluorescent. Such undesirableeffect lacks a satisfactory description in simple terms of electronconjugation and suggests the existence of nonradiative funnelsdominating the excited-state decay. If a design strategy of highlyfluorescent compounds is concerned, then a rationaldescription of the effect of the substituents on the excitedstate of PP needs thorough investigation.Investigated in this paper is the detrimental effect of the

benzoyl group on the fluorescence emission of PPs ascompared with an ester analogue. In this respect, TD-DFTcalculations including solvent effects and steady-state absorp-tion and fluorescence measurements are performed on twopreviously synthesized disubstituted PPs including the highlyfluorescent ester derivative 1 and the nearly nonfluorescentbenzoyl-substituted PP 2. Electronic spectra computations arealso performed on the unsubstituted PP for comparison. Newinsights into the photophysics of pyrrolopyridazines allow us torevise some of the conclusions formulated in the previous workand also to derive a picture of the effects of conformation andsolvent polarity on the first excited states of benzoyl-substitutedPPs.

■ MATERIALS AND METHODSExperimental Section. Dimethyl 5,7-pyrollo[1,2-b] pyr-

idazine-dicarboxylate (1) and methyl 7-(4-chlorobenzoyl)-pyrollo[1,2-b] pyridazine-carboxylate (2) have been preparedusing a microwave-assisted synthetic route previously describedelsewhere.9

Electronic absorption spectra were recorded in a 600−250nm wavelength range on dilute solutions (∼10−5 M) preparedin spectroscopic grade acetonitrile, chloroform, and cyclo-hexane at room temperature using a Shimadzu UV-1800double-beam spectrophotometer using the correspondingsolvent as reference. Recorded absorption spectra weredeconvoluted using a multipeak fitting procedure. A Gaussianprofile function was initially assigned to each observable peak incyclohexane, and the corresponding parameters defining theposition of maximum, the peak area, and it is full width at half-maximum were allowed to vary simultaneously in an iterativenonlinear least-squares minimization. Values of these parame-ters, as refined in cyclohexane, were used as starting values in

subsequent fitting of spectra recorded in chloroform andacetonitrile.Steady-state fluorescence excitation and emission spectra

were measured using a Horiba Fluoromax-4 spectrofluorometerand automatically corrected for instrumental effects. Given thevery weak fluorescence emission, measurements were per-formed in the case of compound 2 at concentrations roughly5−10 times higher than the corresponding solutions of 1.Normalized fluorescence excitation and emission spectra areprovided in the Supporting Information.

Computational Methods. All electronic structure compu-tations were performed using the Gaussian 0911 suite ofprograms. Unless specified, equilibrium geometries and verticaltransition energies were computed at the DFT and TD-DFTlevels using the parameter-free PBE012 hybrid density func-tional (PBE1PBE in Gaussian) and the 6-31+G(d) basis set.This theory level is reported to provide reliable geometries andgood estimates of transition energies at affordable computa-tional costs.13−15 An “ultrafine” integration grid and tightconvergence thresholds were used thoroughly in both self-consistent field (r.m.s density matrix < 10−8 a.u.) and geometryoptimization (r.m.s. force < 10−5 a.u.) cycles.Ground-state geometries were optimized starting from

different initial orientations of the two functional groups withrespect to the planar PP to account for conformationalpreferences arising from hindered rotations about the exocyclicC5−C8 and C7−C10 bonds. Namely, each of the two torsionalcoordinates, dihedral angles φ1 (∠N7a−C7−C10-O11) and φ2(∠C4a−C5−C8-O9), has been initially set at roughly 20 and160° for 1, whereas in 2 a slightly larger deviation fromplanarity has been adopted at C7 in the starting geometries toavoid the creation of too-small interatomic distances. Molecularsymmetry was ignored in all equilibrium geometry calculations.Vibrational frequencies were computed following eachoptimization run to ensure that stationary points locatedrepresent minima of the ground-state (GS) potential energysurface (PES).Subsequent geometry optimization was carried out in

cyclohexane, chloroform, and acetonitrile for each of thestationary points located in the gas-phase computations. Bulksolvent effects were included via the polarizable continuummodel (PCM) in the integral equation formalism (IEFPCM),16

as implemented by default in Gaussian 09.Vertical excitation energies and oscillator strengths were

computed for each conformer both in gas-phase and in PCMsolvent at the corresponding geometries. Two differentapproaches have been adopted in the computation of electronicspectra in the condensed-phase, including a calculation of thefirst up to six singlet excitations in linear-response solvation,followed by a state-specific computation of the excited state ofinterest. The two methods of including implicit solvent effectson electronic transitions basically consist of (i) directcomputation of vertical transition energies, without computingthe excited-state electron density, which is the straightforwardconventional linear-response (LR-PCM) procedure, and (ii) aniterative mutual optimization of the electron density in aspecific excited state with solvent degrees of freedom17,18 in thestate-specific (SS-PCM) approach, respectively. The twoapproaches might yield completely different estimates forelectronic transitions involving a large change in electrondensity or in the case of strong solvent−solute interactions.19,20Excited-state geometry of compound 1 was optimized in a

preliminary step starting from the GS geometry. Next, rigid

Scheme 1. Chemical Structures of Dimethyl 5,7-Pyrrolo[1,2-b]pyridazine-dicarboxylate (1) and Methyl 7-(4-Chlorobenzoyl)-pyrrolo[1,2-b]pyridazine-carboxylate (2)a

aIUPAC and custom numbering scheme adopted for selected atoms isshown for 1.

The Journal of Physical Chemistry A Article

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excited-state PES scans were performed independently on eachof the two torsional coordinates to account for conformationaleffects. Lowest minima located on each PES scan weresubjected to geometry optimizations followed by vibrationalfrequencies calculations. S0/S1 and S0/S2 vibronic couplingswere computed from analytical/numerical vibrational frequen-cies and normal modes21,22 using the FCclasses code23 andincluding solvent (cyclohexane) and finite temperature effects24

to model the band shape at 300 K.In the case of compound 2, successfully converged geometry

optimizations were initiated from GS of the two most stableconformers. Three additional excited states were constantlyincluded in geometry optimization to check for the occurrenceof root-flipping events.To overcome the well-known failure of commonly used

functionals in dealing with long-range charge-transfer (CT)states,25 four additional exchange-correlation functionals wereconsidered. At the (TD-)PBE0/6-31+G(d) ground (excited)state geometries, subsequent calculations of electronicexcitation (emission) energies were performed using (i) thewidely used B3LYP global hybrid, (ii) the long-range correctedCAM-B3LYP,26 (iii) the BH&HLYP hybrid, which has beenrecently recommended by Aittala et al.27 for computing theelectronic spectra of pyridil-indolizines, and (iv) the fullHartree-Fock meta-hybrid functional M06-HF28 reported tooutperform BH&HLYP for charge-transfer excitations.29

Results from configurational interaction singles (CIS) compu-tations are included for comparison. Because the aforemen-tioned caveat of conventional hybrids (including PBE0) affectsthe landscape of the excited-state PES,30,31 with a systematicbias toward twisted (often spurious) intramolecular CTminima, excited-state geometries of compound 2 werecomputed also using the CAM-B3LYP, BH&HLYP, andM06-HF functionals in both gas-phase and PCM solvents.

■ RESULTS AND DISCUSSION

Ground-State Equilibrium Geometries. Ground-stategeometry optimizations yield planar and twisted orientationsof the functional group at C7 with respect to PP in compounds1 and 2, respectively, whereas the ester group from C5 adopts aplanar orientation in all conformations of both compounds.The results are in qualitative agreement with the experimental(X-ray diffraction) geometries of similar indolizine deriva-tives.32,33 A custom notation of the relative orientation of thecarbonyl from the ester groups from C5 and C7 with respect tothe PP ring is adopted in the following for convenience. Heresyn denotes a carbonyl in an orientation facing the heterocycle,namely, either φ1 ≈ 0° or φ2 ≈ 0°, whereas anti refers to φ1 ≈180° or φ2 ≈ 180°. The double notations, extended tononplanar conformations, refer to the corresponding rotamersat C5 and C7, in that order. (See Figure 1.)For both compounds, all of the conformers arising from

hindered internal rotations about the C7−C10 and C5−C8bonds are located in a narrow <2 kcal/mol free energy windowand show little variations from gas phase to bulk solvent andalso from one solvent to another. The syn-syn conformer(Figure 1, left) is predicted as the most stable regardless of theenvironment in molar ratios between 0.43 and 0.69 (forcompound 1) and 0.50 to 0.58 (in case of 2), as computedfrom free-energy differences assuming a Boltzmann distribu-tion. The anti-anti conformer is 1.2 to 1.9 kcal/mol higher infree energy, and the corresponding ratios are <0.1, whereas the

ratio of the two mixed syn-anti and anti-syn conformationsshows little variation with the environment.Conformational preferences may be considered as a result of

an interplay between electronic interactions that favor a planarorientation of the functional groups and steric repulsion havingan opposite effect. It turns that in 1 the former prevails at bothC5 and C7, whereas steric repulsion is more effective incompound 2 at C7 yielding twisted orientations of the benzoylgroup in both syn-syn (φ1 ≈ 24°) and syn-anti (φ1 ≈ 147°)most stable conformers. Overall, computed free-energy barriersto rotation for compound 1 are lower than those reported fornitroso-indolizines34 and lower at C7 (5.8 to 6.2 kcal/mol,depending of the rotamer downhill) than for the correspondingrotation at C5 (9.0 to 10.6 kcal/mol). This is not unexpectedbecause the two ester groups receive, in planar orientation,different effects from the proximity of the diazine ring. At C5, asyn conformation may be favored by a weak intramolecularCH···O interaction between the electron-rich carbonyl oxygenand the hydrogen atom from the arylic C4. In contrast, theproximity of the nitrogen atom, N1, to the ester group of C7induces a steric repulsion with either carbonyl or alkoxy oxygen,which may explain the little energy difference between the synand anti orientations at C7 and also the lower interconversionbarrier (as compared with C5). The same considerations holdfor the GS conformational preferences of 2, although the largebenzoyl group forbids a planar geometry at C7 in both syn andanti orientations and explains the lower barrier to rotation (3.7to 4.1 kcal/mol). Moreover, in both of the most stableconformers of 2, the phenyl group is further twisted withrespect to carbonyl at ∼25° and the exocyclic bond at C7 is∼10° out of the pyrrole ring.

Electronic Spectra and Excited-State Geometries.Ester-Substituted PP (1). Maximum absorption wavelengthsand oscillator strength values for the first three singlettransitions of the most stable syn-syn conformer of 1 (PCMTD-PBE0/6-31+G(d) GS geometry) are listed in Table 1.Computed absorption spectra for the other three conformers(omitted) show little absolute differences of <4 nm inmaximum absorption wavelength of the first band (<0.05 eVin transition energies) and even lower for the second transition.Hence, conformational isomerism is not expected to influencethe structure of the two bands around 350 and 300 nm.Results listed in Table 1 for the syn-syn conformer reveal a

clear correlation of the absorption wavelength for the first twocomputed transitions with the experimental counterparts.

Figure 1. Ground-state equilibrium geometries of the two most stableconformers of 1 (top) and 2 (bottom).



Despite the overall overestimated transition energies (under-estimated absorption wavelengths), the S0 → S1 and S0 → S2transition energies are separated by 0.63 to 0.68 eV incyclohexane, in excellent agreement with the experimentaldifference estimated to 0.6 to 0.7 eV.Excitation character for the first three singlet transitions of

the syn-syn conformer is depicted in Figure 2 by plots of

electron density differences between ground and excited state(Δρ = ρS0 − ρSi). The shapes of the corresponding molecularorbitals are available in the Supporting Information. The firsttwo excited states have ππ* character, S1 arising from an almostpure (95%) HOMO→LUMO excitation, and consists of anincrease in electron density on the pyridazine ring uponexcitation, whereas S2 has a mixed HOMO → LUMO+1 (67%)and HOMO-1→LUMO+1 (26%) orbital character. The latterinvolves a significant electron delocalization in excited stateover the exocyclic bond at C7. The third computed transition(not observed, forbidden by planar symmetry) has nπ*character and consists mainly of excitations from a lone-pair(HOMO-2) with large spatial extent, delocalized between thecarbonyl oxygen from C7 (largest contribution) and the N1nitrogen of the pyridazine ring. Excitation character of the firstpredicted n→π* transition is most likely related to the energygain induced in the syn-syn conformation on the carbonyl lone-pair from C7 (+0.35 eV as compared with the syn-anticonformer and 0.13 eV higher than the carbonyl lone-pair fromposition 5).Overestimated transition energies may be rationalized in

terms of Hartree−Fock (HF) tendency to overestimate theHOMO−LUMO gap. Increasing the amount of HF exchangefrom 20% (25%) in the case of B3LYP (PBE0) to 100% forM06-HF increases the error from +0.14 (0.25) to +0.89 eV,respectively. The largest error in predicting the first transition

energy is +1.17 eV in the case of CIS (not listed). Oscillatorstrength values show the same trend because they dependlinearly on the excitation energy. In effect, among the selectedfunctionals, B3LYP and PBE0 yield the best estimates oftransition energies. Similar consideration regarding theperformances of the two functionals also results from LR-PCM TD-DFT investigations of several related pyrrolopyr-idazines10 and pyridil-indolizines.35 The particular behavior ofthe M06-HF functional in predicting a different ordering of thesecond and third excited states (nπ* < ππ*) may also be noted.Overall, transition energies predicted by the first two hybrid

functionals, in conjunction with a moderate balanced basis setand using the SS-PCM solvation model, are in an acceptableerror of <0.20 eV in cyclohexane, slightly larger (0.21 and 0.23eV) in the case of PBE0 in polar solvents. This is a fairlyaccurate description, provided the strong vibrational couplingbetween the first two electronic states and the fact that we areactually neglecting the zero-point corrections. As shown inFigure 3, explicit inclusion of vibronic effects does not improvethe computed maximum absorption wavelength for the firstband. We note, however, that computation of Franck−Condonfactors in Cartesian coordinates could only recover a smallfraction of the S0 → S1 band (<10% as compared with >99% forS0 → S2), in effect to the nonplanar equilibrium geometry of S1(as shown in the following).36 That makes the reliability of thecorresponding band shape indeed questionable. Despite that,most of the features are well-reproduced even for the firsttransition, including the higher wavelength peak at 364 nm (in+0.16 eV error). In the case of the second band, the differencesin relative peak intensity (as compared with experiment) mayresult from either the low overlap with absorption bands at<270 nm or even the poor performances of PBE0 inreproducing the intensities.37

Excited state geometry optimizations of 1 following theprocedure described in the Computational Methods subsectionyield planar and twisted orientations of the two ester groupsfrom C5 and C7, respectively. In gas phase TD-PBE0/6-31+G(d) geometries, the group at C7 is twisted at 24.9° (syn)and 173.1° (anti) with respect to the planar PP, whereas theester group from C5 adopts a planar syn orientation in bothcases, consistent with electron delocalization over the exocyclicC5−C8 bond in the first excited state. TD-DFT resultsdiscussed in the following refer to the most stable (syn-syn)conformation of S1. Beside the twisted orientation at C7,significant changes in S1 geometry as compared to GS occur at

Table 1. SS-PCM TD-DFT/6-31+G(d)//PBE0/6-31+G(d) Absorption Wavelengths (λabs, nm) and Oscillator Strength Values( f) Computed for the First Three Singlet Electronic Transitions of Compound 1a

B3LYP PBE0 CAM-B3LYP BH&HLYP M06-HF exp.

solvent λabs f λabs f λabs f λabs f λabs f λabsmax

cyclohexane 341 0.07 332 0.07 315 0.10 305 0.11 282 0.13 350 (361b)285 0.07 277 0.08 267 0.10 261 0.08 264 294, 306c

287 271 244 231 250 0.14 not observedchloroform 338 0.07 329 0.08 312 0.10 302 0.11 278 0.13 348 (350b)

286 0.09 278 0.09 268 0.11 261 0.09 258 294, 308c

275 263 239 231 0.42 251 0.16 not observedacetonitrile 333 0.07 324 0.08 307 0.10 297 0.11 273 0.14 345

287 0.10 279 0.10 269 0.12 262 0.10 251 294, 306c

261 253 239 0.41 232 0.43 252 0.17 not observedaMissing values for the oscillator strengths denote n→π* forbidden transitions. Experimental maximum absorption wavelengths (λabs

max, nm) areincluded in the last column. bValues in parentheses are from ref 9. cMultiple values indicate the two most intense vibronic peaks, listed in decreasingintensity.

Figure 2. Electron density differences between ground and first threeexcited states of 1. Green (yellow) surfaces are drawn at the 0.001 auisolevel and represent excess electron density in the ground (excited)state.



the PP ring. N1−C2 and C3−C4 bonds are 0.057 and 0.049 Ålonger than the GS values of 1.310 and 1.370 Å, respectively,while C2−C3 (1.379 Å) and C4a-C5 (1.338 Å) are 0.038 and0.068 Å shorter in S1. Overall, except for the C6−C7 bondwhich is 0.015 Å shorter, bonds of pyrrole ring are slightlylarger in excited state, while for the diazine ring a bond-equalization is predicted to occur in S1, in agreement with thepreviously reported computations preformed on the unsub-stituted PP.4 These results show a clear correlation with theS0−S1 electron density differences, regarding in particular theshort-range charge transfer from the pyrrole ring (donor in GS)to pyridazine (acceptor).Maximum emission wavelengths, computed in the SS-PCM

solvation approach (in the equilibrium regime), and corre-sponding Stokes shifts are listed in Table 2. Experimental values(from current and previous work) are included for comparison.All of the employed functionals yield better estimates ofemission wavelengths, as compared with the absorptionspectrum. PBE0 (B3LYP)-computed emission energies are upto 0.04 (0.07) eV higher (lower) than the experimental values,whereas CAM-B3LYP (BH&HLYP) overestimates the samevalues with up to 0.24 (0.36) eV. M06-HF yields emission

energies overestimated by 0.59 to 0.63 eV depending on thesolvent polarity, comparable to the mean absolute error of 0.58eV reported for this functional in recent benchmarks.38 Thebathochromic shift of fluorescence emission in polar solvents,arising from the charge-transfer character of the first excitedstate, is consistently reproduced by all of the functionals. Stokesshifts are overestimated by PBE0 with 1314 (29%) to 1590cm−1 (28%), as a result of overestimated excitation energies.B3LYP yields slightly better (27−25%) estimates at the expenseof 9−7 nm overestimated maximum emission wavelengths.Results of LR-PCM, omitted for simplicity, give better overallestimates of Stokes shifts in polar solvents but predict insteadan opposite (negative) solvatochromism on fluorescenceemission.

Benzoyl-Substituted PP (2). Absorption Spectra. Assign-ment of bands in the absorption spectra of 2 based on theresults of TD-DFT calculations is much less straightforwardthat in case of 1. This is due to the significant solvent-inducedchanges in the shape of the first band, as shown in thefollowing, but also due to the effects of the conformation on thenature of the first two electronic transition, as predicted bycomputations.TD-DFT computations performed in SS-PCM solvation

using the PBE0 and B3LYP hybrids, proven to provide accuratedescription of transition energies in the previous section,foresee the first three electron transitions to occur in a narrow0.4 eV energy window for all of the conformers of 2.Notwithstanding the systematic overestimation, the long-range corrected CAM-B3LYP and the BH&HLYP functionalsyield similar results although different descriptions of theexcitation character for the first two transitions in the case ofthe latter, as shown in the following.Results of SS-PCM TD-DFT/6-31+G(d) computations for

the syn-syn and syn-anti conformers of 2 using B3LYP andPBE0 are listed in Table 3. The other two conformers haveessentially the same computed spectra with the conformershaving the same orientation of the benzoyl group.Unlike the four conformers of 1 predicted to show essentially

the same electronic spectrum, as mentioned in the previoussection, in the case of compound 2 the two differentorientations of the benzoyl group are found to induce adifferent picture regarding the first three excited states. Namely,computations predict almost identical electronic spectra for thesyn-syn and anti-syn conformers as well as within the syn-antiand anti-anti pair, respectively. The excitation character of thefirst three singlet transitions computed in cyclohexane for thesyn-syn and syn-anti conformers of 2 is depicted in Figure 4 byplots of electron density differences between ground andcorresponding excited state. In nonpolar cyclohexane (and ingas phase), the first excited state of the syn-syn conformer has a

Figure 3. Experimental absorption spectrum of 1 in cyclohexane (top)and computed at the TD-PBE0/6-31+G(d) level including SS-PCMcorrections and vibrational effects for the first two bands (bottom).

Table 2. S0 → S1 Absorption Wavelength (λabs, nm), S1 → S0 Emission Wavelength (λem, nm), and Stokes Shifts (Δν = νabs −νem, cm

−1) for Compound 1, Computed at the TD-DFT/6-31+G(d)//PBE0/6-31+G(d) Level and SS-PCM Solvation

cyclohexane chloroform acetonitrile

method λabs λem Δν λabs λem Δν λabs λem Δν

B3LYP 341 425 5781 338 431 6379 333 437 7127PBE0 332 412 5847 329 418 6507 324 424 7320CAM-B3LYP 315 385 5746 312 393 6632 307 401 7671BH&HLYP 305 372 5857 302 379 6769 297 387 7826exp. 350 416a 4533 348 423a 5095 345 430 5730

aFrom ref 9.



nπ* character (Sn) and involves electron excitation from a lonepair localized on the carbonyl oxygen to the antibondingcarbonyl π* orbital. The predicted negative solvatochromiceffect on the S0 → Sn transition is consistent with the expecteddestabilization induced by polar solvents on nπ* states.The next two singlet transitions of the syn-syn conformer

have π→π* CT character from the pyrrole to the diazine ring(S0 → S2) and from PP to benzoyl (S0 → S3).Consequent to the blue shift of the S0 → Sn transition with

increasing solvent polarity and in conjunction with the littlesolvent effect on the ππ* states, the first two excited statesbecome near-degenerate in chloroform and acetonitrile in the(SS-PCM) PBE0 and CAM-B3LYP description. In the SS-PCM picture, the Sn state is predicted by B3LYP 0.04 eV above(4 nm below) the first ππ* state (Sπ,1) in acetonitrile, whereasin LR-PCM the two states are already interchanged in less polarchloroform. Except for the state-switching predicted by polarsolvents, (LR-PCM) PBE0 and CAM-B3LYP predict the samesolvent effect on the S1 (Sn) and the S2 (Sπ,1) states, the firstthree vertical transition energies predicted by the latter

functional being 0.20 to 0.30 eV larger than in the case ofPBE0. In both chloroform and acetonitrile, the Sπ,1 − Sn energydifferences are ∼0.02 eV in the description of the latterfunctionals and the LR-PCM approach. Consequently, the twostates have mixed nπ*/ππ* character in polar solvents andcomparable oscillator strengths. In contrast with the descriptionof either PBE0 (B3LYP) or CAM-B3LYP functionals,BH&HLYP predicts almost identical spectra irrespective ofthe solvent polarity and in both solvation approaches. In fact,the first π→π* transition energy is estimated by this functionalin cyclohexane at 4.02 eV (309 nm), which is almost outsidethe range of the first absorption band, and 0.08 to 0.11 eVlower than the n→π* transition energy. On the other end,M06-HF results show an opposite trend, overestimating thefirst π→π* transition energy (4.26 eV, 291 nm in SS-PCMcyclohexane), as expected from the results discussed in theprevious section but predicting instead a low-lying n→π*transition (3.21 eV, 387 nm).The first transition of the syn-anti conformer involves

essentially a π→π* excitation taking place in the PP ring(Figure 4). Despite the partial nπ* character, also evidenced toa greater extent in the next two excited states, electron densitychanges in the PP rings in the S0 → S1 and S0 → Sπ,1 transitionsof the syn-anti and syn-syn conformers, respectively, areessentially the same. A similar observation arises whencomparing the electron density changes in S2 and Sn states ofthe two conformations, except for the larger contribution fromthe N1 lone pair in the latter. All of these results suggest thatboth the solvent, as shown in the previous, and the twisting ofthe benzoyl group modulate the order of the first two excitedstates of 2. Namely, we expect a nπ*/ππ* crossing to occur inGS geometry at an intermediate twisting of the benzoyl group,influenced by the solvent polarity.TD-DFT results for the syn-anti conformer listed in Table 3

predict the S0 → S1 (mostly π→π*) transition at roughly 0.10to 0.20 eV below the first (n→π*) computed transition of thesyn-syn conformer, namely, red-shifted with 12 to 22 nm. Inaddition to the estimated ratio of the syn-anti in solution, weexpect the effect of the first transition on the absorption spectraof 2 to be more effective in cyclohexane (x = 0.29) than inpolar acetonitrile (x = 0.07), superimposing in the formersolvent in the same range of the predicted n→π* transition forthe syn-syn conformer. It is noteworthy that in the case of thesyn-syn conformer, the proximity of the dark Sn and the bright

Table 3. SS PCM TD-DFT/6-31+G(d)//PBE0/6-31+G(d) Absorption Wavelength (λabs, nm) and Oscillator Strength Values( f) Computed for the First Three Singlet Transitions of the syn-syn and syn-anti Conformers of 2a

B3LYP PBE0

syn-syn syn-anti syn-syn syn-anti exp.

solvent λabs f λabs f λabs f λabs f λabsmax Arel

cyclohexane 350 0.000 362 0.094 344 0.001 351 0.087 363 0.49345 0.127 339 0.010 336 0.130 332 0.013 332 0.27322 0.174 332 0.100 311 0.191 321 0.144 321 1.00

chloroform 346 0.001 365 0.123 340 0.001 352 0.118 369 0.25344 0.167 335 0.014 334 0.156 329 0.012 336 0.12322 0.174 330 0.022 312 0.190 322 0.121 332 1.00

acetonitrile 342 0.193 365 0.139 336 0.006 352 0.138 370 0.19338 0.001 332 0.087 328 0.173 326 0.010 333 0.11324 0.151 327 0.009 312 0.194 321 0.112 330 1.00

aExperimental maximum absorption wavelengths (λabsmax, nm) and normalized peak absorbance (Arel) relative to the most intense peak as results from

fitting of spectra are listed in the last columns.

Figure 4. Electron density differences between ground and first threesinglet excited states at ground-state geometry of the syn-syn (left) andsyn-anti (right) conformers of 2. Excess electron density surfaces inground and excited states are drawn in green (dark) and yellow (light)at 0.001 au isodensity level. Labels represent PBE0 excitation energiescomputed in SS-PCM cyclohexane.



Sπ,1 states (at most 0.09 eV difference in the gas phase) mayresult in an increase in intensity for the former either by avibrational coupling or via a mixing of their excitation character.Hence, the higher wavelength band evidenced in the absorptionspectra recorded in cyclohexane may result from a mixedcontribution of the first electron transition of the syn-syn andsyn-anti conformers. The blue shift of the n→π* transition ofsyn-syn in polar solvents and also the lower ratio of the syn-anticonformer are both consistent with the almost featureless shapeof the first absorption band, as shown in Figure 5.Figure 5 illustrates the computed solvent effects on the

absorption spectra of 2. Up to 10 transitions for the syn-synand syn-anti conformers were computed at the LR-PCM TD-PBE0/6-31+G(d) level. Then, a Gaussian shape function with aconstant full width at half-maximum of 0.25 eV was used toconvolute the stick spectra. For simplicity, band shape wasconstructed using only the spectra of syn-syn and syn-anticonformers, whereas in conformational averaging the ratio ofthe anti-syn and anti-anti counterparts was also added in thescaling factor of the two spectra, respectively. As shown in theFigure, results of TD-DFT computations are able to reproduce,to some extent, the solvent-induced changes in band shape.Evident discrepancies are likely to result in both solvents fromoverestimated transition energies associated with the combinedperformances of PBE0 and LR-PCM.As already mentioned, the picture regarding the order of the

first two states of the syn-syn conformer in polar solvents is notstrictly consistent among the functionals employed. This is alsothe case with respect to the solvation model. In this aspect,additional experimental and computational investigations arerequired prior to formulating an accurate and conciseconclusion regarding the nature of the first two excited states.On the computational side, convergence tests performed in theTD-PBE0 framework with various Pople basis sets withincreasing complexity show little variation in the energy ofthe first three excited states. At the TD-PBE0/6-311++G(2d,p)level, the relative energies of the first two ππ* states withrespect to GS are 0.03 to 0.05 eV lower than their TD-PBE0/6-31+G(d) counterparts. In contrast, increasing the basis-setcomplexity only affects the relative energy of the Sn state by−0.01 eV. In effect, larger basis sets systematically predict the Snstate at 0.01 to 0.02 eV above Sπ,1 in polar solvents, whereas thenπ* nature of the first excited state in cyclohexane remainsessentially unchanged, regardless of the basis set. Also, careful

examination of the effects of the solvent model (i.e., solutecavity or solvation model) may provide additional insightsregarding the order of the first two states in solution.39

Excited-State Geometry and Fluorescence. The low-lyingSn state of compound 2 in the major syn-syn conformationprovides a feasible explanation for the previously reported weakfluorescence of benzoyl-PP.9 In the following, we focus on theradiative state of 2, reporting two important experimentalobservations that complement our TD-DFT approach. First,fluorescence emission of compound 2 exhibits a significantpositive solvatochromism when passing from cyclohexane (λem

max

= 427 nm, 2.90 eV) to acetonitrile (λemmax = 489 nm, 2.53 eV),

suggesting a large CT character. Second, compound 2 showspractically no fluorescence emission following excitation near orbelow the absorption maxima at 320−330 nm, correspondingto transitions computed for the syn-syn conformer. Instead,excitation spectra show maxima located at λex

max = 374 nm incyclohexane, 370 nm in chloroform, and 361 nm in acetonitrile.Moreover, no dual-fluorescence feature is observable at roomtemperature and maximum emission wavelengths are the same,regardless of the excitation wavelength. These results, in clearcorrelation with the maxima of the absorption band resolved athigher wavelengths, (i) confirm the origin of the largeabsorption band as arising from multiple electronic transitionsand (ii) forbid the assignment of emission as arising fromhigher excited states.Excited-state geometry optimizations performed at the TD-

PBE0/6-31+G(d) level starting from GS geometries of the syn-syn (nπ* character for the first excited state) and syn-anti(ππ*) conformers yield two energy minima represented inFigure 6a. Atomic coordinates for both states are available inTables S12 and S13 of the Supporting Information. TD-CAM-B3LYP yields almost identical geometries and the sameexcitation character, whereas BH&HLYP and CIS could onlylocate the ππ* (twisted) minimum. M06-HF predicts the samenπ* character for both minima, in agreement with the low-lyingnπ* state predicted in the absorption spectra of bothconformers.Electron density differences between ground and excited

state, computed using the PBE0 functional at the correspond-ing excited-state geometries, are plotted in Figure 6b. Thealmost planar geometry (φ1 = 4.5°) corresponds to the Sn stateand consists of a local (dez)excitation (LE) involving mainlythe keto group from benzoyl, whereas the Sπ relaxes into a

Figure 5. Calculated versus experimental absorption spectra of 2 in cyclohexane and acetonitrile. LR-PCM TD-PBE0/6-31+G(d) stick spectra arerepresented as vertical lines. Superimposed are the Gaussian convoluted spectra using a constant 0.25 eV full width at half-maximum. Spectra for thesyn-syn and syn-anti conformers were scaled by their computed ratio in solution. Arrows indicate n→π* transitions.



twisted intramolecular charge transfer (TICT) state in whichthe benzoyl group is oriented at φ1 = 83.5° with respect to PP.Analysis of electron density differences (computed in gas-phase) by means of the charge-transfer index proposedrecently40 reveals that dezexcitation from the TICT statecorresponds to qCT = 0.88 fraction of electron chargetransferred at DCT = 3.82 Å from the benzoyl moiety (donorin excited state) to the pyrrole ring of PP (acceptor). Excesselectron density centroids as defined in ref 40 are plotted inFigure 6c. As computed from the spatial extent of the twocentroids, the H index (2.37 Å) defined as the average spread ofthe two centroids is 1.52 Å lower than the charge-transferindex, DCT. This denotes little spatial extent of excess densitiesalong the direction of CT and results in a small degree ofoverlap between the two centroids (0.22 if normalized tounity). The small overlap, with a similar physical meaning withthe Λ criterion proposed by Tozer,41 is a prerequisite for poorperformances of global hybrid functionals in predicting CTexcitations.40,42,43 In effect, severe underestimation of transitionenergies are expected from B3LYP and PBE0 functionals in thecase of the Sπ (CT) state of 2. Hybrids or meta-hybrids withlarger amounts of HF exchange such as BH&HLYP (50% HF)

and M06-HF (100% HF) and also long-range correctedfunctionals such as CAM-B3LYP (19 to 65% HF) are expectedto be less affected and to predict fewer spurious CT states. Onthe other end, CIS has a systematic bias against CT states, with>1 eV overestimation in vertical excitation energies.44

Furthermore, large amplitude displacement of benzoyl groupbetween CT and GS geometries precludes a reliable vibrationalcomputation to be performed in this case.Computed (LR-PCM) transition energies and wavelengths

for emission from the LE and CT states are listed in Table 4.Oscillator strengths (omitted) are essentially zero for theemission from the LE state and negligibly larger (but <0.01) forthe CT emission, arising from the small orbital overlap. Exceptfor the BH&HLYP functional and the CIS, which both fail tolocate the LE minimum (but predict the same nπ* character),data listed correspond to transitions computed at thegeometries optimized using the same level of theory. Asalready mentioned, the nπ* nature of the first excited statepredicted by M06-HF in both geometries could be anticipatedfrom the computed absorption spectrum. CIS yields twistedCT geometries regardless of the starting geometry, which isparticularly interesting given the above consideration.Data listed in Table 4 expose two important aspects

regarding both the TD-DFT approach and the solvationmodel. First, conventional global hybrid PBE0 yield under-estimated (up to 1 eV lower) transition energies as anticipatedfor the CT state having a small overlap of excess electrondensity. B3LYP functional, omitted in calculations, is expectedto provide even lower transition energies. Second, even in theLR-PCM approach CAM-B3LYP and BH&HLYP givesurprisingly better estimates of transition energies in polarsolvents (+0.01 and +0.19 eV) and −0.30 and −0.57 eV errorin cyclohexane. The failure in predicting solvatochromism ofthe emission process is not unexpected, as we recall that in LR-PCM the solvent degrees of freedom are always equilibratedwith GS electron density. In contrast, SS-PCM transitionenergies, despite reproducing the expected solvatochromism (inthe equilibrium SS limit), are severely underestimated even inthe case of CAM-B3LYP (BH&HLYP) by 0.8 (0.7) eV incyclohexane up to 1.3 (1.2) eV in acetonitrile. We assume thatsuch unexpected discrepancies, which make SS-PCM qual-itatively inadequate in this case, arise from the TD-DFTdeficiency in treating CT excitations, augmented by mutualequilibration of the excited-state electron density with thesolvent degrees of freedom. During the iterative proceduretaking place in the SS step, the electron density in the CTexcited state is allowed to evolve toward lesser or no overlap ofexcess electron densities that correspond to a so-called “trough-

Figure 6. (a) Optimized geometries of the Sn (left) and Sπ,1 (right)excited states of 2, (b) electron density differences between groundand first excited states, and (c) centroids of excess electron density inground and excited stats. Green (yellow) isosurfaces are drawn at±0.001 au level and represent excess electron density in ground(excited) state.

Table 4. Vertical Dezexcitation Energies (ΔE, eV) and Emission Wavelengths (λem, nm) Computed at the LR-PCM TD-DFT/6-31+G(d) Level for LE (Sn) and CT (Sπ) States

a

PBE0 CAM-B3LYP BH&HLYP M06-HFc CIS exp.

solvent state ΔE λem ΔE λem ΔE λem ΔE λem ΔE λem ΔE λemmax

cyclohexane CT 2.00 619 2.43 510 2.60 477 2.17 572 3.58 346 2.90 427LE 2.45 506 2.71 458 2.97b 418 2.46 504 4.09b 303

chloroform CT 2.10 590 2.49 497 2.67 464 2.29 540 3.70 335 2.72 456LE 2.45 505 2.71 457 3.03b 410 2.51 493 4.15b 299

acetonitrile CT 2.17 571 2.54 488 2.72 456 2.40 517 3.79 327 2.53 489LE 2.46 503 2.73 454 3.05b 406 2.57 483 4.21b 294

aExperimental maximum emission wavelengths (λemmax, nm) and the corresponding transition energies are included for comparison. bComputed at

PCM TD-PBE0/6-31+G(d) geometries. cM06-HF foresees nπ* character in both geometries.



space” CT. This assumption, however, needs further inves-tigations on well-established cases. Also, in nonequilibrium SS(excited-state density equilibrated with “fast” degrees of solventpolarization), computations foresee an opposite (blue shift)effect of solvent polarity on emission. This could result fromthe strong CT character of S1 and most likely relates to solventreorganization energy, λ, which is the difference between theequilibrium and nonequilibrium SS-PCM. In acetonitrile, λ ≈ 1eV, whereas the same difference vanishes in cyclohexane.To account for conformational effects, we monitored up to

four excited states at the LR-PCM TD-DFT/6-31+G(d) levelon the linear transit path between the TD-PBE0 optimized LEand CT geometries. Each internal coordinate was linearlyinterpolated between the two excited-state geometries to give atotal number of 21 points, including the optimized ones.Nevertheless, one should note that this approach suffers fromthe monodeterminantal nature of the DFT formalism and alsofrom the increase in underestimation of excitation energy withthe increased benzoyl torsion angle (toward CT minimum).Excited-state energy profiles (twisting curves) along the

linear interpolated path between LE and CT states, ascomputed at the LR-PCM PBE0/6-31+G(d) level in cyclo-hexane, are plotted in Figure 7 against the benzoyl torsionangle, ϕ1. In areas where states are well-separated, we label thestates as nπ* or ππ* based on the major excitation character.The scans recover the avoided crossing feature near GS syn-syngeometry, suggested by the computed of absorption spectra forthe syn-syn and syn-anti conformers. All energy curves haveessentially the same shapes in all three solvents with anadiabatic energy gap between the first two excited states of 0.32eV in cyclohexane, 0.26 eV in chloroform, and 0.21 eV inacetonitrile. At the geometry corresponding to the minimumenergy gap (φ1 = 32°), the first two excited states have mixednπ* and ππ* character, in comparable ratios. One may alsonotice that nπ* and ππ* states show distinct (opposite) trendsalong the path. The former are stabilized in nearly planargeometries consequent to the energy gain of the lone-pairorbital, which appears to be the HOMO in LE geometry. Theenergy of ππ* state, including the expected TD-PBE0 biastoward CT, decreases with the twisting of benzoyl group. CIStwisting curves, also shown in Figure 7b, reveal a more flatsurface for the nπ* state, which may explain why CIS fails tolocate the LE geometry but provides a similar picture regardingthe first two excited states.

A schematic energy diagram of conformational effects onground and the first two excited states of 2 is represented inFigure 7c. Given the underestimated energy in the CT state andalso the poor performances of our PCM approach inreproducing solvent effects, we limit the following discussionwithin the range of consistent (yet rather qualitative)description among the methods employed. According to ourtwo-state model, vertical absorption from the syn-anticonformation to the first electronic excited state (ππ*) isleading directly to TICT. Hence, low-fluorescence intensity ofcompound 2 in general is consistent with the small orbitaloverlap associated with the TICT state, whereas the effects ofsolvent polarity on the intensity of fluorescence emissionshould depend on the ratio of the syn-anti conformer insolution. Our thermochemical estimates of conformationalpreferences predicting a lower amount of syn-anti con former inpolar acetonitrile, are indeed consistent with the lowerfluorescence emission intensity in this solvent. Verticalexcitation from the major (syn-syn) conformation to either S1(nπ*) or S2 (ππ*) leads the system in the dark LE state (nπ*)corresponding to a nearly planar orientation of the carbonylfrom C7 with respect to PP. Within the limits of ourmonodeterminantal approach, one may rather speculate onthe location of a Sn/Sπ crossing along the twisting coordinate.However, provided the proximity of the first two excited statesnear syn-syn GS geometry, a population transfer from thebright S2 (ππ*) to the dark S1 (nπ*) state is likely to occur byinternal conversion. Similar consideration has been maderecently in the case of low-fluorescent pyrene carbonyls ascompared with their highly fluorescent ester derivatives.45

■ CONCLUSIONS AND OUTLOOK

In this article we have approached the fluorescence emissionproperties of two pyrrolopyridazine derivatives by TD-DFTcomputations including solvent effects and steady-state electronspectroscopy in solution, using solvents of different polarities.In TD-DFT, five density functionals were employed and theirperformances were assessed by comparing the computedvertical absorption and emission energies with the experimentalcounterparts. We show that solvent-dependent photophysicalproperties of the highly fluorescent ester-substituted pyrrolo-pyridazine may be accurately predicted by TD-DFT in SS-PCMsolvation (within 0.10 to 0.25 eV error for the first electrontransition) when hybrid functionals such as B3LYP or PBE0 are

Figure 7. Evolution of ground and first four excited states energy along the linear interpolated path (in internal coordinates) between optimized Snand Sπ geometries at the (a) PBE0 and TD-PBE0 and (b) RHF and CIS levels in LR-PCM chloroform. (c) Schematic representation ofconformational effects on the first two excited states.



employed, in conjunction with a moderate basis set. Also, CTcharacter evidenced herein for the first transition in fluorescentpyrrolopyridazines should serve as a basis in further ration-alization of substituent effects. Computations also suggest thatthe ester group from position 7 on the pyrrole ring may induce,in an orientation of the carbonyl group facing the diazinenitrogen, a significant stabilization of the first nπ* state. Such aneffect does not appear to influence the fluorescence of the esterderivative, given the lower (fluorescent) ππ* state predicted forall conformations. In contrast, for the most stable conformationof the benzoyl-substituted derivative, we found that the samenπ* state is predicted lower in energy than the first ππ* state,consistent with the experimentally observed weak fluorescence.Following a thorough computational investigation of theconformational effects on the lowest excited states, we showthat fluorescence emission (with significant CT feature) mayarise upon excitation from a minor conformation of benzoyl-pyrrolopyridazine, whereas the major conformer is trapped intoa nπ* nonfluorescent state. These results add theoreticalsupport to a conclusion from previous experimental studies9

regarding the particular role of the substituent from position 7on the photophysics of pyrrolopyridazines. Results of furtherinvestigations, both computational and experimental (such astime-resolved approaches), on pyrrolopyridazine carbonyls areparticularly interesting, as should allow a quantitative treatmentof dezexcitation pathways. Several approaches are currentlybeing considered by our group. Furthermore, we are confidentthat results reported in this paper would suggest an explicitconsideration of conformational isomerism in organic fluo-rophores bearing carbonyls, esters and other lone-paircontaining groups, with the primary goal toward a rationaldesign of novel efficient fluorescent compounds.

■ ASSOCIATED CONTENT

*S Supporting InformationComputational results: optimized atomic coordinates for thesyn-syn and syn-anti conformers of compounds 1 and 2, resultsof thermochemical estimates of conformational preferences inground state, frontier molecular orbitals for PP, 1, and 2, andoptimized excited state geometries. Experimental: recordedabsorption spectra for 1 and 2 and fluorescence excitation andemission spectra for 2. This material is available free of chargevia the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATION

Corresponding Author*E-mail: [email protected]. Phone: +40 232 201343. Fax: +40232 201313.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

Work supported from the research grant PCCE-9/2010CNCSIS (Romania). Computational resources provided bythe Integrated Platform for Advanced Studies in MolecularNanotechnologies (AMON) from Alexandru Ioan CuzaUniversity of Iasi (Romania). D.M. is grateful to CarloAdamo (ENSCP, Paris) for short yet fruitful discussionregarding the DCT index.

■ REFERENCES(1) Cheng, Y.; Ma, B.; Wudl, F. Synthesis and Optical Properties of aSeries of Pyrrolopyridazine Derivatives: Deep Blue OrganicLuminophors for Electroluminescent Devices. J. Mater. Chem. 1999,9, 2183−2188.(2) Becuwe, M.; Landy, D.; Delattre, F.; Cazier, F.; Fourmentin, S.Fluorescent Indolizine-b-Cyclodextrin Derivatives for the Detection ofVolatile Organic Compounds. Sensors 2008, 8, 3689−3705.(3) Rotaru, A.; Druta, I.; Avram, E.; Danac, R. Synthesis andProperties of Fluorescent 1,3-Substituted Mono and Biindolizines.Arkivoc 2009, 13, 287−299.(4) Mitsumori, T.; Bendikov, M.; Sedo, J.; Wudl, F. Synthesis andProperties of Novel Highly Fluorescent Pyrrolopyridazine Derivatives.Chem. Mater. 2003, 15, 3759−3768.(5) Mitsumori, T.; Bendikov, M.; Dautel, O.; Wudl, F.; Shioya, T.;Sato, H.; Sato, Y. Synthesis and Properties of Highly FluorescentIndolizino[3,4,5-ab]isoindoles. J. Am. Chem. Soc. 2004, 126, 16793−16803.(6) Mitsumori, T.; Craig, I. M.; Martini, I. B.; Schwartz, B. J.; Wudl,F. Synthesis and Color Tuning Properties of Blue Highly FluorescentVinyl Polymers Containing a Pendant Pyrrolopyridazine. Macro-molecules 2005, 38, 4698−4704.(7) Tisler, M. Structure and Reactivity Correlation of Bicyclic 10-πElectron Systems With Bridgehead Nitrogen. Pure Appl. Chem. 1980,52, 1611−1621.(8) Swamy, K.; Park, M. S.; Han, S. J.; Kim, S. K.; Kim, J. H.; Lee, C.;Bang, H.; Kim, Y.; Kim, S.-J.; Yoon, J. New PyrrolopyridazineDerivatives as Blue Organic Luminophors. Tetrahedron 2005, 61,10227−10234.(9) Zbancioc, G. N.; Huhn, T.; Groth, U.; Deleanu, C.; Mangalagiu,I. I. Pyrrolodiazine Derivatives as Blue Organic Luminophores:Synthesis and Properties. Part 3. Tetrahedron 2010, 66, 4298−4306.(10) Vasilescu, M.; Bandula, R.; Cramariuc, O.; Hukka, T.;Lemmetyinen, H.; Rantala, T. T.; Dumitrascu, F. Optical Spectro-scopic Characteristics and TD-DFT Calculations of New Pyrrolo[1,2-b]pyridazine Derivatives. J. Photochem. Photobiol., A 2008, 194, 308−317.(11) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci,B.; Petersson, H.; et al. Gaussian 09, revision B.01; Gaussian, Inc.:Wallingford, CT, 2009.(12) Adamo, C.; Barone, V. Toward Reliable Density FunctionalMethods Without Adjustable Parameters: The PBE0Model. J. Chem.Phys. 1999, 110, 6158−6170.(13) Jacquemin, D.; Perpete, E. A.; Scalmani, G.; Frisch, M. J.;Ciofini, I.; Adamo, C. Fluorescence of 1,8-Naphthalimide: A PCM-TD-DFT Investigation. Chem. Phys. Lett. 2007, 448, 3−6.(14) Jacquemin, D.; Wathelet, V.; Perpete, E. A.; Adamo, C.Extensive TD-DFT Benchmark: Singlet-Excited States of OrganicMolecules. J. Chem. Theory Comput. 2009, 5, 2420−2435.(15) Jacquemin, D.; Perpete, E. A.; Scalmani, G.; Ciofini, I.; Peltier,C.; Adamo, C. Absorption and Emission Spectra of 1,8-NaphthalimideFluorophores: A PCM-TD-DFT Investigation. Chem. Phys. 2010, 372,61−66.(16) Scalmani, G.; Frisch, M. J. Continuous Surface ChargePolarizable Continuum Models of Solvation. I. General Formalism. J.Chem. Phys. 2010, 132, 114110−114124.(17) Improta, R.; Barone, V.; Scalmani, G.; Frisch, M. J. A State-Specific Polarizable Continuum Model Time Dependent DensityFunctional Theory Method for Excited State Calculations in Solution.J. Chem. Phys. 2006, 125, 054103−054111.(18) Improta, R.; Scalmani, G.; Frisch, M. J.; Barone, V. TowardEffective and Reliable Fluorescence Energies in Solution by a NewState Specific Polarizable Continuum Model Time Dependent DensityFunctional Theory Approach. J. Chem. Phys. 2007, 127, 074504−074512.(19) Cammi, R.; Corni, S.; Mennucci, B.; Tomasi, J. ElectronicExcitation Energies of Molecules in Solution: State Specific and Linear



http://pubs.acs.org

mailto:[email protected]

Response Methods For Nonequilibrium Continuum SolvationModels. J. Chem. Phys. 2005, 122, 104513−104524.(20) Corni, S.; Cammi, R.; Mennucci, B.; Tomasi, J. ElectronicExcitation Energies of Molecules in Solution Within ContinuumSolvation Models: Investigating the Discrepancy Between State-Specific and Linear-Response Methods. J. Chem. Phys. 2005, 123,134512−134521.(21) Santoro, F.; Improta, R.; Lami, A.; Bloino, J.; Barone, V.Effective Method to Compute Franck-Condon Integrals for OpticalSpectra of Large Molecules in Solution. J. Chem. Phys. 2007, 126,084509−084521.(22) Santoro, F.; Improta, R.; Lami, A.; Bloino, J.; Barone, V.Erratum: “Effective Method to Compute Franck-Condon Integrals forOptical Spectra of Large Molecules in Solution” [J. Chem. Phys. 126,084509 (2007)]. J. Chem. Phys. 2007, 126, 169903−169903.(23) Santoro, F. FCclasses: A Fortran 77 Code, 2008. http://village.pi.iccom.cnr.it (accessed Jan 19, 2013).(24) Santoro, F.; Lami, A.; Improta, R.; Barone, V. Effective Methodto Compute Vibrationally Resolved Optical Spectra of LargeMolecules at Finite Temperature in the Gas Phase and in Solution.J. Chem. Phys. 2007, 126, 184102−184112.(25) Dreuw, A.; Weisman, J. L.; Head-Gordon, M. Long-RangeCharge-Transfer Excited States in Time-Dependent Density Func-tional Theory Require Non-Local Exchange. J. Chem. Phys. 2003, 119,2943−2946.(26) Yanai, T. A New Hybrid Exchange-Correlation FunctionalUsing the Coulomb-Attenuating Method (CAM-B3LYP). Chem. Phys.Lett. 2004, 393, 51−57.(27) Aittala, P. J.; Cramariuc, O.; Hukka, T. I.; Vasilescu, M.;Bandula, R.; Lemmetyinen, H. A TDDFT Study of the FluorescenceProperties of Three Alkoxypyridylindolizine Derivatives. J. Phys. Chem.A 2010, 114, 7094−7101.(28) Zhao, Y.; Truhlar, D. G. The M06 Suite of Density Functionalsfor Main Group Thermochemistry, Thermochemical Kinetics, Non-covalent Interactions, Excited States, and Transition Elements: TwoNew Functionals and Systematic Testing of Four M06-ClassFunctionals and 12 Other Functionals. Theor. Chem. Acc. 2008, 120,215−241.(29) Jacquemin, D.; Perpete, E. A.; Ciofini, I.; Adamo, C.; Valero, R.;Zhao, Y.; Truhlar, D. G. On the Performances of the M06 Family ofDensity Functionals for Electronic Excitation Energies. J. Chem. TheoryComput. 2010, 6, 2071−2085.(30) Guido, C. A.; Mennucci, B.; Jacquemin, D.; Adamo, C. Planarvs. Twisted Intramolecular Charge Transfer Mechanism in Nile Red:New Hints From Theory. Phys. Chem. Chem. Phys. 2010, 12, 8016−8023.(31) Plotner, J.; Tozer, D. J.; Dreuw, A. Dependence of Excited StatePotential Energy Surfaces on the Spatial Overlap of the Kohn−ShamOrbitals and the Amount of Nonlocal Hartree−Fock Exchange inTime-Dependent Density Functional Theory. J. Chem. Theory Comput.2010, 6, 2315−2324.(32) Gu, W.-J.; Zhuang, J.; Jiang, Y.-L.; Wang, B.-X. DiethylIndolizine-1,3-dicarboxylate. Acta Crystallogr., Sect. E: Struct. Rep.Online 2010, 67, o123−o123.(33) Gu, W.-J.; Zhuang, J.; Jiang, Y.-L.; Wang, B.-X. Ethyl 3-Benzoyl-indolizine-1-carboxyl-ate. Acta Crystallogr., Sect. E: Struct. Rep. Online2011, 67, o3033−o3033.(34) Ghiviriga, I.; El-Gendy, B. E.-D. M.; Martinez, H.; Fedoseyenko,D.; Metais, E. P.; Fadli, A.; Katritzky, A. R. Conformational Equilibriaand Barriers to Rotation in Some Novel Nitroso Derivatives ofIndolizines and 3- and 5-Azaindolizines - an NMR and MolecularModeling Study. Org. Biomol. Chem. 2010, 8, 3518−3527.(35) Aittala, P. J.; Cramariuc, O.; Vasilescu, M.; Bandula, R.; Hukka,T. I. Effect of Substituents on the Absorption Properties of ThreePyridylindolizine Derivatives: A DFT and TDDFT Study. Chem. Phys.2009, 360, 162−170.(36) Up to 1010 Franck−Condon integrals were computed for eachclass of vibronic transitions comprising the first absorption band,whereas only 108 were needed to achieve convergence in the case of

the second band. Vibronic stick spectra have been convoluted usingGaussian functions with 0.03 eV fwhm.(37) Jacquemin, D.; Peltier, C.; Ciofini, I. On the Absorption Spectraof Recently Synthesized Carbonyl Dyes: TD-DFT Insights. J. Phys.Chem. A 2010, 114, 9579−9582.(38) Leang, S. S.; Zahariev, F.; Gordon, M. S. Benchmarking thePerformance of Time-Dependent Density Functional Methods. J.Chem. Phys. 2012, 136, 104101.(39) Improta, R.; Barone, V. PCM/TD-DFT Study of the TwoLowest Excited States of Uracil Derivatives in Solution: The Effect ofthe Functional and of the Cavity Model. J. Mol. Struct.: THEOCHEM2009, 914, 87−93.(40) Le Bahers, T.; Adamo, C.; Ciofini, I. A Qualitative Index ofSpatial Extent in Charge-Transfer Excitations. J. Chem. Theory Comput.2011, 7, 2498−2506.(41) Peach, M. J. G.; Benfield, P.; Helgaker, T.; Tozer, D. J.Excitation Energies in Density Functional Theory: an Evaluation and aDiagnostic Test. J. Chem. Phys. 2008, 128, 044118.(42) Dreuw, A.; Head-Gordon, M. Failure of Time-DependentDensity Functional Theory for Long-Range Charge-Transfer ExcitedStates: the Zincbacteriochlorin-Bacteriochlorin and Bacteriochloro-phyll-Spheroidene Complexes. J. Am. Chem. Soc. 2004, 126, 4007−4016.(43) Magyar, R. J.; Tretiak, S. Dependence of Spurious Charge-Transfer Excited States on Orbital Exchange in TDDFT: LargeMolecules and Clusters. J. Chem. Theory Comput. 2007, 3, 976−987.(44) Subotnik, J. E. Communication: Configuration InteractionSingles Has a Large Systematic Bias Against Charge-Transfer States. J.Chem. Phys. 2011, 135, 071104.(45) Niko, Y.; Hiroshige, Y.; Kawauchi, S.; Konishi, G.-i.Fundamental Photoluminescence Properties of Pyrene CarbonylCompounds Through Absolute Fluorescence Quantum YieldMeasurement and Density Functional Theory. Tetrahedron 2012, 68,6177−6185.



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Conformational Effects on the Lowest Excited States of Benzoyl-Pyrrolopyridazine: Insights from PCM Time-Dependent DFT