Solvent Effect and Two-Photon Optical Properties ofTriphenylamine-Based Donor−Acceptor FluorophoresYilin Zhang,† Meijuan Jiang,‡ Guang-Chao Han,§ Ke Zhao,∥ Ben Zhong Tang,‡ and Kam Sing Wong*,†

†Department of Physics, The Hong Kong University of Science and Technology Hong Kong, 999077, China‡Department of Chemistry, Hong Kong Branch of Chinese National Engineering Research Center for Tissue Restoration andReconstruction, HKUST Jockey Club Institute for Advanced Study, Institute of Molecular Functional Materials, Division ofBiomedical Engineering, State Key Laboratory of Molecular Neuroscience, Division of Life Science, The Hong Kong University ofScience and Technology, Clear Water Bay, Kowloon, Hong Kong, 999077, China§Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids, Institute of Chemistry, ChineseAcademy of Sciences, Beijing, 100190, China∥College of Physics and Electronics, Shandong Normal University, Jinan, 250014, China

*S Supporting Information

ABSTRACT: In this work we present a systematic investigation on the optical properties of twotriphenylamine (TPA)-based donor−acceptor fluorophores: TPA-PA (phenylaldehyde) and TPA-BMO ((Z)-4-benzylidene-2-methyloxazol-5(4H)-one). The two compounds are dissolved in ninedifferent organic solvents as dilute solutions in order to analyze the effect of solvent on their linearand nonlinear optical properties. For each compound under one-photon excitation, its fluorescenceemission spectrum red-shifts more than 160 nm as the solvent polarity increases from hexane toMeCN, while the fluorescence quantum efficiency and lifetime reach maximum magnitudes insolvents with medium polarity. The quantum efficiency reaches as high as 0.72 in dioxane for TPA-PA and 0.69 in Et2O for TPA-BMO, respectively. These TPA-PA and TPA-BMO solutions are alsostrongly emissive upon appropriate two photon excitation, with fluorescence emission spectraidentical to those under corresponding one-photon excitation. The maximum two-photon absorption cross sections are ∼160GM (Goeppert-Mayer units) and 250 GM for TPA-PA and TPA-BMO, respectively, regardless of the solvent identity.Particularly, for TPA-BMO solutions in strongly polar solvents, dual fluorescence peaks are observed in steady state, and distinctrelaxation dynamics are detected in fluorescence decays for the two emission peaks. These dual fluorescence emission spectra anddynamics could be interpreted as signs of charge-transfer state formation.

1. INTRODUCTION

Two-photon fluorescence microscopy is gaining popularity, asits selectivity of excitation within the focus leads to enhance-ment in the excitation penetration depth, reduction of the out-of-focus noise and bleaching, and improvement in 3Dresolution.1,2 Together with other applications such as two-photon lithography3 and two-photon photodynamic therapy,4

these two-photon applications generate the need for novelchromophores with strong two-photon absorption (2PA).Several strategies have been proposed to design new organicmolecules with large 2PA cross sections.3,5,6 Among the generalstrategy to enhance the charge transfer upon excitation, thedonor (D)−π−acceptor (A) type of structure facilitates theprediction of the optimized two-photon excitation wavelengthfrom ground-state absorption spectrum, since its noncentro-symmetric motif relaxes the parity forbidden rule of the 2PAprocess.7

With the D−π−A structure motif, TPA (triphenylamine)-PA(phenyl aldehyde) and TPA-BMO ((Z)-4-benzylidene-2-methyloxazol-5(4H)-one) (Figure 1) are designed andsynthesized, in which the TPA moiety serves as an electron-donating group, whereas PA and BMO are acceptors. To

investigate their optical properties in dilute solutions, the twocompounds are dissolved in nine organic solvents: hexane,toluene, diethyl ether (Et2O), 1,4-dioxane (dioxane), tetrahy-

Received: July 14, 2015Revised: November 3, 2015

Figure 1. Molecular structures of TPA-based molecules studied in thiswork. The moieties colored blue are the electron-donating parts TPA,while those colored red are the electron-accepting groups PA andBMO.

Article

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© XXXX American Chemical Society A DOI: 10.1021/acs.jpcc.5b06762J. Phys. Chem. C XXXX, XXX, XXX−XXX

drofuran (THF), ethyl acetate (EA), acetone, acetonitrile(MeCN), and dimethyl sulfoxide (DMSO).To gain a comprehensive picture of the optical properties of

these two compounds, in this article, first the one-photonoptical properties (absorption and fluorescence emission) oftwo compounds in different solutions are analyzed in steadystate and time-resolved measurements. Then the 2PA proper-ties are investigated by two-photon excited fluorescence(TPEF) experiment and quantum chemical calculation. Theexperiments and calculation details can be found in SupportingInformation.

2. RESULT AND DISCUSSION2.1. One-Photon Optical Properties. One-Photon

Absorption. The ground state absorption spectra are shownin Figure 2 and relevant properties are listed in Table 1. Details

of the physical properties of the solvents such as polarity arelisted in Table S1. The order of solvents is based on thefluorescence emission peak wavelength of the fluorophoresinside. The absorption peaks around 370 nm for TPA-PA and410 nm for TPA-BMO are assigned to the π → π* transitionband.8 Small shoulder features appear to change the slope ofthe absorption spectra at the longer-wavelength band edges ofTPA-BMO in MeCN and DMSO, which indicates a noticeableamount of internal charge transfer (CT) band formation.9 Theabsorption spectra in toluene and DMSO are slightly red-shifted compared to those in other solvents of these twofluorophores, which may be due to the specific interactionsbetween the fluorophores’ π-electrons with surroundingtoluene and DMSO molecules.9−11

The strength and distribution of one-photon absorption(1PA) spectrum of S0 to S1 transition can be described by theequation ε(ν) = A|μ10|

2g1(ν),7 where ε(ν) is the 1PA

(extinction) spectrum, A is a constant, ν stands for thefrequency of the light, μ10 is the electric transition dipolemoment between ground S0 and the first-excited S1 (the 1La

state in Figure 6) states, and g1(ν) represents the line shapefunction of the 1PA on frequency, which is normalizedaccording to ∫ g1(ν) dν = 1. The relatively larger one-photonabsorptivity for TPA-BMO (∼4 × 104 M−1 cm−1 at peakwavelength) than for TPA-PA (∼3 × 104 M−1 cm−1) isconsistent with the larger transition dipole moment μ10 forTPA-BMO indicated from the quantum chemical calculationresults in Table 2.

One-Photon Fluorescence Emission and Solvatochroism.The one-photon excited fluorescence emission shows signifi-cant dependence on solvents, as shown in Figure 3c, Figure S1,and Table 1. There are many aspects of solvent−soluteinteractions. In general, if the solvent environment isconsidered as a polarizable continuous media of uniformdielectric constant (Table S1), for each compound, itsfluorescence emission spectra would red-shift as the solventpolarity increases (Figure 3, Table 1, and Figure S1). Forexample, from nonpolar solvent hexane to strongly polarsolvent MeCN, the emission peak wavelength shifts from 406to 562 nm for TPA-PA, while it shifts from 457 to 646 nm(under 420 nm excitation) for TPA-BMO. Similar solvato-chromism has been observed in asymmetric fluorophores withD−π−A structure in other literature,9 which could be generallyexplained by the Lippert−Mataga equation derived fromOnsager’s reation field theory9,12,13 as follows (here theequation is in SI units; there will be no 4πε0 in the denominatorof this equation if expressed in Gaussian units) Δν =(2ΔμSS2Δf)/(4πε0hca3) + constant, where Δν is the Stokesshift between the spectra peak frequency of emission andabsorption, ΔμSS refers to the effective molecular dipolemoment difference between the ground- and solvent-stabilizedexcited- state, a is the Onsager cavity radius determined by themolecular volume calculated with Gaussian (Table 2), while Δf= [(ε − 1)/(2ε + 1)] − [(n2 − 1)/(2n2 + 1)] is a quantity todescribe the solvent polarity in which ε is the static dielectricconstant, and n is the refractive index of the solvents (seedetails of the solvent parameters in Table S1). Hence, the slopeof the Lippert−Mataga plot (Figure 4) represents the change inStokes shift with the increase of solvent polarity ∂Δν /∂Δf =(2ΔμSS2)/(4πε0hca3), which has quadratic dependence on theΔμSS and inversely proportion to the Onsager volume a3. Asthe structures of TPA-PA and TPA-BMO are asymmetric, ΔμSSis nonzero due to the large charge separation (Table 2); hencethe increase in solvent polarity Δf would render a rise in Stokesshift Δν.The deviations of the experimental data from good linear

fitting in the Lippert−Mataga plot (Figure 4) are probably dueto the complicated nature of solvent−solute interactions. In theLippert−Mataga model the solvent environment is regarded asa polarizable continuum, and only the solvent polarity is takeninto account. However, in reality, the solvent can interact withsolute in aspects such as thermal (e.g., thermal effect),mechanical (e.g., viscosity and rigidity), chemical (e.g.,hydrogen-bonding), and others (e.g., intramolecular chargetransfer), either as a whole medium or as individualmolecules.12

Fluorescence Quantum Efficiency, Lifetime, and Time-Resolved Emission Spectra. Details of the fluorescencequantum efficiency and lifetime of the two compounds indifferent solutions are listed in Table 1. The changes influorescence quantum efficiency ϕ with solvent polarity showsegmented behavior: they remain quite high in low polaritysolvents, such that the highest value is 0.72 for TPA-PA in

Figure 2. Ground state absorption spectra of TPA-PA and TPA-BMOin various solvents at room temperature. Fluorophore concentration is10 μM.

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Table 1. One-Photon and Two-Photon Excited Fluorescence Properties of TPA-PA and TPA-BMO in Various Solventsa

λabs, nm λem, nm ϕ τ, ns kr, 108 s−1 knr, 10

8 s−1 σTPEF, GM σ2PA, GM

TPA-PAhexane 364 406, 427 0.07 0.25 2.48 34.6 9.4 140.3toluene 374 446 0.68 2.24 3.04 1.43 94.9 138.6Et2O 365 456 0.71 2.88 2.46 1.01 126.8 177.9dioxane 365 464 0.72 3.03 2.38 0.92 116.2 160.2THF 366 483 0.67 3.61 1.86 0.91 107.8 161.2EA 371 485 0.61 3.27 1.86 1.19 92.1 152.1acetone 370 529 0.29 2.74 1.06 2.59 50 171.8MeCN 370 562 0.10 1.08 0.93 8.33 15.9 159.3DMSO 371 565 0.17 1.69 1.01 4.91 26 152.7

TPA-BMOhexane 408 457, 482 0.46 1.51 3.05 3.58 136.2 296toluene 419 509 0.68 2.23 3.03 1.46 151.1 223.8Et2O 526 0.69 3.13 2.2 0.99 156.7 227.5dioxane 406 530 0.62 2.88 2.14 1.33 175.7 285.3THF 407 576 0.51 3.74 1.36 1.31 130.9 257.1EA 412 581 0.28 3.2 0.88 2.25 57.4 205acetone 406 550c 0.09b 2.33c 0.39 3.9 6.0e 263e

660d 0.03 0.53d 0.51 18.4 4.8 176.3MeCN 408 580c 0.03b 0.83c 0.36 11.7 1.2e 239e

700d 0.005 0.13d 0.39 76.5 0.2 44DMSO 413 555c 0.04b 2.00c 0.20 4.8 3.4e 241e

685d 0.01 0.15d 0.66 65.6 0.4 40aλabs and λem are the peak wavelength of 1PA and emission spectra. 1 GM ≡ 10−50 cm4 s/photon. The other symbols are explained in the text.bMeasured with a 370 nm SHG excitation. cThe quantities are for LE state. dThe quantities are for TICT state, determined using excitations whosewavelength are so long that the LE emissions in bright yellow regime are hardly observed for TPA-BMO in these strongly polar solvents. eWith 820nm two-photon excitation. Unless otherwise noted, the excitation wavelengths are 380 nm for TPA-PA and 420 nm for TPA-BMO in the one-photon process and are double the corresponding excitation wavelengths in the two-photon process.

Table 2. Theoretical Result of the Typical Electronic Transition Dipole Moment and 2PA Cross Section of Our Compoundsfrom TD-DFT Calculationa

TPA-PA

solvent ε α0, au μ10, D μ00, D μ11, D Δμ10, D λ2pa, nm σ2pacalc, GM

hexane 1.89 399 8.51 5.05 13.7 8.60 656 315toluene 2.38 413 8.67 5.18 14.0 8.82 661 344Et2O 4.33 444 8.56 5.43 13.9 8.52 661 335dioxane 2.25 409 8.58 5.14 13.8 8.70 658 332THF 7.58 465 8.66 5.58 14.1 8.57 665 354EA 6.02 457 8.61 5.53 14.0 8.52 663 345acetone 20.7 487 8.63 5.72 14.1 8.39 665 352MeCN 37.5 493 8.62 5.76 14.1 8.34 665 351DMSO 46.7 495 8.71 5.77 14.4 8.60 668 379

TPA-BMO

solvent ε α0, au μ10, D μ00, D μ11, D Δμ10, D λ2pa, nm σ2pacalc, GM

hexane 1.89 530 11.0 2.60 13.7 11.1 763 522toluene 2.38 549 11.1 2.69 13.9 11.3 770 564Et2O 4.33 588 11.0 2.87 14.3 11.4 770 569dioxane 2.25 543 11.0 2.66 13.9 11.2 768 545THF 7.58 616 11.1 2.99 14.5 11.5 778 597EA 6.02 606 11.0 2.95 14.4 11.5 775 583acetone 20.7 644 11.0 3.11 14.6 11.5 778 602MeCN 37.5 652 11.0 3.14 14.6 11.5 778 603DMSO 46.7 655 11.1 3.15 14.7 11.6 783 636

aα0 is the polarizability in the ground state. μ00 and μ11 are the dipole moments for the ground and the 1La excited state (Figure 6), respectively. μ10refers to the transition dipole moment between the ground state and the 1La excited state, and Δμ10 = μ11 − μ00 is the dipole moment differencebetween the 1La excited state and ground state. All the dipole moments listed here are in the unit of debye (D). λ2pa and σ2pa

calc correspond to the 1Lastate in all solvents. The 1PA properties are also calculated. The calculations show that the same excited states are involved in 2PA and 1PA. TheOnsager radius is 5.75 Å for TPA-PA and 6.07 Å for TPA-BMO in solutions studied, respectively. Details of the quantum chemical calculationinvolved are described in the Supporting Information.

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dioxane and 0.69 for TPA-BMO in Et2O, and then decreasequickly as the solvent polarity becomes stronger (Table 1).The fluorescence decays that are used to estimate the

fluorescence lifetimes of corresponding solutions are plotted inFigure S2. Almost all the decays of TPA-PA solutions can bewell-fitted with a single exponential decay function at theirfluorescence emission peak wavelength, while for TPA-BMO instrongly polar solvents, a fast time component is observedresulting in a biexponential decay at wavelengths longer thantheir emission peak, which could be attributed to the existence

of a lower energy excited state that undergoes either a fasterradiative or nonradiative relaxation process (Table 1 and FigureS2).

Dual Fluorescence for TPA-BMO in Strongly PolarSolvents. It should be noted that the emission spectra ofTPA-BMO in acetone, MeCN, and DMSO vary with differentexcitation wavelengths, with dual fluorescence emission peaksas shown in Figure 5 and Figure S1(c,d). Among the dual

fluorescence peaks in Figure S1(c,d), the peak at shorterwavelength (∼550−580 nm) is assigned to the locally excited(LE) state, while the peak at longer wavelength (∼660−700nm) is probably ascribed to the twisted intramolecular charge-transfer (TICT) state.14 Details about this TICT state will bediscussed in the following text. Among the intramolecularcharge transfer (ICT) processes that describe the changes inoverall charge distribution in a molecule, the TICT is a specificprocess in the excited state related to the twist in molecularconformation that lowers the energy of the excited state andresults in a red-shifted emission band.a The method forobtaining the LE/TICT spectra is illustrated in SupportingInformation.To investigate this faster decay process, a series of steady

state and time-resolved fluorescence measurements areperformed. For TPA-BMO in strongly polar solvents, thesteady-state one-photon fluorescence emission spectra varywith the change of excitation wavelength from 410, 420 to 440nm (Figure S1c). When observed with the naked eye, thefluorescence emission is bright yellow under 410 nm excitation

Figure 3. (a, b) TPEF emission spectra for TPA-PA and TPA-BMO.Fluorophore concentration is 40 μM. Excitation wavelength is 760 nmfor TPA-PA and 840 nm for TPA-BMO. (c) Photographs of the TPA-PA and TPA-BMO in various solvents under UV irradiation.

Figure 4. Lippert−Mataga plot of the relation between Stokes shift Δνwith solvent polarity Δf for TPA-PA and TPA-BMO, in which a refersto the Onsager radius. The fluorescence emission peaks for TPA-BMOin acetone, MeCN, and DMSO are 622, 646, and 646 nm under 420nm excitation.

Figure 5. (a−c) Contour plots of time-resolved fluorescence spectra ofTPA-BMO in MeCN, THF, and TPA-PA in MeCN. The instrumentalresponse function of the system is ∼40 ps. (d−f) Time-resolvedfluorescence spectra (smoothed by average) for TPA-BMO in MeCN,THF, and TPA-PA in MeCN. Fluorophore concentration is 400 μM.

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while it is dim orange under 420 and 440 nm excitation. Twokinds of emission bands can be extracted by comparing theemission spectra under these different excitation wavelengths(Figure S1d), in which one is peaked at around 550−580 nmwhile the other peak is around 660−700 nm. According to thisphenomenon, several assumptions such as excimer, dimer, orTICT state formation16 could be the possible reason for thedual fluorescence. Further studies show that the ratio ofintensities between the two fluorescence peaks is independentof fluorophore concentration (here concentration of 40 μMand 400 μM are studied), while the Δμ10 from the calculationsin Table 2 indicates that the largest charge separation occurswhen the TPA-BMO molecules are being excited in stronglypolar solvents. According to this additional informationtogether with the donor−acceptor molecular structure, itbecomes very likely that it is the addition of TICT statewhich is responsible for the dual fluorescence.14,16 In this case,according to fluorescence decays of TPA-BMO in stronglypolar solvents, the fluorescence bands at the short wavelengthwith decay lifetimes similar to the corresponding TPA-PA onesin the same solutions are mainly from the LE states, while thefluorescence bands at long wavelengths are mainly from theTICT states. In many systems, the TICT state emission isforbidden in nature, with rapid nonradiative decay due to theintramolecular fluorescence quenching. However, in our TPA-BMO system, the number of π-electrons is large enough torelax the forbidden nature of the TICT emission, making theTICT state emissive with a relatively longer lifetime.16

To know whether there is TICT emission in other solutions,the time-resolved emission spectra are captured as shown inFigure 5. According to Figure 5(a and d) for TPA-BMO inMeCN, under the time resolution determined by theinstruments, the fluorescence emission peak first appears ataround 520 nm, and then another peak becomes dominant ataround 680 nm and gradually shifts to around 580 nm as the

fluorescence gradually decays. The fluorescence intensityaround 580 nm decays much more slowly than the one around680 nm, indicating that distinct excited states are involved inthese two fluorescence bands. In Figure 5 (b, e, c, and f) forTPA-BMO in THF and TPA-PA MeCN, though the bluer sideof the emission spectra rise slightly earlier than the redder side,there is no obvious spectral shift after the fluorescence intensityreaches maximum, indicating that the general solvent relaxationtakes place in a time scale comparable to or even faster than thetemporal resolution of the system. Comparing the time-resolved emission spectra of the three different solutions shownin Figure 5, it can be observed that for the same solute TPA-BMO, the TICT emission is much stronger in MeCN than inTHF, while for the same solvent MeCN, the TICT emission ismuch stronger for solute TPA-BMO than with TPA-PA. Onthe basis of the relative strength of the TICT emission amongthe three solutions, indications could be made that the strengthof the TICT emission will generally increase with increasingsolvent polarity and also increase by substituting the PA withBMO moiety.These trends above could possibly be explained according to

the Jablonski diagram as proposed in Figure 6, in which thestates labeled 1La and

1Lb correspond to the two lowest singletexcited states using Platt’s notation with weak transition dipoleswhose polarizations are parallel and perpendicular to the longaxis of the solute molecule, respectively,17 and the state labeled1B is the 1Lb-type state in the charge-transferred (CT)geometry.16 In ground state geometry, the energies of 1Laand 1Lb states are close and nearly degenerate18 so that thefluorophores can be excited to both 1La and

1Lb states when theexcitation wavelength is around the longest absorption peakwavelength. It should be noted that the lowest-energy Franck−Condon excited states are 1Lb states for TPA-PA and 1La forTPA-BMO.18 The distribution of the excited populationsbetween the 1La and 1Lb states depends on the excitation

Figure 6. Schematic presentation of the photophysical processes. In this diagram, thicker lines indicate relatively higher population in states andstronger transitions. Other details of the diagram are discussed in the text.

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wavelength and thermal equilibrium, so the excited populationsare larger on the excited state with relatively lower energy (i.e.,1Lb states for TPA-PA and 1La for TPA-BMO).The solvent−solute interaction includes a series of processes

such as solvent relaxation, intramolecular rotation, chargetransfer, and reorientation of the solvent molecules. Withoutintramolecular rotation, the conformations of the fluorophoresremain in the ground state geometry, and the energy of boththe 1La and 1Lb states will be lowered by solvent−soluteinteraction: the decrease in energy is generally more significantin solvent with stronger polarity. On the other hand, afterintramolecular rotation, the fluorophore conformation is alteredto the CT geometry.16 During this conformational change, theenergy of 1Lb state (i.e., the 1B band) rises to a higher energythan that of the ground state geometry, while the energy of 1Lastate (denoted as TICT states) changes differently in varioussolvents: it may slightly increases in solvent with weak polarity,while it decreases in solvent with medium to high polarity.Therefore, the 1La bands in solvent with medium to highpolarity are more stabilized in CT geometry than in ground-state geometry. As the system tends to relax to lower energystates during solvent−solute interaction, in solvent with weakpolarity, the excited populations for both TPA-PA and TPA-BMO tend to relaxed to the LE state after solvent−soluteinteraction, whereas in solvent with medium to high polarity,the excited populations have divergent behaviors for TPA-PAand TPA-BMO. For TPA-PA in strongly polar solvent, only theminor excited populations on the 1La state can relax to theTICT state, while for TPA-BMO in strongly polar solvent, themajor excited populations on the 1La state will relax to the morestabilized TICT state. Considering the fluorescence quantumefficiency of the TICT band is relative lower than those of theLE band, the TICT emission remains negligible for TPA-PA instrongly polar solvents, whereas it becomes discernible forTPA-BMO in strongly polar solvents. Therefore, thefluorescence emissions for all TPA-PA solutions studied andTPA-BMO solutions in solvent of polarity lower than that ofTHF are mainly assigned to the LE bands after general solventeffect, while those of TPA-BMO in strongly polar solvent aremainly from the stabilized TICT band.The internal rotation of the fluorophores from ground state

geometry to CT geometry during solvent−solute interactionupon excitation is a dynamic relaxation process. Hence,radiation from ground state and CT geometry can be resolvedin time-resolved spectra, while both are captured as a time-averaged value in steady state fluorescence. According to Figure5, taking TPA-BMO in MeCN as an example, its generalsolvent relaxation takes place much faster than the timeresolution of the streak camera system, ∼40 ps (peak around580 nm), while the TICT process of the internal rotationtogether with solvent reorganization (peak around 680 nm)occurs after a time scale of ∼91 ps.Radiative and Nonradiative Decay Rates. The radiative

decay rate kr and nonradiative decay rates knr of thesefluorophores are estimated from their ϕ and fluorescencelifetime τ according to the equations kr = ϕ/τ and knr = (1 −ϕ)/τ. The general decrease of radiative decay rate withincreasing solvent polarity (Table 1) is also observed in otherliterature,9,12,14,19,20 which could be explained by the reductionin transition probability for the new π-electron distribution ofthe excited states after solvent relaxation.16,17 The increase ofthe nonradiative decay rate in polar solvent is related toprocesses like quenching, energy transfer, and solvent−solute

interaction.12 Particularly, for TPA-PA it can be explained by arapid depopulation pathway to the ground state12 such as self-absorption, while for TPA-BMO in strongly polar solvents, theextra large nonradiative decay rates are probably attributed tothe formation of an TICT state.12

2.2. Two-Photon Optical Properties. The two-photonoptical properties of these fluorophores are investigated with arelative two-photon excited fluorescence (TPEF) technique21

(see the details of this measurement in SupportingInformation). The intensity of TPEF shows quadratic depend-ence on the excitation power (Figure S4), indicating that theprocess involved in absorption is a 2PA process. The TPEFemission spectra for these fluorophores are identical to those inthe one-photon excitation process (Figure S1 and Figure 3c),while their TPEF excitation spectra are close to double thewavelength of their one-photon absorption (1PA) spectra(Figure 7a). The similarity of the absorption and emission

bands in the one- and two-photon processes indicates that theexcited states involved in these processes are the same, sincethe parity selection rule17 is relaxed as the molecular structuresare asymmetric.7,22

The peak strength of 2PA cross sections for the S1 stateestimated from TPEF measurement is around 160 GM forTPA-PA and 250 GM for TPA-BMO in all nine solventsstudied as depicted in Figure 7b. According to Figure 7b, the

Figure 7. (a) 2PA spectra (left axis) compared with 1PA spectra (rightaxis) of TPA-PA and TPA-BMO solutions in THF. For easycomparison, 2PA wavelength (bottom) is plotted as double of the1PA wavelength (top). (b) Maximum 2PA cross sections in differentsolvents determined by TPEF experiment (scattered points, left axis)and by quantum chemical calculation (lines, right axis). Experimen-tally, for TPA-BMO solutions in weak polar solvents such as THF, the2PA cross sections reach maximum at around 840 nm, while for theTPA-BMO in polar solvents, the 2PA cross sections peak at around820 nm and drop quickly as the two-photon excitation wavelengthreaches 840 nm.

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relative maximum strength of 2PA between TPA-PA and TPA-BMO estimated by the TPEF is consistent with the relativestrength indicated by quantum chemical calculations (Table 2),and the results from both TPEF and calculation of differentsolvents show that the maximum 2PA strengths are notsensitive to the change of solvents, with minor fluctuationwithin the 21% experimental error.22 If the two-photonexcitation wavelength is longer than double of the longestabsorption peak wavelegnth, the 2PA cross sections of TPA-BMO dramatically drop (Figure 7), since the ∼250 GMmaximum 2PA cross sections for TPA-BMO are attributedfrom the weighted average of 2PA strength of the 1Lb and

1Labands (Figure 6), and the excitation of longer wavelengths withsmaller photon energy can only reach the 1La state with weaker2PA strength (∼600 GM) rather than the 1Lb state which haslarger 2PA cross sections (∼1200 GM) estimtated fromcalculation. The smaller 2PA strength for the 1La state isprobably due to its preference to take on configurations withlarge torsional angles between the donor−acceptor plane inpolar solvents such as acetone, MeCN, and DMSO, as theenergy is lower in CT configuration. This conformationalchange reduces the planarity of the π-electron system, as aresult jeopardizing the π-electron coherence of the wavefunction and segmenting the π-electrons into shorter chains,which then leads to the decrease of 2PA strength.13,23−25 ForTPA-PA, the 2PA strength does not show this dramatic drop,since the excited population on the 1La state with weaker 2PAstrength is either minor or small at the two-photon excitationwavelength. When Δμ10 estimated from quantum chemicalcalculation (Table 2) and ΔμSS estimated by a Lippert−Matagaplot (denoted as ΔμTPA‑PA and ΔμTPA‑BMO in Figure 4) arecompared, although they both refer to the difference betweenexcited- and ground-state dipole moments, their distinctmagnitudes reveal that they are intrinsically different. TheΔμ10 influences the strength of the 2PA transition, while ΔμSSaffects the degree of excited state energy shift after solventrelaxation. Hence, Δμ10 is related to the excited state right afteran absorption process, while the ΔμSS corresponds to theexcited state just before the fluorescence emission, i.e., aftersolvent relaxation. There are relations ΔμSS = μSS − μ00 andΔμ10 = μ11 − μ00 by definition, where μSS is the dipole momentvector for the solvent-stabilized excited state while μ11 is theone for the 1La-type Franck−Condon excited state as labeled inFigure 6. Under the assumption that the directions of dipolemoments before and after excitation are nearly parallel forlinear-structured D−π−A molecules, the vector relations abovecan be regarded as scalar relations. The relation that ΔμSS >Δμ10 indicates that the solvent relaxation processes finally resultin a solvent-stabilized state (for TPA-PA when estimated fromFigure 4 and Table 2, μSS = μ00 + ΔμSS ≈ 23.5 D) with largerdipole moment than the 1La state (for TPA-PA when estimatedfrom Figure 4 and Table 2, μ11 = μ00 + Δμ10 ≈ 14 D).To analyze the structure−property relationship, the molecule

TPA-BMO is compared with TPA-PA. The difference in theirone- and two-photon absorption transition probabilities isattributed to the fact that the BMO acceptor is physically largerin π-electron system than the PA acceptor, resulting in a greatertransition dipole moment. In terms of the TICT stateformation in TPA-BMO rather than TPA-PA, it is probablydue to accessibility from the excited states due to symmetry.The underlying reason is that the slopes of energies decreasevesus Hammett substituent constants26 are different for thelowest two excited states 1La and

1Lb so that the energy of 1La

state is higher than that of 1Lb for TPA-PA and gets reversed forTPA-BMO with stronger substituents.14 Therefore, the 1Lastate related to TICT emission is more accessible for TPA-BMO. In addition, the maximum 2PA cross section in THFsolution of TPA-PA (161.2 GM) is much larger than the one ofDPA-TPE-PA (51 GM) in our previous study22 (in thepublication the DPA-TPE-PA is named as DPA-TPE-CHO),although the molecular structures of TPA-PA and DPA-TPE-PA are similar to a certain extent. It has been found in theliterature2,3,27−40 that many monomers containing the TPAmoiety have 2PA cross sections of hundreds to thousands ofGM units. TPA (triphenylamine) seems to be a very promisingbackbone structure to compose molecules with large 2PA crosssections. The plausible reason could be that the TPA group hasa relatively greater number of π electrons3 and strongerelectron-donating ability41 than many other organic structures.To summarize, in terms of fluorophore design for TPEFapplications in general, using donor/acceptor groups withlarger π-electron system could enlarge the two-photonabsorptivity, and using substituents with larger Hammettconstants could increase the possibilities to observe TICTemission.

■ CONCLUSIONIn this work the one- and two-photon optical properties ofTPA-PA and TPA-BMO in nine different solvents areinvestigated. General solvent effect, i.e., the solvent interactionwith the fluorophore dipoles as a continuous medium, is themain cause of the basic bathochromic shift in fluorescenceemission as the solvent polarity increases. The major excitedstate populations will relax to the TICT state rather than the LEstate for TPA-BMO solutions in strongly polar solvents likeacetone, MeCN, and DMSO. The general solvent effect due tosolvent polarity has minor effect on the 2PA strength, while thestabilization of the TICT state correlates with the decrease in2PA cross section for TPA-BMO in strongly polar solvents.When the excited-state dipole moments indicated from 2PA arecompared with those estimated from solvatochroism, asignificant change in excited-state dipole moment is presentedin solvent−solute interaction upon excitation.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.jpcc.5b06762.

Details of the solvent properties, UV/vis spectroscopy,method to obtain LE/TICT state, fluorescence quantumefficiency, time-resolved spectroscopy, TPEF measure-ment, quantum chemical calculation, and materialinformation such as synthesis, preparation, and character-ization details (PDF)

■ AUTHOR INFORMATIONCorresponding Author*E-mail: phkswong@ust.hk.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSWe express our appreciation to Prof Zhigang Shuai for thecoordination of the molecular calculation and to Prof. JiannongWang, Ya Yi, and Yiqun Xiao for the support in time-resolved

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fluorescence emission spectra measurements. This work issupported by Research Grants Council of Hong Kong (ProjectsHKUST2/CRF/10 and CUHK1/CRF/12G), the UniversityGrants Committee of Hong Kong (Projects AoE/P-03/08 andAoE/P-02/12), and Shandong Provincial Natural ScienceFoundation, China (Grant ZR2014AM026).

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