ELSEVIER

6December 1996

Chemical Physics Letters 263 (1996) 209-214

CHEMICAL PHYSICS LETTERS

Role of higher excited states in radiative and nonradiative processes of coordination compounds of Ru(II) and Rh(III)

in crystal

Ashraful Islam, Noriaki Ikeda, Koichi Nozaki, Takeshi Ohno Department of Chemistry. Graduate School of Science, Osaka University, Toyonaka, Osaka 560. Japan

Received 29 July 1996; in final form 30 September 1996

Abstract

3MLCT of [Ru(bpy)3](PF6) 2 (bpy : 2,2'-bipyridine) in solid state above 350 K decays via the lowest excited d-d state lying higher with a frequency of 7 × 1013 S- 1 at the high-temperature limit. Crystal water of [Ru(bpy)3]Cl 2 • 6H20 lowers the energy level of an intermediate for the nonradiative decay by -- 2000 cm- i. Crystalline sample of [Rh(bpy)2Cl2](PF 6) at 536 K emits an unresolved phosphorescence with a fwhm of 6000 cm- 1, which comes from two closely lying excited d-d states. Nonradiative decays of the lowest excited d-d state occur via the second lowest excited d-d state with a more displaced potential curve in a higher-temperature region of 350-586 K.

1. Introduction

Photoinduced electron transfer (ET) reactions within donor-acceptor compounds consisting of d 6

metal ions - Ru(II), Os(II), Rh(III), and Co(Ill) - in both solution and crystal at room temperature have been intensively studied [1-7]. Rates of the initial ET between an excited metal ion of Ru(II) or Os(II) and an acceptor metal ion of Co(Ill) or Rh(III) are controlled by energetics and dynamics of the reorga- nization of a toms-in-molecules and solvated molecules. Studies of the excited-state characteris- tics, however, were limited to those in either rigid glass or crystal below 77 K [8-17], because heat a n d / o r fluid polar solvents increase fluctuation in solvation structure and open decay channels of the excited state resulting in a broad spectrum and a chemical reaction [18-26]. Since excited states lying closely to the lowest one might participate in both

radiative and nonradiative processes at higher tem- peratures, it is quite important to know the character- istics of higher excited d - d states of d 6 metal ions and solvation effects on them to interpret ET dynam- ics.

Studies of high resolution spectroscopy have re- vealed that the higher lying metal-to-ligand charge transfer (MLCT) states of Ru(II) compounds emit a well resolved emission below 20 K. We have ex- tended the study of temperature dependence of the emission up to 480 K, to clarify the nonradiative decay of it. A well displaced excited d - d state was found to be thermally excited and undergo rapid internal conversion to the ground state in solid state. As for Rh(III) compound in solid state, a higher excited d - d state was found to exhibit a more dis- placed emission and play the major role in the nonradiative process in a wide temperature range of 330-586 K.

0009-2614/96/$12.00 Copyright © 1996 Elsevier Science B.V. All rights reserved PII S0009-2614(96)01188-8

2 1 0 A. Islam et a l . / Chemical Physics Letters 263 (1996) 209-214

2. Experimental

[Ru(bpy) 3 ]CI 2 • 6H 20 (bpy = 2,2'-bipyridine) was prepared and purified according to a literature method [27]. [Ru(bpy)3](PF6) 2 was prepared by the addition of NH4PF 6 to the solution of [Ru(bpy)3]Cl 2 • 6H20. A diluted crystal of [Ru(bpy)3](PF6) 2 in the host of [Zn(bpy)3](PF6) 2 was prepared from the mixed aque- ous solution of guest and host. Cis-[Rh(bpy)2Cl2]- (PF 6) was synthesized and purified according to a literature method [28]. The emission spectrum of a single crystal of the Ru(II) compound was recorded on a Hitachi MPF-4 spectrofluorometer with an R928 photomultiplier. The emission spectrum of the Rh(III) compound was measured by using a grating monochromator (Jasco CT250) with a silicon diode- array (Hamamatsu $3901-512Q) [29]. The detector sensitivity was corrected by using a bromine lamp (Ushio IPD 100V500WCS). For the lifetime mea- surement of emission of Ru(II) compounds, the in- tensity of SHG of a Q-switched YAG laser (Quatltel YG580) was as low as possible (less than 1 ix J /pulse) [30]. For the lifetime measurement of Rh(III) emission, the THG with intensity of 100 ix J /pulse was used for the excitation. The sample crystals on a copper holder were retained in a cryo- stat (Oxford ND-1740) controlled by a controller (Oxford ITC4). Above 353 K, the samples in a capillary quartz cell were put into an aluminum block and heated by a hot plate.

3. Results

3.1. Temperature effect on emission spectra

3.1.1. [Ru(bpy)31(PF 6 )2 and [Ru(bpy)31Cl 2 • 6HeO Guest compounds of [Ru(bpy)3](PF6) 2 (0.1%) in

the host of [Zn(bpy)3](PF6) 2 emit a 3MLCT phos- phorescence with a vibronic feature as is shown in Fig. 1. There seems a progression of vibrational frequency ( = 1400 c m - ~). Rise in temperature changed neither the first band energy nor the vi- bronic feature of the emission. The full width at half maximum (fwhm) of the first band is dependent on temperature; the fwhm of the first band changed from --- 550 c m - ~ at 77 K to = 950 cm- ~ at 300 K.

50O i . . . . i

1

o~

u~

,X/nm • , , . 6 o 9 . . . . . . . 7 .9 .0 . . . . . . ~ '

//~ 77 K

, 1 J , t . , I , , I

20 ;4 "~" / 103cm -1

Fig. 1. Emission spectra of [Ru(bpy)3](PF6) 2 in [Zn(bpy)3](PF6) 2. Solid line: 77 K; broken line; 300 K.

A similar temperature dependence of emission spectra was observed for a single crystal of [Ru(bpy)3](PF6) 2 and [Ru(bpy)3]Ci 2 • 6H20 for which the emission at 300 K was detected from the front side of a single crystal to prevent deformation of the emission spectrum due to reabsorption.

3.1.2. Cis-[Rh(bpy) 2 CI 2 I (PF 6 ) The anhydrous crystal of cis-[Rh(bpy)2Clz](PF 6)

emits a broad emission with a fwhm of 2850 cm- at 77 K. The intensity distribution among vibronic lines is reconstituted by taking a Huang-Rhys factor (11) and a vibrational frequency (400 cm- l ) , respec- tively. Since individual vibronic lines are not seen in the emission spectrum, the fwhm of the vibronic lines is larger than 500 cm -1. The Franck-Condon energy of emission is estimated to be 3700 cm- from the 0 - 0 energy (18300 cm - I ) and the emission maximum (14600 cm-~). Raising the temperature from 77 to 536 K changed the emission spectrum. The emission maximum was shifted from 14600 to 12800 cm-~ and the fwhm of the whole spectrum got larger from 2850 c m - l to 5450 cm-~ as is shown in Fig. 2.

3.2. Temperature dependence o f emission decay

3.2.1. Neat crystals o f [Ru(bpy)3l(PF6) 2 and

[Ru(bpy) 3 ]Cl 2 • 6H 2 0 The decay rate of 3MLCT emission (k d) gradually

increased with temperature for the neat crystal of

A. Islam et al . / Chemical Physics Letters 263 (1996) 209-214 211

[Ru(bpy)3](PF6) 2 up to 350 K. In the high-tempera- ture region between 350 and 480 K, the nonradiative decay rate increases abruptly as temperature in- creases. The dependence of emission decay rate on temperature can be analyzed by using a double Ar- rhenius equation in the region of 77-480 K. The activation energy (5200 cm - I ) and frequency factor (7 .6× 1013 s - ] ) for the high-temperature decay channel are much larger than those for the low-tem- perature one. Neat crystal of [Ru(bpy)3]Cl2- 6H20 exhibits a different temperature dependence of k d from the anhydrous crystal as is shown in Fig. 3. The activation energies are 80 c m - l for the low-tempera- ture decay channel and 3300 cm- ~ for the high-tem- perature decay channel. The latter figure is close to those measured for the solution samples [21-23]. Above 360 K, a reduction in the decay rate of emission was found for the hydrous crystal due to dehydration.

3.2.2. Cis-lRh(bpy)eCle l (PF 6 ) The decay rate of the broad emission at 77 K was

a little slower (1.0 × 104 s- 1) than that in rigid glass [26]. Below 370 K, double Arrhenius type tempera-

10 ' I

i 0 ~ / ' ~ 1 77 v 297K 441K

0.5 ',

. ~ - ,,

0.0 i 20 12

?z j 1%,

16 V / 1 0 3 c m -1

Fig. 2. Emission spectra of cis-[Rh(bpy)2Cl2](PF6). Circles: 77 K; dotted line: 297 K; broken line: 441 K; solid line: 536 K. The inset shows that the observed spectrum (solid line) at 536 K is decomposed into an emission spectrum (1) from the lowest ex- cited state and an emission spectrum (2) from the second lowest excited state.

ago 2o0 r/K 16 7

° . ~

,.~o 1 4 ,~ ~ o • 6 ~ '

t"-"

o • o •

o

o eg oe o a

12

, , , , g , , , , I , , , , , 0 0 10 1

10 3 / T K -1

Fig. 3. Temperature dependence of emission decay (kob s) of neat crystals of [Ru(bpy) 3 ] (PF 6 )2 ( 1:3 ) and [Ru(bpy) 3 ]CI z" 6H zO (O).

ture dependence of emission decay was observed; the smaller extent of frequency factors ( < 2 × 106 s - t ) implies that the emitting state undergoes radia- tive transition and radialtionless transition via weak coupling• In the 370-586 K range, the decay rate of emission increased with temperature from 1.3 × 105 s - l at 377 K to 1.7 × 107 s -1 at 586 K. The Arrhenius parameters, i.e. a frequency factor of 7 × l0 II s - I and an activation energy of 4400 cm - ] , were obtained.

4. Discussion

4.1.3MLCT of lRu(bpy) 3 I(PF 6)2 and [Ru(bpy) 3 lCl 2 • 61-120

3MLCT emission of [Ru(bpy)3](PF6) 2 in the host of [Zn(bpy)3](PF6) 2 exhibits a vibronic feature which is nearly independent of temperature in the region of 77-300 K. The MLCT states lying higher than the lowest one are more emissive than the lowest one [8-13], though 3MLCT have such a nondisplaced structure as the lowest excited state does [8,9]. The temperature-dependent fwhm of the first band arises from both the Franck-Condon energy along the low-frequency mode and inhomogeneous broaden- ings [30]. The former (h) is roughly estimated as 380

212 A. Islam et al. / Chemical Physics Letters 263 (1996) 209-214

cm - t by using the following temperature depen- dence of fwhm,

(fwhm) 2 = 16 In 2kBTA.

The vibronic feature of emission is retained in the presence of crystal water even at 300 K, while the vibronic feature disappears completely in solution above the melting point of the solvent [21-24].

The temperature dependence of emission decay of neat crystal of [Ru(bpy)3](PF6) 2 in the lower-temper- ature region than 330 K is interpreted by the faster radiative decay of the excited state lying 61 cm- l [8] above the lowest one. The small activation energy (24 cm- ] ) for the low-temperature decay channel of the anhydrous crystal may be ascribed to a phase change of the crystal at 190 K [31]. The temperature dependence of emission decay in the high-tempera- ture region can be interpreted in terms of a nonradia- tive decay via the strong coupling scheme. Since the large frequency factor (7 × 1013 S - t ) implies the strong coupling case, the nonradiative transition to the ground state does not occur directly but via a displaced intermediate compared to the ground state (gs). The most probable candidate for the intermedi- ate has been considered to be the lowest triplet excited d-d state (3(d-d)) [21,23,24]. The nonradia- tive transition of 3MLCT proceeds via the following

s u c c e s s i v e p r o c e s s , 3 M L C T ~ 3 ( d - d ) a n d 3 ( d - d ) - - ~ I gs as is depicted in Fig. 4. The endoergonic internal

conversion to a strongly displaced excited d -d state needs a high activation energy. Provided that the extent of reorganization energy (A) of 3(d-d) is similar to that obtained for Rh(III) compounds (vice versa), for which 3(d-d) is the lowest excited state with the electronic configuration of (dTr) 5 do' *, the Gibbs function difference between 3MLCT and 3(d-d) (AG °) for [Ru(bpy)3](PF6) 2 is estimated to be 5150 cm-i from the activation energy (Era = 5200 cm-i ) by using the relation Era = (AG O + A)2/4A. Meanwhile, the highly exoergonic intersystem cross- ing of 3(d-d) to gs occurs via potential crossing seams with a vibronically excited gs in addition to the classical potential crossing with the gs. The semiquantum mechanical intersystem crossing should reduce the frequency factor. Consequently, the rate- determining step of the nonradiative decay of 3 MLCT is the internal conversion to 3(d-d).

The crystal water lowers the activation energy of the nonradiative process, which is identical to those observed in the solution [22,24-26]. The intermedi- ate thermally populated is not the 3(d-d) of Ru(bpy)~ ÷ but a seven-coordinated state, which may be formed in the solution phase at higher tempera- tures.

0

50

40

T

~ 3 0 o o

-2 - I 0 1 2 2 -1 0 1

Normal ized Coordinate Normalized Coord inate

4 !64 4 3.7

2

Fig. 4. Schematic potential curves of excited states and the ground state (gs). (a) [Ru(bpy)3](PF6) 2. Nonradiative transition of 3MLCT to the ground state occurs as the following successive process with an activation energy of 5200 cm- ~ and a frequency factor of 7.6 × 1013 s - l, endoergonic internal conversion from 3MLCT to 3(d-d) and exoergonic intersystem crossing from 3(d-d) to gs. (b) [Rh(bpy)2Cl2](PF6). 3(d-d) l denotes the lowest excited d -d state and 3(d-d)2 denotes the second lowest excited d -d state. The Franck-Condon energies of emissions are 3700 cm- i for 3(d-d)j and 6400 cm ] for 3(d-d) 2 along a low-frequency mode and a medium-frequency mode.

A. Islam et al./ Chemical Physics Letters 263 (1996) 209-214 213

4 .2 .3 (d -d) o f cis-lRh(bpy) 2 C1 z I (PF 6 )

The broad emission with a large fwhm of 2850 c m - 1 at 77 K is very similar to the phosphorescence from the lowest excited d - d state of cis- [Rh(bpy)2Cl2] + [14] and [Rh(NH3)6] 3+ [15] in low- temperature rigid solvent. The appearance of vi- bronic bands for the emission of [Rh(NH3)6]- [Rh(CN)6] crystal at 4.2 K showed the broad band at higher temperatures to be decomposed into progres- sions of vibronic bands [16]. The broad emission spectrum of [Rh(bpy)2Cl2](PF 6) observed at 77 K can be decomposed to a progression with a large Huang-Rhys factor (11) and the 0 - 0 band at 18300 cm -z . On raising the temperature from 77 K to 536 K, a large shift of the emission peak (1800 c m - i) is accompanied by an extraordinary increase of the fwhm (2850 cm -j to 5450 cm-~) . The unusual spectral change with temperature is interpreted in terms of a two emitting states model; while the emission at 77 K comes from the lowest excited state (3(d-d)l) , the broader emission at higher tempera- tures comes from both 3(d-d)l and the second lowest one (3(d-d)2). Reduction in the symmetry of the compound splits the degenerate level of 3(d-d), 3T2g of O h, into 3A2, 3B l and 3B 2 of C2v, The 0 - 0 band, the peak energy and the Huang-Rhys factor of the second emission are estimated to be 18400 cm -~, 12000 cm- ~ and 18, respectively, by means of spec- tral fitting. The vibrational frequency of the accept- ing mode is presumed the same as that of the low- temperature emission. The fwhm of vibronic lines is assumed to increase in proportion to the square root of temperature. An increased fwhm of the whole band (5400 cm -I at 528 K) for cis-[Rh(l,10- phenanthroline)2Cl2](PF 6) [32] indicates the dual

emission from closely lying excited d - d states at higher temperatures.

The 3(d-d)l undergoes nonradiative transition via weak interaction below 250 K. Above 350 K, the nonradiative intersystem crossing of 3(d-d)J3occurs through the curve crossing between gs and (d -d) 2 with a small frequency factor of 7 × 101~ s -~. The probability of 3(d-d)1 excited to the curve crossing seam ( e x p [ - ( E + A)2/4AkBT]) can be roughly esti- mated from the energy gap (E) between 3(d-d)l and gs (18300 c m - t ) , and the Franck-Condon energy of the emission (either 3700 c m - l for 3(d-d)1 or 6400 c m - t for 3(d-d)2) by assuming harmonic potential wells. The potential crossing between 3(d-d)2 and gs is more adequate for the activation energy of 4400 c m - ~.

Such a large Franck-Condon energy of a d - d emission was estimated to be 7500 cm -~ for [Rh(bpy)2 (N_methyl_2_(2.pyridyl)benzimidazole)]3 + in butyronitrile from a difference in the energy be- tween the 0 - 0 peak of a well resolved 7r-Tr ~ emis- sion and the peak of a broad d - d emission at 300 K [33].

5. C o n c l u s i o n

Higher excited states lying above the lowest one for [Ru(bpy)3 ](PF6)2 and cis-[Rh(bpy) 2 CI 2 ](PF6) play an important role in the radiative a n d / o r nonradia- tive processes.

(1) The closely lying MLCT states of [Ru(bpy)3]- (PF6) 2 above 350 K decay nonradiatively via a dis- placed intermediate, 3(d-d), with a frequency factor of 7 × 1013 S -1 and an activation energy of 5200 cm -1. Crystal water tunes the characteristics of an

Table 1 The 0-0 energy (~o-o), maximum (~ma~), Huang-Rhys factor (S), Franck-Condon energy (A), and lifetime of emission (r) at 77 K. Frequency factor (A H ) and activation energy (E H) are obtained for the high-temperature decay channels

Neat crystals ~0 0 ~max S A ~'77 g An EH (cm-1) (cm- 1) (cm- i) (10 -6 s) (10 II s- 1) (cm- t)

[Ru(bpY)3](PF6)2 - 17400 - - 4.3 760 5200 [Ru(bpy) 3 ]C12 " 6H 20 - 17400 - - 5.3 31 3300 [Rh(bpy) 2 CI 2 ](PF6) (18300) 14600 11 3700 98 7 4400

(18400) a 12000 a 18 a 6400 a

a Emission from a higher excited d-d state.

214 A. Islam et al. / Chemical Physics Letters 263 (1996) 209-214

intermediate for nonradiative transition of 3MLCT, which may be involved in the solution phase.

(2) An increased emission width (5450 cm - t ) of cis-[Rh(bpy)2C12](PF6) 2 on raising the temperature to 536 K is ascribed to superposition of two emis- sions from the lowest and the second lowest excited d - d states. The second lowest excited d -d state (3(d-d)2) is so displaced that the Franck-Condon energy of the emission is as large as 6400 cm -1. Nonradiative intersystem crossing of the lowest ex- cited state occurs via a potential crossing between the second lowest excited d -d state and the ground state.

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