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Decay pathways of the first excited singlet state of cis-1-styrylpyrene
Vdume 186, number 2,3
CHEMICAL PHYSICS LETTERS
8 November I99 I
Decay pathways of the first excited singlet state of cis-1 -styrylpyrene Anna SpaIletti, Giampiero Bartocci, Ugo Mazzucato Dipurtimento dr Chimica, Universitd di Perugia. 06100 Perugia, ltaly
and Guido Galiazzo Dipartimento di Chimica Organica, Clniversitd di Podova. 35’131 Padua, Italy
Received 6 August I99 I
The cis isomer of I-styrylpyrene displays an unusual fluorescence emission in fluid media at room temperature. The photophysical properties of the fluorescent state were studied in non-polar solvents in a large temperature range.The fluorimetric and photochemical results evidenced a substantial contribution of an adiabatic singlet mechanism to the cis-&ans photoisomerization.
1. Introduction
The main competitive processes of the first excited singlet state S, of 1,2_diarylethylenes (DAEs, Ar-CH=CH-Ar’) are fluorescence, geometrical isomerization and intersystem crossing (ISC), the latter followed by isomerization in the triplet manifold. The relative weight of these decay processes depends on the nature of the aryl groups linked to the ethylenic bridge. With regard to the trans isomers, stilbene has a high trans+cis quantum yield, little fluorescence and a negligibleISC (for review articles, see ref. [ 1 ] ), while in the analogous compounds containing polycyclic group(s) of low S, energy, the isomerization yield decreases (e.g., Ar= phenyl, Ar’= phenanthryl) or comes to naught (e.g., Ar = phenyl, Ar’=anthryl) and fluorescence is the main deactivation pathway, sometimes accompanied by substantial ISC (for review articles, see ref. [ 21). Much less information is available about ,the excited states of cis-DAEs because they are very shortlived. Usually, they do not fluoresce, at least in fluid solutions, and isomerize easily through a generally non-activated twisting. Only in a rigid matrix, be-
cause of the constraints imposed by the medium, iso-
merization becomes less efficient and fluorescence can be observed [ 11. The S, state of the cis isomers of 2-styrylanthracene (2-StA) [ 3,4] #I, 9-styrylanthracene (9-StA) [ 5-8 ] and 1-styrylpyrene ( 1-StPy ) [ 9,10 ] has been reported to display a typical behaviour. In fact, the presence of a sizeable energy barrier to twisting in S, for these compounds makes them relatively longlived. Therefore, they are fluorescent also in fluid media at room temperature and may have a substantial ISC yield, which is responsible for their isomerization in the triplet manifold. The triplet behaviour of 2-S& 1-StPy and related molecules has been extensively studied by Tokumaru and co-workers [3,4,10]. In order to extend our study of the fluorescent and photochemical behaviour of trans-DAEs containing polycyclic groups [ 21, we recently started an investigation of the decay properties of the corresponding cis isomers, particularly those displaying the unusual fluorescence behaviour strongly differing from that of cis-stilbene. RIRef. [ 41 is a review article.
0009-26 14/91/$ 03.50 0 1991 Elsevier Science Publishers B.V. All rights reserved.
297
Volume 186,number 2,3
CHEMICAL PHYSICS LETTERS
We report here a brief description of the decay properties of cis- I -StPy. The results obtained for its radiative decay also gave information on the contribution of the singlet mechanism to the cis-+trans photoisomerization.
\\”
‘/ %
\
,YH
’/
cis-1-StPy I’
2. Experimental A mixture of the two stereoisomers of I-StPy was prepared by a Witting-type reaction from L-pyrenaldehyde and benzyltrisphenylphosphonium salt. The trans and cis isomers were then separated by liquid chromatography (Waters apparatus) with a semipreparative column (uBondapak Cls, 125 A, 10 pm) and UV detector. A 15: 85 water:acetonitrile mixture was used as eluant and 350 nm as analysis wavelength. The compounds were characterized by elemental analysis and UV, ‘H NMR and mass spectra. The solvent was a mixture of methylcyclohexane and 3-methylpentane (MCH: 3MP, 9: 1). Both of
ABSORPTION
8 November
I99
I
them were RS grade from Carlo Erba, purified by standard procedures. The fluorescence spectra and quantum yields (&) were measured by a Perkin Elmer MPF-66 spectrofluorimeter using 9,10-diphenylanthracene in cyclohexane (I& = 0.98 ) as standard. The fluorescence lifetimes (7F) were measured by an Edinburgh Instruments 199 spectrometer using the single-photon counting (SPC) technique. For the measurements as a function of temperature (in the range 150 to 354 K), an Oxford cryostat was used. The cis-trans photoreaction quantum yield (@I_~) was measured in dilute solutions (2x 10m5 to 2~ 10m4M) with a Perkin Elmer Lambda 5 spectrophotometer (A,,,=335 nm) and controlled by an analytical HPLC Waters chromatograph.
3. Results and discussion 3.1. Fluorimetric behaviour Direct photoexcitation of cis-1-StPy in MCH : 3MP (9: 1) at room temperature gave the fluorescence spectrum reported in fig. 1. This emission is very similar in shape of the corresponding trans isomer, shown by comparison. The fluorescence excitation spectrum was independent of the emission wave-
FLUORESCENCE I-stpy in MCH: 3t.l~ (9 :l) T=300 K
Fig. 1. Absorption and fluorescence spectra of the cis (-) and trans (- - -) isomers of I-StPy in MCH :3MP (9: 1) at room temperature. Stars refer to fluorescence excitation of cis-I-StPy at A,,=425 nm. The fluorescence spectrum of cis-1-StPy is reported as experimentally obtained by direct excitation of the cis isomer and contains a large contribution by the tram isomer (see text).
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length and completely overlapped the absorption spectrum (fig. 1). Single-photon counting measurements of the decay curves showed a b&exponential trend with a growth of the first term (negative preexponential factor, A,) and a decaying term (positive pre-exponential factor, A,) (fig. 2a). The corresponding lifetimes were 1.3 and 4.4 ns, respectively, the latter value corresponding to that measured by direct excitation of the trans isomer (~,=4.4 ns in n-hexane at room temperature). Lack of fluorescence of the cis isomer at room temperature had been reported by Kovalenko et al. [ 91, who recorded a spectrum in n-hexane at 77 K (in substantial agreement with that shown in fig. 1) and found a lifetime of 3 ns, probably measured at
the same temperature. Our present results show that excitation of cis-lStPy gives rise to two fluorescent species. In fact, a negative amplitude factor in the first term of the SPC decay measurements indicates that the decay of the short-lived component leads to direct formation of the longer-lived component according to the processes ociss’cis+‘trans-+“trans (superscripts 0 and 1 refer to the ground and first excited singlet states, respectively). A similar behaviour has been reported for cis-9-StA [6]. In an attempt to obtain the emission properties of the cis isomer alone, a study was carried out by sta-
8 November 1991
tionary and pulsed techniques as a function of temperature. Fig. 3 shows the emission spectra recorded at decreasing temperatures in the range 344 to 150 K. The contribution of the trans component gradually decreases and disappears below 200 K, where the spectrum no longer changed with temperature. A similar temperature effect on the decay curves was observed by pulsed SPC measurements (table 1) . A bi-exponential decay was found above 200 K, where the contribution of the trans emission is not negligible, while a mono-exponential decay was operative at lower temperatures ( 150 and 180 K), where only the cis isomer emits (fig. 2b). Perusal of table 1 shows that the 7F.2values, pertaining to the trans isomer, remain practically constant, in agreement with the high-energy barrier to twisting in S, which prevents isomerization of ‘ttXttS (the slight decrease of TF,2 with temperature can be justified by the changes in the refractive index). On the other hand, the temperature-dependent tF,] values of the cis isomer lead to an activation energy of 7.7 ( IL 3%) kcal mol-’ and a frequency factor of 1.9~ lOI ( k 2%) for the ‘cis+ ‘trans process (fig. 4). These data are collected in table 2 along with other photophysical and photochemical parameters at room and low temperature. The eF measured at 300 K is the sum of the contributions by both the directly excited cis isomer and the excited tram isomer formed by adiabatic
Fig. 2. Fluorescence decaycurvesofcis-l-StPy in MCH:3MP (9: 1) at (a) 300 K (biexponential decay: TV,,= 1.31 ns, ~~,~=4.38ns) and (b) 180 K (monoexponential decay: sF,,= 3.85 I-IS).
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186, number 2,3
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8 November 1991
T-344K
L L
400
450
T.235K
T=265K
500
550
Fig. 3. Temperature effecton thefluorescence spectrumobtainedbyexcitationof cis-l-StPy in MCH: 3MP (9: I). Dashed spectrum at 150 K refers to the tram isomer.
Table 1 Lifetimes, pre-exponential factors and fitting parameters as obtained by deconvolution of the decay curves of cis-IdtPy in MCH: 3MP (9: 1) at different temperatures
T(K)
7F.l (ns)
354 344 334 324 314 300 290
0.27 0.33 0.44 0.79 0.89 1.30
280 265 235 205 180 150
1.83 2.11 2.71 3.82 3.85 3.85 3.22
7F,z (ns)
A,
A2
X2
4.72 4.66 4.59 4.46 4.40 4.36 4.26 4.08 3.92 3.83 3.92
-0.097 -0.095 -0.096 -0.064 -0.089 -0.081 -0.094 -0.130 -0.183 -7.59
0.090 0.091 0.103 0.107 0.127 0.144 0.148 0.192 0.251 1.67 2.91
1.28 1.18 1.30 1.23 1.12 1.04 1.15 1.05 1.15 1.33 1.19 1.03 .0.98
isomerization, while the value at 180 K pertains to the cis isomer alone. From the latter value and the corresponding lifetime, measured at the same temperature, where only the emission of ‘cis was observed, the radiative rate parameter ( kF= 1.5X 10’ s- ’ ) was calculated. By assuming that this value does not change with temperature, and using ?F,] at 300 K, we obtained the fluorescence quantum yield of 300
-2.83 0.085 0.093
the cis isomer ($9). This shows that the experimental value of 0.74 found at 300 K contains, in addition to the value pertinent to the cis isomer (0.19), a substantial contribution (0.55) from its isomerization product. Considering that &““” is 0.9 in the same solvent at room temperature, one can estimate a population of 6 1W for ‘trans.
Volume 186, number 2,3
CHEMICAL PHYSICS LETTERS
4
.2 22 c
0 50
250
150
350
T/K Fig. 4. Fluorescence lifetimes ofcis-I-StPy in MCH: 3MP (9: I ) as a function of temperature. The inset shows the corresponding Arrhenius plot. Table 2 Photophysical and photochemical parameters of the S, state pop ulated by direct excitation of cis-1-StPy in MCH: 3MP (9: I ) at 300 K (data in parentheses refer to I80 K) Experimental parameters QF 7F.l
tns)
@E-l AE,,, (kcal mol-‘) A,,, ( lOI s-‘)
3.2. Photochemical
Derived parameters 0.74 (0.58)
@$
I.30 (3.85 0.50 7.7
#k:;,OY)
1.9
0.19
1 $d%%ution) k_,
(I@ s-1)
0.55 0.61 1.5 4.7
behaviour
The T, state produced by photosensitization of lStPy has been reported to isomerize in both directions leading to a photostationary state much enriched (98%) in the trans isomer [ lo]. A cis+trans quantum yield much higher than unity (increasing with the cis- 1-StPy concentration) was reported and explained by two decay channels (a 3cis+3perp twisting followed by decaying to trans and cis ground states and an adiabatic 3cis+3trans isomerization followed by triplet energy transfer to the cis isomer through a chain process) [ lo]. On the other hand, the S, state is less photoreactive due to high-energy barriers to twisting for both geometrical isomers. An energy barrier > 12 kcal mol-’ for the
8 November 199I
‘trans+‘cis process was estimated by the constancy in @km”” with temperature and by theoretical calculations [ 111 while a barrier of 7.7 kcal mol --I was found for the ‘cis-t ‘trans isomerization (a similar energy difference of 5-6 kcal mbl- ’ between the two stereoisomers has been reported in the triplet state [ lo] ). The ‘trans form does not isomerize and decays mainly by fluorescence and slight dimerization [9]. The small quantum-yield value (0.04) for the sum L + @dimer. reported previously [ 9 ] seems to be practically due to dimerization, the cis production being below our detectability limits. We have now measured the isomerization yield of the cis isomer in dilute solutions (table 2) where the chain process (energy transfer from 3trans to Otisregenerating 3cis [ lo] ) should not be operative. In fact, &heyield remained practically constant (0.50 + 0.04) in the concentration range explored (2x lop5 to 2 x 10e4 M). In similar conditions, Kovalenko et al. [ 91 found a yield of 0.7 ( + 30%). These values reflect the sum of the adiabatic mechanisms operative in both the singlet (this work) and triplet [lo] states. From the kC,t value calculated by the Arrhenius parameters for twisting in S, and the fluorescence lifetime, the expected &_, value for the singlet mechanism was found to be 0.61, a value equal to that estimated for the ‘trans population by the & values at room temperature. This agreement supports the decay scheme proposed to account for our experimental data. The value, which was estimated with an uncertainty of 2596,is found to be even higher than the experimental one. This indicates that, under direct excitation and in the absence of the chain contribution, the isomerization via the triplet mechanism has a very small quantum yield. A &_, estimated by the same procedure for T= 180 K gave a negligible value of 3 x 10-4, in agreement with the observation that only the cis isomer emits at this temperature.
Acknowledgement This work was supported by the Consiglio Nazionale delle Ricerche (Rome) and Minister0 per l’llniversith e la Ricerca Scientifica e Tecnologica (Rome). The technical assistance of Mr. D. Pannacci is gratefully acknowledged. 301
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CHEMICAL PHYSICS LETTERS
References [ I ] J. Saltiel, J. D’Agostino, E.D. Me&y,
L. Metts, K.R. Neuberger, M. Wrighton and O.C. Zafiriou, Org. Photochem. 3 (1973) I; J. Saltiel and J.L. Charlton, in: Rearrangementsin ground and excited states, Vol. 3, ed. P. De Mayo (Academic Press, New York, 1980); J. Saltiel and Y.-P. Sun, in: Photochromism: molecules and systems, eds. H. Dlirr and H. Bouas-Laurent (Elsevier, Amsterdam, 1990) ch. 3. t2) U. Mazzucato, Pure Appl. Chem. 54 (1982) 1705: Gazz. Chim. Ital. I17 (1987) 661. [ 31 T. Karatsu, T. Arai, H. Sakuragi and K. Tokumaru, Chem. Phys. Letters I I5 (1985) 9.
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[4] T. Arai, T. Karatsu, H. Misawa, Y. Kuriyama, H. Gkamoto, T. Hircsaki, H. Furuuchi, H. Zeng, H. Sakuragi and K. Tokumaru, Pure Appl. Chem. 60 ( 1988) 989. [S] H.-D. Becker and K. Andersson, 3. Org. Chem. 48 (1983) 4542. [6] K. Sandros and H.-D. Becker,J. Photochem. 39 ( 1987) 301; 43 (1988) 291. [7] H. Garner, J. Photochem. Photobiol. A 43 ( 1988) 263. [S] G. Galiazzo, A. Spalletti, F. Elisei and G. Gennari, Gazz. Chim. Ital. I I9 (1989) 277. [9] N.P. Kovalenko, A.T. Abdukadyrov, V.I. Gerko and M.V. Alfimov, J. Photochem. 12 (1980) 59. [IO] T. Arai, H. Okamoto, H. Sakuragi and K. Tokumaru, Chem. Phys. Letters I57 (1989) 46; Bull. Chem. Sot. Japan 63 (1990) 2881. [ 111G. Poggi, A. Spalletti et al., unpublished results.
CHEMICAL PHYSICS LETTERS
8 November I99 I
Decay pathways of the first excited singlet state of cis-1 -styrylpyrene Anna SpaIletti, Giampiero Bartocci, Ugo Mazzucato Dipurtimento dr Chimica, Universitd di Perugia. 06100 Perugia, ltaly
and Guido Galiazzo Dipartimento di Chimica Organica, Clniversitd di Podova. 35’131 Padua, Italy
Received 6 August I99 I
The cis isomer of I-styrylpyrene displays an unusual fluorescence emission in fluid media at room temperature. The photophysical properties of the fluorescent state were studied in non-polar solvents in a large temperature range.The fluorimetric and photochemical results evidenced a substantial contribution of an adiabatic singlet mechanism to the cis-&ans photoisomerization.
1. Introduction
The main competitive processes of the first excited singlet state S, of 1,2_diarylethylenes (DAEs, Ar-CH=CH-Ar’) are fluorescence, geometrical isomerization and intersystem crossing (ISC), the latter followed by isomerization in the triplet manifold. The relative weight of these decay processes depends on the nature of the aryl groups linked to the ethylenic bridge. With regard to the trans isomers, stilbene has a high trans+cis quantum yield, little fluorescence and a negligibleISC (for review articles, see ref. [ 1 ] ), while in the analogous compounds containing polycyclic group(s) of low S, energy, the isomerization yield decreases (e.g., Ar= phenyl, Ar’= phenanthryl) or comes to naught (e.g., Ar = phenyl, Ar’=anthryl) and fluorescence is the main deactivation pathway, sometimes accompanied by substantial ISC (for review articles, see ref. [ 21). Much less information is available about ,the excited states of cis-DAEs because they are very shortlived. Usually, they do not fluoresce, at least in fluid solutions, and isomerize easily through a generally non-activated twisting. Only in a rigid matrix, be-
cause of the constraints imposed by the medium, iso-
merization becomes less efficient and fluorescence can be observed [ 11. The S, state of the cis isomers of 2-styrylanthracene (2-StA) [ 3,4] #I, 9-styrylanthracene (9-StA) [ 5-8 ] and 1-styrylpyrene ( 1-StPy ) [ 9,10 ] has been reported to display a typical behaviour. In fact, the presence of a sizeable energy barrier to twisting in S, for these compounds makes them relatively longlived. Therefore, they are fluorescent also in fluid media at room temperature and may have a substantial ISC yield, which is responsible for their isomerization in the triplet manifold. The triplet behaviour of 2-S& 1-StPy and related molecules has been extensively studied by Tokumaru and co-workers [3,4,10]. In order to extend our study of the fluorescent and photochemical behaviour of trans-DAEs containing polycyclic groups [ 21, we recently started an investigation of the decay properties of the corresponding cis isomers, particularly those displaying the unusual fluorescence behaviour strongly differing from that of cis-stilbene. RIRef. [ 41 is a review article.
0009-26 14/91/$ 03.50 0 1991 Elsevier Science Publishers B.V. All rights reserved.
297
Volume 186,number 2,3
CHEMICAL PHYSICS LETTERS
We report here a brief description of the decay properties of cis- I -StPy. The results obtained for its radiative decay also gave information on the contribution of the singlet mechanism to the cis-+trans photoisomerization.
\\”
‘/ %
\
,YH
’/
cis-1-StPy I’
2. Experimental A mixture of the two stereoisomers of I-StPy was prepared by a Witting-type reaction from L-pyrenaldehyde and benzyltrisphenylphosphonium salt. The trans and cis isomers were then separated by liquid chromatography (Waters apparatus) with a semipreparative column (uBondapak Cls, 125 A, 10 pm) and UV detector. A 15: 85 water:acetonitrile mixture was used as eluant and 350 nm as analysis wavelength. The compounds were characterized by elemental analysis and UV, ‘H NMR and mass spectra. The solvent was a mixture of methylcyclohexane and 3-methylpentane (MCH: 3MP, 9: 1). Both of
ABSORPTION
8 November
I99
I
them were RS grade from Carlo Erba, purified by standard procedures. The fluorescence spectra and quantum yields (&) were measured by a Perkin Elmer MPF-66 spectrofluorimeter using 9,10-diphenylanthracene in cyclohexane (I& = 0.98 ) as standard. The fluorescence lifetimes (7F) were measured by an Edinburgh Instruments 199 spectrometer using the single-photon counting (SPC) technique. For the measurements as a function of temperature (in the range 150 to 354 K), an Oxford cryostat was used. The cis-trans photoreaction quantum yield (@I_~) was measured in dilute solutions (2x 10m5 to 2~ 10m4M) with a Perkin Elmer Lambda 5 spectrophotometer (A,,,=335 nm) and controlled by an analytical HPLC Waters chromatograph.
3. Results and discussion 3.1. Fluorimetric behaviour Direct photoexcitation of cis-1-StPy in MCH : 3MP (9: 1) at room temperature gave the fluorescence spectrum reported in fig. 1. This emission is very similar in shape of the corresponding trans isomer, shown by comparison. The fluorescence excitation spectrum was independent of the emission wave-
FLUORESCENCE I-stpy in MCH: 3t.l~ (9 :l) T=300 K
Fig. 1. Absorption and fluorescence spectra of the cis (-) and trans (- - -) isomers of I-StPy in MCH :3MP (9: 1) at room temperature. Stars refer to fluorescence excitation of cis-I-StPy at A,,=425 nm. The fluorescence spectrum of cis-1-StPy is reported as experimentally obtained by direct excitation of the cis isomer and contains a large contribution by the tram isomer (see text).
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length and completely overlapped the absorption spectrum (fig. 1). Single-photon counting measurements of the decay curves showed a b&exponential trend with a growth of the first term (negative preexponential factor, A,) and a decaying term (positive pre-exponential factor, A,) (fig. 2a). The corresponding lifetimes were 1.3 and 4.4 ns, respectively, the latter value corresponding to that measured by direct excitation of the trans isomer (~,=4.4 ns in n-hexane at room temperature). Lack of fluorescence of the cis isomer at room temperature had been reported by Kovalenko et al. [ 91, who recorded a spectrum in n-hexane at 77 K (in substantial agreement with that shown in fig. 1) and found a lifetime of 3 ns, probably measured at
the same temperature. Our present results show that excitation of cis-lStPy gives rise to two fluorescent species. In fact, a negative amplitude factor in the first term of the SPC decay measurements indicates that the decay of the short-lived component leads to direct formation of the longer-lived component according to the processes ociss’cis+‘trans-+“trans (superscripts 0 and 1 refer to the ground and first excited singlet states, respectively). A similar behaviour has been reported for cis-9-StA [6]. In an attempt to obtain the emission properties of the cis isomer alone, a study was carried out by sta-
8 November 1991
tionary and pulsed techniques as a function of temperature. Fig. 3 shows the emission spectra recorded at decreasing temperatures in the range 344 to 150 K. The contribution of the trans component gradually decreases and disappears below 200 K, where the spectrum no longer changed with temperature. A similar temperature effect on the decay curves was observed by pulsed SPC measurements (table 1) . A bi-exponential decay was found above 200 K, where the contribution of the trans emission is not negligible, while a mono-exponential decay was operative at lower temperatures ( 150 and 180 K), where only the cis isomer emits (fig. 2b). Perusal of table 1 shows that the 7F.2values, pertaining to the trans isomer, remain practically constant, in agreement with the high-energy barrier to twisting in S, which prevents isomerization of ‘ttXttS (the slight decrease of TF,2 with temperature can be justified by the changes in the refractive index). On the other hand, the temperature-dependent tF,] values of the cis isomer lead to an activation energy of 7.7 ( IL 3%) kcal mol-’ and a frequency factor of 1.9~ lOI ( k 2%) for the ‘cis+ ‘trans process (fig. 4). These data are collected in table 2 along with other photophysical and photochemical parameters at room and low temperature. The eF measured at 300 K is the sum of the contributions by both the directly excited cis isomer and the excited tram isomer formed by adiabatic
Fig. 2. Fluorescence decaycurvesofcis-l-StPy in MCH:3MP (9: 1) at (a) 300 K (biexponential decay: TV,,= 1.31 ns, ~~,~=4.38ns) and (b) 180 K (monoexponential decay: sF,,= 3.85 I-IS).
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186, number 2,3
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T-344K
L L
400
450
T.235K
T=265K
500
550
Fig. 3. Temperature effecton thefluorescence spectrumobtainedbyexcitationof cis-l-StPy in MCH: 3MP (9: I). Dashed spectrum at 150 K refers to the tram isomer.
Table 1 Lifetimes, pre-exponential factors and fitting parameters as obtained by deconvolution of the decay curves of cis-IdtPy in MCH: 3MP (9: 1) at different temperatures
T(K)
7F.l (ns)
354 344 334 324 314 300 290
0.27 0.33 0.44 0.79 0.89 1.30
280 265 235 205 180 150
1.83 2.11 2.71 3.82 3.85 3.85 3.22
7F,z (ns)
A,
A2
X2
4.72 4.66 4.59 4.46 4.40 4.36 4.26 4.08 3.92 3.83 3.92
-0.097 -0.095 -0.096 -0.064 -0.089 -0.081 -0.094 -0.130 -0.183 -7.59
0.090 0.091 0.103 0.107 0.127 0.144 0.148 0.192 0.251 1.67 2.91
1.28 1.18 1.30 1.23 1.12 1.04 1.15 1.05 1.15 1.33 1.19 1.03 .0.98
isomerization, while the value at 180 K pertains to the cis isomer alone. From the latter value and the corresponding lifetime, measured at the same temperature, where only the emission of ‘cis was observed, the radiative rate parameter ( kF= 1.5X 10’ s- ’ ) was calculated. By assuming that this value does not change with temperature, and using ?F,] at 300 K, we obtained the fluorescence quantum yield of 300
-2.83 0.085 0.093
the cis isomer ($9). This shows that the experimental value of 0.74 found at 300 K contains, in addition to the value pertinent to the cis isomer (0.19), a substantial contribution (0.55) from its isomerization product. Considering that &““” is 0.9 in the same solvent at room temperature, one can estimate a population of 6 1W for ‘trans.
Volume 186, number 2,3
CHEMICAL PHYSICS LETTERS
4
.2 22 c
0 50
250
150
350
T/K Fig. 4. Fluorescence lifetimes ofcis-I-StPy in MCH: 3MP (9: I ) as a function of temperature. The inset shows the corresponding Arrhenius plot. Table 2 Photophysical and photochemical parameters of the S, state pop ulated by direct excitation of cis-1-StPy in MCH: 3MP (9: I ) at 300 K (data in parentheses refer to I80 K) Experimental parameters QF 7F.l
tns)
@E-l AE,,, (kcal mol-‘) A,,, ( lOI s-‘)
3.2. Photochemical
Derived parameters 0.74 (0.58)
@$
I.30 (3.85 0.50 7.7
#k:;,OY)
1.9
0.19
1 $d%%ution) k_,
(I@ s-1)
0.55 0.61 1.5 4.7
behaviour
The T, state produced by photosensitization of lStPy has been reported to isomerize in both directions leading to a photostationary state much enriched (98%) in the trans isomer [ lo]. A cis+trans quantum yield much higher than unity (increasing with the cis- 1-StPy concentration) was reported and explained by two decay channels (a 3cis+3perp twisting followed by decaying to trans and cis ground states and an adiabatic 3cis+3trans isomerization followed by triplet energy transfer to the cis isomer through a chain process) [ lo]. On the other hand, the S, state is less photoreactive due to high-energy barriers to twisting for both geometrical isomers. An energy barrier > 12 kcal mol-’ for the
8 November 199I
‘trans+‘cis process was estimated by the constancy in @km”” with temperature and by theoretical calculations [ 111 while a barrier of 7.7 kcal mol --I was found for the ‘cis-t ‘trans isomerization (a similar energy difference of 5-6 kcal mbl- ’ between the two stereoisomers has been reported in the triplet state [ lo] ). The ‘trans form does not isomerize and decays mainly by fluorescence and slight dimerization [9]. The small quantum-yield value (0.04) for the sum L + @dimer. reported previously [ 9 ] seems to be practically due to dimerization, the cis production being below our detectability limits. We have now measured the isomerization yield of the cis isomer in dilute solutions (table 2) where the chain process (energy transfer from 3trans to Otisregenerating 3cis [ lo] ) should not be operative. In fact, &heyield remained practically constant (0.50 + 0.04) in the concentration range explored (2x lop5 to 2 x 10e4 M). In similar conditions, Kovalenko et al. [ 91 found a yield of 0.7 ( + 30%). These values reflect the sum of the adiabatic mechanisms operative in both the singlet (this work) and triplet [lo] states. From the kC,t value calculated by the Arrhenius parameters for twisting in S, and the fluorescence lifetime, the expected &_, value for the singlet mechanism was found to be 0.61, a value equal to that estimated for the ‘trans population by the & values at room temperature. This agreement supports the decay scheme proposed to account for our experimental data. The value, which was estimated with an uncertainty of 2596,is found to be even higher than the experimental one. This indicates that, under direct excitation and in the absence of the chain contribution, the isomerization via the triplet mechanism has a very small quantum yield. A &_, estimated by the same procedure for T= 180 K gave a negligible value of 3 x 10-4, in agreement with the observation that only the cis isomer emits at this temperature.
Acknowledgement This work was supported by the Consiglio Nazionale delle Ricerche (Rome) and Minister0 per l’llniversith e la Ricerca Scientifica e Tecnologica (Rome). The technical assistance of Mr. D. Pannacci is gratefully acknowledged. 301
Volume 186, number 2.3
CHEMICAL PHYSICS LETTERS
References [ I ] J. Saltiel, J. D’Agostino, E.D. Me&y,
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