- Email: [email protected]
Photoisomerization dynamics of alkyl-substituted stilbenes in supersonic jets
Volume
125, number
1
CHEMICAL
PHOTOISOMERIZATION IN SUPERSONIC JETS Klaus
RADEMANN
School of Chemrstty, Received
DYNAMICS
‘, Uzi EVEN,
PHYSICS
LETTERS
21 March 1986
OF ALKYL-SUBSTITUTED
Shlomo ROZEN
and Joshua
STILBENES
JORTNER
Tel-Aviv Umoersuy, 69978 Tel Avrv, Israel
17 December
1985
We report time-resolved fluorescence decay lifetimes from photoselected states of jet-cooled 4-ethyl-trans-stilbene (4ETS) enhances and 4-n-propyl-trans-stilbene (4PTS) in the energy range E, = O-2800 cm-’ above the S, origin. Alkyl substitution the non-radiative decay rates. These data can be fitted by the statistical RRKM theory in conjunction with the reduction of the to Et = 1100 f 100 cm-’ for 4ETS. and E, = lOOO+ photoisomerization threshold from E, = 13OOk 50 cm-’ for trans-stilbene 100 cm-’ for IPTS.
With the advent of modern experimental techniques of picosecond spectroscopy and spectroscopy in supersonic jets, a novel area of intramolecular dynamics has emerged, which pertains to photochemistry in collision-free, isolated, large molecules [l-3]. The dynamics of tram-cis isomerization of stilbene, which provides a prototype for photoisomerization in an isolated molecule, has been explored in the low-pressure gas phase [3,4] and in seeded supersonic expansions [5-131. In contrast to cis-trans photoisomerization of stilbene, which is a barrierless process characterized by a subpicosecond lifetime [ 121, the trans-cis isomerization involves barrier crossing [6, 7,9-l 1,13,14]. The lifetimes of photoselected states above the barrier in jet-cooled tram-stilbene, which was estimated to be located at Et = 900-1300 cm-l [6,7,9-l 1,141 above the S1 origin, are sufficiently long (2.5 ns-30 ps) [6,7,9-l 1,131 to permit their interrogation by fluorescence lifetime measurements [6,11,13], fluorescence quantum yields [7] and multiphoton ionization [IO]. These experimental methods have provided extensive information on the energy dependence of the rate constant, krir, for the nonradiative unimolecular decay process. A central issue ’ Permanent address: Fachbereich Physikalische lips-Universitat Marburg, D-3550 Marburg Germany.
Chemie, Phil(Lahn), West
0 009-2614/86/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
pertaining to isolated-molecule photochemistry involves the role of intramolecular vibrational energy redistribution (IVR), which relates to the following major points: (1) Do the reaction rates carry the signature of mode selectivity? (2) What are the implications of IVR in determining the reaction rates? Regarding point (l), it has been established that efficient non-radiative decay at E > Et in tram-stilbene [5--l 1,131 and some of its derivatives [ 111 essentially occurs in the energy range where the Su +S, transition exhibits a structureless absorption spectrum and where the energy dependence of b is smooth, indicating that efficient IVR has eroded all mode selectivity. Concerning the effects of IVR on the photoisomerization rates (point (2)), it is still an open question whether photoisomerization in the isolated molecule involves a “democratic” IVR process among all vibrational modes. Most of the evidence pertaining to this cardinal point rests on a quantitative analysis of the energy dependence of & The RRKM calculations of Khundkar et al. [9] indicated that the experimental k, values are lower by about an order of magnitude than those obtained for the statistical model, and it was suggested [9,13] that an efficient reverse process has to be invoked. Subsequently, Troe has demonstrated [ 141 that an optimized RRKM calculation, 5
Volume125, number 1
CHEMICAL PHYSICSLETTERS
which uses a fitting procedure for the activated complex frequencies, can quantitatively account for the rates, providing necessary but by no means sufficient evidence for full energy r~dom~ation in the isolated molecule. An alternative approach to this issue involves the effect of the change of the total number of vibrational degrees of freedom on the isomerization dynamics [ 111. In this context, Majors et al. [ 1l] did not observe a marked dependence of k, on methyl substitution of tram-stilbene, while the RRKM theory predicts a decrease of kar by about 35% [ 141 upon methyl substitution for the same molecular parameters. As slight changes in the barrier height Et and the activated complex frequencies scaling factor F canbe induced by methyl substitution [ 141, the discrepancy between the calculated drop of k, by 35% for 4methyl-tram-stilbene relative to trans.stilbene, and the slight experimental increase of w 15% [l 11,does not provide a critical scrutiny of the app~cab~~ of the statistical theory. In this note we report on the time-resolved fhroresence decay lifetimes from photoselected states of jet-cooled 4ETS and 4pTS. It was established in the studies of Smalley et al. [ 151 that alkyl substitution of the benzene ring results in considerable enhancement of intrastate vibrational level mixing. It is interesting to inquire what the implications are of IVR in ~1.substituted tr~s-st~bene on photoisome~~tion dynamics. We have obtained the “counterintuitive” result that ethyl and propyl substitution of tram-stilbene enhances the photoisomerization rates, while general arguments based on the role of IVR and the implications of statistical theories in dicate that the rate should be retarded. It appears that our experimental results can be accounted for in terms of the mo~~cation of the moIecular parameters, i.e. the threshold energy, by aIky1 substitution. The study of the isomerization dynamics of alkyl-substituted stilbenes will further clarify the applicability of statistical theories for the description of photochemistry in isolated molecules. The time-resolved fluoresence decay lifetime for photoselected vibrational states in the S, manifold of jet-cooled tr~s-st~bene (TS), 4-rne~yl-tr~~stilbene (4MTS), 4.ethyl-tram-stilbene (4ETS) and 4-n-propyl-trans-stiene (4FTS) were excited by a mode-locked picosecond pulsed dye laser. The seeded supersonic jet was produced by a continuous expansion in Ar (stagnation pressure p = 150-300 Tori) through 6
21 March1986
a 150 Mm nozzle. The jet was excited by a cavitydumped dye laser (Coherent 599 laser equipped with a Coherent 7200 dumper), which was synchronously pumped by the 5 14 nm line of a rn~e-liked .Ar laser (Coherent CR12). The output of the dye laser (pulse length 30 ps and 250 mW average power) was frequency doubled by a KDP crystal. The W light could be scanned in the range 320-280 nm. The time-resolved fluorescence signal was interrogated using a fast photon-counting system, which consists of a two-stage microchannel plate photom~tip~er whose output pulse was detected by a pair of consent-fraction discriminators (Tennelec 453) and a microcomputer [ 1I]. The response curve, F(t), of the excitation pulse was characterized by a width (fwhm) of 180 ps. The timeresolved excited-state decay of the seeded molecule was determined by a reconvolution technique, simulating the experimental decay curve, I(t), by the convolution I(t) = F(t) cxt exp(- t/r). A mean least-squares fit of the expe~ment~ data was performed over a time domain of O-5 r to extract the lifetime r. Lifetimes were averaged over several different measurements. The accuracy of the lifetime data was 280 ps for r values in the range 2500- 1000 ps and f 50 ps for r values in the range 700- 100 ps. The shortest r values that could be measured were T = 100 f 50 ps. Decay lifetimes were obtained for excess vibrations energies& = O-3000 cm-l above the S, origin. The alkyl stilbenes were prepared by the Grignard reaction between the corresponding 4-alkyl benzaldehydes with phenylmagnesium bromide. The resulting alcohols were thermally dehydrated in boiling xylene to produce the known corresponding 4-alkyl transstilbenes. Increasing the size of the alkyl group con~derably enhances complexlng of Ar atoms with the alkyl stilbenes, as the longer alkyd chain provides an efficient sink for vibrational energy released by the attachment of Ar atom(s) to the large molecule. Some care had to be exerted to separate the isolated-molecule photochemistry from some (interesting) artifacts originating from the photophysics of its van der WaaIs(vdW) complexes. For TS and 4MTS the effects of vdW complexing with Ar are minor under our experimental conditions in the pressure range p = 160-250 Torr of Ar, as is evident from the laser-induced fluorescence (LIF) spectra of these molecules cooled in Ar and in He [ IX]. The time-resolved decay
Volume 125, number 1
CHEMICALPHYSICS LETTERS
of TS and 4MTS cooled at p = 160-250 Torr of Ar were characterized by a single exponential. For 4ETS internal cooling in Ar is efficient under our experimental conditions at p 2 160 Torr, while complexing with Ar sets in at p > 220 Torr, as is evident from the appearance of a broad spectral background in the LIF spectra in the range Ev = O-1200 cm-l. In the narrow pressure range p = 180-220 Torr, the timeresolved decay of 4ETS for Ev = O-2900 cm-l consists of a single exponential with a lifetime 7, which corresponds to the decay of the isolated molecule. At p > 250 Torr, the fluorescence decay following excitation in the “quasi-continuum” (Ev= 1000-2800 cm-l) was biexponential: 1(t) = F(t) 0 [d exp(- t/To) + B exp(- r/r)] ,
(1)
where r. = 2300 +-100 ps corresponds to the decay lifetime in the range Ev = O-1000 cm-l, while the amplitude ratio B/A decreases with increasing p. The slow exponential is attributed to the vibrational predissociation (VP) of 4ETS *Ar, complexes of an unidentified composition, which results in 4ETS’ Ar, (m < n) complexes or in bare 4ETS with internal excess vibrational energy Ev < 1000 cm-l below the photoisomerization threshold. The fast component in eq. (1) corresponds to the decay of the bare molecule. For 4PTS, complexing was severe. At p = 150 Torr Ar, the decay was a single exponential with r = 2200 +-100 ps in the range Ev = 04300cm-l. At p = 190 Torr, the decay curves in the range Ev = 1000-2800 cm-l were biexponential (eq. (1)) with 7. = 2300 f 100 ps andA/B=O.l-0.2. The longdecay component corresponds again to the product of VP of 4F’TS. Ar, * . The values of r for 4F’TSobtained from the short-decay component at p = 190 Torr are identical with the single lifetime obtained atp = 150Torr. The LIF excitation spectra of the alkyl stilbenes * Alternative mechanisms for the reduction of the photoisomer&&on rate by AI complexing, which involve: (i)
IVR involving vibrational energy flow to the relative motion of the AI atom telative to the large molecule, and (ii) modification of the potential surface of the TS, resulting in steric hindrance of isomerization, can be discarded. The VP mechanism, which inhibits reactive isomerization in Ar-alkyl TS complexes, is analogous to the effects of VP on non-reactive intersystem crossing in h--dichloroanthracene complexes [ 191.
21 March 1986
were obtained in pulsed He jets (nozzle diameter 300 pm, gas pulse rate 12 s-l and He stagnation pressure p = 2-3 atm) and excited by frequency-doubled radiation from a pulsed dye laser (Molectron DLII). Table 1 summarizes the energetics of the S, electronic origin of these compounds. The LIF spectrum of 4MTS was identical to that previously reported [ 111. The LIF spectrum of 4ETS (fig. 1) reveals a set of low-frequency vibrations (starting at 22 and 32 cm-l), which may be due to the excitations of the ethyl group. A prominent vibrational progression of 191 cm-l is observed in 4ETS, being presumably analogous to the 200 cm-l vibration in TS, which corresponds to the C-C-Q symmetric in-plane bending. The LIF spectrum of 4FTS (fig. 2) reveals two lowest-energy intense spectral features separated by 60 cm-l, which may originate from the tram and the gauche (or eclipsed) isomers of the n-propyl group, in analogy to the spectrum of n-propylbenzene [ 151. The decay lifetimes of TS, 4MTS, 4ETS and 4PTS over the energy range Ev = O-2800 cm-l are portrayed in fig. 3. The lifetimes of TS over the range Ev = O-2800 cm-l and the lifetimes of 4MTS over the range Ev = 1200-3000 cm-l are in good agreement with previous data, revealing the practical invariance of r on methyl substitution [ 111. The lifetimes of the alkyl stilbenes reveal three domains: (i) An energyindependent region Ev T==S O-1200 cm-l for TS and 4MTS and Ev = O-900 cm-l for 4ETS and 4FTS. These energy-independent lifetimes, which are identical with r values for the electronic origins of S, (table l), are assigned on the basis of absolute quantum yield data [ 161 to the pure radiative decay lifetimes of the vibronic states in this region, i.e. r = r,d. As is apparent from table l, the effect of alkyl substituents and of isomeric conformation of the n-propyl chain on the pure radiative lifetime is minor, as expected. (ii) The onset of non-radiative decay is exhibited at Et% 1200 cm-l for 4MTS and at Ev * 1000 cm-l for 4ETS and 4PTS. (iii) A region of effective non-radiative decay at Ev > Et.The energy-dependent decay lifetimes calculated from the relation h = l/r - l/r& for isolated 4ETS and 4PTS are portrayed in fig. 4, where we have presented also the experimental data for TS together with Troe’s optimized RRKM calculation for TS with the parameters Et= 1300cm-1 and F = 1.2 [ 141, which provide an excellent fit of the TS experimental data. Our new experimental
Volume 125, number 1
CHEMICAL PHYSICS LETTERS
21 March 1986
Table 1 Energetics and decay dynamics of the Sr electronic origin of alkyl tram-stilbenes Molecule
ha) (A)
u a) (cm-t )
6 v b) (cm-’ )
7 @s)
trans-stilbene 4-methyl-trans~~bene $-ethyl-trans-stilbene 4-npropyl-trans-stilbene
3101.7 3147.3 3150.2 3159.2 c) 3Is3.2d)
32240 31773 31744 31654 31714
0 -467 -496 -586 -526
2500 rt 40 2650 f 80 239.5 i 50 2320 i 40 2300 + 40
b, Spectral shift upon alkyl substitution. a) Absolute accuracy f 0.5 A and f 5 cm-l. d, n-trans-propyl-trans-stilbene. c, n-gauche-propyl-trans-stilbene.
results for the decay rates of 4ETS and 4FTS are inconsistent with the RRKM calculations with the molecular parameters of TS. Troe’s calculations for 4MTS indicate that r) = k4MTS ~ (E)/kT,SQ = 0.74 at Ev = 2500-3000 cm- [ 141. On the basis of conI
I
3110
1 3020
’
I
~deration of ~yl-substitution effects on unimolecular rates, it is ex ected [I’? that ~4ETs(~)~~~(~) 1 =:02 =0.55 andkBPTS(E)/k,S(E)*q3 =0.41 in this energy range. Thge estimates are incompatible with the experimental data (fig. 4), which yield
I 3040
3120
WAVELENGTH
I 3060 WAVELENGTH
3150
1 3orjo
I
(A,
I
I 3100
(ii,
Fig. 1. LIF spectrum of 4ethyl-trans-stilbene in pulsed supersonic expansions of He. The numbers in parentheses are the energies (cm-r) above the electronic origin marked O-O, and the numbers in square brackets are the fluorescence lifetimes (ns) of the corresponding spectral features. Total fluorescence is normalized to the laser intensity.
8
ill0
3120
3130 WAVELENGTH
3070 I
3080 1
I
3150
3140 6)
I
3090
I
I
WAVELENGTH
I
1
3100
:c 0
$1
Fig. 2. LKF spectrum of 4-n-propyl-ttansstene in pulsed supersonic expansions of He. Notation as in fig. t The two electronic origins of the trans and gauche configurations of the n-propyl chain are marked as O-O(t) and O-O(g) respectively. Energies are given relative to O-O@).
4000
I
I
0
METHYL
6
ETHYL
v
PROPYL
1
I
1
Fig. 3. The dependence of the experimental fiuorescence decay lifetimes of TS, 4MTS, 4ETS and 4PTS over the energy range EV = O-2800 cm-’ above the Sx origin. The solid ties were drawn for visual representation of the data,
CHEMICAL PHYSICS LETTERS
Volume 125, number 1
E&m-1)
Fig. 4. The dependence of the non-radiative decay rates of TS, 4ETS and 4PTS on the excess vibrational energy above the Sr origin. - OptimizedRRKMcalculationfor TS [ 141. --- An estimate of the RRKM fit for 4PTS obtained from k&s(E)[k$MTS(E)/k&s(~]2. -.-.A rough estimate of th;sRRKM tit for 4ETS obtained from kTrs(,!?)[k$MTS(&‘)/ k,, (E)] 3. The rates kTSQ and k4MT$Q with Et = 1300 cm-’ and F = 1.2 werenfaken fromy’ef. [14].
k4ETS(E)/kTS(E) = k4PTS(E)/kT,S(E)* 1.7. We assert tl% the eneygy dependence of the non-radiative rates of alkyl TS (fig. 4) reveals a marked increase of the k, values relative to those of TS, which is apparent contrast to the expectation of the REXM calculations [ 141.This discrepancy cannot be taken as evidence for non-statistical effects on the unimolecular dynamics, as a reasonable modification of the molecular parameters may cure the discrepancy between our data and the RRKM calculations. It is reasonable to assume that the threshold energy Et is reduced by alkyl substitutions, as is evident from a visual inspection of fig. 3. On the basis of Troe’s calculations [ 141, we infer that k,(E; Et - 100 cm-l)/k,(E; Et) = 1.7. Accordingly, 10
21 March 1986
the discrepancy of a numerical factor of 3.1 between the RRKM theory and the experiment for kETS(E)/ kT,S(E)(for Ev = 2500-3000 cm-l) can be accounted for by a shift of 200 cm-l in Et towards lower energies, while the numerical discrepancy of 4.1 for 7c~S(E)/~~(E) (for Ev = 2500-3000 cm-l) can be reconciled by a shift of -300 cm-l in Et. Thus, the isomerization threshold for the Calkyl stilbenes are Et = 1100 cm-l for 4ETS and Et w 1000 cm-l for 4PTS. The accuracy of these estimates appears to be f 100 cm-l. With this modification of the threshold energy, the RRKM model does provide an adequate account of the experimental data. We conclude by emphasizing that the experimental data for alkyl benzenes are consistent with the statistical theory. The cardinal question of whether fully randomized IVR prevails in these isolated molecules has not been resolved. The major technical result of the present work involves the observation of the reduction of the threshold energy for photoisomerization of TS by alkyl substitution, with a lowering of Et by about 100 cm-l per CH2 group. An analogous effect of intermolecular interactions was invoked by Troe [14,18], who proposed the lowering of Et of TS by van der Waals clustering in the gas and liquid phase in order to account for solvent effects on photoisomerization dynamics.
Acknowledgement
We are indebted to Professor J. Troe for stimulating correspondence and for prepublication information. This research was supported in part by the United States Army through its European Research Office. References [ 1] J. Jortner , S.A. Rice and R.M. Hochstrasser, in: Advances in photochemistry, eds. B.O. Pitts and G. Hammond (Wiley, New York, 1969). [ 21 W.M. Gelbart and S.A. Rice, J. Chem. Phys. 50 (1969) 4715. [ 31 W.M. Gelbart, K.F. Freed and S.A. Rice, J. Chem. Phys. 52 (1970) 2460. (41 B.I. Greene, R.M. Hochstrasser and R.B. Weisman, Chem. Phys. 48 (1979) 544.
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CHEMICAL PHYSICS LETTERS
[ 51 B.I. Greene, R.M. Hochstrasser and R.B. Weisman, Chem. Phys. 48 (1980) 289. [6] J. Syage, W. Lambert, P. Felker, A.H. Zewail and R.M. Hochstrasser, Chem. Phys. Letters 88 (1982) 266. [ 71 A. Amirav and J. Jortner, Chem. Phys. Letters 95 (1983) 295. [ 81 T. Zwier, E. Carrasquillo and D.H. Levy, J. Chem. Phys. 78 (1983) 5493. [ 91 L.R. Khundar, R.A. Marcus and A.H. Zewail, J. Phys. Chem. 87 (1983) 2473. [lo] N.F. Scherer, J.P. Shepanski and A.H. Zewail, J. Chem. Phys. 81 (1984) 2181. [ll] T.J. Majors, U. Even and J. Jortner, J. Chem. Phys. 81 (1984) 2330. [12] B.I. Greene and R.C. Farrow, J. Chem. Phys. 78 (1983) 3336.
21 March 1986
[13] J.A. Syage, P.M. Felker and A.H. Zewail, J. Chem. Phys. 81 (1984) 4685,4706. [ 141 J. Troe, Chem. Phys. Letters 114 (1985) 241. [ 151 D. Powers, J.B. Hopkins and R.E. Smalley, J. Chem. Phys. 72 (1980) 5039. 1161 M. Sonnenschein, A. Amirav and J. Jortner, J. Phys. Chem. 88 (1984) 4214. [ 171 J. Troe, private communication. [ 181 J. Schroeder and J. Troe, Chem. Phys. Letters 116 (1985) 453; G. Maneke, J. Schroeder, J. Troe and F. Voss, Ber. Bunsenges. Physik. Chem. 89 (1985) 896. [ 191 A. Amirav, M. Sonnenschein and J. Jortner, Chem. Phys. 88 (1984) 199.
11
125, number
1
CHEMICAL
PHOTOISOMERIZATION IN SUPERSONIC JETS Klaus
RADEMANN
School of Chemrstty, Received
DYNAMICS
‘, Uzi EVEN,
PHYSICS
LETTERS
21 March 1986
OF ALKYL-SUBSTITUTED
Shlomo ROZEN
and Joshua
STILBENES
JORTNER
Tel-Aviv Umoersuy, 69978 Tel Avrv, Israel
17 December
1985
We report time-resolved fluorescence decay lifetimes from photoselected states of jet-cooled 4-ethyl-trans-stilbene (4ETS) enhances and 4-n-propyl-trans-stilbene (4PTS) in the energy range E, = O-2800 cm-’ above the S, origin. Alkyl substitution the non-radiative decay rates. These data can be fitted by the statistical RRKM theory in conjunction with the reduction of the to Et = 1100 f 100 cm-’ for 4ETS. and E, = lOOO+ photoisomerization threshold from E, = 13OOk 50 cm-’ for trans-stilbene 100 cm-’ for IPTS.
With the advent of modern experimental techniques of picosecond spectroscopy and spectroscopy in supersonic jets, a novel area of intramolecular dynamics has emerged, which pertains to photochemistry in collision-free, isolated, large molecules [l-3]. The dynamics of tram-cis isomerization of stilbene, which provides a prototype for photoisomerization in an isolated molecule, has been explored in the low-pressure gas phase [3,4] and in seeded supersonic expansions [5-131. In contrast to cis-trans photoisomerization of stilbene, which is a barrierless process characterized by a subpicosecond lifetime [ 121, the trans-cis isomerization involves barrier crossing [6, 7,9-l 1,13,14]. The lifetimes of photoselected states above the barrier in jet-cooled tram-stilbene, which was estimated to be located at Et = 900-1300 cm-l [6,7,9-l 1,141 above the S1 origin, are sufficiently long (2.5 ns-30 ps) [6,7,9-l 1,131 to permit their interrogation by fluorescence lifetime measurements [6,11,13], fluorescence quantum yields [7] and multiphoton ionization [IO]. These experimental methods have provided extensive information on the energy dependence of the rate constant, krir, for the nonradiative unimolecular decay process. A central issue ’ Permanent address: Fachbereich Physikalische lips-Universitat Marburg, D-3550 Marburg Germany.
Chemie, Phil(Lahn), West
0 009-2614/86/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
pertaining to isolated-molecule photochemistry involves the role of intramolecular vibrational energy redistribution (IVR), which relates to the following major points: (1) Do the reaction rates carry the signature of mode selectivity? (2) What are the implications of IVR in determining the reaction rates? Regarding point (l), it has been established that efficient non-radiative decay at E > Et in tram-stilbene [5--l 1,131 and some of its derivatives [ 111 essentially occurs in the energy range where the Su +S, transition exhibits a structureless absorption spectrum and where the energy dependence of b is smooth, indicating that efficient IVR has eroded all mode selectivity. Concerning the effects of IVR on the photoisomerization rates (point (2)), it is still an open question whether photoisomerization in the isolated molecule involves a “democratic” IVR process among all vibrational modes. Most of the evidence pertaining to this cardinal point rests on a quantitative analysis of the energy dependence of & The RRKM calculations of Khundkar et al. [9] indicated that the experimental k, values are lower by about an order of magnitude than those obtained for the statistical model, and it was suggested [9,13] that an efficient reverse process has to be invoked. Subsequently, Troe has demonstrated [ 141 that an optimized RRKM calculation, 5
Volume125, number 1
CHEMICAL PHYSICSLETTERS
which uses a fitting procedure for the activated complex frequencies, can quantitatively account for the rates, providing necessary but by no means sufficient evidence for full energy r~dom~ation in the isolated molecule. An alternative approach to this issue involves the effect of the change of the total number of vibrational degrees of freedom on the isomerization dynamics [ 111. In this context, Majors et al. [ 1l] did not observe a marked dependence of k, on methyl substitution of tram-stilbene, while the RRKM theory predicts a decrease of kar by about 35% [ 141 upon methyl substitution for the same molecular parameters. As slight changes in the barrier height Et and the activated complex frequencies scaling factor F canbe induced by methyl substitution [ 141, the discrepancy between the calculated drop of k, by 35% for 4methyl-tram-stilbene relative to trans.stilbene, and the slight experimental increase of w 15% [l 11,does not provide a critical scrutiny of the app~cab~~ of the statistical theory. In this note we report on the time-resolved fhroresence decay lifetimes from photoselected states of jet-cooled 4ETS and 4pTS. It was established in the studies of Smalley et al. [ 151 that alkyl substitution of the benzene ring results in considerable enhancement of intrastate vibrational level mixing. It is interesting to inquire what the implications are of IVR in ~1.substituted tr~s-st~bene on photoisome~~tion dynamics. We have obtained the “counterintuitive” result that ethyl and propyl substitution of tram-stilbene enhances the photoisomerization rates, while general arguments based on the role of IVR and the implications of statistical theories in dicate that the rate should be retarded. It appears that our experimental results can be accounted for in terms of the mo~~cation of the moIecular parameters, i.e. the threshold energy, by aIky1 substitution. The study of the isomerization dynamics of alkyl-substituted stilbenes will further clarify the applicability of statistical theories for the description of photochemistry in isolated molecules. The time-resolved fluoresence decay lifetime for photoselected vibrational states in the S, manifold of jet-cooled tr~s-st~bene (TS), 4-rne~yl-tr~~stilbene (4MTS), 4.ethyl-tram-stilbene (4ETS) and 4-n-propyl-trans-stiene (4FTS) were excited by a mode-locked picosecond pulsed dye laser. The seeded supersonic jet was produced by a continuous expansion in Ar (stagnation pressure p = 150-300 Tori) through 6
21 March1986
a 150 Mm nozzle. The jet was excited by a cavitydumped dye laser (Coherent 599 laser equipped with a Coherent 7200 dumper), which was synchronously pumped by the 5 14 nm line of a rn~e-liked .Ar laser (Coherent CR12). The output of the dye laser (pulse length 30 ps and 250 mW average power) was frequency doubled by a KDP crystal. The W light could be scanned in the range 320-280 nm. The time-resolved fluorescence signal was interrogated using a fast photon-counting system, which consists of a two-stage microchannel plate photom~tip~er whose output pulse was detected by a pair of consent-fraction discriminators (Tennelec 453) and a microcomputer [ 1I]. The response curve, F(t), of the excitation pulse was characterized by a width (fwhm) of 180 ps. The timeresolved excited-state decay of the seeded molecule was determined by a reconvolution technique, simulating the experimental decay curve, I(t), by the convolution I(t) = F(t) cxt exp(- t/r). A mean least-squares fit of the expe~ment~ data was performed over a time domain of O-5 r to extract the lifetime r. Lifetimes were averaged over several different measurements. The accuracy of the lifetime data was 280 ps for r values in the range 2500- 1000 ps and f 50 ps for r values in the range 700- 100 ps. The shortest r values that could be measured were T = 100 f 50 ps. Decay lifetimes were obtained for excess vibrations energies& = O-3000 cm-l above the S, origin. The alkyl stilbenes were prepared by the Grignard reaction between the corresponding 4-alkyl benzaldehydes with phenylmagnesium bromide. The resulting alcohols were thermally dehydrated in boiling xylene to produce the known corresponding 4-alkyl transstilbenes. Increasing the size of the alkyl group con~derably enhances complexlng of Ar atoms with the alkyl stilbenes, as the longer alkyd chain provides an efficient sink for vibrational energy released by the attachment of Ar atom(s) to the large molecule. Some care had to be exerted to separate the isolated-molecule photochemistry from some (interesting) artifacts originating from the photophysics of its van der WaaIs(vdW) complexes. For TS and 4MTS the effects of vdW complexing with Ar are minor under our experimental conditions in the pressure range p = 160-250 Torr of Ar, as is evident from the laser-induced fluorescence (LIF) spectra of these molecules cooled in Ar and in He [ IX]. The time-resolved decay
Volume 125, number 1
CHEMICALPHYSICS LETTERS
of TS and 4MTS cooled at p = 160-250 Torr of Ar were characterized by a single exponential. For 4ETS internal cooling in Ar is efficient under our experimental conditions at p 2 160 Torr, while complexing with Ar sets in at p > 220 Torr, as is evident from the appearance of a broad spectral background in the LIF spectra in the range Ev = O-1200 cm-l. In the narrow pressure range p = 180-220 Torr, the timeresolved decay of 4ETS for Ev = O-2900 cm-l consists of a single exponential with a lifetime 7, which corresponds to the decay of the isolated molecule. At p > 250 Torr, the fluorescence decay following excitation in the “quasi-continuum” (Ev= 1000-2800 cm-l) was biexponential: 1(t) = F(t) 0 [d exp(- t/To) + B exp(- r/r)] ,
(1)
where r. = 2300 +-100 ps corresponds to the decay lifetime in the range Ev = O-1000 cm-l, while the amplitude ratio B/A decreases with increasing p. The slow exponential is attributed to the vibrational predissociation (VP) of 4ETS *Ar, complexes of an unidentified composition, which results in 4ETS’ Ar, (m < n) complexes or in bare 4ETS with internal excess vibrational energy Ev < 1000 cm-l below the photoisomerization threshold. The fast component in eq. (1) corresponds to the decay of the bare molecule. For 4PTS, complexing was severe. At p = 150 Torr Ar, the decay was a single exponential with r = 2200 +-100 ps in the range Ev = 04300cm-l. At p = 190 Torr, the decay curves in the range Ev = 1000-2800 cm-l were biexponential (eq. (1)) with 7. = 2300 f 100 ps andA/B=O.l-0.2. The longdecay component corresponds again to the product of VP of 4F’TS. Ar, * . The values of r for 4F’TSobtained from the short-decay component at p = 190 Torr are identical with the single lifetime obtained atp = 150Torr. The LIF excitation spectra of the alkyl stilbenes * Alternative mechanisms for the reduction of the photoisomer&&on rate by AI complexing, which involve: (i)
IVR involving vibrational energy flow to the relative motion of the AI atom telative to the large molecule, and (ii) modification of the potential surface of the TS, resulting in steric hindrance of isomerization, can be discarded. The VP mechanism, which inhibits reactive isomerization in Ar-alkyl TS complexes, is analogous to the effects of VP on non-reactive intersystem crossing in h--dichloroanthracene complexes [ 191.
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were obtained in pulsed He jets (nozzle diameter 300 pm, gas pulse rate 12 s-l and He stagnation pressure p = 2-3 atm) and excited by frequency-doubled radiation from a pulsed dye laser (Molectron DLII). Table 1 summarizes the energetics of the S, electronic origin of these compounds. The LIF spectrum of 4MTS was identical to that previously reported [ 111. The LIF spectrum of 4ETS (fig. 1) reveals a set of low-frequency vibrations (starting at 22 and 32 cm-l), which may be due to the excitations of the ethyl group. A prominent vibrational progression of 191 cm-l is observed in 4ETS, being presumably analogous to the 200 cm-l vibration in TS, which corresponds to the C-C-Q symmetric in-plane bending. The LIF spectrum of 4FTS (fig. 2) reveals two lowest-energy intense spectral features separated by 60 cm-l, which may originate from the tram and the gauche (or eclipsed) isomers of the n-propyl group, in analogy to the spectrum of n-propylbenzene [ 151. The decay lifetimes of TS, 4MTS, 4ETS and 4PTS over the energy range Ev = O-2800 cm-l are portrayed in fig. 3. The lifetimes of TS over the range Ev = O-2800 cm-l and the lifetimes of 4MTS over the range Ev = 1200-3000 cm-l are in good agreement with previous data, revealing the practical invariance of r on methyl substitution [ 111. The lifetimes of the alkyl stilbenes reveal three domains: (i) An energyindependent region Ev T==S O-1200 cm-l for TS and 4MTS and Ev = O-900 cm-l for 4ETS and 4FTS. These energy-independent lifetimes, which are identical with r values for the electronic origins of S, (table l), are assigned on the basis of absolute quantum yield data [ 161 to the pure radiative decay lifetimes of the vibronic states in this region, i.e. r = r,d. As is apparent from table l, the effect of alkyl substituents and of isomeric conformation of the n-propyl chain on the pure radiative lifetime is minor, as expected. (ii) The onset of non-radiative decay is exhibited at Et% 1200 cm-l for 4MTS and at Ev * 1000 cm-l for 4ETS and 4PTS. (iii) A region of effective non-radiative decay at Ev > Et.The energy-dependent decay lifetimes calculated from the relation h = l/r - l/r& for isolated 4ETS and 4PTS are portrayed in fig. 4, where we have presented also the experimental data for TS together with Troe’s optimized RRKM calculation for TS with the parameters Et= 1300cm-1 and F = 1.2 [ 141, which provide an excellent fit of the TS experimental data. Our new experimental
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Table 1 Energetics and decay dynamics of the Sr electronic origin of alkyl tram-stilbenes Molecule
ha) (A)
u a) (cm-t )
6 v b) (cm-’ )
7 @s)
trans-stilbene 4-methyl-trans~~bene $-ethyl-trans-stilbene 4-npropyl-trans-stilbene
3101.7 3147.3 3150.2 3159.2 c) 3Is3.2d)
32240 31773 31744 31654 31714
0 -467 -496 -586 -526
2500 rt 40 2650 f 80 239.5 i 50 2320 i 40 2300 + 40
b, Spectral shift upon alkyl substitution. a) Absolute accuracy f 0.5 A and f 5 cm-l. d, n-trans-propyl-trans-stilbene. c, n-gauche-propyl-trans-stilbene.
results for the decay rates of 4ETS and 4FTS are inconsistent with the RRKM calculations with the molecular parameters of TS. Troe’s calculations for 4MTS indicate that r) = k4MTS ~ (E)/kT,SQ = 0.74 at Ev = 2500-3000 cm- [ 141. On the basis of conI
I
3110
1 3020
’
I
~deration of ~yl-substitution effects on unimolecular rates, it is ex ected [I’? that ~4ETs(~)~~~(~) 1 =:02 =0.55 andkBPTS(E)/k,S(E)*q3 =0.41 in this energy range. Thge estimates are incompatible with the experimental data (fig. 4), which yield
I 3040
3120
WAVELENGTH
I 3060 WAVELENGTH
3150
1 3orjo
I
(A,
I
I 3100
(ii,
Fig. 1. LIF spectrum of 4ethyl-trans-stilbene in pulsed supersonic expansions of He. The numbers in parentheses are the energies (cm-r) above the electronic origin marked O-O, and the numbers in square brackets are the fluorescence lifetimes (ns) of the corresponding spectral features. Total fluorescence is normalized to the laser intensity.
8
ill0
3120
3130 WAVELENGTH
3070 I
3080 1
I
3150
3140 6)
I
3090
I
I
WAVELENGTH
I
1
3100
:c 0
$1
Fig. 2. LKF spectrum of 4-n-propyl-ttansstene in pulsed supersonic expansions of He. Notation as in fig. t The two electronic origins of the trans and gauche configurations of the n-propyl chain are marked as O-O(t) and O-O(g) respectively. Energies are given relative to O-O@).
4000
I
I
0
METHYL
6
ETHYL
v
PROPYL
1
I
1
Fig. 3. The dependence of the experimental fiuorescence decay lifetimes of TS, 4MTS, 4ETS and 4PTS over the energy range EV = O-2800 cm-’ above the Sx origin. The solid ties were drawn for visual representation of the data,
CHEMICAL PHYSICS LETTERS
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E&m-1)
Fig. 4. The dependence of the non-radiative decay rates of TS, 4ETS and 4PTS on the excess vibrational energy above the Sr origin. - OptimizedRRKMcalculationfor TS [ 141. --- An estimate of the RRKM fit for 4PTS obtained from k&s(E)[k$MTS(E)/k&s(~]2. -.-.A rough estimate of th;sRRKM tit for 4ETS obtained from kTrs(,!?)[k$MTS(&‘)/ k,, (E)] 3. The rates kTSQ and k4MT$Q with Et = 1300 cm-’ and F = 1.2 werenfaken fromy’ef. [14].
k4ETS(E)/kTS(E) = k4PTS(E)/kT,S(E)* 1.7. We assert tl% the eneygy dependence of the non-radiative rates of alkyl TS (fig. 4) reveals a marked increase of the k, values relative to those of TS, which is apparent contrast to the expectation of the REXM calculations [ 141.This discrepancy cannot be taken as evidence for non-statistical effects on the unimolecular dynamics, as a reasonable modification of the molecular parameters may cure the discrepancy between our data and the RRKM calculations. It is reasonable to assume that the threshold energy Et is reduced by alkyl substitutions, as is evident from a visual inspection of fig. 3. On the basis of Troe’s calculations [ 141, we infer that k,(E; Et - 100 cm-l)/k,(E; Et) = 1.7. Accordingly, 10
21 March 1986
the discrepancy of a numerical factor of 3.1 between the RRKM theory and the experiment for kETS(E)/ kT,S(E)(for Ev = 2500-3000 cm-l) can be accounted for by a shift of 200 cm-l in Et towards lower energies, while the numerical discrepancy of 4.1 for 7c~S(E)/~~(E) (for Ev = 2500-3000 cm-l) can be reconciled by a shift of -300 cm-l in Et. Thus, the isomerization threshold for the Calkyl stilbenes are Et = 1100 cm-l for 4ETS and Et w 1000 cm-l for 4PTS. The accuracy of these estimates appears to be f 100 cm-l. With this modification of the threshold energy, the RRKM model does provide an adequate account of the experimental data. We conclude by emphasizing that the experimental data for alkyl benzenes are consistent with the statistical theory. The cardinal question of whether fully randomized IVR prevails in these isolated molecules has not been resolved. The major technical result of the present work involves the observation of the reduction of the threshold energy for photoisomerization of TS by alkyl substitution, with a lowering of Et by about 100 cm-l per CH2 group. An analogous effect of intermolecular interactions was invoked by Troe [14,18], who proposed the lowering of Et of TS by van der Waals clustering in the gas and liquid phase in order to account for solvent effects on photoisomerization dynamics.
Acknowledgement
We are indebted to Professor J. Troe for stimulating correspondence and for prepublication information. This research was supported in part by the United States Army through its European Research Office. References [ 1] J. Jortner , S.A. Rice and R.M. Hochstrasser, in: Advances in photochemistry, eds. B.O. Pitts and G. Hammond (Wiley, New York, 1969). [ 21 W.M. Gelbart and S.A. Rice, J. Chem. Phys. 50 (1969) 4715. [ 31 W.M. Gelbart, K.F. Freed and S.A. Rice, J. Chem. Phys. 52 (1970) 2460. (41 B.I. Greene, R.M. Hochstrasser and R.B. Weisman, Chem. Phys. 48 (1979) 544.
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[ 51 B.I. Greene, R.M. Hochstrasser and R.B. Weisman, Chem. Phys. 48 (1980) 289. [6] J. Syage, W. Lambert, P. Felker, A.H. Zewail and R.M. Hochstrasser, Chem. Phys. Letters 88 (1982) 266. [ 71 A. Amirav and J. Jortner, Chem. Phys. Letters 95 (1983) 295. [ 81 T. Zwier, E. Carrasquillo and D.H. Levy, J. Chem. Phys. 78 (1983) 5493. [ 91 L.R. Khundar, R.A. Marcus and A.H. Zewail, J. Phys. Chem. 87 (1983) 2473. [lo] N.F. Scherer, J.P. Shepanski and A.H. Zewail, J. Chem. Phys. 81 (1984) 2181. [ll] T.J. Majors, U. Even and J. Jortner, J. Chem. Phys. 81 (1984) 2330. [12] B.I. Greene and R.C. Farrow, J. Chem. Phys. 78 (1983) 3336.
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[13] J.A. Syage, P.M. Felker and A.H. Zewail, J. Chem. Phys. 81 (1984) 4685,4706. [ 141 J. Troe, Chem. Phys. Letters 114 (1985) 241. [ 151 D. Powers, J.B. Hopkins and R.E. Smalley, J. Chem. Phys. 72 (1980) 5039. 1161 M. Sonnenschein, A. Amirav and J. Jortner, J. Phys. Chem. 88 (1984) 4214. [ 171 J. Troe, private communication. [ 181 J. Schroeder and J. Troe, Chem. Phys. Letters 116 (1985) 453; G. Maneke, J. Schroeder, J. Troe and F. Voss, Ber. Bunsenges. Physik. Chem. 89 (1985) 896. [ 191 A. Amirav, M. Sonnenschein and J. Jortner, Chem. Phys. 88 (1984) 199.
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