Optical spectra and excited-state dynamics of cis-thioindigo - The

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J. Phys. Chem. 1989, 93, 7081-7087 paring the results gathered in Tables I11 and IV, one may observe that the substitution of hydrogen by a fluorine atom causes an increase of the energetic gap between the 2-oxo-4-hydroxy and dioxo tautomers by 1.6 kJ mol-'. At the same time, the energy gap between the 2-hydroxy-4-oxo and dioxo forms decreases by 14.6 kJ mol-'. The 2-oxo-4-hydroxy tautomer (FU3) can potentially occur in the nucleic acids exposed to mutagenic agents. Although we cannot exclude its role in the mismatches, we may say that the 5-fluorouracil will adopt the rare 2-oxo-4-hydroxy form almost half as frequently as uracil. Such a conclusion contradicts an early hypothesis on the greater propensity of 5halogeno uracils incorporated into the nucleic acids to adopt the hydroxy form.20 On the other hand, the 5-fluor0 substituent considerably increases the stability of the 2-hydroxy-4-oxo form (FU2). This form cannot appear in nucleic acids because both the N(1) and C(5) positions are blocked. Such a form can potentially be observed in the gas phase ( T - 500 K) if the resolution of spectrometers were high enough to detect as small an amount of a rare form of only 0.1% of the main dioxo form. Actually, we are slightly surprised that Tsuchiya et a1.I7*'*have observed in their UV studies the rare hydroxy tautomers of uracil and thymine. They even estimated the energy gap of about 40 kJ mol-' or less than this value, which agreed well with the semiempirical estimation^.^^ In our opinion, the detection of the rare forms by spectroscopic methods does not seem to be possible. A similar remark recently came from Brady et a1.,I9 who suggested that the spectra attributed by Tsuchiya et al.'7*18to rare tautomers correspond instead to some unidentified impurities.

Conclusions Several conclusions can be drawn from the present study: The most stable tautomeric form of uracil and 5-fluorouracil in the

7081

gas phase is the 2-0x0-4-0x0 form, in agreement with numerous experimental data.12-14J9*42The energy splittings between the rare hydroxy forms and the dioxo form depend on the position of the protons. The 2-hydroxy-4-oxo (N(3)-H) tautomeric forms of uracil and 5-fluorouracil are less stable than the dioxo forms by 43.9 and 29.3 kJ mol-', respectively. The 2-oxo-4-hydroxy (N( 1)-H) tautomeric forms are considerably less stable than the dioxo forms, and the corresponding values of energy splitting are 50.2 and 51.8 kJ mol-' for uracil and its 5-fluor0 derivative. At room temperatures the uracil residue of nucleosides (the glycosidic bond at N(1)) may adopt the rare tautomeric forms with the frequency of about lo* which falls into the region of the observed frequency of spontaneous mutations ( 10-8-10-'1). The 5-fluorouracil residue should tautomerize twice less frequently. The unusual for nucleic acids 2-hydroxy-4-oxo (N( 3)-H) tautomers of uracil and 5-fluorouracil should not be detectable in the gas phase at T = 500 K with the present-day spectrometers. A possible exception might be the UV fluorescence spectroscopy. The important contributions to the tautomer's relative stability arise from the electron correlation and zero-point nuclear vibration effects. The dioxo forms of both uracil and fluorouracil are destabilized by those contributions in relation to the respective 2-hydroxy-4-oxo and 2-oxo-4-hydroxy forms by a few kilojoules per mole.

Acknowledgment. The present study was supported by an institutional grant from the National Cancer Institute. We thank Dr. K. Szczepaniak for valuable comments. (42) Katritzky, A. R., Linda, P., Eds. The Tautomerism of Heterocycles; Advances in Heterocyclic Chemistry, Supplement No. 1 ; Academic Press: New York, 1976.

Optical Spectra and Excited-State Dynamics of cis-Thioindigo A. Corval and H. P. Trommsdorff* Laboratoire de SpectromPtrie Physique, associP au C.N.R.S.,UniversitP Joseph Fourier, Grenoble I , B.P. 87, 38402 St. Martin d'H?res Cedex, France (Received: January 1 1 , 1989; In Final Form: May 9, 1989)

The metastable cis isomer of thioindigo has been characterized by absorption, emission, and resonance Raman spectroscopy in low-temperature crystalline matrices. The spectroscopic data indicate that the change of the electronic structure upon excitation to the first excited singlet m*state is similar to the one in the stable trans conformation. The main limitation of all measurements on the cis isomer is the ultrafast nonradiative decay of the excited state, which was found to be increased by about 5 orders of magnitude as compared to the trans isomer. It is proposed that this relaxation is due to more efficient intersystem crossing to the triplet manifold, linked to the presence of a n r * triplet state close to the excited ?TX* singlet state in the cis isomer. The fast decay of the excited singlet state in the cis isomer rules out a singlet mechanism for the cis trans isomerization but all data are consistent with a triplet route.

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Introduction The strong visible absorption of indigo dyes (I) involves the lowest excited singlet state.' The dynamics following excitation in SI is interesting in many regards: in addition to competition between different deactivation processes (fluorescence, internal conversion, and intersystem crossing) photoisomerimtion ocCurs.2

* X

/

0

X=NH,S,Se,or 0

The relative yields of these processes depend strongly upon the nature of the dye and the environment. The photoisomerization involves a 180' rotation around the central c=c bond and is reversible. The cis isomer is metastable and, in liquid solutions at room temperature, reverts back to the stable trans h n e r on the time scale of tens of minutes. Molecular orbital calculations3 indicate that the changes of the electronic structure upon excitation of SI are very similar for both isomers. The oscillator strength of the transition is also predicted to be nearly unchanged in going from trans to cis,4 in agreement with experimental e v i d e n ~ e . ~ * ~ The excitation to SI involves to a minor extent only the six-

I

(1) Liittke, W.; Klessinger, M. Chem. Ber. 1964, 97, 2342.

(2) Wyman, G . M.; Brode, W. R. J . A m . Chem. SOC.1951, 73, 1487. (3) Liittke, W., unpublished results. (4) (a) Liittke, W.; Hermann, H.; Klessinger, M. Angew. Chem. 1966,12, 638. (b) Luhmann, U.; Liittke, W. Chem. Ber. 1978, 111, 3246. (5) Blanc, J.; Ross, D. L. J . Phys. Chem. 1968, 72, 2817.

0022-3654/89/2093-708 1$01.50/0 0 1989 American Chemical Society

7082 The Journal of Physical Chemistry, Vol. 93, No. 20, 1989

Corval and Trommsdorff

membered rings and is predominantly located at the central The present paper is concerned with the emission properties structure, which is to be considered as the basic chromophore.'S6 of the unstable cis-thioindigo. Vibrationally resolved electronic and resonance Raman scattering spectra have been obtained for Indigo' as well as thioindigo in protic solvents8v9have a low this compound in low-temperature n-alkane matrices. At longer quantum yield of fluorescence. No transient triplet-triplet abwavelengths the emission from the ever present trans isomer in sorption is observed in indigolo indicating that the intersystem the samples completely dominates, and, even at wavelengths crossing rate also is very small. Photoisomerization is not observed So transition, the unrelaxed shorter than the origin of the S, in indigo and is strongly quenched by protic solvents in thioindigo. emission from trans as well as Raman scattering from the matrix This underlines the influence of hydrogen bonding in the decay is much stronger than the emission from cis. These weak emissions of SI but the exact nature of the internal conversion process from the cis isomer could be quantified and thus an evaluation is controversia1.l' Polar solvents also reduce both the fluorescence of the emission quantum yield and the excited-state lifetime has and the isomerization quantum yield, but to a lesser extent. In been made. Mechanisms for the extremely rapid decay of SI are aprotic nonpolar solvents (e.g., benzene), on the other hand, discussed in particular with regard to the cis trans isomerization trans-thioindigo fluoresces with a 71% quantum yieldi2 (the process. lifetime is 13.4 ns at room temperaturei2 and 15 ns in low-temperature matrices13), the remaining deexcitation process being Experimental Section intersystem crossing to Ti. The yield for isomerization is 11%,12 that is, nearly one half of the triplet yield. For cis-thioindigo, in Sample Preparation. Thioindigo was prepared and purified contrast, fluorescence emission is very weak and has only recently in the laboratory of W. Liittke, University of Gottingen. The been observed in a low-temperature matrix,I4 while the yield of solvents used were BDH n-nonane (standard for chromatography) isomerization is very high (45%).12 or Fluka n-nonane, purified by the method described in ref 27 (stirred with sulfuric acid during 12 h, washed, dried, and fracIt is generally accepted that the trans to cis reaction occurs via tionally distilled). 2,5-Diphenyloxazole was purified by zone the triplet state.I5-l8 At room temperature, the lifetime of the refining. The purification of this product was limited by its low trans triplet state lies, depending upon the solvent, in the range ability to crystallize and an emission background was therefore of 135-300 ns.I5-I7 The reaction then proceeds by a torsion in always observed. Samples containing a mixture of both the cis the triplet state leading to a twisted conformation (neither emission and trans isomers in n-nonane were prepared as follows: saturated nor absorption from this phantom triplet state has been reported), solutions of thioindigo ( ~ X3 lo-' M) were degassed by freezewhich in turn relaxes to the ground-state surface producing the pumpthaw cycles. These solutions were subsequently irradiated two isomers with similar effi~iencies.'~J~ The mechanism of the reverse reaction from cis to trans is controversial: both ~ i n g l e t ' ~ * ~ ~at room temperature with a 150-W quartz iodine incandescent lamp for about 20 min using a color filter which eliminated all and triplet16*2imechanisms have been proposed. wavelengths shorter than 500 nm in order to minimize the phoFor trans-thioindigo, vibrationally resolved, sharp absorption toinduced back reaction. The ratio of the concentrations of cis and emission spectra have been reported in low-temperature to trans in these samples is about 2: 1. This solution was quickly d~~,~~ single-crysta122and polycrystalline mat rice^.^^,^^ I ~ ~ f r a r eand transferred into a glass-metal cell for low-temperature experiments resonance-enhanced Raman s p e ~ t r ahave ~ ~ ,also ~ ~been recorded. (optical path length, 2 mm) and plunged into liquid nitrogen. From the available spectral and structural data, a potential energy Samples, containing the trans isomer only, were prepared in the surface for the torsion of the molecule around the central C=C same way except for the degassing and irradiation. Solutions in bond has been proposed for both the So and the Si states.22 For 2,5-diphenyloxazole with both isomers present were prepared in cis-thioindigo in SI this potential is very shallow with no significant the melt at temperatures above 72 OC under the same irradiation barrier toward the trans conformation. conditions as described above, but without previous degassing. These solutions were rapidly crystallized by sputtering on a cold glass surface. For both types of solutions containing the cis isomer it was (6) Hermann, H.; Liittke, W. Chem. Ber. 1968, 101, 1715. verified that the photoinduced species reverted back in the dark (7) (a) Haucke, G.; Paetzold, R. Nova Acta Leopoldina, Suppl. 1978, No. I I . (b) Haucke, G. Thesis, Friedrich-Schiller Universitit, Jena, DDR, 1977. to the trans isomer. The lifetime of the cis isomer is ca. 10 min (8) (a) Wyman, G. M . J . Chem. SOC., Chem. Commun. 1971, 1332. (b) in n-nonane at room temperature and 1 min in the 2,S-diWyman, G. M.; Zarnegar, B. M. J . Phys. Chem. 1973, 77, 1204. phenyloxazole melt at 75 OC. (9) (a) Windhager, W.; Schneider, S.; WIT,F. Z . Naturforsch. 1977,32a, Spectral Measurements. Absorption spectra were recorded by 876. (b) Schneider, S.; Lill, E.; Hefferle, P.; Dorr, F. II Nuovo Cimento 1981, using a JY THR monochromator and a 55-W quartz iodine lamp. 6 3 8 , 411. Emission spectra were obtained by exciting with the lines of an (10) Kobayashi, T.; Rentzepis, P. M. J . Chem. Phys. 1979, 70, 886. argon ion laser and using a JY Ramanor monochromator. (1 1 ) (a) Siihnel, J.; Gustav, K. Mol. Photochem. 1977, 8, 437. (b) ElThe intensities of the weak emissions (Raman scattering from saesser, T.; Kaiser, W.; Liittke, W. J . Phys. Chem. 1986, 90, 2901. the solutes and the weak fluorescence of the cis isomer) were (12) Kirsch, A. D.; Wyman, G. M. J . Phys. Chem. 1975, 79, 543. quantified by using the intensity of the 890-cm-' Raman line of (13) Clemens, J. M.; Hochstrasser, R. M.; Trommsdorff, H. P. J . Chem. Phys. 1984, 80, 1744. the n-nonane matrix as an internal standard. As the samples are (14) Corval, A,; Trommsdorff, H. P. J. Phys. 1985, 46, C7-447. strongly scattering it can safely be assumed that all emissions are (15) Kirsch, A. D.; Wyman, G. M. J . Phys. Chem. 1977, 81, 413. isotropic in the polarization of the light. The total scattering cross (16) Grellmann, K. H.; Hentzschel, P. Chem. Phys. Lett. 1978, 53, 545. section of the 890-cm-' mode of n-nonane was calibrated against (17) Gorner, H.; Schulte-Frohlinde, D. Chem. Phys. Lett. 1979, 66, 363. the 992-cm-l line of benzene by using an equimolar mixture of (18) Karstens, T.; Kobs, K.; Memming, R. Ber. Bumenges. Phys. Chem. both compounds at 77 K, Le., under conditions comparable to the 1979, 83, 504. recording of the emission spectra of the doped samples. The total (19) Memming, R.; Kobs, K. Ber. Bumenges. Phys. Chem. 1981,85,238. cross section (integrated over all directions and polarizations) of (20) Klages, C. P.; Kobs, K.; Memming, R. Chem. Phys. Letr. 1982, 90, the 992-cm-' mode of liquid benzene, excited at 4880 8,is 2.83 46. X A2-molecule-1.28~29 For the 890-cm-I line of n-nonane (21) Krysanov, S . A.; Alfimov, M. V. Laser Chem. 1984, 4, 121. we find 2.32 X 82.molecule-1. The values for other excitation (22) Corval, A.; Trommsdorff, H. P. J . Phys. Chem. 1987, 91, 1317. wavelengths are obtained by extrapolation as 2.56, 2.16, and 1.86

-

,899


-

(23) Gastilovich, E. A.; Tskhai, K. V.; Shigorin, D. N. Opt. Spectrosc.

1977, 41, 332.

(24) Aleksandrov, I. V.; Bobovich, Ya; Vartanyan, A. T. Opt. Spectrosc. 1978, 45, 341.

(25) Klessinger, M.; Liittke, W. Chem. Ber. 1966, 99, 2136. (26) Baranov, A. V.; Veniaminov, A. V.; Petrov, V. I. J . Appl. Spectrosc. 1986, 44, 46.

(27) Perrin, D. D.; Armarego, W. L. E.; Perrin, D. R. Purification of Laboratory Chemicals, 2nd ed.; Pergamon Press: New York, 1980; p 364. (28) Myers, A. B.; Harris, R. A,; Mathies, R. A. J . Chem. Phys. 1983, 79, 603. (29) Kato, Y.; Takuma, H. J. Chem. Phys. 1971, 54, 5398.

The Journal of Physical Chemistry, Vol. 93, No. 20, 1989 7083

Spectra and Excited-State Dynamics of cis-Thioindigo

A

/

gl

V

TRANS

,

J

I

I

I

I

20000

19000

FREOUENCY

1

CIS

21000 icm-11


Figure 1. Absorption spectra of a mixture of cis- and trans-thioindigo in a n-nonane matrix at 77 and 1.6 K. In the region below 19 500 cm-I the spectra are distorted by the strong emission of the trans isomer.

i

I

I

20 000

I

21 000 FREOUENCY fcm-'l

I

I

22 000

Figure 2. Absorption spectrum of cis-thioindigo only in a n-nonane matrix at 1.6 K (obtained by difference, see text). X A*.molecule-' for excitation at 4765, 4965, and 5145 A, respectively.

Results and Discussion We shall discuss here in detail only results obtained in the n-nonane matrix. A preliminary account of measurements in the ' ~ present 2$diphenyloxamle matrix has been given p r e v i o ~ l y ;the work confirms these results with more precision. Absorption Spectra. We have previously recorded and analyzed in detail the spectra of trans-thioindigo in a single-crystalline benzoic acid matrix.22 Thioindigo occupies in this matrix a well-defined substitutional site and the spectra at low temperatures are well resolved with inhomogeneous line widths of 1 cm-' and less. For the present work this matrix could not be used as the trans to cis photoisomerization is strongly quenched in molten benzoic acid. It was therefore not possible to trap cis-thioindigo in sufficient concentration in this matrix. n-Alkane matrices have previously been used to record the spectra of the trans isomer and our observations concerning this compound are in general agreement with the reported ~ p e c t r a . ~ ' . ~ ~ Figure 1 shows the spectra of a mixture of both isomers in a n-nonane matrix at 77 and 1.6 K. At 77 K a vibrational structure is resolved for both isomers; the full widths at half-maximum of the spectral features range from 300 to 400 cm-' in the cis compound and are about 100 cm-' for trans. Further cooling to 1.6 K sharpens the spectrum of trans: the multiplet site structures are resolved and the line widths reach their inhomogeneous limit of about 15 cm-I. The lines attributed to the cis isomer, in contrast, remain virtually unchanged. The spectrum of cis-thioindigo alone, obtained by numerical subtraction of the contributions of the trans-thioindigo and the matrix to the absorption, is shown in Figure 2. The observation that the spectrum of cis has the same resolution at 77 and 1.6 K and the fact that the inhomogeneous contribution to the line width is expected to be similar to the value measured for trans (15 cm-I) strongly suggests that homogeneous broadening makes a large contribution to the observed line profiles. Because of the uncertainties associated with the determination of the base line, obtained by numerical subtraction of two spectra, and as the underlying site structure is unresolved and unknown for cis-

thioindigo, any deconvolution of the Lorentzian contribution to the line shapes is subject to considerable error. If we assume that the site structure and the inhomogeneous broadening as well as the electron phonon coupling for the two isomers are similar, as might be expected in view of the similarity of the electronic structure (see below), then the homogeneous contribution to the line broadening of the cis isomer must be at least 70 cm-I (fwhm) in order to account for the low resolution and the absence of narrow spectral features in the spectra. A good upper limit of this contribution can nevertheless be given from an analysis of the slope of the spectrum at the low-energy side. This yields a value of 100 cm-' for the maximum value of the damping parameter (Le., the half-width at half-maximum of the Lorentzian). The overall intensity distribution in the spectra of cis and trans is similar as is evident from the spectra at 77 K but is known to be strongly influenced by the site distribution in trans. There the splitting of two predominant sites equals 499 cm-', a value similar to the frequency of the strongest vibrational progression in the excited state (500 cm-'). The peaks of the broad lines in cisthioindigo show a mean spacing of about 485 cm-' and the vibrational mode responsible for this progression is presumably similar to the mode active in trans-thioindigo and which corresponds to an in-plane distortion of the five-membered rings. The line spacings, on the other hand, are somewhat irregular as is their width, indicating a more complex underlying site distribution and possibly the activity of other modes. The recordings of the absorption spectra are also used to evaluate the concentrations of the two isomers present in the matrix: these values are needed for the quantitative measure of the different emission yields. The integrated oscillator strengths of the SI Sotransitions of both isomers are known and can be used for this calibration. As, however, part of the solute is rejected from the saturated solution during the freezing process, the precise determination of the total concentration of thioindigo in the liquid solutions gives an upper limit only for the true value of the conmol/mol. centration of the matrix-isolated species: 1.3 X The direct measure in the solid suffers from the fact that the light is strongly scattered in the sample, making the actual path length greater than the nominal value and leading to an overestimation of the concentration, but the measure of the relative concentrations is not affected by this. Emission Spectra. In solution at room temperature the emission of trans-thioindigo is very broad and even at energies above the pure electronic transition, in the spectral region of the emission of cis-thioindigo, the intense fluorescence of the trans isomer completely masks any emission from cis. As in solution the cis isomer cannot be isolated from trans, it is only by lowering the temperature that this emission background can be reduced sufficiently to make the emission of cis-thioindigo observable. At 77 K resonant Raman and fluorescence barely become visible above the reduced emission background and most of the spectral observations have therefore been made at liquid helium temperatures. Measurements have been performed on many different samples (as described in the Experimental Section) but the spectra shown in Figure 3 are all obtained from two different samples only, one containing the pure trans isomer and the other a mixture of both isomers. As we are interested here in the emission of cis and as the relaxed emission of trans-thioindigo in this matrix has already been reported by other groups,30the spectral region showing this relaxed emission, which is more intense by 3-4 orders of magnitude, is omitted in the figures. The spectra in Figure 3 correspond to four different excitation wavelengths: 4765,4880,4965, and 5 145 A, the positions of which are indicated in the absorption spectra of Figures 1 and 2. The comparison of these spectra and the spectra of other samples (e.g., the pure matrix) made it possible to identify the following different emissions: nonresonant Raman scattering from the matrix; resonant-enhanced Raman scattering from both isomers of thioindigo; unrelaxed fluorescence from

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(30) Tskhai, K. V.; Gastilovich, E. A.; Shigorin, D. N. Russ. J . Phys. Chem. 1976, 50, 161 1.

7084 The Journal of Physical Chemistry, Vol. 93, No. 20, I989

1

Corval and Trommsdorff L

m

Aexc~5145A

1

I

I

18800

19200


Aexcz4965A

1

T

m

18 800

I

1

I

19 600

19200

I

I

19000

19400

.

19100

19800

I

1

19800

20200

I

2 0 000

I

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20200

20 6 0 0 f r e q u e n c y (cm-1)

Figure 3. Emission spectra of cis- and trans-thioindigo in a n-nonane matrix a t liquid helium temperature, excited as indicated a t 5145, 4965, 4880, and 4765 A. The upper and lower traces correspond to samples containing the trans isomer and both isomers, respectively. Except for the small spectral region, marked with a diamond, all spectra are recorded with the same two types of samples. The meaning of the symbols is as follows: t, c, and m mark Raman lines of trans-thioindigo, cis-thioindigo, and the matrix, respectively; asterisks mark Raman lines of both isomers too close in frequency to be separated in the spectra; F designates unrelaxed fluorescence from the trans isomer; inverted triangles mark fluorescence lines of the laser. Lines without any marks could not be assigned unambiguously. The very strong relaxed fluorescence from the trans isomer lies to the red of the spectral region shown here.

trans-thioindigo; fluorescence from cis-thioindigo; a fluorescence background from "irreducible" impurities of the matrix. The spectral ranges over which it is possible to observe these emissions are limited toward higher energies by the wavelength of the exciting laser line and toward lower energy by the onset of the relaxed emission from the highest energy site of trans-thioindigo at about 5410 A. For the four different excitation wavelengths these

frequency ranges are approximately 2500, 2000, 1650, and 950 cm-I. Quantitative Determination of the Emissions. As the samples are weakly absorbing only, and as the emissions are measured from the excited front surface of the sample, reabsorption can be neglected. The intensity of emission from the solute, E,, in the n-nonane matrix is therefore given by the following expression:

The Journal of Physical Chemistry, Vol. 93, No. 20, 1989 7085

Spectra and Excited-State Dynamics of cis-Thioindigo

TABLE I: Raman Cross Sections (8,2molecule-1)of trans- and cis-Tbioindigo in a n-Nonane Matrix at Liauid Helium Temwrature

Here N,(r) is the number density of solute molecules at the position r in the sample, their absorption cross section at the excitation wavelength, and a, the quantum yield of the emission considered. ZEx is the intensity of the exciting laser beam and S(r) is a function which takes into account the actual structure of the sample (which is strongly diffusing) so that 6(r)Z& represents the actual intensity of the light field experienced by the solute molecules at the position r . The integration extends over the volume elements dV(r) of the sample. For the Raman emissions of either the matrix or the solutes the total intensity, Rts is given by an expression analogous to eq 1


R', = J6(r)ZExNs(r)uRs dV(r) where N,(r) and uRsare the number density and the total Raman cross section of the species considered (n-nonane molecules of the matrix or either isomer of thioindigo). If the spatial distribution of all molecules within the sample is the same on a scale over which the light intensity varies, then the integral lS(r)N,(r) dV(r) is proportional to the relative concentration of the different species (solvent or solute molecules). It is therefore possible to quantitatively compare the intensities of the fluorescent and Raman emissions of the solute and matrix molecules, provided that the spectra are recorded under strictly identical conditions. From the known Raman cross sections of the n-nonane molecules of the matrix, the resonant Raman cross sections of the solutes and the emission quantum yields can be evaluated if both the concentrations and the absorption cross sections at the exciting laser wavelength are known. The emission quantum yield of cis-thioindigo, for example, is given by +cis

= (Ecis/Rtm) ( U R m / #cis)(Nm/Ncis)

(3)

The absorption cross section, ."&, can be evaluated with reasonable accuracy by using the observed spectral distributions of the lowtemperature spectra and assuming that the integrated absorptions are not changed significantly from the values determined in solution. The in situ determination of the concentrations is, as explained above, not possible from the absorption spectra because the optical path length is unknown in the strongly scattering samples. The relative concentrations, on the other hand, can be obtained accurately and, together with the value of the total concentration in the solution prior to freezing, lead to evaluations which represent upper limits. Another evaluation of the emission yields and Raman cross sections can be made using the known fluorescence quantum yield of the trans isomer for calibration. The ratio of the amount of light, I,, absorbed by the two isomers at the wavelength of excitation is given by the following expression: PcidPtrans

= (."cis/ 8trans)(Ncis/Ntrans)

(4)

The emission quantum yield of cis-thioindigo is therefore

The Raman cross sections of the solutes are given analogously by

As four different wavelengths are used, and as some parameters, such as the concentration, must be the same when all measurements are made on the same sample, the other unknown parameters are actually overdetermined, and the consistency of the procedure can be checked. Matrix Emissions. The by far strongest features in the spectra correspond to nonresonant Raman scattering from the n-nonane matrix. The relative intensities of these Raman lines are, within experimental accuracy, the same for all excitation wavelengths; this observation is in agreement with the fact that the first

freq shift, cm-I

5145A trans 212 2.4 X 233 4.0 X 10" 413 460 490 1.2 X 502 673 720 4.0 X 1002 1020 1533 1685

cis

223 240 413 466 487 502 673 966 973 1527

excitation wavelength 49658,

4765 8,

48808,

6.1 X IO4

d 4.2 X 2.6 X

1.4 X

d d 2.1 x 10-8 1.0 x 10-7

3.3 x 10-8 2.9 X

1.1 x 10-7

7.6 x 10-8 7.6 X

d 4.7 x 10-7

5.1 x 10-7

2.7 x 10-7 2.7 x 10-7

d d 2.0 x 10-76 1.1 x 10-7

1.5 x 10-7* 1.3 x 10-7

'Overtone band? 2 X 233 cm-l. bOvertone band? 2 X 487 cm-I. CCombinationband? 233 + 490 cm-I. dRaman emission from both isomers. electronic transitions of n-nonane are located at much higher energies in the vacuum ultraviolet and justifies the extrapolation used in the evaluation of the Raman cross sections. The intensity of this emission sets a limit to the observation of the weaker emissions originating from the solute molecules. In addition to the sharp Raman lines of the matrix a broad, weak, emission background is present in all samples; a much stronger background is observed in unpurified solvents and the residual level very likely represents the emission from impurities which could not be removed by the purification procedure. Emissions from the Solutes. Unrelaxed Fluorescence from trans- Thioindigo. trans-Thioindigo occupies several energetically inequivalent sites in the matrix and the laser excitation mostly occurs into broad featureless regions of the spectrum: phonon sidebands and overlapping weaker vibronic bands (see Figure 1). The excess vibrational energy deposited in the molecules by the excitation above the electronic origin of the Sl So transition ranges from about 1000 to 3000 cm-I. The competition between vibrational relaxation and emission from the populated vibronic levels governs the intensity distribution of the unrelaxed emission. It is beyond the scope of this paper to analyze in detail this emission but we note that the general features can be understood in terms of the known frequency intervals of the absorption and emission spectra of trans-thioindigo. A quantitative analysis of our spectra, however, is difficult as the observed spectral features in general represent the sum of numerous transitions originating from many of the populated excited-state vibronic levels. Such an analysis would become feasible for spectra obtained by exciting the molecule selectively into specific excited state levels. From Figure 3 it is evident that, as expected, the complexity of the spectral distribution increases and the spectra become broader with the increase of excess energy deposited in the molecules. The unrelaxed emission spectra, shown in Figure 3, represent of course only part of the emission of the excited levels, the longer wavelength region of these spectra being masked by the strong relaxed fluorescence originating from the pure electronic S1 state. A quantitative estimate (analogous to the quantum yield evaluations explained below) of the intensity of the unrelaxed emission of trans-thioindigo can nevertheless be made and is consistent with lifetimes of the vibronic levels in the range of 10 ps and shorter. Resonant-Enhanced Raman Emissions. A number of bands observed in the spectra of Figure 3 can unambiguously be at-

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7086 The Journal of Physical Chemistry, Vol. 93, No. 20, 1989

tributed to resonant-enhanced Raman scattering from the two thioindigo isomers. These are listed in Table I, together with their cross sections, which have been evaluated as explained above, using eq 6. For trans-thioindigo, resonance-enhanced Raman lines are unambiguously assigned with the three lowest energy laser lines. When the excitation is closer to the electronic origin, the observed lines and their relative intensities closely match the fluorescence spectrum. Excitation at higher energies results in significant changes of the spectrum. As in the absorption and emission spectra, no long progressions in a specific mode are observed. The most significant deviations from this general behavior occur for the 490-cm-I mode under 5145-A excitation and the 1533-cm-’ mode under 4965-A excitation. In both cases the extra intensity can be traced back to a near-resonance with a vibronic level of a specific site which involves the corresponding mode in absorption. The observation, under 4880-A excitation, of the 1685-cm-I Ceo stretch mode is presumably due to a similar mechanism, but the congestion of the absorption spectra in this range precludes any specific assignments. For cis-thioindigo, excitation at 5145 A is too far off resonance and no Raman bands of cis are therefore visible in the corresponding spectrum of Figure 3, while excitation with the other three laser lines gives rise to resonance-enhanced Raman scattering. The lines observed, and their relative intensities, are remarkably close to the ones also seen for the trans isomer, without showing any unexpected extra enhancement; this result clearly confirms the picture of the excited electronic state as obtained from molecular orbital calculations, which, as mentioned in the Introduction, predicts that the changes of the electronic charge distributions upon excitation to SIare very similar for both isomers. Fluorescent Emission from cis-Thioindigo. While the above-discussed emissions are fairly easily identified by comparison of the spectra obtained for different samples and excitation wavelengths, fluorescence from cis-thioindigo is more difficult to uncover. This fluorescence is very weak, and being nearly unstructured, contributes only to the background of the spectra. It is therefore identified as the difference (by numerical subtraction) of spectra obtained for different samples. Even though strictly the same procedures are followed to fabricate these samples they are not identical as samples containing the cis isomer were irradiated prior to freezing. The presence of minute amounts of impurities in the “pure” matrix as evidenced by a very weak emission (see above) could contribute to the difference of the spectra if these impurities were to undergo a photochemical reaction. In order to eliminate this possibility, spectra were also recorded for samples which, after irradiation, were left in the dark during a sufficient length of time for the cis isomer to convert back to trans. Because of the other interfering emissions the measure of the background representing fluorescence from cis-thioindigo was made over a limited spectral range between 19 550 and 20 400 cm-I (4880-A excitation) or 20 900 cm-’ (4765-A excitation). From the intensity distribution in absorption it can be estimated that at least 2/3 and 3/4, respectively, of the total emission lies in these spectral ranges. It must be realized that, because of the limitations of these measurements, the quantitative evaluation of the emission made here represents an upper limit only of the fluorescence of cis-thioindigo. For the sample studied at all wavelengths we find that the ratio of the integrated fluorescence equals at most 2.0 X lo4 and 5.0 of the two isomers, Ecis/Etrans, X lo4 for excitation at 4880 and 4765 A, respectively. The ratio of the number of photons absorbed by the two isomers at these two wavelengths, Pcis/Ptrans, is 6.2 and 10, so that we find from eq 5 and using the value of 0.71 for the fluorescence quantum yield of the trans isomer an upper limit of the quantum yield for cis-thioindigo of 2.3 X lom5and 3.5 X from the two measurements. If the intensity of the 890-cm-l Raman line of the matrix is used for calibration (eq 3), and the concentration is taken to be the same as in the liquid solution prior to freezing, then these values become a factor of almost 10 smaller; we think that this difference is due to the fact that a large part of the solute is rejected and it is the concentration which is lower in the solid sample. A lower limit is given by the total intensity of the resonance Raman

Corval and Trommsdorff lines; this gives a lower limit of the emission quantum yield which is about a factor of 10 smaller than the upper limit stated above. The Decay of S1of cis-Thioindigo. The radiative decay rate, krcis,of cis-thioindigo can be evaluated from the integrated oscillator strength of the SI So transition as krk = (21 ns)-’; this value is very similar to the corresponding rate determined for the trans isomer from the measured values of the lifetime and the quantum yield: krtrsns= (20 ns)-l. The values of the emission quantum yield obtained here for the cis isomer predict therefore that the lifetime of SI lies in the range of 70-700 fs. This gives a homogeneous contribution to the line width of the transition of 7-70 cm-’ as compared to the interval of 70-200 cm-’ estimated from the observed width and structure of the absorption spectra. This suggests that the lifetime of the first excited singlet state of cis-thioindigo is not much longer than 100 fs. A direct time domain measurement of this lifetime should be possible by using advanced femtosecond technology but is complicated in the present situation by the interfering emissions discussed above. The observation that the radiationless decay rate of SIincreases from 2 X lo7 s-l in trans-thioindigo by more than 5 orders of magnitude in going to the cis isomer is not easy to rationalize. Both molecular orbital calculations’ and the spectral measurements presented here indicate that there is no drastic difference in the electronic structure of the excited SI state of the two isomers so that the cause of the anomalously short lifetime for the cis isomer has to be sought elsewhere. The main structural difference of the two isomers is the proximity of the oxygen atoms in the cis isomer. As was discussed in previous work,22there is considerable steric hindrance, the 0-0 distance being only 1.87 A when the rigid molecular moieties are twisted around the central C=C bond. However, we have found that this hindrance is not sufficient to lead to a nonplanar structure of the cis isomer as most of the strain can be relaxed by an in-plane distortion of the molecule which increases the 0-0 distance to about 2.38 A. The torsional potential becomes nevertheless quite flat in the cis configuration and the molecule, in the excited state, is susceptible to an out-of-plane distortion in a condensed-phase environment; this geometry change could provide an efficient pathway of energy relaxation and therefore increase the rate of internal conversion. The relatively small 0-0distance also makes energetically available electronic configurations corresponding to a structure (11) in which a chemical bond between the two oxygen atoms is established, and these electronic degrees of freedom also could play a role in the relaxation process. An other important dif-

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10-0,

It ference of the electronic structure of the two isomers is the increase, by about 2200 cm-l in the cis isomer, of the energy separation of the excited aa* state with respect to the ground state. For the first excited na* configurations, on the other hand, it can be expected that the interaction of the spatially nearby C = O groups leads to a lowering of the excitation energy such that the corresponding triplet state could lie close to or below the singlet aa* state; such an energy level structure would provide a very efficient route of intersystem crossing. Preliminary results of ab initio molecular orbital calculations made for the basic chromophore of thioindigo support this suggestion; while the experimentally observed blue shift of the .na*transition is confirmed, the first na* transition is red-shifted in going from the trans to the cis isomer.” Another interesting issue is the significance of the ultrafast decay of cis-thioindigo with respect to the cis-trans isomerization. This reaction is of course completely blocked under our experi(3 I ) Cardy, H., private communication.

7087

J. Phys. Chem. 1989, 93, 7087-7091 mental conditions (low-temperature solid-state environment), but we can expect that the nonradiative decay mechanisms, which are active under these conditions, are not slowed down at higher temperatures in the liquid state. It therefore appears that the isomerization reaction cannot compete with the decay processes of SI (already active at low temperatures) and will not take place on the excited-state singlet surface. It is also very unlikely that the isomerization can take place on the ground-state surface; this would require that the reaction speed for the hot molecule created by the internal conversion from SI increase by about 15 orders of magnitude from its value measured for a thermalized molecule at room t e m p e r a t ~ r e(5.4 ~ ~ x IO4 s-l at 120 "C). While our results rule out a singlet mechanism for the cis to trans isomerization, they are fully compatible with a triplet route. The occurrence of this isomerization on the triplet surface was demonstrated by the observation of this reaction after triplet sensitizing.l2 A triplet mechanism is also consistent with a number of time-resolved experiments: the observation, after excitation of the cis isomer, of a long-lived transient absorption attributed TI t r a n s i t i ~ n , ' ~and , ~ ~the * ~recovery ~ of the trans ground to a T, state on the time scale of the decay of this TI state.16 In nanosecond experiments, the same transient absorption, attributed to a planar trans triplet species, was observed after excitation of either isomer,16*18 while recent picosecond experiments on thioindigo derivatives show the appearance of different transient absorptions after exciting the cis or the trans isomer. In the case of a perinaphthothioindigoid dye, the buildup of the absorption is much more rapid when the cis isomer is excited, (3.7 f 1.2) X 1Olos-l, as compared to (1.4 f 0.3) X lo9 s-I for the trans isomer,21


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(32) Erler, M.; Haucke, G.; Paetzold, R. Z . Phys. Chem. ( k i p z i g ) 1977, 258, 315. (33) Krysanov, S.A.; Alfimov, M. V. Dokl. Phys. Chem. 1981,258,460.

consistent with our observation of an increased intersystem crossing in the cis isomer of thioindigo. Conclusion

In this work we have shown the following: The comparison of the resonance Raman spectra (and, in a more restricted way, of the absorption spectra) of the two isomers of thioindigo confirms predictions, based on molecular orbital calculations, of a very similar electronic structure of the first excited singlet state. The lifetime of this state in the cis configuration is in the order of 100 fs, in contrast to a value of 15 ns for the trans isomer. We suggest that the rapid decay of S1of cis-thioindigo reflects an increase of the intersystem crossing rate, due to a nearby nx* triplet state and possibly also related to the proximity of the two oxygen atoms. Even then this observation is difficult to rationalize and presents a challenge for a theoretical treatment of the question. Preliminary results of molecular orbital calculations support the hypothesis put forward here of the presence of a n i * triplet state close to or below the a x * singlet state. The rapid decay of the excited singlet state of cis-thioindigo should not be slowed down in liquid solutions as compared to the solid-state environment; this observation, therefore, excludes a singlet pathway for the cis trans isomerization, while all evidence is consistent with a triplet route.

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Acknowledgment. We are grateful to W. Liittke for the gift of the dye samples and many fruitful exchanges concerning indigo dyes. Stimulating discussions with P. Barbara during the completion of this work have been very helpful. We thank H. Cardy for performing the MO calculations and communicating the unpublished results. Registry No. Thioindigo, 522-75-8; nonane, 11 1-84-2.

Time-Resolved ESR Studies on the Excited Triplet States and Photoenolization of 2-Methylacetophenone and Related Molecules Tadaaki Ikoma, Kimio Akiyama, Shozo Tero-Kubota, and Yusaku Ikegami* Chemical Research Institute of Non-Aqueous Solutions, Tohoku University, Katahira 2-1 -1, Sendai 980, Japan (Received: January 23, 1989)

Nonphosphorescent excited triplet (%ri*) states of photoenols (3E)generated from the intramolecular hydrogen transfer of o-methylacetophenone (OMAP) and related compounds were measured, together with their phosphorescent excited triplet by time-resolved ESR techniques at low temperatures. The triplet ESR spectrum with zero-field splitting parameters, state (3K), 1 0 1 = 0.060 and IEI = 0.0025 cm-', clearly different from those of the parent molecules (3K), was assigned to 'E. The observed small ID1 value suggests the biradical character of 'E. It was pointed out from the examination of substituent and matrix effects that the ease of photoenol generation clearly depends on the character of the T1state of the parent molecules and also on the coplanarity of the carbonyl group with the aromatic ring. The presence of water induces a striking change in the character of the TI state of OMAP; i.e., n?r* of Tl in nonpolar or absolute ethanol glassy matrices is altered by T X * in 0.5% HzO/ethanol. CIDEP spectra observed from the quenching reaction with methylviologen proved that 'E behaves as an excellent electron donor.

Introduction During recent decade, photoenolization1-3 as well as the Nomish particular, a Type 113-5 has received considerable attention.

number of studies on the reaction mechanism and kinetics have been reported for the transient species in the photoenolization Of o-alkyl-substituted aromatic ketones6-lZ and o-alkylbenzo-

( 1 ) Haag, R.; Wirz, J.; Wagner, P. J. Helu. Chim. Acta, 1977, 60, 2595. (2) Sammes, P. G.Tetrahedron 1976, 32, 405. (3) Scaiano, J. C. Acc. Chem. Res. 1982, 15, 252. (4) Wagner, P. J. Acc. Chm. Res. 1971, 4, 168. (5) Scaiano, J. C.; Lissi, E. A.; Encina, M. V. Reu. Chem. Inrermed. 1978, 2, 139.

(6) Findlay, D. M.; Tchir, M. F. J . Chem. SOC.,Faraday Trans. 1 1976, 72, 1096. (7) Lutz, H.; Breheret, E.; Lindqvist, L. J . Chem. SOC.,Faraday Trans. 1 1973, 69, 2096. (8) Wagner, P. J.; Chen, C.-P. J . Am. Chem. SOC.1976, 98, 239. (9) Small, R. D.; Scaiano, J. C. J . Am. Chem. SOC.1977, 99, 7713.

0022-3654/89/2093-7087$01.50/0

0 1989 American Chemical Society

Optical spectra and excited-state dynamics of cis-thioindigo - The

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