7078
J. Phys. Chem. 1989, 93, 7078-7081
Oxo-Hydroxy Tautomerism of Uracil and 5-Fluorouracll Andrzej Lest and Ludwik Adamowicz* Department of Chemistry, The University of Arizona, Tucson, Arizona 85721 (Received: January 4, 1989; In Final Form: March 14, 1989)
The three lowest energy tautomeric forms of uracil and 5-fluorouracil have been studied by using the second-order many-body perturbation theory (MBPT(2)) with Gaussian DZP basis sets. The zero-point nuclear energy has been estimated by means of the harmonic approximation using analytical derivatives of the SCF 3-21G energy. The uracil and 5-fluorouracil molecules have been predicted to exist in the gas phase in the dioxo forms in agreement with the majority of experimental data. The 2-hydroxy-4-oxo and 2-oxo-4-hydroxy tautomeric forms of uracil have been found to be less stable than the main dioxo form by 44 and 50 kJ mol-', respectively. The corresponding values for 5-fluorouracil are 29 and 52 kJ mol-'. The relevance of the present results to recent IR and fluorescence spectroscopical experimental works is discussed. Some aspects of the point mutation theory in view of the present work are also considered.
Introduction Uracil belongs to a group of the most important pyrimidines playing a fundamental role in the structure and functioning of nucleic acids, enzymes, and drugs. Uracil and its 5-methyl derivative, thymine, are components of DNA and R N A helices, and various nucleoside analogues involving uracil or thymine are potent inhibitors of the DNA viruses, R N A viruses, and retroviruses.' On the other hand, thymine is a product of the spontaneous deamination of 5-methylcytosine, which may cause a formation of mutational hot spot^.^,^ Furthermore, a potent chemical mutagen, the nitrous acid, converts cytosine into uracil when acting on nucleic acids. The biological relevance of various halogen derivatives of uracil is also well-recognized. For example, the 5-substituted halogen derivatives of uracil nucleosides have useful physiological activity for antitumor and antiviral drugs.4d Since the proposal of the double helical structure of DNA; the hypothesis of the occurrence of pyrimidine and purine bases in rare tautomeric forms has attracted considerable attention (ref 8 and 9 and references therein). Tautomer studies were given a great impetus when Lowdin suggested a molecular mechanism of spontaneous mutations.'O In his theory the rare (unusual) tautomeric forms of nucleic bases played an essential role in an alteration of the normal base-base pairing, thus leading to the transition-type point mutations. At present, one can find numerous evidence that cytosine and guanine may tautomerize much easier ~ ability of uracil and than adenine, uracil, or t h ~ m i n e . The thymine to adopt rare tautomeric forms still remains an unsolved problem, although numerous efforts have been undertaken. A majority of experimental studies (NMR," IR,12 UV,I3 microwavei4) clearly state that the most stable form of uracil is the 2-0XO-4-0XO form, either in the gas phase, or in the solution of various polarity and in the crystalline state. The question arises on the estimation of the relative stability of the rare tautomeric forms. The experimental studies are spread, and in some cases inconclusive. Beak et al.Is argued that the 2-oxo-4-hydroxy tautomer of uracil is by 79.5 f 25.1 kJ mol-' less stable than the dioxo form. Shugar, Szczepaniak, and co-workers12J3did not find in their matrix isolation and vapor IR spectra any traces of the rare forms of uracil, and its 5-flUOrO and 5-chloro derivatives, although they could not exclude a possibility of appearance of the rare forms in concentrations below 10% of the main form. Ruterjans et al." presented evidence for a tautomerism in nucleic acid bases, including uracil, based on an investigation of imino proton resonances by means of a proton N M R of the I5N-labeled tRNA. However, their announcement has been criticized,I6 and an alternative explanation of the observed resonances without a proton jump was suggested. Very recently, Tsuchiya et al."J* reported the electronic spectra of uracil in supersonic jets and
suggested that the energy splitting between the hydroxy form and the main form should be less than 40 kJ mol-'. This last work was strongly criticized by Brady et a1.I9 who presented a multiphoton ionization spectra of uracil in the supersonic beam and argued that the spectra recorded by Tsuchiya et al.'*Jg are not due to uracil, but probably to an unidentified impurity. According to Brady et al.,I9the uracil sample in the gas phase at T = 200-300 O C does not contain any detectable amount of rare tautomeric forms. Very little is known about the tautomerism of 5-halogeno derivatives of uracil. It has been suggested that those derivatives may adopt rare hydroxy forms easier than the nonsubstituted uraci120and therefore can potentially act as stronger mutagenic agents. However, it is known that 5-bromouracil may replace thymine completely in some D N A s , ~ but ~ ~ *surprisingly ~ the replication is not affected. In the recent IR matrix isolation study was found of methyl derivatives of 5 - f l u o r o ~ r a c i l , ' ~no' ~evidence ~ for the appearance of any detectable absorption corresponding to the hydroxy rare tautomers. The results of theoretical studies with semiempirical methods of uracil tautomers have been rather uncertain as to the relative energy differences. Using the MNDO method, Buda and S y g ~ i a ~ ~
*Author to whom correspondence should be addressed. 'Permanent address: Department of Chemistry, University of Warsaw, Pasteura 1, 02-093 Warsaw, Poland.
2082. (23) Buda, A,; Syguka, A. J. Mol. S t r c t . (THEOCHEM) 1983,92, 255, 267.
(1) (2) 349. (3) (4) (5)
Mansuri, M. M.; Martin, J. C. Ann. Rep. Med. Chem. 1987.22, 147. Adams, R. L. P.; Burton, R. H. CRC Crit. Rev. Biochem. 1982, 13,
Fix, D. F.; Glickman, B. W. Mutur. Res. 1986, 175, 41. Drach, J. C. Annu. Rep. Med. Chem. 1980, 15, 149. Burger, A. A Guide to the Chemicul Busis of Drug Design; Wiley: New York, 1983. (6) Bodor, N.; Kaminski, J. J. Annu. Rep. Med. Chem. 1987, 22, 303. (7) Watson, J. D.; Crick, F. H. C. Nature 1953, 171, 737. (8) Ts'o, P. 0. P. In Busic Principles in Nucleic Acids Chemistry; Ts'o, P. 0. P., Ed.; Academic Press: New York, 1974. (9) Kwiatkowski, J. S.;Zielinski, T. J.; Rein, R. Ado. Quuntum Chem. 1986, 18, 85. (10) LBwdin, P. 0. Rev. Mod. Phys. 1963, 35, 724. (1 1) Riiterjans, H.; Kaun, E.; Hull, W. E.; Limbach, H. H. Nucleic Acids Res. 1982, 10, 7027. (12) Shugar, D.; Szczepaniak, K.Int. J. Quuntum Chem. 1981,20, 573. (13) (a) Nowak, M. J.; Szczepaniak, K.; Barski, A.; Shugar, D. Z . Nuturforsch. 1978, C33, 876. (b) Szcz@niak, M.; Nowak, M.J.; Szczepaniak, K.; Person, W. B.; Shugar, D. J. Am. Chem. Soc. 1983, 105, 5969. (14) Brown, R.D.; Godfrey, P. D.; McNaughton, D.; Pierlot, A. P. J. Am. Chem. SOC.1988, 110, 2329. (15) Beak, P.; White, J. M. J. Am. Chem. SOC.1982, 104, 7073. (16) Redfield, A. G., personal communication quoted by Riiterjans et al. (ref 11). (17) Tsuchiya, Y . ;Tamura, T.; Fujii, M.; Ito, M. J. Phys. Chem. 1988, 92, 1760. (18) Fujii, M.; Tamura, T.; Mikami, N.; Ito, M., Chem. Phys. Lett. 1986, 126, 583. (19) Brady, B. B.; Peteanu, L. A.; Levy, D. H. Chem. Phys. Lett. 1988, 147, 53820. Mandel, H. G. Prog. Mol. Subcell. Biol. 1969, 1 , 82. (21) Kimball, R. F.; Perdue, S.W. Mutut. Res. 1977, 44, 197. (22) Bick, M. D.; Davidson, R. L. Proc. Nutl. Acud. Sci. USA 1974, 71,
0022-365418912093-7078$01.50/0 0 1989 American Chemical Society
Oxo-Hydroxy Tautomerism of Uracil and 5-Fluorouracil correctly predicted the largest stability of the hydroxyamino form of cytosine. With the same method they obtained, however, the largest stability of the hydroxy forms of uracil and thymine which is now known to be incorrect. N ~ r i n d e r , with * ~ the semiempirical AM1 method, correctly predicted the highest stability of the dioxo form of uracil and the next two hydroxy forms, but his prediction of the relative order of cytosine tautomers was incorrect (the oxoamino form being the most stable, next being the oxoamino N(3)-H, the hydroxyamino, and the oxoimino forms). At the same time, Zielinski and Reinz5 predicted the 2-oxo-4-hydroxy form of uracil as the most stable tautomer by means of the semiempirical MIND0/2 method. On the other hand, Czermiiiski et aLZ6correctly obtained the largest stability of the dioxo form of uracil using the M I N D 0 / 3 method, but their prediction of the next stable 2-oxo-4-hydroxy tautomer of 21 kJ mol-] above the dioxo form does not seem to be reasonable. Using the INDO method and including some solvation effects, SaundersZ7predicted the most stable forms of uracil to be the dioxo form for nonpolar solvents and the 2-oxo-4-hydroxy form for weakly polar and polar solvents. There have been some low-level ab initio studies on the uracil tautomeric equilibrium. The S C F calculations with the splitvalence basis set predicted the energy difference between the dioxo and oxo-hydroxy tautomers consistently at about 70-90 kJ .28-31 Similar energy differences have been obtained for 5 - f l u o r o ~ r a c i l .There ~ ~ are several factors which determine the reliability of ab initio calculations for larger molecular systems. The most important one is the quality of the basis set. The ab initio S C F studies with the split-valence basis sets augmented by polarization functions located on heavy atoms (6-3 1G*) and hydrogens (6-3 1 G**, DZP), are considerably more trustworthy than basis sets without polarization functions. Moreover, it has been recognized that, to obtain qualitatively correct results of the tautomer relative stability, it is necessary to include the contribution arising from the electron correlation effects and the zero-point nuclear vibration^.^ In a recent paper Kwiatkowski et al. suggested, however, that the electron correlation effects are unimportant for predicting the tautomer relative stability of uracil tautomers.32 This is a somewhat surprising conclusion considering the degree of the bond structure alternation during the tautomerizations process. A high-quality ab initio calculation could certainly help to clarify these problems, and this is the intent of this study. In the present work we calculated the SCF, electron correlation, and nuclear vibration contributions to the relative stability of uracil and 5-fluorouracil.
Methodology The present post-SCF calculations were performed with the many-body perturbation theory at the second-order level (MBPT(2)) using DZP-quality basis sets (for a description of the basis sets, see Tables I and 11). Considering that for closed-shell molecular systems at the equilibrium geometries the second-order correction usually accounts for about 90% of the total correlation energy, one should obtain the most important correlation component of the tautomerization energy at this level of theory. There could be, however, some contributions from higher orders which we neglected in this work due to limitations of the computer power available to us at the present time. The MBPT(2) level of the theory is the most computationally expeditious procedure because it requires only a small subset of all two-electron molecular in(24) Norinder, U. J. J . Mol. struct. 1987, 151, 259. (25) Zielinski, T. J.; Rein, R. Int. J . Quantum Chem. 1978, 14, 851. (26) CzermiAski, R.; Lesyng, B.; Pohorille, A. Int. J . Quantum Chem. 1979, 16, 605. (27) Saunders, M.; Webb, G. A.; Tute, M. S. J. Mol. Struct. 1987, 158, 69. (28) Mondragon, A.; Ortega Blake, I. Int. J. Quantum Chem. 1982.22, 89. (29) Zielinski, T. J. J . Comput. Chem. 1983, 4, 345. (30) Scanlan, M. J.; Hillier, I. H. J . Am. Chem. SOC.1984, 106, 3737. (31) Leg, A.; Ortega Blake, I. Int. J. Quantum Chem. 1986, 30, 225. (32) Kwiatkowski, J. S.; Bartlett, R. J.; Person, W. B. J. Am. Chem. SOC. 1988, 110, 2353.
The Journal of Physical Chemistry, Vol. 93, No. 20, 1989 7079 TABLE I: Various Contributions to the Total Energy of the 2-oxo-4-oxo (Ul), 2-Hydroxy-4-oxo (UZ), and 2-Oxo-4-hydroxy (U3) Tautomers of Uracil, with the DZP Basis Set‘ and with the Optimal Geometries Obtained at the 3-21G SCF Level (Energies in au) u1 u2 u3 -412.538984 -412.535106 SCF -412.557762 ZPEb 0.095316 0.094029 0.094069 -1.18 1976 -1.183489 MBPT(2)c -1.181 107 “ D Z P basis set (148 basis functions) of Dunning34composed of 9s5p primitive Gaussians contracted to 4s2p and augmented by one d-function on carbon, nitrogen, and oxygen 4s contracted to 2s and augmented by one p-function on hydrogen atoms. The exponent (0.7) of the hydrogen p-function has been interpolated from the MBPT-optimized basis sets.35 Zero-point energy calculated with 3-21G basis set by using analytical second derivatives of the S C F energy. CThe second-order correlation energy calculated with the DZP basis set (1 19 virtual and 21 occupied orbitals). The frozen-core approximation assumed (16 core electrons not correlated).
TABLE 11: Various Contributions to the Total Energy of the 2-oxo-4-oxo (FUl), 2-Hydroxy-4-oxo (FUZ), and 2-Oxo-4-hydroxy (FU3) Tautomers of 5-Fluoro-uracil,with the DZP Basis SeP (Energies in au) SCF FU 1 FU2 FU3 SCF -511.392817 -511.378963 -511.369649 ZPEb 0.086655 0.085432 0.085378 MBPT(2)‘ -1.315914 -1.317481 -1.318174 DZP basis set (1 50 basis functions) of Dunning34 composed of 9s5p primitive Gaussians contracted to 3s2p and augmented by one d-function on carbon, nitrogen, oxygen, and fluorine atoms;34 the exponent (1.581) of the d-function of fluorine has been interpolated form the MBPT-optimized basis sets for first-row atoms.3s 4s contracted to 2s and augmented by one p-function on hydrogen atoms (see footnotes to Table I). bZero-point energy calculated with 3-21G basis set and analytical first and second derivatives of the S C F energy. cThe second-order correlation energy calculated with the D Z P basis set (1 17 virtual and 24 occupied orbitals). The frozen-core approximation assumed (18 core electrons not correlated).
tegrals (with two occupied and two virtual indexes). In the present calculations, these integrals were generated by our selective four-index transformation procedure. This kind of transformation usually requires much less computer time than the S C F calculation and has almost the same demand for the disk space. The optimal molecular structures and harmonic vibrational frequencies, as well as the zero-point energy (ZPE) of the nuclear vibrational motions were obtained with the 3-21G basis set by means of the S C F analytical derivative technique.33
Results and Discussion Our results for the three lowest energy tautomers of uracil and 5-fluorouracil (Figure 1) are presented in Tables I-V. Other rare forms have considerably higher total energies30 and are not considered here. In Tables I and I1 we gathered the contributions to the total energy arising from the electronic (SCF + electronic correlation) and nuclear (zero-point vibrations) motions for uracil and 5-fluorouracil, respectively. Our S C F energy values are considerably lower than the recent 6-31G* results of Kwiatkowski (33) Binkley, J. S.; Frisch, M.; Raghavachari, K.; De Frees, D.; Schlegel, H. B.; Whiteside, R.; Fluder, E.; Seeger, R.; Fox, D. J.; Head-Gordon, M.; Pople, J. A,, GAUSSIAN 86, release C, Carnegie Mellon University. (34) Dunning, Th. H. J. Chem. Phys. 1970, 53, 2823. (35) Redmon, L. T.; Purvis, G. D., 111; Bartlett, R. J. J . Am. Chem. SOC. 1979, 101, 2856. (36) Shibata, M.; Zielinski, T. J.; Rein, R. Int. J . Quantum Chem. 1980, 18, 323. (37) Kwiatkowski,J. S.; Person, W. B.; Szczepaniak, K.; SzczgSniak, M. Acta Biochim. Polon. 1987, 34, 165. (38) SzczgSniak, M.; Szczepaniak, K.; Kwiatkowski, J. S.;Ku Bulat, K.; Person, W. J. Am. Chem. SOC.1988, 110, 8319. (39) Hes, B. A,, Jr.; Schaad, L. J.; Ca’rsky, P.; Zahradnik, R. Chem. Reu. 1986, 86, 709.
7080 The Journal of Physical Chemistry, Vol. 93, No. 20, 1989
Le3 and Adamowicz
OB
u3
u2
ul
Figure 1. The 2-0XO-4-0XO (U1, FUl), 2-hydroxy-4-oxo(U2, FU2), and 2-oxo-4-hydroxy (U3, FU3) tautomers of uracil and 5-fluorouracil, respectively. TABLE III: Relative Stabilities of Uracil Tautomers' (Energies in kJ mol-')
u2-u 1 SCF
ZPE MBPT(2) SCF + 0,91*ZPE+MBPT(2) experiment
DZP 6-3lG* 3-21G AM1 3-21G MIND0/3 DZP 6-31G*
this paper ref 32
49.3
ref 30
72.1
ref 24 this paper ref 36 this paper ref 32
39.0 -3.4
Cumulative Relative Stabilityb this paper ref 32 ref 17 ref 15
-2.3
43.9 53 (estim) 540 (?)
U3-U1 59.5 62.5 81.8 -3.3 0.8 -6.3 -0.6 50.2 62.7 79 f 25
'Zero energy corresponds to the 2-0x0-4-0x0 ( U l ) form. BThe 0.91 scale factor for the ZPE energy has been adopted from ref 37 and 38. For general references to this problem see, ref 39. TABLE I V Relative Stability of 5-Fluorouracil Tautomers' (Energies in kJ mol-')
SCF
ZPE MBPT(2) SCF + 0,91*ZPE+MBPT(2)
DZP 3-21G 3-21G DZP
this paper
ref 30 this paper this paper
Cumulative Relative Stabilityb this paper
FU2-FUl 36.4 66.2 -4.1
FU3-FUI 60.8 83.8 -3.4 -5.9
29.3
51.8
-3.2
"Zero energy corresponds to the 2-0XO-4-0XO (FUI) tautomer. bSee footnotes to Table 111. et due to the better basis set employed with polarization functions on the hydrogens. The relative stabilization energies for different uracil and 5-fluorouracil structures are presented in Tables 111 and IV, respectively. In both cases the dioxo tautomer has been chosen as the reference. From the biophysical point of view, the 2-oxo-4-hydroxy tautomers are particularly interesting because they can form the N( 1)-glycosidic bonds characteristic for all nucleic acids. The present estimation of the energetic gap of 50.2 kJ mol-l between the U1 and U3 uracil tautomers suggests rather low probability of the Occurrence of uracil in the rare U3 form when incorporated in biological systems. However, if we calculate the equlibrium constant for the U1 U3 transition according to the exponential formula exp(-50.2/kT), k = 0.008 314 51 kJ mol-I K-I, T = 298 K, we obtain the value of which falls into the region of estimated frequency of the spontaneous point mutations. Thus, we cannot exclude the possibility of the formations of mismatches which involve the rare hydroxy tautomer of uracil nucleosides and nucleotides. According to our calculations, the 2-hydroxy-4-oxo tautomer of uracil (U2) is even more stable than the U3 form. Although the U2 uracil tautomer cannot appear in the normal nucleic acids, it can be found in those modified nucleic acids which
-
contain the pseudouridine-an unusual nucleoside with the sugar moiety bound to the C5 atom of the pyrimidine ringsa In pseudouridine, both oxygen atoms (O(2) and O(4)) tnay act as proton acceptors, which increases the probability of tautomerization (to about We would like to point out a particular role of the electron correlation and zero-point vibration effects in estimating of the tautomer relative stabilities. Recently, Kwiatkowski et al.32have argued that those effects are not important. In contrast, in the present work we found that neither of these contributions can be neglected. For uracil, as well as for fluorouracil, both the zeropoint vibration energy and the correlation effect destabilizes the 2-0XO-4-0XO form. This appears to be somewhat different from whose 2-oxo-4-imino form is cytosine, recently studied by stabilized by the electronic correlation contribution but destabilized by the zero-point vibration energy effect. The influence of the halogen substitution on the uracil tautomerism can be investigated considering 5-fluorouracil. Com(40) Dirheimer, G . Modified Nucleosides and Cancer; Springer-Verlag: Berlin, 1983. (41) LeS, A.; Adamowicz, L.; Bartlett, R. J. J . Phys. Chem. 1989, 93,4001.
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 S t . 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.
-
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 . Am. 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
