J . Phys. Chem. 1985, 89, 5 156-5 160
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ARTICLES Evidence from Multimode Vibrational Theory for a Low-Lying Forbidden Transition in the Uracil Chromophore of Uridine 5’-Monophosphate Pierre-Yves Turpin and Warner L. Peticolas* Department of Chemistry, University of Oregon, Eugene, Oregon 97403 (Received: August 16, 1984; In Final Form: July 16, 1985)
The multimode theory of molecular vibrations has been used previously to calculate the absorption spectrum, the Raman excitation profile, and the relative Raman overtone intensities of the uracil chromophore in uridine 5’monophosphate (UMP). In this paper the corresponding theory of the fluorescence of this chromophore is discussed for the first time. With the values of the Franck-Condon factors for the five resonance Raman active modes determined from the overtone resonance Raman spectrum it is possible to calculate the absorption and fluorescence profiles. The calculated absorption and fluorescence profiles are in good agreement with the experimental line shapes but, while the observed Stokes shift between the absorption and fluorescence line shapes is enormous (ca. 6500 cm-’ from absorption peak to fluorescence peak), the calculated Stokes shift is only 3500 cm-I. Furthermore, the apparent 0-0 transition is calculated to occur at 36 500 cm-’ but is observed at 35 100 cm-I. So that these results as well as the intrinsic lifetime of this chromophore can be explained, it is suggesed that, after absorption of a photon, the excited-state energy is relaxed into a forbidden electronic state located in the red wing of the strongly allowed electronic transition. This explanation is consistent with the discrepancy between the best experimental estimation of the radiative lifetime and the value obtained for the excited-statelifetime calculated from the observed absorption profile. It is also consistent with the recent observation of greatly altered fluorescence lifetimes and yields for uracil and other nucleotides at 77 K in an aprotic polar solvent. The possibility of a solvent-induced relaxation of the allowed excited electronic state energy before fluorescence cannot be ruled out by our calculations. However, it can be definitively shown that, in order to have a consistent theoretical description of the Raman scattering, the absorption and the fluorescence of the latter must originate from a lower energy electronic state than that to which absorption has occurred.
Introduction In a recent series of papers’-5 (for a review see ref 5 ) it has been shown how to calculate the resonance Raman intensities of fundamental and overtone vibrations as a function of the incident laser frequency from the measured values of the absorption profile of a molecule by using a Kramers-Kronig transform technique. In these calculations, the geometric shift of the potential curve along the jth normal coordinate in an electronic excited state, Aj, is obtained in a natural way for each of the resonance Raman active modes of the uracil chromophore in uridine 5’-mOnOphosphate (UMP).]s2v4From the values obtained in the resonance Raman calculation one can also calculate the absorption profile. When the technique is applied to the uracil chromophore in UMP excellent agreement is obtained for the relative intensities of the overtone progression and the absorption p r ~ f i l e . l * Recently, ~,~ detailed measurements have been made of the fluorescence profiles of uracil derivatives.6-10 It seems worthwhile to see if the fluorescence profile can also be calculated from exactly the same parameters as used for the resonance Raman and absorption profile (1) (2) (3) (4)
D. Blazej, Ph.D. Dissertation, University of Oregon, 1978. D. Blazej and W. L. Peticolas, J . Chem. Phys., 72, 3134 (1980). D. L. Tonks and J . B. Page, Chem. Phys. Lett., 66, 449 (1979). L. Chinsky, A. Laigle, W. L. Peticolas, and P. Y. Turpin, J . Chem.
Phys., 76, 1 (1982). ( 5 ) P. M. Champion and A. C. Albrecht, Ann. Rev. Phys. Chem., 33,353 (1982). (6) J. W. Longworth, R. 0. Rahn, and R. G. Shulman, J . Chem. Phys., 45, 2930 (1966). (7) M. Daniels and W. Hauswitz, Science, 171, 675 (1971).
(8) M. Gueron, J. Eisinger, and A. A. Lamola in “The Basic Principles in Nucleic Acid Chemistry”, P. 0. P. Ts’O, Ed., Academic Press, New York, 1974, p 311. (9) P. Vigny, Ph.D. Dissertation, University of Paris, 1974. (10) R. S. Becker and G.Kogan, Photochem Phorobiol., 31, 5-13 (1980).
0022-365418512089-5156$01.SO10
calculations. Exmimental measurements show that for the uracil chromophore in’UMP the fluorescence has the following rather interesting set of properties: (1) At low temperatures (77 K) in ethylene glycol/water glasses, the fluorescence intensity profile is approximately a mirror image of the absorption.8 (2) The fluorescence is extremely weak: the quantum yield, &, is 3 X This low quantum yield is of the order of resonance Raman quantum yield i n t e n ~ i t i e s .(3) ~ ~ The ~ lifetime of the fluorescence is extraordinarily short, certainly less than 50 P S . ” - ’ ~ (4)The fluorescence maximum is enormously red shifted: it does not start until well past the resonance Raman band of water which is 3500 cm-’ from the exciting line, and the distance the absorption peak and the fluorescence peak is about 6500 cm-l. Indeed it is the weak greatly Stokes shifted fluorescence that allows the easy observation of the resonance Raman spectrum from the low-lying 260-nm band.'^^,^ The question naturally arises as to the origin of this large Stokes shift and the extraordinary weakness of the fluorescence from such a strongly allowed electronic state. The textbook explanation for a large Stokes shift in the fluorescence is that it is due to the large displacement of the excited-state potential energy along the multidimensional space of the Raman active normal coordinates. Since this explanation is untenable for U M P we must seek another explanation. One possibility for the large Stokes shift in the fluorescence is the existence an unallowed state whose energy lies below that of the first allowed excited state. If the energy of the allowed state is funneled into this lower state by intersystem crossing then (1 1) J. Eisenger and A. A. Lamola in “Excited States of Proteins and Nucleic Acids”, R. F. Steiner and I. Weinryb, Ed., Plenum Press: New York, 1971. (12) J. P. Morgan and M. Daniels, Photochem. Photog. Biol. 2, 73 (1978). (13) J. P. Morgan and M . Daniels, Chem. Phys. Lett., 67, 533 (1979). (14) P. R . Callis, Chem. Phys. Lett., 61, 563 (1979).
0 1985 American Chemical Society
Forbidden Transitions in U M P the subsequent fluorescence would be very strongly red shifted. There recently has been a great deal of interest in the experimental observation of forbidden molecular states which lie lower in energy than the first allowed excited state and therefore are not seen in normal one-photon spectroscopy. (For recent reviews of this field consult ref 16-20.) In order to be able to observe the resonance Raman effect it is necessary that the resonant electronic state must be allowed and that some means must be present which suppress the intensity and/or lowers the frequency of the fluorescence from such a state. If no such means exist then the fluorescence will overwhelm the Raman signal since the quantum yield of fluorescence is orders of magnitude greater than the resonance Raman effect. It is obvious that the properties of the fluorescence and the absorption present a rather contradictory set of values. The absorption band has a moderately large extinction coefficient which together with quantum mechanical calculation^'^ certainly seems to indicate an electronic transition with a rather large transition dipole moment. Such an excited electronic state should have a strong fluorescence. If it is argued that the fluorescence comes not from this band but from a nonallowed lower energy electronic transition to which the excitation energy is transferred, then the question arises why is the fluorescence so nearly a mirror image of the strong absorption band a t 77 K? One possible explanation which must be ruled out at is that the excited-state dipole moment is 2 D larger or smaller than the dipole moment in the ground state. Both theoretical and experimental work has shown that, if the excited state has a dipole moment which is 2 D larger or smaller than the ground-state dipole moment, the absorption band as measured in a solvent of high dielectric constant could shift to the blue by 3000 cm-' causing an apparent 3000-cm-I shift in the fluorescence maximum from the absorption maximum as measured in low dielectric solvents. (For a discussion of measurements on the solvent-dependent shift in the absorption maxima with little change in the fluorescence of pyrazine and pyrimidine in various solvents with many relevant references to the literature see ref 26.) Unfortunately no absorption and fluorescence measurements in different solvents have been made to our knowledge on N-1-substituted uracils in nonhydrogen-bonding solvents. There is then no direct evidence for the presence or absence of a large change in dipole moment between the ground and first excited electronic states in UMP. However, a very detailed study was recently made of the adsorption and fluorescence of the isolated base uracil along with the 3'methyl- and 3', 1'-dimethyluracil in a number of different solvents by Becker and Kogan.'O (As we discuss below, these authors came to the conclusion that in an aprotic polar solvent at 77 K uracil, thymine, and thymidine all fluorescence from a forbidden nr* state.) Unfortunately, direct comparison of our calculations with the experimental measurements of Becker and Kogan is not strictly possible since they work with uracils substituted in a different position than the N-1-substituted compound UMP which we have studied. Methyl substitution greatly changes the normal-mode structure in both the ground and excited states. The changes in the ground state give rise to frequency changes in the Raman spectrum while changes in the excited-state displacements give rise to changes in observed Raman intensities of the ground-state vibrations. Thus great differences exist in the observed Raman intensities and frequencies in uracil, 1-methyluracil, 3-methyluracil, and 5-methyluracil (thymine). However, the results of these authorslo are very helpful in ruling out the possibility of a dipole moment change in U M P between the ground and first allowed excited state. These authors have shown that there is not a (15) (16) (1974). (17) (1982). (18) (19) (20)
W. Hug and I. Tinoco, Jr. J . Am. Chem. Soc., 96,665-673 (1974). B. S. Hudson and B. E. Kohler, Annu. Reu. Phys. Chem., 25, 437
B. S. Hudson, B. E. Kohler, and K. Schulter, Excited Stares, 6, 1-95
R. R. Birge, Annu. Reu. Biophys. Bioeng., 10, 315 (1981). M. Ottolenghi, Adu. Photochem., 12, 97 (1980). R. R. Birge, "One-Photon and Two-Photon Excitation Spectroscopy" in 'Ultrasensitive Laser Spectroscopy", D. S. Kliger, Ed., Academic Press, New York, 1983, pp 109-174.
TI;re Journal of Physical Chemistry, Vol. 89, No. 24, 1985 5157 250
1
300
I
45
40
35
350
400
I
I
30
25
nm
kK
Figure 1. Absorption and fluorescence profiles of the uracil chromophore taken from the literature. The solid line is the fluorescence from the chromophore in a pH 7.0 phosphate buffer at room temperature9 while the dashed line is from the chromophore in a water/ethylene glycol glass at 7 1 K.*
significant difference in the absorption and fluorescence band positions in the isolated base uracil between those observed in alcohol and in an aprotic substituted furan. Furthermore, the absorption maximum of uracil reported by them is not far from that reported for UMP. Since it seems unlikely that there is a great difference between the dipole moments in the first allowed electronic states of uracil and 1-methyluracil (since both excited states are T T * states which involve the C5=C6 double bond) it appears possible to rule out a large difference in the dipole moment of the excited state of UMP as the cause of the observed very large Stokes shift in water. In order to determine the origin of the large Stokes shift in UMP, we have used the standard equations for absorption and fluorescence and the previously obtained values for the Aj's, the shift in the excited-state potential energy minima along each of the normal to calculate the expected fluorescence profile from the allowed electronic state. This calculation fails to give the observed value for the apparent 0-0 transition (Le., the point where the observed absorption and fluorescence bands cross). However, it does show reasonable agreement with the shape of the observed fluorescence band. We have found that although a broad absorption or fluorescence profile depends on the values of the Raman active vibrational frequencies it is rather insensitive to the exact values of the Franck-Condon factors, when five or more modes are resonance Raman active. Indeed, one can calculate the experimental absorption profile of the strongly allowed band from an assumption of five resonance Raman active modes each of which has a shift parameter equal to the geometric mean of the five values obtained from Raman measurements using the same damping factor. Consequently, if we assume that there exists a low-lying, nonallowed electronic transition with five or so normal modes whose potential energy curves are displaced along these normal coordinates in the excited state then we can obtain a reasonable explanation of the experimentally observed properties of the fluorescence.
Theory The object of this theoretical treatment is to calculate the observed absorption and fluorescence spectra of the uracil chromophore of U M P by using an approach which is consistent with that used to calculate the absorption and resonance Raman spectrum including the overtone resonance Raman spectra. As we shall see this proves to be impossible without the assumption of a low-lying unallowed state. In this paper, it is assumed that the strongly allowed band at 260 nm of UMP is a single electronic transition, as has been shown by polarized a b s o r p t i o r ~and ~~~~~ reflection experiment^.^^^^^ Figure 1 shows the observed absorption and fluorescence spectra of U M P both at room temperature and at 77 K. Although the (21) S. J. Strickler and R. A. Berg, J . Chem. Phys., 37, 814 (1962). (22) P. F. Stewart and N. Davidson, J . Chem. Phys., 39, 255 (1963). (23) W. A. Eaton and T. P. Lewis, J . Chem. Phys., 53, 2164 (1970). (24) P. R. Callis and W. T.Simpson, J . Am. Chem. SOC.,92,3593 (1971). (25) P. R. Callis, Annu. Rev. Phys. Chem., 34, 329 (1983). (26) H. Baba, L. Goodman, and P. C. Valletti, J . Am. Chem. Soc., 88, 5410 (1966).
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room temperature fluorescence spectrum is broadened, the lowtemperature fluorescence spectrum appears to be a rather good mirror image of the absorption spectrum but both the low- and high-temperature spectra show a very large displacement of the maximum intensity of the fluorescence spectrum to lower frequencies, i.e., a large Stokes shift. Since both the resonance Raman scattering and absorption are dominated by the FranckCondon factors it is a simple matter to provide equations that describe in the multimode theory the absorption profile, the fluorescence profile, and the resonance Raman excitation profile which depend upon the same parameters. Inhomogeneous broadening can be introduced into the equations for absorption, fluorescence, and R R excitation profiles in a semiempirical way by increasing the value of the damping factor, r. We start with the assumption that the absorption spectrum of a molecule can be expressed by a series of Lorentzian functions each centered about the excited-state energies, wej
Turpin and Peticolas TABLE I: Excited State Displacements A j for the Five Resonance Raman Active Modes of Uracil
n,, cm-'
Ai
n,. cm-'
785 1335 1390
1.o
1630 1680
1.3 0.6
Ai 0.5 0.7
3N-6
wej =
+ jc= I u p j
where w& is the frequency of the 0-0 transition from the ground to excited electronic state, Qj is the frequency of thejth normal mode (assumed to be the same in the ground and excited electronic state), and vj is the corresponding vibrational quantum number in the excited state. This formula assumes that all of the absorption starts from the 0th vibrational level of the ground state. Similarly, the fluorescence spectrum can be expressed as a similar set of energies
32
36
34
38
42
40
4 4 kK
Figure 2. Calculation of the absorption profile of the uracil chromophore in UMP using eq 2 (solid triangles) which assumes a sum of Lorentzians replaced by the Gaussians. The excited-state displacement of the five resonance Raman active modes are taken from ref I , 2, and 4. The solid line is the experimental result. 6600
ern.' I
I
3600 cm-'
I
if it is assumed that all of the fluorescence originates from the 0th vibrational level of the excited electronic state and ends in the multivibrational level of the ground state. (It should be noted that this mirror image effect between the absorption and fluorescence is a result of the equivalence of the squares of the = ( O , J U ~ for ) ~ the displacement Franck-Condon factors (O,~V,)~ of the potential curve of a harmonic oscillator from the ground state g to the excited state e.) With these assumptions the absorption spectrum €+(a), the nonrelaxed fluorescence spectrum t - ( w ) , and the A term of the as defined by Albrecht5 Raman scattering tensor element m 1(a), can be written as 28
30
32
34
36
38
40
42
44kK
Figure 3. The calculated absorption profile (solid line) and fluorescence (filled square on dotted line) obtained by using the excited-state displacements of the five resonance Raman modes of the uracil chromophore of UMP. The dashed line (filled triangles) is obtained by arbitrarily displacing the dotted curve until the observed crossover point a t 35 100 cm-I between fluorescence and absorption is obtained.
c c ... c ">
"3
3rtf
where ( 1(vi)( vjlO,) and ( Ojlvj)2are the well-known products of Franck-Condon factors between the zeroth of first vibrational level of the ground state and the vth level of the excited states, and the other symbols have their usual significance. Thus the equations for the absorption profile e+ and the fluorescence profile e- differ by a sign in the denominator which is shown in eq 3. In order to use these equations one simply uses the formulas for the
Franck-Condon overlap factors as a function of the displacement in the excited state along the normal coordinates, Qj, for each of the five observed resonance Raman active mode~.'J*~ Table I gives the shift values for those five modes. Figure 2 shows the observed (solid line) and calculated absorption profiles for the uracil chromophore in UMP, using IO00 cm-' for I',the damping factor. The solid triangles show the results obtained by using the sum of Lorentzian band shape formulas given by eq 3 while the solid circles gives the values calculated by replacing the series of Lorentzian band shape functions with a sum of Gaussian band shape formulas.'S2 Although this is a new calculation it confirms the earlier result of Blazej and one of the present authors that the sum of Gaussian formula more closely agrees with the experimental results.1q2 This result may be due to inhomogeneous broadening. The apparent 0point can be easily calculated and is found to be 36 500 cm-'. Figure 3 shows the calculated absorption and fluorescence profiles normalized so as to have the same maxima which is taken as unity. The calculation exhibits a good "mirror effect" between the absorption and the fluorescence. Again the 0-0 frequency
The Journal of Physical Chemistry, Vol. 89, NO. 24, 1985 5159
Forbidden Transitions in U M P
i
6200 cm.’
..
05.
1 \
1
0 ’
,
28
i\ L.
30
32
34
36
38
40
42
44 C K
Figure 4. The observed absorption profiles (solid line) and fluorescence
profile (dashed line) and the calculated absorption profile (filled circles) and calculated fluorescence profile (solid triangles). Both the calculations of the absorption profile and the fluorescence profile assume a sum of Gaussian band shapes arising from five resonance Raman active modes each displaced 0.8 dimensionless units in the excited state. The 0-0 transition for absorption is taken as 36 500 cm-’ as in Figure 2, but the 0-0 transition for the fluorescence is taken to be that of a lower energy electronic state with a 0-0 transition of 35 300 cm-’ in order to obtain a calculated result in agreement with experiment. or the crossover point of 36 500 cm-’ is obtained. The Stokes shift between the two calculated curve maxima is about 3600 cm-’. Figure 3 also shows the fluorescence profile which is obtained by in the formula for e- in such a way changing the energy factor as to obtain a calculated fluorescence shifted along to the red until it crosses the absorption profile at 35 100 cm-’ which is the experimentally observed crossover point. In this case the agreement between the “calculated” and the observed low-temperature fluorescence profiles is very good. However, it is obvious that one cannot simply shift the calculated fluorescence profile to lower frequencies without a sound theoretical reason. Consequently, we have addressed the theoretical problem of explaining this apparently curious result. One obvious question is how can the fluorescence be the mirror image of the absorption if it comes from a different (i.e. lower energy) excited electronic state. In order to answer this question we have repeated the calculation of the absorption profile to see if good agreement could be obtained without using the exact values for the five displacement parameters given in Table I. Instead, we have taken the five values to all be the same, i.e. 0.8, which is the geometric mean of the five values obtained from the resonance Raman result. Figure 4 shows the calculated absorption and fluorescence profiles and the agreement is good with the experimentally observed profiles. The good agreement between the observed and the calculated absorption profiles which is obtained when the five actual excited state displacements are each replaced with the mean value appears to indicate that in the condensed phase, when there is little or no vibronic structure to the absorption band, then the absorption profile (and necessarily also the fluorescence profile) must be relatively insensitive to the exact structure of the excited-state geometry. Thus if it is assumed that there exists a nonallowed lower energy excited electronic state in uracil, and that this state is also deformed in the excited state along five normal modes as is the allowed state (without specifying which normal modes) with an average value similar to that of the allowed electronic state, then one can calculate the observed fluorescence profile easily using the approximations discussed above. In conclusion to this section, we can state that it appears to be impossible to calculate the observed fluorescence profile from the observed absorption profile without making the assumption of a lower-energy excited state, because the 0-0 level is determined primarily by the frequencies of the Raman active modes and these cannot be varied from their experimental values. There appears to be no values for the Franck-Condon factors nor for the damping parameters which will shift the calculated fluorescence profile down to the observed profiles. Some further comments can be made about the fluorescence
measurements on the unsubstituted uracil reported by Becker and KoganlO in the light of our calculation. Although we do not have exact Stokes shift parameters for uracil itself there are some observations of these authors which we can interpret by making reasonable assumptions. First of all, the fluorescence of uracil in alcohol at 77 K is structured. Presumably this structure would also show up in the excitation profile measured in this solvent at the same temperature. To calculate this structure one would simply decrease the half-width or damping which is reasonable to do as the temperature is lowered. Fluorescence maxima are found near 33 000 cm-I which are in close agreement to what we have calculated here using the excited-state shift values for the five resonance Raman active modes of UMP. If we assume that unsubstituted uracil also has about five resonance Raman active vibrations with about the same excited-state displacements then we would suggest for unsubstituted uracil that either the fluorescence comes from the 0-0 level of the lowest allowed electronic state or that there exists an unallowed state which lies just below the allowed state. In either case the Stokes shift of the fluorescence is essentially what is calculated from the resonance Raman parameters and hence it is not so large as in the case of UMP. This analysis can be further supported by the fact that the fluorescence lifetime measurements appear to show that the fluorescence comes from a nonallowed state in one solvent (a furan) and from an allowed state in another solvent (alcohol). This is quite in line with our analysis because unless the two electronic states are quite close to each other in energy it is unlikely that solvent effects would be able to change the relative energy of the allowed and nonallowed electronic states. Thus it seems apparent that in order to use the resonance Raman parameters to obtain evidence of a low-lying nonallowed state it is necessary to have an anomalously large Stokes shift in the observed fluorescence over that calculated from the Raman parameters, and evidence must exist that this large Stokes shift is not due to a blue shift in the absorption in a polar solvent because of a large change in the excited-state dipole moment over that of the ground state. It seems apparent that when the fluorescence is not anomalously Stokes shifted over that calculated from the resonance Raman data one cannot use this method as a criteria for the existence of a lower unallowed state and one must resort to the measurement of fluorescence lifetimes in the various solvents.1°
Fluorescence Lifetime Measurements Having shown that it appears impossible to calculate the observed fluorescence profile from the known properties of the lowest allowed electronic state we now wish to show that the assumption of the existence of a lower energy nonallowed electronic state is consistent with the estimations of both the radiative and fluorescence lifetimes. Fluorescence lifetime measurements have been attempted by two different sets of investigators. Using the Eu3+energy transfer method, Eisinger and Lamola” have estimated an upper limit to the fluorescence lifetime of the order of 2 X lo-’’ s. From measurements of fluorescence depolarization, Callis14 has estimated the fluorescence lifetime to be about 2 ps. The radiative lifetime, ‘T,~, may be obtained from the ratio of the fluorescence lifetime, rF,to the quantum yield, dF: lrad
=
TF/dF
Using this formula one obtains a range from 7.0 X lo-* to 70 X s depending upon the value of the fluorescence lifetime one chooses. One can calculate the excited-state lifetime of the allowed excited electronic absorption band from its known profile and molar absorption using the method of Strickler and Berg.*’ For the uracil chromophore the value we have obtained is s. Thus, the calculated value of the radiative lifetime is at least 7 and at most 70 times longer than the estimated lifetime of the electronic state. Thus, it seems unlikely that the fluorescence originates in the allowed electronic state, but rather from some lower-lying nonallowed electronic state to which the energy is transferred. Loss of energy during the transfer step would account for the very low fluorescence quantum yield of 3 X For convenience, all of
J . Phys. Chem. 1985, 89, 5bO-5167
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assumption that the fluorescence originates from a low-lying nonallowed state to which the energy is partially transferred after absorption occurs in the allowed excited state. The above arguments for the existence in uracil of an unallowed electronic state which lies below the first allowed state is very similar to those given for polyenes by Hudson and K ~ h l e r ~who ~J' observed a similar large displacement in the weak fluorescence of these materials. Subsequently, two-photon spectroscopic studies have shown the existence of a low-lying two-photon band (for a review, see ref 17). However, it is not clear in uracil if the unallowed state could also be two-photon active since this molecular has very low symmetry.
TABLE 11: Fluorescence Lifetime Properties of the UMP Chromophore" 71%s
2 2
X X
IO-" (ref 11) (ref 14)
@i
Trad
3 X IO-' (ref 9) 4.5 X IO-' (ref 7)
70
= rf/df
X 5.0 X
TO,
(ref 21)
7,. fluorescence lifetime; bf, fluorescence quantum yield; Trad, rade, lifetime. cifine lifetime; T ~ excited-state
the fluorescence lifetime properties are tabulated in Table 11. In conclusion, it can be said that the absorption, fluorescence, and resonance Raman intensities can be quantitatively calculated from the assumptions discussed in our previous papers plus the
Pulsed Laser Studies of Molecular Interactlons and Reorientation of CS, in Organic Liquids via Phase Conjugation M. Golombok, G. A. Kenney-Wallace,* and S. C. Wallace Lash Miller Laboratories, University of Toronto, Toronto, M5S 1AI Canada (Received: December 5, 1984:
In Final Form: July 23, 1985) We describe the application of phase conjugation using degenerate four-wave mixing (4WM) to the study of interactions and dynamics in liquids via modification of the nonlinear polarizability, third-order in the applied laser field, of a probe molecule in a host fluid. The experimental observables of the conjugate wave intensity and x$, are explicitly linked to the anisotropy in the orientational distribution of molecules, which depends on the intermolecular forces and angular-dependent molecular correlations that are characteristic of each binary system. New data are presented on the interactions of CS2in a wide range of organic liquids at 298 K which illustrate the versatility of the 4WM approach in revealing microscopic information on the preferred solvation structures of CS2, from which ultimately pair correlation functions can be deduced.
Introduction While nonlinear laser spectroscopy has now become a powerful approach toward the study of gas-phase phenomena, ranging from multiphoton absorption,',2 optical coherent transient p h e n ~ m e n a , ~ . ~ and laser field effects in atomic and molecular dynamics5 to select and name but a few areas, it is only in recent years that the dynamics and interactions of molecules in liquids have begun to be explored through their nonlinear optical behavior. Considerable effort has been focused on the study of molecular vibrational dephasing mechanisms in solids and liquids via coherent antistokes Raman spectroscopy (CARS) and its analogues,&* the results of which have stimulated comparable theoretical interestg-" in elucidating the mechanisms for dephasing. Experiments in nonlinear optical phase conjugation have been concerned predominantly with macroscopic phenomenlogy.'* In this work, we will demonstrate the application of phase conjugation as a probe of solvation interactions in binary fluids. (1) P. M. Johnson and C. E. Otis, Annu. Rev. Phys. Chem., 32, 139 (1981). (2) R. M. Hochstrasser, G. R. Meredith, and H. P. Trommsdorf, Chem. Phys. Lett., 53, 423 (1978). (3) R. Beach, S. R. Hartmann, and R. Friedberg, Phys. Reu. A , 25,2658 (1982). (4) W. S. Warren and A. H. Zewail, J . Chem. Phys., 78, 3562 (1983). (5) (a) W. R. Green, J. Lukasik, J. R. Wilson, M. D. Wright, J. F. Young, and S. E. Harris, Phys. Rev. Lett., 42, 970 (1979). (b) J. Lukasik and S. C. Wallace, Phys. Reu. Lett., 47, 240 (1981). (6) M. D.Levenson and N. Bloembergen, J . Chem. Phys., 60,1323 (1974). (7) J. 0. Bjarnason, B. Hudson, and H. C. Anderson, J . Chem. Phys., 70, 4130 (1979). (8) W. Kaiser and A. Laubereau, Reu. Mod. Phys., 50, 608 (1978); W. Zinth et al., Appl. Phys. E , 26, 77 (1981). (9) R. M. Hochstrasser and H. P. Trommsdorf, Acc. Chem. Res., 16, 376 (1983), and references therein. (IO) D. W. Oxtoby, A h . Chem. Phys., 40, 1 (1979). (1 1 ) For example, D. W. Oxtoby, D. Levesque, and J. J . Weis, J . Chem. Phys., 72,2744 (1980); K. Schweizer and D. Chandler, J . Chem. Phys., 76, 2296 (1982); S. M. George and C. B. Harris, Phys. Reu. A , , 28,863 (1983), and references therein. ( 1 2) R. A. Fisher, Ed., "Phase Conjugation", Academic Press, New York, 1984. A bibliography of phase conjugation papers up to 1981 is included.
0022-3654/85/2089-5160$01 S O / O
Phase conjugation, which may operate through a wide range of optical nonlinearities, has rapidly developed over the past five years as part of a major research thrust into nonlinear optical device-oriented applications, based on its demonstrated properties for optical image reconstruction.I2 Applications of both transient grating effects and of phase conjugation to problems of chemical interest in liquid systems have revealed vibronic to thermal relaxation processes.13 We have now applied phase conjugation techniques to a study of the molecular interactions and dynamics in liquids, utilizing a four-wave mixing process based on nonresonant Kerr-type interactions, in which the dynamical and spectroscopic character of the conjugate wave generated in the medium carries information on the molecular properties of the system.14J5 Although much insight can be gained by viewing phase conjugation as a grating process13this is essentially a bulk description, and one has to develope a molecular intepretation of nonlinear optical phenomena in order to derive microscopic intermolecular properties from the data. In this paper, we report new experiments on the phase conjugate interactions in organic liquids, as part of a comprehensive study of nonlinear optical properties of liquids in which the versatility of phase conjugation and access to unique information on ground-state solvation structures and correlated molecular motion is illustrated. We demonstrate that local solvation effects in binary fluids are reflected in observable changes in the nonlinear optical signals. Such macroscopic phenomena (13) (a) E. J. Heilweil, R. M. Hochstrasser, and H. Souma, Opt. Commun., 35,227 (1980); H. Souma et al., J . Chem. Phys., 76, 5693 (1982). (b) G. Eyring and M. D. Fayer, J . Chem. Phys., 81, 4313 (1984); R. S. Moog et al., J . Phys. Chem., 86, 4694 (1982); R. Trebino and A. E. Siegman, J . Chem. Phys., 79, 3621 (1983). (14) (a) G. A. Kenney-Wallace and S . C. Wallace, IEEE J . Quantum Electron, JQE-19, 719 (1983); (b) C. Kalpouzos et al. in 'Picosecond Phenomena", Vol. 111, K. B. Eisenthal, R. M. Hochstrasser, W. Kaiser, and A. Laubereau, Ed., Springer-Verlag, West Berlin, 1982, p 221. (1 5) M. Golombok and G. A. Kenney-Wallace in "Ultrafast Phenomena", Vol. IV, D. H. Auston and K. B. Eisenthal, Ed., Springer-Verlag, West Berlin, 1984, p 383; and to be submitted for publication.
0 1985 American Chemical Society
