Intramolecular excited-state proton transfer in 3-hydroxyflavone

Chem. , 1984, 88 (11), pp 2235–2243. DOI: 10.1021/ ... View: PDF | PDF w/ Links ... Ultrafast Elementary Photochemical Processes of Organic Molecule...
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J. Phys. Chem. 1984, 88, 2235-2243 was imposed is that kz should be independent of the surfactant surface coverage, as found for k l . With these constraints, a trial-and-error procedure led to kz = 2.5 X lo7 s-I, in agreement with previous estimation^.^^ With the k l and k, values estimated in that way, only two parameters had to be optimized to fit the

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experimental decays. In the continuous-phase model (eq 7 and 8), only one parameter was optimized (k,”), using the same kl and k , values as above. Registry No. Triton X 100, 9002-93-1; silica, 7631-86-9.

Intramolecular Excited-State Proton Transfer in 3-Hydroxyflavone. Hydrogen-Bonding Solvent Perturbations Dale McMorrow and Michael Kasha* Department of Chemistry and Institute of Molecular Biophysics, Florida State University, Tallahassee, Florida 32306 (Received: July 27, 1983; In Final Form: January 23, 1984)

The phenomenon of excited-state proton transfer in 3-hydroxyflavoneis shown to depend sensitively on traces of H-bonding impurities in hydrocarbon solvents. In extremely dry and highly purified hydrocarbon solvents, a unique tautomer yellow-green fluorescence (region I) is observed from 298 to 77 K, independent of solvent temperature and viscosity, in contradiction to the results of previous research. With traces of water present, three regions of fluorescence of 3-hydroxyflavone (2.0 X M in methylcyclohexane (MCH)) can be observed, the tautomer yellow-green fluorescence (maximum at 523 nm) (region I), another green fluorescence (maximum at 497 nm) (region 11) attributed to the solute anion, and a blue-violet fluorescence (maximum at 400 nm) (region 111) attributed to the normal electromer of 3-hydroxyflavone. Excitation spectroscopy confirms the presence of a series of ground-state solvates which are correlated with the diverse luminescence behavior observed with water, alcohol, and ether both as trace impurities and as pure solvents. Potential energy curves for the various molecular species studied, and for various solvation modes, are used to reinterpret laser kinetic studies previously published. In particular the reported biexponential “normal” molecule fluorescence (111) decay, and tautomer fluorescence (I) rise time, are shown to represent a slow solvent-reorganizationstep from the polysolvated 3-hydroxyflavoneand an ultrarapid intrinsic proton-transfer step for the intramolecularly H-bonded 3-hydroxyflavone.

1. Introduction The observation of the intramolecular proton-transfer tautomer luminescence of 3-hydroxyflavonel has aroused a great deal of interest in the exploration of the double-well potential model for the transformations between defined normal and benzopyrylium tautomer structures. Low-temperat~re,’-~ laser picosecond kin e t i ~ , ~and - ~ h i g h - p r e ~ s u r e studies ~,~ all have been applied in exploring the mechanism of excitation, tautomerization, and relaxation to the normal form. The present research indicates, however, that stoichiometric traces of H-bonding impurities (water, alcohols, ethers) in the supposedly pure and dry hydrocarbon solvents dominate and can prevent proton transfer in the 3hydroxyflavone solute. Our preliminary communication’ presented evidence for the effect of traces of water in the dilute solutions ( 5 X 10-5-1 X IO-’ M) of 3-hydroxyflavone in methylcyclohexane. We report here further details of those studies and extension of the work to the effects of alcohol and ether solvates on proton transfer, and also the role of larger trace amounts of water in inducing anion formation of 3-hydroxyflavone as a further interference to intramolecular proton transfer. In highly purified and extremely dry hydrocarbon solvents we observe intramolecular proton transfer in 3-hydroxyflavone, with the subsequent fluorescence of the tautomer species, equally well in fluid solutions at 298 K and in rigid glass solutions a t 77 K , or in polycrystalline (hydrocarbon) Shpol’skii matrices (77 K). Thus, we find that in the absence of external perturbations, the excited-state intramolecular proton transfer appears to be temperature independent in the range 298-77 K and that the tautomer fluorescence is the only luminescence observed regardless of the ~

~~~~

(1) P. K. Sengupta and M. Kasha, Chem. Phys. Lett., 68, 382 (1979). (2) G. J. Woolfe and P. J. Thistlethwaite, J . Am. Chem. Soc., 103, 6919 (1981). (3) M. Itoh, K. Tokumura, Y. Tanimoto, Y.Okada, H. Takeuchi, K. Obi, and I. Tanaka, J . Am. Chem. Soc., 104, 4146 (1982). (4) A. J. G. Strandjord, S. H. Courtney, D. M. Friedrich, and P. F. Barbara, J . Phys. Chem., 87, 1125 (1983). ( 5 ) 0. A. Salrnan and H . G. Drickamer, J . Chem. Phys., 75, 572 (1982). (6) 0. A. Salman and H. G. Drickamer, J . Chem. Phys., 77, 3329 (1982). (7) D. McMorrow and M. Kasha, J . Am. Chem. Soc., 105, 5133-4 (1983).

0022-3654/84/2088-2235.$01.50/0

physical phase, in the rigorous absence of water, alcohol, and other perturbing impurities. This observation is in contradiction to all of the published literature on proton-transfer spectroscopy of these molecules, since all previous researchers reported that at 77 K, fluorescence of the “normal”, nontautomerized molecule occurs.14 We shall show that the variation of details of proton-transfer spectroscopy on the 3-hydroxyflavones and 3-hydroxychromones in the literature is the result of varying traces of H-bonding impurities in the hydrocarbon solvents used. 2. Protron-Transfer Spectroscopy of 3-Hydroxyflavone Proton-transfer spectroscopy in 3-hydroxyflavone was first reported and interpreted by Sengupta and Kasha.’ Their principal observations were the following: (1) A green (proton-transfer) fluorescence (A, = 520 nm, onset at -490 nm) is observed at 297 K for 3-hydroxyflavone (2.0 X M) in 2-methylbutane solution. (2) The UV absorption spectrum of 3-hydroxyflavone in 2methylbutane solution at 297 K has an onset at -370 nm, with A, at 335 nm, and exhibits some vibrational structure, and has further electronic transitions at A, 303 nm, and a strong band with onset at -270 nm. (3) A violet “normal” fluorescence (A,, = 410 nm, onset at -370 nm) was observed at 77 K in the 3-hydroxyflavone (2.0 X M) 2-methylbutane (rigid glass) solution, with a trace of the residual green fluorescence of the tautomer. Also a 470-nm hump was observed and assumed to be the triplet-singlet emission of the normal molecule. (4) Methanol solutions of 3-hydroxyflavone (2.0 X M) at 297 K were observed to exhibit both the “normal” violet 360 nm, A, = 405 nm) and the taufluorescence (onset = tomer green fluorescence (onset = 480 nm, A,, = 528 nm), as previously known,8 with the green to violet intensity ratio -2.0. In methanol-d solvent, with OD exchange in the 3-hydroxyflavone, the violet emission was enhanced, with this ratio decreasing to

-

-

(8) Y. L. Frolov, Y. M. Sapozhnikov, S. S. Barev, N. N. Pogodaeva, and. N. A. Tyukavkina, Izu. Akad. Nauk SSSR, Ser. Khim., 10, 2364 (1974).

0 1984 American Chemical Society

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The Journal of Physical Chemistry, Vol. 88, No. 11, 1984

McMorrow and Kasha

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-0.8 (green/violet) as expected for proton tunneling.' The large spectral shift of the tautomer band ( 8400 cm-' from absorption to emission onsets) was attributed by Sengupta and Kasha' to formation of a benzopyrylium ion structure of the proton-transferred tautomer (structure I-tautomer; R = phenyl

I

(normal)

*.O.' H/

- *;I

(tautomer)

9"

in 3-hydroxyflavone and R = methyl in 2-methyl-3-hydroxychromone) compared with the benzopyrone structure of the normal molecule (structure I-normal). The intramolecular proton transfer across the internal hydrogen bond of the hydroxy group to the carbonyl oxygen was assumed to involve a double-minimum potential barrier, with a normal Boltzmann temperature dependence taken for granted. It was also assumed that phenyl torsion of the B ring of the flavone could result in an additional viscosity dependence to the barrier. Our current results now indicate that observations 1, 2, and 4 are intrinsically correct; in contrast we shall demonstrate that observation 3 is caused by solvation of the 3-hydroxyflavone by traces of hydrogen-bonding impurities in the hydrocarbon solvent. (Throughout this work we loosely use the term "solvation" to describe H-bonding interactions of the 3-hydroxyflavone solute with the bulk solvent, as well as with trace H-bonding impurities in hydrocarbon solvents.) Frolov et ale8 first reported the dual fluorescence of 3hydroxyflavone in alcoholic solutions. Their interpretation of the spectra attributed the violet fluorescence to the uncomplexed normal molecule, and the green fluorescence to the alcohol solvate. Sengupta and Kasha' reversed this interpretation, assigning the violet fluorescence to the externally H-bonded solvate, analogous to the 3-hydroxyflavone polysolvate (structure 111, vide infra), In such a polysolvate it was assumed that proton transfer could not occur directly and that normal molecule fluorescence thus could be observed. On the other hand, with internal H bonding (structure I), intramolecular proton transfer could occur upon excitation; then, the molecule would exhibit intrinsically the tautomer green fluorescence. Salman and Drickamer tested the viscosity dependence of the excited-state proton-transfer potential barrier. They used highpressure spectroscopic techniques with hexamethylnonane, and also a rigid polyisobutylene film, as s ~ l v e n t .They ~ recently extended these studies to include n-octane,6 which crystallizes at high pressure at 298 K. Their general observation is that only the tautomer fluorescence of 3-hydroxyflavone occurs in all these cases. These results are in accord with our new experiments in which we find that in hydrocarbon environments (glass or Shpol'skii matrix) the barrier to proton transfer in the excited state is nonexistent, or so low that proton transfer dominates even at 77 K. Woolfe and Thistlethwaite: who presented the first laser kinetic studies of 3-hydroxyflavone, repeated all of the Sengupta and Kasha observations with essentially identical results (except that their absorption curve was for methanol solutions, without partial vibronic resolution). These authors accepted all of the Sengupta and Kasha interpretations, except for the argument concerning a viscosity-dependent barrier; they chose a poly(methy1 methacrylate) (PMMA) rigid matrix as a solvent for luminescence studies of 3-hydroxyflavoneat 290 K in order to separate viscosity from temperature effects. In agreement with Salman and Drickamer, they did find that at 290 K proton transfer in 3hydroxyflavone in the rigid matrix occurred freely upon electronic excitation, confirming the absence of a viscosity-dependent barrier. Unfortunately, the choice of poly(methy1 methacrylate) introduced a complication: at lower temperatures, down to 80 K, an increasing amount of normal molecule fluorescence was observed. Woolfe and Thistlethwaite recognized that H-bonding sites in the

WAVELENGTH, nm

Figure 1. Fluorescence spectra of 3-hydroxyflavoneat 77 K: (upper) 2.0 X M in highly purified dried methylcyclohexane;(lower) 3.26 X M in commercial spectroquality MCH (Eastman). Region I is the tautomer fluorescence (structure I, tautomer); region I11 is the normal molecule fluorescence (structure 111). Cf. section 4.

PMMA matrix yielded the same intermolecular interference to excited-state proton transfer which Sengupta and Kasha had adduced in alcohol solutions of 3-hydroxyflavone.

3. Spectroscopic Properties of Nonassociated 3-Hydroxyflavone Molecules We shall now demonstrate that the luminescence phenomena of nonassociated 3-hydroxyflavone differ strikingly from its behavior when specific H bonding can occur to water, alcohols, ethers, or other trace components in hydrocarbon solvents. Figure 1 presents spectroscopic proof of the effect of solvation on the excited-state proton transfer in 3-hydroxyflavone. The lower curve M of Figure 1 shows the fluorescence spectrum of a 2.0 X solution of 3-hydroxyflavone in commercial "spectroscopic-quality" methylcyclohexane (MCH, Eastman) rigid glass at 77 K. This spectrum consists primarily of the violet normal molecule emission (labeled region 111) and resembles that published by previous investigators for low-temperature hydrocarbon The upper curve for Figure 1 is for a 2.0 X M solution of 3hydroxyflavone in highly purified and extremely dry MCH glass at 77 K. (The removal of the violet (region 111) luminescence depends on which H-bonding impurity is present (water, alcohols, ether) and the method of purification. Details are discussed in the Appendix.) This spectrum consists predominantly of a yellow-green tautomer fluorescence (region I) which is slightly better resolved than the room-temperature spectrum' but is otherwise identical in both position and contour. A trace of region I11 luminescence is detectable in this spectrum. The curves of Figure 1 clearly indicate that nonassociated 3-hydroxyflavone molecules tautomerize freely in rigid systems at 77 K and that the previously observed spectral transformations upon lowering the temperature are caused by external perturbations arising from impurities in the solvents used. The lack of any significant normal molecule blue-violet fluorescence at 77 K indicates that little or no intrinsic barrier exists in the isolated molecule, contrary to what was assumed in the mechanistic interpretations in previously reported 4. Luminescence Spectroscopy in the Stepwise Hydration of 3-hydroxy flavone

Region Z Luminescence at 77 K . The lowest curve of Figure 2 shows again the 77 K region I luminescence spectrum of a 2.0

The Journal of Physical Chemistry, Vol. 88, No. 11, 1984 2237

Intramolecular Excited-State Proton Transfer

demonstrated that 7-azaindole can exhibit excited-state tautomerization in cylindrically H-bonded water monohydrate complexes, analogous to structure 1', in ether solutions containing small quantities of water.I0 All of our experience, however, leads us to consider that a perturbed tautomer species of 3-hydroxyflavone should suffer a red shift in the A,, of the tautomer fluorescence as, e.g., in the case of alcohol solutions relative to that in hydrocarbon. Thus, if structures such as I' (monosolvate) undergo excited-state tautomerization, we would expect the fluorescence to be hidden under the yellow-green fluorescence of the unperturbed tautomer species, structure I (tautomer). We are thus forced to conclude that the region I1 fluorescence originates neither from internally H-bonded nor from a cyclically H-bonded monohydrate species. Parallel studies of 3-hydroxyflavone in aqueous solutions suggest that excited-state anion formation from hydrates such as structure I1 (monohydrate, as (monohydrate)

400

500

EXCITED STATE ANION monohydrate

600

WAVELENGTH, NANOMETERS

Figure 2. Fluorescence spectra of 2.0 X M 3-hydroxyflavone in quick-frozen MCH glass at 77 K as a function of addition of traces of water. Lowest curve shows unique tautomer fluorescence in the anhydrous solvent; highest curve is for solvent saturated by addition of a drop of water. Excitation wavelength 335 nm; the curves are intensity normalized at 523 nm for clarity. Regions I and I11 as in Figure 1; region I1 fluorescence attributed to anion (structure 11). Cf. section 6 . X low5M solution of 3-hydroxyflavone in highly purified methylcyclohexane (purification procedures given in the Appendix). The strongest fluorescence peak is at 523 nm, with onset at -510 nm; we observe the room-temperature fluorescence peak at 524 nm in methylcyclohexane solvent, with onset at -490 nm. The region I fluorescence emission was assigned by Sengupta and Kasha' to the structure I benzopyrylium tautomer S,(T,T*) So emission. This emission is shifted -8400 cm-I (emission to absorption onsets) from the S l ( a , t * ) So normal molecule absorption. The large spectral shift and uniqueness of emission correlate well with the benzopyrylium tautomer assignment. Region IZ Luminescence at 77 K . The detailed study of the stepwise hydration of 3-hydroxyflavone is shown in Figure 2. As minute traces of water are added (see Appendix) to the same extremely dry methylcyclohexane solution, a region I1 fluorescence develops with increasing water concentration, bottom to top of Figure 2. The region I1 fluorescence (Arnx = 494 nm, onset at -480 nm) appears to consist of several components, judging by its variable band contours and water concentration dependence. In our preliminary communication we assigned this emission band to a perturbed tautomer fluorescence and suggested that a cyclically H-bonded monohydrate species, structure 1', might be capable

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of generating the benzopyrylium tautomer upon excitation, resulting in a blue-shifted tautomer emission. The existence of biprotonic phototautomerization is established for the case of 7-azaindole/ethanol monosol~ates,~ and recently it has been

.. "OYi

a prototype for water clusters) are the most probable origin of the region I1 fluorescence. This topic of anion fluorescence is discussed further in section 6 . Itoh et aL3 were the only previous investigators among the group of four who studied low-temperature spectra to specifically observe region I1 fluorescence (their Figure 7), an indication that their hydrocarbon solvent was less contaminated. Their observation of an apparent isosbestic point at -510 nm is indirect corroboration of a two-species equilibrium such as our comparison of structures I and 11. Itoh et aL3 did not comment on region I1 as a separate emission region. The results of Itoh et alS3were obtained by temperature lowering, and the results of our Figure 2 by water addition. It is clear that at low temperatures the solvation equilibrium shifts, with stabilization of solvates 1', 11, and 111, and

?l(polysolvate) I

-\

ij

R

R

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their analogues. Region III Luminescence at 77 K . The blue-violet fluorescence in region I11 is assignable simply and concretely from its spectral position as the Sl(n,a*) So emission of the normal nonproton-transferred 3-hydroxyflavonemolecule (structure 111). The onset of absorption is observed at -365 nm at 298 K in methylcyclohexane (curve 1A, Figure 3). The blue-violet fluorescence has an onset of -380 nm at 77 K in methylcyclohexane (Figure 2, region 111); we shall show in the next section that the excitation

-

(9) C. A. Taylor, M. A. El-Bayoumi, and M. Kasha, Proc. Natl. Acad. Sei. U.S.A.,63, 253 (1969). (10) S. Collins and M. Kasha, in preparation. (1 1) M. Kasha, in "Symposium on Excited States of Matter, Texas Technological University Graduate Studies", No. 2, April 1973, C. W. Shoppee, Ed., Texas Tech Press, Lubbock, TX, pp 5-19.

2238 The Journal of Physical Chemistry, Vol. 88, No. 11, 1984 spectrum onset is at -385 nm for this (impure) hydrocarbon solution at 77 K. As Figures 1 and 2 indicate, the blue-violet fluorescence appears only in impurity-containing hydrocarbon solvents at low temperatures. It is this behavior which was mistaken by all previous investigators as the intrinsic behavior of 3-hydroxyflavone in hydrocarbon solvents. At 298 K 3hydroxyflavone exhibits only the yellow-green tautomer fluorescence (region I) in the same wet hydrocarbon. It is clear that the production of the normal molecule violet fluorescence requires external H-bonding perturbation of the 3-hydroxyflavonemolecule. The blue-violet region 111 fluorescence of 3-hydroxyflavone varies in a subtle fashion with water traces, alcohol traces, or ether traces, each impurity yielding a characteristic fluorescence contour and characteristic wavelength position and intensity of emission maximum. Thus, in methyl alcohol glass solution the (77 K) region I11 normal fluorescence has a band maximum at 406 nm, in (purified) ethyl ether glass it has one at 395 nm, while in extremely pure methylcyclohexane no region 111 fluorescence is observed. In purified, extremely dry methylcyclohexane glass (77 K) with a trace of ethyl ether, the fluorescence band maximum is at 398 nm; with a trace of methyl alcohol, it is at 408 nm; and with a trace of HzO, it is at 415 nm. Commercial spectrograde methylcyclohexane gives blue-violet emission clearly indicative of one of the specific trace impurities; or sometimes an intermediate spectrum is obtained, indicating a mixture of trace impurities. These specific solvation effects are evident in a comparison of the spectra of Figures 1 and 2. As Sengupta and Kasha indicated,' H-bonding perturbation by externally associating molecules can prevent proton transfer in the excited state, resulting in the blue-violet normal molecule (region 111) emission. It is now evident that such an association is necessary for observation of the normal fluorescence, and we may consider that the polysolvate, structure 111, accounts for the region I11 fluorescence in low-temperature hydrocarbon solutions containing H-bonding impurities, as well as in alcohol and ether solutions at room temperature. An analogous structure was considered" to be necessary to account for the failure of 7azaindole to exhibit excited-state proton transfer in aqueous solvent at 298 K. Although at 77 K in methyl alcohol solution 3hydroxyflavone exhibits predominantly region I11 fluorescence, both the blue-violet fluorescence and the tautomer yellow-green fluorescence are observed at 298 K, as reviewed earlier.',* It is likely that with stoichiometric traces of water, alcohol, ether, or other H-bonding species, various structures which could prevent excited-state proton transfer could exist, such as structures 111' and 111". Such structures interfere with the intramolecular

III' (solvo t e

E'' (ether

complex)

:0' 'R

H bonding of the 3-hydroxy group essential for excited-state proton transfer.

5. Excitation Spectra of the Three Fluorescence Regions of 3-hydroxy flavone Figure 3 compares absorption spectra and fluorescence excitation spectra for the three fluorescence regions of 3-hydroxyflavone in MCH at various temperatures. Curve A is the near-UV absorption spectrum of 3-hydroxyflavone in methylcyclohexane at 298 K. The absorption onset is at -370 nm with the first peak at 355 nm. Curve Ia is the excitation spectrum for the yellow-green proton-transfer fluorescence (region I, Figure 2) of 3-hydroxyflavone at 298 K. The onset and first peak agree with those for absorption curve A.

McMorrow and Kasha

300

350 WAVELENGTH, NANOMETERS

4 00

Figure 3. Excitation and absorption spectra for 3-hydroxyflavone in methylcyclohexaneunder various conditions: curve A, 298 K absorption M; curve Ia, 298 K excitation spectrum of tauspectrum, 1.67 X tomer fluorescence, monitored at 523 nm, 3.0 X 10" M; curve Ib, 77 K excitation spectrum of tautomer fluorescence, monitored at 523 nm, M; curve 11, 77 K excitation spectrum of region I1 fluorescence in a water-saturated solution, monitored at 495 nm, 5 X M; curve III,77 K excitation spectrum of region 111fluorescence,monitored at 430 nm, M, in commercial spectrograde solvent; curve B, 77 K absorption spectrum in commercial spectrograde solvent, M.

-

-

-

Curve Ib is the excitation spectrum for the proton-transfer fluorescence (region I) at 77 K for 3-hydroxyflavone in methylcyclohexane glass. The first excitation peak is now sharper and is red shifted to 361 nm, with the onset remaining at -370 nm. Curve I1 is the excitation spectrum for the region I1 fluorescence (Figure 2) at 77 K for water-saturated methylcyclohexane solution. The first peak is now strongly shifted to 375 nm (cf. 361-nm curve IC), thus confirming a strong solvation perturbation. Curve I11 is the excitation spectrum for the region 111blue-violet normal molecule fluorescence. The diffuseness and wavelength position of this excitation spectrum clearly indicate a still different solvent perturbation. The first peak occurs at about 372 nm, with onset at -385 nm. It is also noteworthy that the first curve 111 peak is blue shifted relative to that for curve 11, indicating a difference in the solvation mode. This slight relative shift is a general result for solutions exhibiting both region I1 and 111 fluorescence. The spectra of Figure 3 clearly indicate the existence of several distinct ground-state species in 77 K rigid glass solutions. Itoh et al.3 gave fluorescence excitation spectra analogous to those for our region 1-111 assignments. Specifically, our experimental results

The Journal of Physical Chemistry, Vol. 88, No. 11, 1984 2239

Intramolecular Excited-State Proton Transfer 0

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a ' " ' " '

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Figure 4. Solvation-fluorescence correlation diagram for 3-hydroxyflavone. First peak positions of absorption, excitation, and fluorescence spectra are shown for water traces in hydrocarbon solvent.

agree in essence, especially in the differences between the excitation spectra for region I1 (their curve e, Figure 8, for 495-nm emission) and region I11 fluorescence (their curve d, Figure 8 for 430-nm emission), in contrast to that for region I fluorescence (their curve a, Figure 8, for 530 nm). In our experiments the excitation curves were all obtained at 77 K but from different samples, each maximizing conditions for the structure species sought, and thus affording the best opportunity for unique excitation spectra. In the experiments of Itoh et aL3 one solution was used at different wavelengths and temperatures; thus, their curves (their Figure 8) represent a trend of excitation spectra as the ground-state equilibrium mixture of solvates changes with temperature. Itoh et aL3 did not consider solvation in their passing discussion of excitation spectra. Curve B of Figure 3 shows the low-temperature near-UV absorption spectrum of 3-hydroxyflavone (-5 X M) in methylcyclohexane glass a t 77 K. The first absorption peak is at 373 nm, the second (and strongest) peak is at 354 nm, and the absorption onset occurs at -390 nm. These observations are in concordance with excitation curve III, indicating that the lowtemperature absorption spectrum is for solvated 3-hydroxyflavone. The commercial spectrograde methylcyclohexane used in this experiment apparently contained a trace of methyl alcohol. A schematic correlation of the excitation spectroscopy of 3hydroxyflavone as a function of solvation is given in Figure 4. The unsolvated molecule (structure I) yields invariably only the proton-transfer tautomer yellow-green fluorescence (region I). If 3-hydroxyflavone is solvated by traces of water (structure 11), a region I1 blue-green fluorescence is observed. If the 3-hydroxyflavone is highly solvated by larger traces of water at lower temperatures (structure 111), a region I11 blue-violet fluorescence is observed as a normal molecule S 1 ( r , r * ) So fluorescence, in addition to an enhanced and finally dominating region I1 emission. Ethyl ether as a solvent always gives some region I11 fluorescence (structure III"), at both 77 and 298 K. As a trace impurity in hydrocarbon solvent, ether yields region I11 fluorescence at 77 K but not at 298 K, indicating that the ground-state equilibrium is shifted toward ether complex formation at 77 K in the ether glass (see Appendix).

-

6. Excited-State Anion Formation Figure 5 (lower) shows the total emission spectrum at 77 K of a water-saturated 3-methylpentane solution of 3-hydroxyflavone excited at 375 nm. If we consider the similarities in general Franck-Condon envelopes and the close relation in absolute wavelength of t ! x region I1 emission of Figure 5, compared with the region I emission (Figure 2, lower), both at 77 K, we could come to the conclusion that region I1 represents perturbed tautomer emission.

500

400

WAVELENGTH, nm Figure 5. Fluorescence spectra of 3-hydroxyflavone: (upper) in pure water at 298 K, excited at 350 nm; and (lower) in water-saturated 3methylpentane glass at 77 K, excited at 375 nm. Regions I11 and I1 correspond to structures I11 and 11.

It is now evident that the appearance of this region I1 fluorescence in water-containing hydrocarbon solutions is the result of excited-state anion formation, as is depicted for the monohydrate (structure 11). This assignment is in contrast to that presented in our preliminary communication' in which the region I1 fluorescence was attributed to a perturbed tautomer species. The 3-hydroxyflavone anion fluorescence is identified unambiguously at room temperature in water solutions at pH 12.7 as a broad structureless bandI2 with A,, = 515 nm. Wolfbeis et al.I3 have recently published similar results. In neutral water solutions at room temperature 3-hydroxyflavone exhibits two emission regions, as is shown in the upper part of Figure 5. This spectrum is superficially similar' to that in methanol solvent at 298 K. The very broad green region band labeled IIw with A,, at 510 nm is assignable to the anionic species, analogous to the 505-nm band at pH 12.7, with the possibility of some tautomer emission being buried under the long-wavelength tail. This assignment is in essential disagreement with the discussion given by Wolfbeis et al.,13 in which the 510-nm band (their 512 nm) in neutral water solution is assigned to a tautomer luminescence. Their argument, based solely on dielectric constant effects, fails to include the specific H-bonding characteristics of water, and also its ability to accept free protons. Furthermore, a blue shift (-20 nm, -740 cm-l) of a room-temperature emission band on going from methanol to water solvent is the reverse of that to be expected. A more detailed discussion, as well as further work on 3-hydroxyflavone anion spectroscopy, will be published e1~ewhere.l~ The anion fluorescence band in pure water at 298 K (Figure 5,II,) seems at first sight to bear little resemblance to the much narrower, blue-shifted region I1 emission band (77 K) shown in Figure 2. We may compare the emission spectrum of 3hydroxyflavone in water-saturated 3-methylpentane glass at 77 K given in the lower part of Figure 5. Selective excitation at 375 nm (curves I1 and 111, Figure 3) produces a spectrum with ~

~~~

~~

(12) P. Chou, unpublished work, this laboratory. (13) 0. S. Wolfbeis, A. Knierzinger, and R. Schipfer, J . Photochem., 21, 67 (1983). (14) P. Chou, D. McMorrow, and M. Kasha, to be submitted for publi-

cation.

2240 The Journal of Physical Chemistry, Vol. 88. No. 11. 1984

N

r

N

H-BONDED 3-HYDROXYFLAVONE

N

T

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Qo: H - O + I. INTERNALLY

McMorrow and Kasha

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a.EXTERNALLY

CYCLICALLY H-BONDED 3-HYDROXYFLAVONE MONOSOLVATE

H-BONDED

3- HYDROXYFLAVONE POLY SOLVATE

Figure 6. Schematic potential energy curves for the two lowest singlet states of the various 3-hydroxyflavonespecies. N is the normal molecule electromer; T is the tautomerized electromer (cf. structures I, 1‘, normal and tautomer). QO+O represents the coordinate for single-proton motion within the represents the coordinate for concerted two-proton motion in the cyclic H-bonded complex. intramolecular hydrogen bond;

probably very little contamination from the unperturbed tautomer (region I) fluorescence band (cf. curve Ib, Figure 3). A comparison of the curves in Figure 5 reveals that the bands I1 and IIw are located in the same general spectral region, with their origins being very nearly coincident. A blue shift in the band maximum accompanied by the appearance of some vibronic structure is a general observation when solvent cage relaxational processes of a high dielectric and strongly interacting solvent are frozen out at low temperatures in rigid solutions. Thus, the identification of the region 11 fluorescence in water-containing hydrocarbon solutions at 77 K arising from excited-state anion formation may be made with a reasonable degree of certainty. The nonmonotonic behavior of the 3-hydroxyflavone fluorescence in these systems (cf. Figure 2 ) would then be associated with variations in the proton-stabilizing capability for different species at various stages of hydration. It is noteworthy in connection with our present region I1 assignment that this band makes its appearance only in the presence of water as an impurity in hydrocarbon solvents, and not with other H-bonding impurities such as methanol or ethyl ether. Addition of methanol to a water-saturated hydrocarbon solution of 3hydroxyflavone shows at 77 K only the tautomer (region I) and normal (region 111) fluorescences, with the region I1 fluorescence now having been suppressed. Such observations argue strongly in favor of structure I1 hydrates (water clusters grouped about the “monohydrate” prototype), yielding a specific interaction (anion formation), as compared with structure I’ solvates, which can lead to proton transfer, or structures 111’ and III”, which can inhibit proton transfer. We should indicate that the quantum yield of fluorescence in aqueous and alcohol solutions is significantly lower than in hydrocarbon solutions. It is possible that exciplex formation is present, with a consequent primary fluorescence quenching.ls On a slow time scale there is observed a fluorescence diminution indicative of a subsequent photodegradation process, which is dramatically accelerated with intense laser excitation. (15)

S. Collins, J . Phys. Chem., 87, 3202 (1983).

7. Potential Diagrams for Proton Transfer in 3-Hydroxyflavone and Various Solvates The schematic potential energy diagrams given in Figure 6 will permit a distinction to be made in the excited-state proton-transfer mechanisms for the various structures of 3-hydroxyflavone and its solvates. We tacitly omit the excited-state anion species from the following discussions. Curve I of Figure 6 represents the double-minimum potential function for intrinsic proton transfer for the internally H-bonded (nonassociated) molecule. In the Sl(r,7r*)excited state for the normal molecule N(1) (structure I) the barrier is so low as to be nonrestrictive to proton transfer at both 77 and 298 K. The Sl(r,.rr*) So tautomer T(1) emission is thus observed with a large energy decrease with respect to excitation energy. In constructing such double-minimum potential diagrams, there is generally expected to be a barrier of electronic In the present case the barrier may become significant only at 20 or 4 K. Curve I’ of Figure 6 represents the double-minimum potential diagram in So and S1states for structure I’, the cyclic monosolvate of 3-hydroxyflavone. The excited-state barrier to proton transfer is still expected to be small enough to permit tautomer (region I) fluorescence to be observed easily without viscosity dependence down to 77 K. The cooperativity of the two-proton transfer in this case is permitted by inductive effects in the cyclic monos~lvate.~ Curve I11 of Figure 6 shows the deep single-well potential appropriate to the polysolvate, structure 111. Excited-state proton transfer cannot occur directly in this case, since cooperativity for two-proton transfer within the lifetime of the excited state is lost owing to the lack of any electronic connection between the separate solvation chains. It is necessary to consider the mechanism of solvation and desolvation in order to complete the picture of mechanisms available for excited-state proton transfer for the various solvate species. Figure 7 represents the dual coordinate potential diagram

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(16) V. I. Goldanskii, Annu. Rev. Phys. Chem., 27, 85 (1976). (17) R. W. Somorjai and D. R. Hornig, J . Chem. Phys., 36,1980 (1962).

Intramolecular Excited-State Proton Transfer

The Journal of Physical Chemistry, Vol. 88, No. 11, 1984 2241

4

E

Figure 7. Schematic dual-coordinate potential energy diagram for the S,(a,a*) excited states of the cyclically H-bonded monosolvate species in relation to the Sl(a,a*)state of the internally H-bonded 3-hydroxyare the intrinsic potential barriers to proton flavone molecule. PIand PI, is the H-bonding solvation-desoltransfer in structures I and 1’; vation barrier between structures I and I’ (cf. Figure 6 for other labels).

for the SI excited state of the cyclic monosolvate (structure 1’) in relation to the internally H-bonded 3-hydroxyflavone molecule (structure I). The coordinate Q0:+,,represents proton-transfer motion within a hydrogen bond, and its effect on the potential energy of the electronic system. The solvation coordinate represents the H-bonding potential of interaction between the 3hydroxyflavone and an R O H molecule, using methyl alcohol as a prototype for cyclic monosolvate complex formation. The S1 excited state for the normal monosolvate molecule structure I1 has two independent paths for excited-state proton transfer: (a) proton transfer can occur directly over or through to form tautomer TI,; (b) the monosolvate the (low) barrier PI,, can overcome the barrier 7fICIby desolvation and then pass over or through the very small barrier PIto form the tautomer TI. The polysolvate structure 111, as represented by the dual coordinate potential diagram of Figure 8, must surmount a very large barrier %111-1 to reach the internally H-bonded species N(1) for proton transfer to occur to state T(1). This large barrier represents the solvent reorganization energy (and H-bond strength) which traps the polysolvate against intrinsic proton transfer. This barrier should exhibit a viscosity dependence analogous to that described by the Dellinger-Kasha viscous-flow solvent cage An alternative mechanism for excited-state proton transfer from the polysolvate would be to reorganize solvent to the monosolvate, as discussed by Woolfe and Thistlethwaite; the diagram would be analogous to that of Figure 8, but the species N(1’) in the S1 state would then have the two subsequent proton-transfer alternative paths discussed for N(1’) in Figure 7 . It is expected that reorganization over %111-1 to form the intramolecularly H-bonded species N(I), as is depicted in Figure 8, is the predominant pathway for tautomerization following excitation of externally H-bonded 3-hydroxyflavone polysolvates. The formation of a cyclically H-bonded monosolvate during the lifetime of the excited state should be considerably less probable due to the rather stringent configurational requirements of this species. 8. Kinetic Mechanism for Excited-State Proton Transfer in 3-Hydroxyflavone and Its Solvates The potential energy curves of Figures 6-8 allow a precise interpretation of the various processes which occur in 3hydroxyflavone and its solvates following electronic excitation. To interpret fully the observed luminescence properties of 3hydroxyflavone it is necessary also to consider the temperaturedependent ground-state solvation equilibrium which occurs when (18) B. Dellinger and M. Kasha, Chem. Phys. Lett., 36, 410 (1975). (19) B. Dellinger and M. Kasha, Chem. Phys. Lett., 38, 9 (1976).

c;,

’ Q 0:H-O-

Figure 8. Schematic dual-coordinate potential energy diagram for the Sl(?r,r*)excited state of the externally H-bonded polysolvate in relation to the Sl(n,**) state of the intramolecularly H-bonded species. Front potential is for N(III), the normal molecule polysolvate (structure 111). The H-bonding solvation-desolvation barrier %111-1 is for conversion between structures 111 and I (cf. Figures 6 and 7 for other labels).

H-bonding impurities are present in the hydrocarbon solvents. We will restrict this discussion to the equilibrium between internally and externally H-bonded species, since these appear to dominate the luminescence behavior. It is realized fully, however, that the multitude of solvated species which are possible for 3-hydroxyflavone requires consideration of a complicated multiple equilibrium. The existence of several different H-bonding impurities (water, alcohol, ether) in the same solution introduces a further complication. The relaxation kinetics of 3-hydroxyflavoneand its solvates may be discussed with reference to the dual coordinate potential diagram of Figure 8. Nonassociated 3-hydroxyflavone molecules are excited directly to the SI internally H-bonded potential minimum, for N(1). On the basis of the present results in highly purified hydrocarbon solvents, we conclude that tautomerization is then rapid, efficient, independent of viscosity, and largely independent of temperature. The validity of this conclusion could be tested with picosecond and low-temperature (C77 K) spectroscopic techniques in rigorously purified hydrocarbon solvents. Externally H-bonded molecules when excited to the SI state, reach the solvation minimum, for N(III), of Figure 8. Following excitation, the solvated molecules may undergo (1) radiative decay to the ground state via SI So fluorescence, (2) radiationless decay to So, (3) intersystem crossing to the triplet manifold, or (4) solvent and solute reorganization over the desolvation barrier 7fII1+ to form the intramoleculely H-bonded proton-transfercapable species, N(1). We will not consider explicitly the radiationless processes 2 or 3 in the present general discussion. Because the proton-transfer step is rapid once an intramolecular H-bond is formed, the observed luminescence will depend largely on the rate of the desolvation process 4, although process 2 could exhibit a strong temperature dependence and must be included in a more quantitative discussion. In contrast to the rapid intrinsic tautomerization step, the necessary breaking of intermolecular H bonds and reorganization of the solvent cage to facilitate formation of an intramolecular H bond requires that this desolvation process be strongly temperature and viscosity dependent. It is evident, then, that the buildup time for tautomer fluorescence, which was identified by earlier workers with the rate of tautomerization, must now be. related to the rate of those relaxation processes which occur prior to intramolecular H-bond formation. Similarly, the activation energies deduced from earlier measurements must be related to the H-bonding desolvation barrier %111-1, rather than the in-

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The Journal of Physical Chemistry, Vol. 88, No. 11, 1984

trinsic barrier for proton transfer P I ,as was assumed by previous workers. We may now discuss the published temperature-dependent kinetic measurements of 3-hydroxyflavone, which we recognize as having been obtained in the presence of H-bonding solvent i m p ~ r i t i e s . ~At- ~higher temperatures (near room temperature) the ground-state solvation equilibrium strongly favors nonassociated 3-hydroxyflavonemolecules; thus, the luminescence behavior in both unpurified and highly purified hydrocarbon solvents consists only of the yellow-green tautomer fluorescence (region

1).

McMorrow and Kasha of a molecule. The 3-hydroxyflavone molecule is especially susceptibleto external H-bonding perturbation because of the weak hydrogen bond to the carbonyl group via a five-membered ring. This research focuses attention on the need for an exhaustive spectroscopicstudy and interpretation, as a prelude to laser kinetic measurements and mechanistic deductions. Another research horizon in this subject, now that qualitative aspects of the intraand intermolecular potential functions are delineated, lies in the possibility of quantitative study of these potentials: energy minima, barrier heights, proton-tunneling characteristics, and the shapes of potentials deduced from vibrational anharmonicities. 16-19 Studies related to this project which have been completed include the recognition of proton-transfer potentials in four-level laser action,z0the picosecond laser kinetics of the intrinsic proton transfer in 3-hydroxyflavone in anhydrous hydrocarbon solvents,z1 and the study of solid-state Shpol’ski matrix spectroscopy of 3-hydroxyflavone.22

As the temperature is lowered, the equilibrium shifts in favor of the solvated species. As this occurs, and as solvent relaxation times become significant relative to the intrinsic excited-state lifetime, the blue-violet region 111fluorescence becomes observable. The rise time for the tautomer fluorescence continually increases with decreasing temperature, with a close parallelism observed between the tautomer fluorescence rise time and the violet Acknowledgment. We thank Professors Thijs Aartsma, Robert fluorescence decay time.z-4 Recently, Strandjord et ale4reported Fulton, and Harry Walborsky for valuable commentary which two-component kinetic behavior for both the tautomer (region helped in the development of this research. This research was 1) fluorescence rise time and the normal (region 111) fluorescence sponsored in part by a grant from the National Science Foundecay time, each consisting of an unresolvably fast (