Fluorescence studies of the excited-state proton transfer in substituted 3-hydroxychromones in supersonic jet

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1 Pure &Appl. Che., Vol. 65, No. 8, pp ,1993. Printed in Great 1993 UPAC Fluorescence studies of the excitedstate proton transfer in substituted 3hydroxychroones in supersonic jet Michiya toh Departent of Physical and Cheical Biodyanics, Graduate School of Kanazawa University, and Faculty of Pharaceutical Sciences, Kanazawa University, Kanazawa 920, Japan Abstract ntraolecular excited state proton transfer (ESPT) of 3hydroxychroone (3HC), 3hydroxyflavone (3HF) and 2(2naphthyl)3hydroxychroone (2NHC), and of their water clusters were investigated in a supersonic expansion. The visible tautoer fluorescence excitation spectra of these copounds exhibit considerably well resolved vibrational structures, while no significant uv fluorescence excitation spectru due to the noral for was observed. Upper liit of rate constant of the tautoer foration was estiated to be 1.17 x 1012 slfor 3HC, 6.5 x 1011slfor 3HF and 1.54 x 10l1 s for 2NHC in supersonic expansion by siulation of line width of the respective origin bands in the visible fluorescence excitation spectra. The fluorescence excitation and dispersed fluorescence spectra deonstrate 1;l and 1;2 water coplex forations of 3HF and 2NHC and no ESPT in these water clusters. However, no evidence of water coplex foration was obtained in jetcooled 3HC. NTRODUCTON Usual intraolecular hydrogen bonding systes such as ethyl salicylate and ohydroxyacethophenone in hydrocarbon solution are believed to undergo a barrierless proton transfer in their singlet excited state. Since Sengupta and Kasha (ref. 1) reported an energy barrier in the excited state proton transfer (ESPT) of 3hydroxyflavone at the first tie, soe research groups including us concerned in intraolecular excited state proton transfer of this copound. n 1982 (ref. 2) and 1986 (ref. 3), we reported rather slow rise of tautoer fluorescence in carefully purified hydrocarbon solution of 3HF, while no slow rise was detected in 3hydroxychroone (3HC) which lacks the 2phenyl group of 3HF. We suggested that the different feature of the intraolecular ESPT between 3HF and 3HC was ascribed to the torsional effect of the phenyl group of 3HF. However, McMorrow and Kasha (ref. 41, and soe other research groups that the different feature between these copounds is the results of varying traces of Hbonding ipurities in the hydrocarbon solvents. Further, Kelley and his coworkers (ref. 5) reported picosecond fluorescence studies of 3HF in low teperature Ar atrix, and they suggested fast ESPT in a onosolvate of 3HF with water in a 10K Ar atrix. n spite of nuerous investigations including recent picosecond and feotosecond fluorescence studies, there are still several and serious controversies aong the. These controversies are concerned with potential barrier of the excited state proton transfer and also 1629

2 1630 M. TOH of the ground state reverse proton transfer. These controversies are attributable to a liitation of the experiental condition in the condensed phase. Recent supersonic jet spectroscopy provides us with environent of obtaining a saple of the olecules in the cooled and isolated olecular condition to investigate weak inter and intraolecular interactions. n 1989, Ernsting and Dick (ref. 6) reported the tautoer fluorescence and excitation spectra of 3HF in Ar atrix and in supersonic jet. At the alost sae tie, we have reported a preliinary result of supersonic jet spectroscopy of SHF in coparison with 3HC. This paper presents the coparative studies of ESPT of several'substituted 3HC, and their water clusters in supersonic jet. EXPERMENTAL Fluorescence excitation and dispersed fluorescence spectra of the supersonic free jet were easured with the sae ethod and procedures as those described in previous papers (ref. 7). Two at He gas was introduced into a stainless steel saple reservoir leading to a pulsed nozzle (0.4 diaeter). The coplex foration with H O/D2 in the free jet was perfored by ixing H20/D20 vapor (l?2 Torr) with He carrier gas. RESULTS AND DSCUSSON Figure l(a) shows a visible fluorescence (> 510 n) excitation spectru of 3hydroxyflavone in supersonic expansion. The visible fluorescence spectra is due to the tautoer generated by the intraolecular excited ytate proton transfer. A ain vibrational progression of 45 c ay be attributable to the phenyl torsional otion, since siilar progressions are reported in phenylanthracene. However, no significant uv fluorescence excitation spectru was observed. Figure 2a shows a dispersed fluor scence spectru in the excitation of the origin band at ern?. The spectra are alost identical with the tautoer fluorescence in static vapor and in nonpolar solution. The decay ties of the tautoer fluorescence were 1415 ns, which are alost independent of excitati n wavelength in the excess vibrational energy region less than 500 c'. Unfortunately, picosecond fluorescence rise of the tautoer was not obtained because of very weak fluorescence in the jet cooled condition. The fluorescence excitation and dispersed fluorescence spectra were observed in the siilar supersonic expansion of several 3HF derivatives with F, C1, Br, and ethyl substituents at the para position of 2phenyl group. The vibrational progressions and decay ties of these copounds are alost siilar to those of 3HF. n 3HF JHF(H&)s F so zeoeo zez90 zoo WAVENUMBER / crn1 Fig. 1. (a) Fluorescence ( > 510 n) excitation spectra of jetcooled 3HF, (b) uv fluorescence ( n) excitation spectra of 3HF water clusters WAVELENGTH / n Fig. 2. Dispersed fluorescence spectra of la) 3HF excited at c, a uv fluorescence is attributable to a v d ~ coplex with a trace aount of water containated. (b) and (c) are spectra of water clusters of 3HF.

3 Fluorescence studies of excitedstate proton transfer 1631 The 3ethyl pentane solution of 2naphthyl3hydroxychroone (2NHC) exhibits the ESPT tautoer fluorescence at ag538 n. The tautoer fluorescence excitation spectru in supersonic expa sion exhibits a doinant low frequency vibrational ode of 36 c f which is ascribed to naphthyl torsional otion. Siilar to 3HF, no significant uv fluorescence excitatian spectru was detected. n the origin and vibronic band excitation, the dispersed tautoer fluorescence spectra were observed. The decay of this tautoer fluorescence exhibits a single exponential decay with decay ties of 1112 ns. Figure 3 shows a visible fluorescence excitation spectru of jet cooled 3HC onitored at >480 n. The spectru exhibits very diff sed bands without the low frequency vibrational progression of 451c r. However, a broad lowfrequency band with a spacing of 182 c was observe. n jetcooled ethyl salicylate, a low frequency ode of 176 c' was reported in he blue tautoer fluorescence excitation spectra. Further, 180 c' vibrational progression was reported in the dispersed blue fluorescence of this jetcooled copound. These low frequency odes in ethyl salicylate were ascribed to an outofplane bending otion containing an intraolecular hydrogen bonding in the ground and excited states Therefore, it is likely that broad low frequency ode of 182 c' in this 3HC is attributable to the outofplane bending otion in this copound. n the excitations of the origin and ain vibronic bands, the large Stokesshifted tautoer fluorescence was observed (see Fig. 4). The decay tie of the tautoer fluorescence was obtained to be 211 ns depending on the excitation wavelength (313329n). The decay tie decreases with increasing excess vibrational energy of excitation. These fluoresce ce excitation spectra lack low frequency vibrations with 45 and 36 c' spa ings observed in those of 3HF and 2NHC. Therefore, 45 and 36 c spacing are ascribed to the phenyl and naphthyl torsional otions. The iportant role of the phenyl and naphthyl torsional otion in the ESPT will be entioned later. z 3 Lx 5 > z W z 3HC WAVELENGTH / nrn WAVENUMBER / c1 Fig. 3. Fluorescence (>480 n) excitation spectru of jetcooled 3HC. Fig. 4. Dispersed fluorescence spectru of jetcooled 3HC excited at c'. 0 n the excitation spectru of 3HC, we can look at a rearkable broad line width of the origin band, copared with those of other related copounds. The observed band widths of the origin bands of tautoer fluorescence excitation spectra of 3HC, 3HF, and 2NHC were copared with each other, as shown in Figure 5. The spectral line shape of the origin band were siulated by deconvolution ethod taking account of Lorentzian line shapes:n 1989, Ernsting and Dick have

4 1632 M. TOH reported the estiation of electronic decay tie of isolated 3HF ( 1.8 ps) in its first singlet excited state fro a line width of the S1 So transition. These line widths indicate significant dependence on substituents at 2position of pyrone ring. Since it is not likely that rotational contour of origin bands of these three copounds is very uch different fro each other within this experiental condition, the rearkable difference of line widths between these copounds ay be attributable to the reflection of the respective decay rate constant of the S1 state of these copounds. Further, the decay rate constant ay reflect the upper liit of proton transfer rate constant. n this discussion on the ESPT, however, rate constant of the ESPT only eans rate of the tautoer foration fro the noral for in the excited state. The rate constant obtained 0.69 x 10l2 sl HF is consistent with that estiated by Ernsting and Df"," within experiental error. n our previous papers, the intraolecular ESPT in the 3HC solution was '' reported to take place uch faster than those of 3HF and related copo ds 'n hydrocarbon solutions. The estiated rate constant 1.8 x 10 sit for 3HC provides us with the concrete confiration of faster ESPT than 3HF. Further, the rate constant for 2NHC indicates considerably slow proton transfer copared with the other related copounds. The slow rate constants in the related copounds ay reflect the naphthyl and phenyl torsional otion as well as electronic effect at 2position of rpyrone ring. The yubstituted 3HF shows alost sae torsional vibration (4446 c ) of parasubstituted phenyl group at 2position and decay tie ( ps) of the noral S1 state, though substituted phenyl groups see to ake soe electronic effect on the excited state proton transfer. Therefore, it sees that the torsional otion of 2phenyl and naphthyl groups is responsible not only to the nonradiative process of the noral S1 state but also to the excited state proton transfer. \ 3HC t U 3.5cl6' \ 2NHC 14 Fig. 5. (a) Band shapes of origin bands of fluorescence excitation spectra and (b) siulation of linewidths by Lorentzian line shapes. n jetcooled ethyl salicylate, the vibrationaly well resolved origin band was reported in the blue tautoer fluorescence excitation spectra, and the large Stokes shifted tautoer fluorescence exhibiting excess vibrational energy dependence was detected. The facts deonstrated an existence of a potential barrier in the ESPT of this jetcooled copound. n jetcoolede3hc, 3HFJ and 2NHC reported here, appreciably resolved vibrational structures starting fro their respective strong origin bands in the excitation spectra see to

5 Fluorescence studies of excitedstate proton transfer 1633 suggest a sall potential iniu in the Franck Condon excited state of the noral for. f there is no appreciable potential iniu in the S1 state of the noral for, the origin band of the tautoer fluorescence excitation spectru can not be practically observed because of very sall Franck Condon factor. The initially excited state ay be optically active intraolecular ode, and initial excitation energy redistributes into the reactive ode. The reactive states ay strongly ix with the vibronic anifold of the tautoer leading to the tautoer foration. The ixing of the reactive states of noral for with the vibronic levels of the tautoer ay be sufficiently strong, probably because of high level density of vibronic levels of the tautoer. Consequently, it sees that the tautoer foration takes place within picosecond tie doaine. n the intraolecular hydrogen bonding syste exhibiting ESPT, Hbonding ipurities in hydrocarbon solution used soeties induce a atter of controversy. Further, in the Hbonding solution of intraolecular ESPT syste, it is considerably difficult to distinguish whether an observed ESPT is attributable to interolecular interation with the solvent olecules or intraolecular one. As entioned above, Brucker and Kelley suggested that a onosolvate of 3HF with ethanol exhibits fast ESPT in 10 K Ar atrix while a disolvate does not proton transfer by a sall spectral change of vibrationally unresolved fluorescence/excitation spectra in the absence and presence of ethanol in the Ar atrix. However, there is a liit to deterine nuber of solvent olecule concerned in the condensed phase spectroscopy. cn Z 3 K a Y > cn Z W Z 3H F Hz WAVENUMBER / c Fig. 6. Fluorescence excitation spectra of jetcooled 3HF in the absence and presence of water vapor in He carrier gas: (a) H20 p = 2 Torr, (b) p = 4 Torr, (c) p = 6 Torr, (d) p = 8 Torr, (e) p = 10 Torr. a > + v, z 0.5 w + = 0.2 y L A /.,=2.1 ll PRESSURE OF H20 / Torr Fig. 7. Log plots of relative intensities of uv fluorescence excitation spectra of (a) 3HF and (b) 2NHC vs. H20 pressure. 0 and are corresding to ono and dihydrates. All of 3HF and related copounds exhibit no significant uv fluorescence attributable to the noral for, which eans the fast ESPT and predoinant nonradiative relaxation of the S1 state. Figure 6 shows uvvisible total fluorescence excitation spectra in the presence of water vapor of 3HF. n the absence of H20 vapor in seeded He gas, no significant excita ion spectru was observed in the lower energy region than c k. n the presence of water vapor, the longer wavelength bands appeared and rearkably increased in intensity with increasing water vapor pressure. However, no

6 1634 M. TOH significant difference of tautoer fluorescence excitation spectra was observed in the absence and presence of water vapor. Siilar water cluster foration was observed in jetcooled 2NHC in the alost identical experiental condition with that of 3HF. Figure 7 shows logarithic plots of these band intensities versus vapor pressure of HzO in the He carrier gas. Slopes of both plots suggest that vibrational envelopes are attributable to 1;l coplexes with water. n jetcooled 3HC, however, no uv fluorescence excitation spectru was detected even by seeding water vapor (26 Torr) in the He carrier gas. ncreasing yapor pressure of H20 to 612 Torr, further red shifted vibrational structures gradually appeared and increased in intensity of both 3HF and 2NHC. Slope of these log plots of further redshifted band intensities indicates approxiately 2. The fact iplies that the further redshifted bands are attributable to clusters of twg water olecules coplexed of these copounds. The ono and dihydrate forations of 3HF and 2NHC, and their spectral features are very siilar each other. Figure lb shows the uv fluorescence excitation spectra attributable to these water clusters of 3HF, n = 1 and 2 for 3HF(H20), in coparison with the vis fluorescence excitation spectru of bare 3HF. Figure 2b and 2c show dispersed fluorescence spectra of ono and dihydrates of 3HF in the excitation of the origin bands of clusters. The dispersed fluorescence spectra of water clusters of 2NHC are alost identical with those of 3HF. These spectra exhibits no tautoer fluorescence but only uv noral for fluorescence. The fact indicates that the ESPT in these copounds does not take place in interolecularly Hbonded clusters, but does occur only in the isolated olecular condition. That eans that the ESPT of 3HF and related copounds takes place only in the isolated olecular condition. Acknowledgeent The work is indebted to Dr. Y. Fujiwara and A. to for their wellexperienced experients and valuable discussion. REFERENCES 1. P. K. Sengupta and M. Kasha, Che. Phys. Lett. 68, 382 (1979). 2. M. toh, K. Tokuura, Y. Tanioto, H. Okada, K. Takauchi, K. Obi, and. Tanaka, J. A. Che. SOC. 104, 4146 (1982). 3. M. toh, Y. Fujiwara, M. Suitani, and K. Yoshihara, J. Phys. Che. 90, 5672 (1986). 4. D. McMorrow and M. Kasha, J. Phys. Che. 88, 2235 (1984) and references therein. 5. G. A. Brucker and D. F. Belley, J. Phys. Che. 91, 2856 (1987); J. Phys. Che (1988), and references therein.