Indian Journal ef Chemistry Vol. 34A,Jarn¢ry 1995, pp. 55-58

Photophysics of 2-(2' -hydroxyphenyl) benzoxazole in the presence of cx-cyclo- dextrin: Deactivation of ESIPT through back protonation ( II) (I) Santi Kundu, Subhas Chandra Bera & Nitin Chattopadhyay" Department 'Of Chemistry, Jadavpur University, Calcutta 700 032, India Received 30 May 1994; revised and accepted 31 August 1994

The complex photophysics of 2-(2'-hydroxyphenyl)- benzoxazole (HBO) in the presence of o-cyclodextrin (III) (ClCD) has been investigated using steady state as well as time-resolved spectroscopy. The 480 nm emission ofHBO in aqueous medium has been found to be suppressed presence of rx-cyclodextrin. The results show- that meaningfully in the presence of cyclodextrin. Steady state (XCDreduces the tautomer fluorescence yield as well study as well as decay analyses suggest that the tautomer as the corresponding lifetime and this has been undergoes back protonation reaction in the excited state argued in the light that the keto form ofHBO, in its through the participation of ClCD. excited singlet state, undergoes back protonation reaction through the involvement of cyclodextrin. Considerable interest is recently being provoked by 2-(2'-hydroxyphenyl)benzoxazole and related Experimental thiazole, indazole and triazole due to their complex HBO (Aldrich) was purified through chrornato- photoprocesses including excited state graphy on acidic alumina using 1:1toluene-hexane as eluent, sublimation and finally recrystallisation from intramolecular proton transfer (ESIPT )1-14. ethanol. Fine needle shaped crystals were obtained. Photophysics of these compounds are important because they can serve as ultraviolet stabilisers!", e-Cyclodextrin (Aldrich) was used as received. liquid laser materials'> and models for keto-enamine Triply distilled water was used throughout the experiment. Fresh 2 x 10-6 mol dm - 3 HBO solution tautomerisation occurring in enolamines". Rotamers, tautomers and anions of the compounds have been was prepared in water. Only aerated solutions were taken for the experiments. proposed. Tautomer formed through ESIPT has been proposed from the very large Stokes shifted Absorption and emission spectra were recorded on (~ 10000 em -1) emission of the compounds in Shimadzu MPS 2000 spectrophotometer and aprotic as well as protic solvents. That ESIPT is Perkin-Elmer MPF 44B spectrofluorimeter extremely fast has been established from the respectively. The time-resolved experiments using time-resolved studies2•3•11. Involvement of triplet time-correlated single-photon counting (TCSPC) states have also been indicated by Mordzinski et technique+", was performed using nitrogen flash aP·s. lamp (Edinburgh Instruments, 199 fluorescence The fluorescence of HBO depends largely on the spectrometer) as a source of excitation beam. FWHM nature of the solvent. Emission ofHBO is observed of the exciting pulses were 1.2 nanoseconds. principally in three regions: (i) around 360 nm, ascribed to the normal orenolic form of the molecule Results and discussion (I); (ii) around 480 nm, attributed to the tautomeric Absorption study species (III) and (iii) around 440 nm, more The absorption spectra of HBO in aqueous prominently found in protic solvents, corresponding solution as a function of (XCDare shown in Fig. I. to the ground state rotamer II having an absorption With increase in cyclodextrin concentration, 360 nm near 360 nm. band increases slightly whereas the shorter This note reports a study of the complicated wavelength band is decreased giving an isosbestic photoprocesses of HBO in aqueous medium in the point (at 343 nm). This reflects the interaction ofHBO 56 INDIAN J CHEM. SEe. A, JANUARY 1995

'c-.. ~ Q~ CII :- ..:>. ••c '...c f e - t t 0 a 31tO 380 1t20 1+60 500 5ItO 580 Wavelength (nm) .

Fig. 2-Fluorescence spectra of HBO solutions as a function of 250 ~ 350 400

Fig. I-Absorption spectra of HBO solution as a function of Table I-Decay analyses ofthree emissions of HBO as a function oxygen>". The 440 nm emission is taken to phenolic part of HBO. (XCD has been taken originate from the rotamer (II)3. The lowest energy intensionally to avoid the possibility of any deviation emission (F3) with a large Stokes shift (;::;10400cm -I) from the aforesaid 1:1 stoichiometrvt+!". from the corresponding excitation can be ascribed to the tautomer species formed through ESIPT, in Fluorescence study which the proton goes to the nitrogen atom of the The fluorescence spectra of aqueous HBO heterocyclic ring. solutions as a function of (XCDconcentration are Figure 2 shows that as a.CD concentration in the presented in Fig. 2. In the absence of CD, the spectra solution increases F3 band intensity gradually consists of three distinct bands, consistent with the diminishes. At a.CD > 10- 2 mol dm - 3, the 480 nm literature, the maxima are at around 360 nm (F 1),440 emission is practically masked by the 440 nm

nm (F2) and 480 nm (F3).The intensity of the bands are emission. Time resolved study also provide same in the order F3 > F2 > F l- The yields of the bands are lifetime values at the two wavelengths reflecting that in conformity with some other findings for similar 480 nm emission is only the tail of the F2 emission. The other compounds":", As consistent with the gradual decrease in the tautomer emission yield with absorption, the excitation spectrum corresponding addition of (XCD may be due to two reasons. (i) to 440 nm emission shows a band around 360 nrn. The Ground state trapping of transferable proton within normal Stokes shifted emission (FI) is ascribed to the CD cavity: as clear from the absorption spectra, originate from the excitation of the normal HBO HBO forms a I:1 inclusion complex with a.CD and NOTES 57

1Channel = 0.1ns 2.50

<, 2.00 . o ,} - 1.50 1.00 .-CII Z ::I 0.50 +'-TT"rT'1"TTTT'rrTTT"rnrr"TTT"r1 e o 2 4 6 a 10 12 u cone. of CD (mM) u, I e

Fig. 3=---Plot of 10/1 for the tautomer emission against (lCD 8..J concentration

because of the cyclodextrin encapsulation; the - OH proton may not be accessible to the nitrogen atom of 100 the heterocycle. (ii) Non-radiative decay of the ChOnntl no. excited tautomer: species with oxygen atom doubly Fig. 4-'-Decay profiles of the 480 nm emission as a function of bonded.to a phenyl ring (as is the keto form ofHBO) oeCD.The different concentrations ofcxCD are: (a) 0, (b) I, (c) 4, (d) are known to abstract hydrogen from suitable donors 6 and (e) 15 m mol dm.-J .~ in the pfiotoexcited states. In the presence of a cyclodextrin, which is known to be a good hydrogen for MB05 and 1.5ns for HPBI9). The r value obtained donor18.20.21,the tautomer, in its excited singlet state,. by us for F3 emission, though a bit higher, is ?f.the undergoes a back protonation reaction to produce order to those reported in literature for similar the enolic species. This non-radioactive channel is compounds in some other solvents, viz., 4. I ns f?r • expected to diminish the fluorescence yield of the keto HBT in aqueous ethanol? or 4 ns for HPBI m form (480 nm emission) as well as the corresponding ethanol". The decays of the first two emissions are excited state lifetime (Table I). The observation practically indifferent to cyclodextrin concentration matches exactly with the expectation. A gradual (Table I). This constancy in r values reflects that both increase in the enol fluorescence (360 nm band) with F\ and F2 come from the direct excitation of the the addition of (XCD, further substantiates the ground state rotamers I and II and (XCDhas hardly formation of enol from the keto form. We have also any effect on the excited photophysics of these followed the steady state quenching of the tatitomer rotamers. Decay of the 480 nm emission is, however, fluorescence and noticed that it obeyed the found to be sensitive to (XCD concentration. The Stern- Volmer equation very well yielding a straight tautomer lifetime goes on decreasing with the line when 10/1was plotted against (XCDconcentration addition of cyclodextrin (Fig. 4). At (XCD (Fig. 3). The values of the Stern-Volmer constant concentration> 10-lmoldm-3tvaluesfor480nm and the quenching rate constant came out to be and 440 nm emissions are equal indicating that the 7 1 0.13 dm-rnol" and 1.9 x 10 dm ' mol-1s- tautomer emission is masked by the other band. Had respectively. The above discussion, in association ground state CD-encapsulation of the phenolic with the decay analyses, led us to conclude that the proton, responsible for ESIPT, been the only factor back prototropic reaction occurs in the excited then with an increase in (XCDconcentration, one singlet state. would expect, not a decrease in the tautomer.lifetime but a decrease in the tautomer fluorescence intensity. Time-resolved study The decrease in tautomer lifetime with increasing Time-resolved analyses of the fluorescence decays (XCD reveals that CD influences the decay indicate that all three emissions are single exponential channels of the excited singlet species. The above in nature. Table I summarises the lifetime data for discussion proposes that the tautomer, in its excited HBO emissions in different (XCDconcentrations. In singlet state, undergoes back protonation reaction the absence of

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