J. Phys. Chem. 1995,99, 17711-17714
17711
Excited-State Intramolecular Proton Transfer of 2-(2'-Hydroxyphenyl)benzimidazole in Micelles Nilmoni Sarkar, Kaustuv Das, Swati Das, Anindya Datta, Debnarayan Nath, and Kankan Bhattacharyya* Department of Physical Chemistry, Indian Association for the Cultivation of Science, Jadavpur, Calcutta 700 032, India Received: August I I , I995@
Excited-state intramolecular proton transfer (ESIPT) of 2-(2'-hydroxyphenyl)benzimidazole (HPBI) has been studied using steady-state and time-resolved emission spectroscopy in neutral (Triton X- lo), anionic (sodium dodecyl sulfate, SDS) and cationic (cetyltrimethylammonium bromide, CTAB) micelles. It is found that the tautomer intensity and the lifetime of the tautomer emission are sensitive monitors to estimate the critical micellai concentration (cmc) for all three micelles. Above the cmc the intensity of the normal emission band decreases while there is a large enhancement of the intensity of the tautomer emission along with a red shift of the emission maxima by about 20 nm. The decay of the normal emission of HPBI remains similar below and above the cmc for all three micelles. However, the lifetime of tautomer emission of HPBI increases sharply above the cmc. The enhanced tautomer emission is ascribed to the HPBI molecules residing in the less polar and aprotic interior of the micelles.
SCHEME 1
1. Introduction
In recent years the excited-state intramolecular proton transfer (ESIPT) phenomenon has been the subject of very active research due to its widespread implications.'-'' Molecules undergoing ESIPT exhibit dual emission, with the normal one bearing a mirror image relation to the absorption spectrum, arising from an excited species similar in geometry to that in the ground state while the Stokes' shifted emission originates from a tautomer formed in the excited electronic state. The normal and the tautomer emissions depend crucially on the conformation of the molecule, the polarity and hydrogenbonding ability of the solvent, and the temperature.l4-'' In the case of 2-(2'-hydroxypheny1)-benzimidazole (HPBI) in the ground state two intramolecularly-hydrogen-bonded rotamers, I and 11, exist in equilibrium. On excitation, I gives normal emission while 11undergoes ultrafast ESIPT in the femtosecond time scale to produce the tautomer III in the excited state from which the tautomer emission results (Scheme l).I43l5 In the strongly hydrogen-bonding aqueous medium intermolecular hydrogen bonding favors normal emission through the formation of species like IV.I4 Since the excited-state proton transfer phenomenon is highly sensitive to the environment, organized assemblies affect it markedly.'8-26 A number of recent studies have reported on the influence of micelles, reverse micelles, and cyclodextrins on the inter- and intramolecular proton transfer processes.18-26 Using time-resolved spectroscopy, Fleming et al. showed that, in aqueous media, the rate of intermolecular proton transfer of protonated 1-aminopyrene increases considerably on binding with B-cyclodextrin.'* Warner et al. studied the ESIPT phenomenon with 10-hydroxybenzoquinoline(HBQ) in cyclodextrins and micelles using steady-state spectroscopy and found that the steady-state yield of the tautomer emission increases slightly on binding to cyclodextrins and micelles.20 In the present study we report on how the dual emission of HPBI is affected when the probe molecule, HPBI, is transferred from the highly polar bulk water to the relatively nonpolar and aprotic interior of the cationic (cetyltrimethylammonium bro@
Abstract published in Advance ACS Abstracts, November 1, 1995.
0022-365419512099-17711$09.0010
I
I1
IV
A 111'
mide, CTAB), anionic (sodium dodecyl sulfate, SDS), and neutral (Triton X-100) micelles. The main impetus of this work is to elucidate the effect of hydrophobic microenvironments on the ESIPT phenomenon.
2. Experimental Section HPBI was synthesized and purified as described p r e v i ~ u s l y . ' ~ - ' ~CTAB ~ * ~ and SDS (Aldrich) were recrystallized before use. Triton X-100 (Aldrich) was used as received. Absorption and emission spectra were recorded in JASCO 7850 and Perkin Elmer MPF 44B spectrophotometers, respectively. The quantum yields were determined with respect to that of quinine sulphate in 1 N H2S04 as 0.55. The fluorescence decays were recorded in a picosecond setup in which the sample is excited by the second harmonic (300 nm) of a Coherent cavitydumped rhodamine 6G dye laser (702) synchronously pumped by a Coherent CW mode-locked Antares Nd:YAG laser (76s). The emissions are collected at magic angle (54.7') polarization 0 1995 American Chemical Society
17712 J. Phys. Chem., Vol. 99, No. 50, 1995
Sarkar et al.
TABLE 1: Effect of Surfactant on the Emission Properties of HPBI" lifetime (ns) fluorescence emission (nm) surfactant
concentration (mM)
CTAB
0 0.1 0.3 0.5 0.75 0.90 1.25 1.75 2.5 3.5 2.3 4.6 6.9 8.0 16.0 19.6 24.0 0.025 0.10 0.13 0.20 0.40 0.80 1.20
SDS
Triton X- 100
fl
fluorescence quantum yield
,IF
,y
4;
4:
rT
r]
t2
a]
a2
435 435 435 445 450 450 450 455 45 5 455 435 440 450 450 450 450 450 435 435 435 445 450 455 455
350 350 350 350 355 355 355 355 355 355 350 350 355 355 365 365 370 350 350 350 350 355 355 355
0.30 0.30 0.27 0.26 0.26 0.33 0.425 0.51 0.52 0.58 0.33 0.33 0.42 0.45 0.45 0.47 0.5 1 0.30 0.35 0.39 0.39 0.56 0.60 0.58
0.045 0.045 0.045 0.037 0.017 0.015 0.0 15 0.016 0.016 0.020 0.036 0.036 0.029 0.030 0.039 0.039 0.034 0.045 0.028 0.025 0.033 0.033 0.034 0.030
1.8 2.1 2.2 2.2 2.3 2.7 4.4 4.6 4.6 4.8 1.7 1.7 1.8 1.9 4.0 4.0 3.9 2.1 2.1 2.1 2.4 4.1 4.9 4.9
0.29 0.27 0.22 0.24 0.24 0.20 0.25 0.22 0.19 0.18 0.22 0.20 0.20 0.22 0.19 0.19 0.2 0.25 0.26 0.23 0.23 0.23 0.20 0.20
1.5 1.0 0.7 1.0 1.0 0.7 1.0 0.7 0.8 0.8 1.3 1.3 1.2 1.4 0.74 0.7 0.72 1.1 1.4 1.0 1.0 1.0 1.0 1.1
0.90 0.95 0.90 0.94 0.95 0.90 0.83 0.70 0.60 0.50 0.90 0.93 0.90 0.93 0.7 0.6 0.6 0.95 0.98 0.93 0.94 0.90 0.80 0.75
0.10 0.05 0.10 0.06 0.05 0.10 0.17 0.30 0.40 0.50
AF,&, and tT are the emission maxima, quantum yield, and lifetime of the tautomer, and A?,
320
560
440
w a v e l e n g t h (rim 1
__j
Figure 1. Emission spectra of HPBI in water (-), in 24 mM SDS in 1.2 mM Triton X-100 (- - -), and in 3.5 mM CTAB (.*a),
t;
(-*-e-).
by a Hamamatsu MCP PM tube (R-2809U). The time resolution of this setup is about 50 ps. Deconvolution of the fluorescence decays was done using global lifetime analysis software.28 All the measurements were done at pH = 7. 3. Results 3.1. Steady-State Emission Properties of HPBI in Micelles. As reported earlierI4 in aqueous media HPBI exhibits a moderately strong tautomer emission at ca. 435 nm with quantum yield 4; = 0.3 and a weak normal emission at ca. 350 nm (4; = 0.045). On addition of the surfactants, the absorption and excitation spectra remain unaffected, except for slight changes (510%) in absorbance. The quantum yield of the normal and the tautomer emission of HPBI remain more or less unaffected up to the reported critical micellar concentration (cmc) (Figure 1 and Table 1). Around the cmc, a sharp increase in the intensity and a marked red shift of the tautomer emission
4:. and
ty
0.1 0.07 0.10 0.07 0.3 0.4 0.4 0.05 0.02 0.07 0.06 0.10 0.20 0.25
denote those of the normal
are observed while the intensity of the normal emission decreases. Figure 2 describes the variation of the quantum yields of the tautomer (4:) and the normal (4:) emissions of HPBI with the concentration of the surfactants. For CTAB, SDS, and Triton X-100 the yield of the tautomer emission (4:) of HPBI exhibits a sharp change of slope around the reported cmc's of these micelles of 0.9, 8, and 0.2 mM, re~pectively.~~ This indicates that the steady-state quantum yield of the tautomer emission and to a lesser extent the yield of the normal emission of HPBI are sensitive indicators of the micellization process. Above the cmc the quantum yield of the tautomer emission (4:) of HPBI saturates to a value higher than that in the case of water by a factor of 1.93, 1.7, and 1.93 for CTAB, SDS, and Triton X-100, respectively. The quantum yield of the normal emission at the highest concentration of the surfactants decreases by a factor of 2.25, 1.33, 1.49, respectively, in CTAB, SDS, and Triton X-100. It should be noted that the magnitudes of the changes in the steady-state emission quantum yields of HPBI are much larger than that reported for HBQ in micelles by Warner et al.*O 3.2. Time-Resolved Studies of HPBI in Micelles. In aqueous media the decay of the tautomer emission of HPBI is single exponential with a lifetime of 1.8 ns while the decay of the normal emission is biexponential with a major component of 290 ps and a minor component with a 1.5 ns lifetime.I4 On addition of the three surfactants the decay of both the normal and tautomer emission of HPBI remains similar up to the cmc (Table 1). Above the cmc the decay of the normal emission continues to be very similar to that in water, with the lifetime of the major component being 200-290 ps and that of the minor component being 5 1 ns. The lifetime of the tautomer emission of HPBI, however, increases very sharply around the cmc and quickly reaches a value higher than that in water by a factor of two or more (Table 1). As a representative, Figure 3 depicts the decay of the tautomer emission of HPBI in water and in 1.2 mM Triton X. Figure 4 shows the variation of the lifetime of the tautomer emission of HPBI with the concentration of
J. Phys. Chem., Vol. 99, No. 50, 1995 17713
2-(2'-Hydroxyphenyl)benzimidazole in Micelles
3
1
4
[CTAB] (n*l 1 - 4
4
0.4
