J. Phys. Chem. 1993,97, 13625-13628
13625
Photochemistry of Squaraine Dyes. 6. Solvent Hydrogen Bonding Effects on the Photophysical Properties of Bis( benzothiazoly1idene)squaraines Suresh Das,'-+K. George Thomas, R. Ramanathan, and M. V. George'** Photochemistry Research Unit, Regional Research Laboratory (CSIR), Trivandrum 695 019, India
Prashant V. Kamat' Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556 Received: August 26, 1993"
The photophysical properties of two bis(benzothiazoly1idene)squaraine dyes have been studied in aromatic hydrocarbon and alcoholic solvents. These dyes exhibit fluorescence quantum yields of 0.06-0.52 and excited singlet lifetimes of 0.48-2.5 ns in these solvents. Although the photophysical properties of the dyes are independent of solvent polarity in the hydrocarbon solvents, in alcoholic solvents a marked hyposchromic shift in the absorption and emission spectra and a reduction in lifetimes of excited state have been observed. These effects have been correlated with the hydrogen bond donating ability of the alcoholic solvents.
Introduction The absorption and emission properties of molecules that undergo large changes in charge distribution, following photoexcitation due to intramolecular electron-transfer processes, are quite often highly sensitive to the nature of the solvent These properties make them very useful as probes in the study of a variety of heterogeneous and biological sy~tems.43~ Squaraine dyes have a strong absorption in the visible and near-infrared region arising out of intramolecular donor-acceptor-donor type charge-transfertransitions? The sensitivityof their photophysical properties to environmental factors has been utilized to probe heterogeneous system^.^^^ Recent studies by Law on the spectroscopic properties of bis[4-(dimethylamino)phenyl]squaraine and its derivatives have shown that these dyes are capable of forming solute-solvent complexes which lead to a bathochromic shift in the absorption and emission ~ p e c t r a . ~ Here we report on the photophysical properties of bis(3ethylbenzothiazol-2-y1idene)squaraine(SQ1) and its surfactant derivative, bis(3-octadecanebenzothiazol-2-ylidene)squaraine (SQZ), in two classes of solvent: aromatic hydrocarbons and alcohols. 0-
Experimental Section All new compounds were fully characterized on the basis of analytical results and spectral data. All melting points are uncorrected and were determined on a Btichi melting point apparatus. IR spectra were recorded on a Perkin Elmer Model 882 IR spectrometer and the UV-visible spectra on a Shimadzu 2100 spectrophotometer. IH NMR spectra were recorded on a JEOL EX90 spectrometer and the mass spectra on a Finnigan MAT Model 8430 or JEOL JMS AX 505 HA mass spectrometer. f
Also at the Radiation Laboratory of the University of Notre Dame.
f
Also at the Radiation Laboratory of the University of Notre Dame and
Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore 560 012, India.
Abstract published in Advance ACS Absrracrs. November I S , 1993.
0022-365419312097- 13625$04.00/0
Emission spectra were recorded on a SPEX-fluorolog F112-X spectrofluorimeter. Quantum yields of fluorescence were measured by the relative method using optically dilute solutions. Bis[4-(dimethylamino)-2-hydroxyphenyl]squaraine was used as r e f e r e n ~ e . ~Spectroscopic J~ solvents were used throughout, Picosecond Laser Flash Photolysis. Picosecond laser flash photolysis experimentswere performed with a mode-locked 532nmlaser pulsefromaQuante1 YG-5OlDPNd:YAGlaser (output = 4 mJ/pulse, pulse width 18 ps). The white continuum picosecond probe pulse was generated by passing the residual fundamentaloutput througha D20/H20 solution. The excitation and the probe pulse were incident on the sample cell at right angles. The output was fed to a spectrograph (HR-320, ISDA Instruments, Inc.) with fiber optic cables and was analyzed with a dual diode array detector (Princeton Instruments, Inc.) interfaced with an IBM AT computer. The details of the experimental setup and its operation are described in detail elsewhere.lla*bTime zero in these experiments corresponds to the end of the excitation pulse. All the lifetimes and rate constants reported are within the experimental error of AS%. StartingMaterials. 3-Ethyl-2-methyllbenzothiazoliumiodide, mp 190-191 OC, and 2-methyl-3-octadecanebenzothiazolium iodide, mp 1 1 6 118 OC, were prepared by reported procedures.12 Synthesis of SQ1 and SQ2. SQ1 was synthesized from squaric acid and 3-ethyl-2-methylbenzothiazoliumiodide by a reported procedure13 and purified on a silica gel (230-400 mesh) flash column by eluting with a mixture (1:4) of chloroform and methanol. SQ2 was synthesized by a similar procedure by mixing 2-methyl-3-octadecylbenzothiazolium iodide (0.53 g, 1.O mmol), squaric acid (0.057 g, 0.5 mmol), and quinoline (0.13 g) in a solvent mixture containing 6 mL of benzene and 15 mL of 1-butanol. The mixture was refluxed for 20 h a t 120 OC, and the water formed during the reaction was distilled off azeotropically. After cooling, the mixturewas filtered and the residue was washed with ether. Purification by chromatography on a silica gel (230400 mesh) flash column, using methanol as eluent, gave 140 mg (24%) of SQ2, mp 148-149 OC: IR vmax2925,2853 (CH), 1583 (C-0) cm-I; UV A, (CHC13) 670 nm (c 27 000 M-I cm-I); IH NMR (CDCls) 6 0.8-1.8 (70 H, m, aliphatic), 4.05 (4 H, t, 2 CH2), 5.09 (2 H, s, 2 CH), 7-7.7 (8 H, m, aromatic). Molecular wt calcd for C5&4N202S2 880.59742, found 880.5980 (FAB high-resolution mass spectrometry).
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0 1993 American Chemical Society
Das et al.
13626 The Journal of Physical Chemistry. Vo1. 97, No. 51, 1993 0.8
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Wavelength, nm
F w e 3 . Effect of trifluoroethanol (TFE) concentrationon theabsorption spectrum of SQ1 in toluene. TFE concentration: (a) 0, (b) 1.4, (c) 2.7, (d) 4.1, and (e) 5.5 mM.
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Wavelength (nm)
by Bigelow and Freund show that the charge transfer during the
SO SItransition of bis[4-(dimethylamino)pheny1]squaraine is
I
Wavelength (nm) Figure 1. (A) Absorption spectra of SQ1 in hydrogen bonding solvents: (a) trifluoroethanol, (b) methanol, (c) ethanol, (d) 2-methoxyethanol. (B) Normalized emission spectra of SQI in hydrogen bonding solvents: (a) trifluoroethanol, (b) methanol, (c) ethanol, (d) 2-methoxyethanol.
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670
ui n
a
Y
s
: x lo
630 0.3
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0.4
0.5
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0.6 T*
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0.7
0.0
Figure 2. Plot of Taft parameter r* vs absorption maximum of SQ1 in different solvents: (1) trifluoroethanol, (2) methanol, (3) ethanol, (4) 2-propanol, ( 5 ) I-butanol, ( 6 ) 2-methoxymethanol, (7) terf-butanol, (8) benzene, (9) toluene, (10)chlorobenzene, (1 1) bromobenzene.
Results and Discussion Absorption and Emission Properties. Hydrocarbon Solvents. The absorption and emission spectra of SQ1 in different solvents are shown in Figure 1A and 1B. The fluorescence spectrum is a reasonably good mirror image of the absorption spectrum, and the Stoke shift is also quite small, indicating negligible reorganizational effects and small geometry changesbetween theground and excited singlet state. Earlier MNDO and CNDO calculations
primarily confined to the (2402 unit.6 Such a transition would lead to negligible reorganizational and structural changes. Table I lists the absorption and emission data as well as the excited singlet state lifetimes of SQ1 and SQ2, in various aromatic and alcoholic solvents. A plot of A, of absorption versus the solvent Taft parameter, ?F* is shown in Figure 2. The r* sacle is a solvent polarity-polarizability parameter, which provides a measure of the ability of the medium to stabilize a charge or dipole by virtue of its dielectric effect.14 From Figure 2, it is clear that no change in the absorption maximum occurs for the aromatic hydrocarbon solvents. Also as can be seen from Table I, the fluorescencemaxima as well as fluorescenceyields of SQ1 are independent of ?F* for these solvents. AlcoholicSolvents. The absorptionandemissiondata for SQ1 and SQ2 in alcoholic solvents are also included in Table I. Since there were no significant differences between the absorption and emission properties of SQ1 and SQ2 in the solvents that we have examined, SQI was chosen for further detailed studies. As in the case of the aromatic hydrocarbon solvents, the emission spectra are good mirror images of the absorption spectra with very low Stoke shifts, indicating negligible reorganization of solvent and solute between the ground and excited state. However, the A, of absorptionand emission show a significanthypsochromic effect with increasingsolvent polarity (Table I, Figure 2). Concurrently there is a significant decrease in the fluorescence lifetime and quantum yield with increasing solvent polarity. As discussed above, studies in the hydrocarbon solvents have shown that the photophysicalpropertiesof SQ 1 are independent of bulkdielectric effects. Hence theeffect of alcoholicsolvents on the photophysical properties of SQ1 is attributed to specific solute-solvent interaction. Figure 3 shows the effect of addition of small amounts of 2,2,2trifluoroethanol (TFE) on the absorption spectrum of SQ1 in toluene. With increasingconcentrationof alcohol, theabsorption spectrum shifts to shorter wavelengths, accompanied by an increase in the intensity of absorption. Addition of equivalent amounts of polar aprotic solventssuch as acetonitrile to a solution of SQ1 in toluene did not lead to any changes in the absorption spectrum. The substantial effects in the absorption spectrum observed upon addition of TFE further suggest complexformation between the alcohol and SQ1. TFE and the other alcoholic solvents are known to form hydrogen bonded complexes with dyes,lsJ6 and in the present case this is most likely to occur at the oxygen atom sites on the cyclobutane ring. The strength of such interaction is expected to depend upon the relative acidity of the alcohols. As discussed above, the electron density at the oxygen site will be strongly reduced in changing from the ground to the excited singlet state. Such a change is expected to weaken the hydrogen bond between the alcohol and the excited state of the dye.
H-Bonding Effects on Photophysics of Squaraine Dyes
The Journal of Physical Chemistry, Vol. 97, No. 51, 1993 13627
TABLE I: Absorption a d Emission Data for Bis(benzotbirzo1ylidene)squaraines SQ1 and SQ2 in Different Solvents
~~
2,2,2-trifluorocthanol methanol ethanol 2-propanol 1-butanol 2-methoxyethanol
635
646
0.06
649 654 656
662 666 667 668 670 673
0.09 0.21 0.3 1 0.35 0.25 0.40
678 678
690 690 693
0.54 0.53
678
693
0.52
657 657 659 679
tert-butanol
benzene toluene chlorobenzene bromobenzene
0.14
660
673
0.35
68 1
696
0.57
TABLE Ik Singlet Excited State Properties of SQ1 abs max/nm solvent (SISA T,/nsa &,/io9 1-1 b methanol 525 0.48 1.9 ethanol 526 1.35 0.59 2-propanol 520 1.45 0.48 1-butanol 521 1.72 0.38 tert-butanol 516 1.94 0.31 benzene 493 2.50 0.18 a Measured from the first-order decay of transient absorption (Sl-S,) at its maximum. Determined from T~ by using cqs 1 and 2.
0.05
4.00
p
663
0.51
0.10
f
653
.0.05
4.10
a a 4.15
4.m 4.25
400
450
500
550
0 0
Bso
WAVELENGTH, nm
Figure4. Transient absorption spectra of the singlet excited state of SQ1 in benzene. The spectra were recorded following the 532-nm laser pulse excitation of SQ1 in benzene at time intervals of (a) 0.1, (b) 0.2, (c) 1, (d) 4, and (e) 8 ns.
0.25
t
400
450
500
550
600
WAVELENGTH (nm)
Figure 5. Transient absorption spectra of the singlet excited state of SQ1, recorded immediately following the 532-nmpulse in excitation (pulse width 18 ps) of SQ1 in benzene, 2-propanol, 1-butanol, ethanol, and
methanol. PicosecondLaser Flash Photolysis Studies. Picosecond laser flash photolysis was used to probe the singlet excited state properties of SQ1. ISQ 1* was generated by direct excitation with 532-nm laser pulses. The transient absorption spectra recorded in benzene at different times following the laser pulse (1 8 ps) are shown in Figure4. %Q1* showed an absorption maximum at 493 nm and decayed by first-order kinetics with a lifetime of 2.5 ns in benzene. Figure 5 shows the spectrum of the excited singlet state of SQ1 in benzene and alcoholic solvents. In the alcoholic solvents, the spectra show broad absorption maxima around 520 nm and indicate a very small shift toward longer wavelengths with increasing solvent polarity, while the absorption maximum
of the excited singlet state in benzene is substantially blue shifted. The properties of the excited singlet state in various solvents are summarized in Table 11. These results indicate that the solvatochromiceffects on the absorption spectrum of the singlet excited state are the oppositeof those observed for the absorption in the ground state, confirming that the excited state of the dye is less polar than the ground state. The hydrogen bond donating ability of the alcohols also has a significant effect on the lifetime of SQ1. Since these dyes exhibit very low intersystem crossing efficiency (#triplet < lO-3), as determined by laser flash photolysis, the rate of nonradiative decay via internal conversion can be obtained from the following equations:
where #f is the quantum yield of fluorescence; kf and knr are the rate constants for flu0 rescence and nonradiative decay, respectively; and is the lifetime of the excited singlet state. The values of k,, are shown in Table 11. As seen from Table 11, the rate constant for nonradiative decay increases from 3.1 X lo8 s-l in tert-butanol to 1.9 X lo9s-l in methanol. Thus, even though the excited state is only weakly bound to alcohol by a hydrogen bond, this interaction brings about a significant increase in the nonradiative decay. Since the solvatochromic effects on the absorption spectra of SQ1 appear to depend upon the formation of hydrogen bonded complexes between solute and solvent, the existence of a relationship between the hydrogen bond donating ability of the alcohols and the observed effects was explored. Neerinck and Lamberts have measured the shift in the OH frequency (A;) of alcohols complexed to pyridine in CC14.17 These values can be correlated to the hydrogen bond donating ability, since for alcohols there is a linear relationship between A; and AH of hydrogen bonding.I5 Figure 6 shows the plot of Av versus the absorption maxima of SQ1, indicating a linear relationship between the two. This confirms that hydrogen bonding by the alcohols to the 0 atoms of the cyclobutane ring does strongly stabilize the ground state of SQ1 in comparison to its e_xcitedsinglet state. The linear relationship between Av and the absorption maxima of SQ1 indicates that the relative hydrogen bond donating ability
13628 The Journal of Physical Chemistry, Vol. 97, No. 51, 1993
Das et al.
Acknowledgment. The authors thank the Council of Scientific and Industrial Research, Government of India (S.D., K.G.T., M.V.G.); Jawaharlal Nehru Centre for Advanced Scientific Research (R.R. for the award of a Summer Research Fellowship for the Summer of 1992 and M.V.G.); and the Office of Basic Energy Sciences of the U S . Department of Energy (P.V.K., S.D. (in part), and M.V.G. (in part)). This is ContributionNo. RRLTPRU-36 from RRL, Trivandrum, and No. NDRL-3641 from the Notre Dame Radiation Laboratory. 260
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220 630
References and Notes I
I
640
650
660
670
hnl, ( A W Figure 6. Plot of A i of OH vibration vs-absorption maxima in various hydrogen bonding solvents. Values for Av are taken from measurements of complexation of the given alcohols with pyridine: (a) TFE, (b) methanol, (c) ethanol, (d) 1-butanol, (e) 2-propanol, and (f) terr-butanol.
of the alcohols is fairly independent of the nature of the base being utilized. It would therefore be possible to use the hydrogen bond strength sensitive solvatochromic property of SQ1 and its surfactant derivative, SQ2, to probe hydrogen bonding environments in a variety of homogeneous and heterogeneous systems.
Conclusion The photophysical properties of bis(benzothiazo1yidene) squaraines, SQ1 and SQ2, are independent of solvent polarity in aromatic hydrocarbon solvents. In alcoholic solvents, specific hydrogen bonding between the solvents and dyes leads to changes in the absorption and emission properties which are related to the hydrogen bond donating ability of the alcoholic solvents. These dyes can thus serve as excellent probes for monitoring hydrogen bonding environments. The photophysical properties of thin films and monolayers of the surfactant dye SQ2 are currently being investigated.
(1) Reichardt, C. Chem. SOC.Reu. 1992, 21, 147. (2) Buncel, E.; Rajagopal, S. Acc. Chem. Res. 1992, 23, 226. (3) Effenberger F.; Wurthner, F. Angew Chem. Int. Ed. Engl. 1993,32, 719. (4) Kalyanasundaram, K. In Photochemistry in Organized and Constrained Media; Ramamurthy, V., Ed.; VCH: New York, 1991; p 39. (5) Krasovitskii, B. M.; Bolton, B. M. Organic Luminescent Materials, translated by V. G. Vopian; VCH: Wcinheim, 1988. ( 6 ) Bigelow, R. W.; Freund, H. J. J . Chem. Phys. 1986, 107, 159. (7) Das, S.; Thomas, K. G.; George, M. V.; Kamat, P. V. J . Chem. Soc., Faraday Trans. 1991, 88, 3419. (8) Das, S.; De la Barre, B.; Thomas, K. G.; Ajayaghosh, A.; Kamat, P. V.; George, M. V. J. Phys. Chem. 1992, 96, 10327. (9) Law, K. Y. J. Phys. Chem. 1989, 93, 5925. (10) Kamat, P. V.; Das, S.;Thomas, K. George; George, M. V. J . Phys. Chem. 1992, 96, 195. (11) (a) Ebbesen, T. W. Reu. Sci. Instrum. 1988,59, 1307. (b) Kamat, P.V.; Ebbesen, T. W.; DimitrijeviC, N. M.; Nozik, A. J. Chem. Phys. Lett. 1989,157, 384. (12) Mills, W. H. J . Chem. Soc. 1922, 121,455. (13) Sprenger, H.-E.; Ziegebien, W. Angew Chem. Inr. Ed. Engl. 1967, 6 , 553. (14) Kamlet, M. J.; Abboud, J.-L. M.; Abraham, M. H.; Taft, R. W. J. Org. Chem. 1983, 48, 2871. (IS) Cramer, L. E.; Spears, K. G J . Am. Chem. SOC.1978, 100, 221. (16) Moog, R. S.: Burozski, N. A.; Desai, M.M.; Good, W. R.; Silvers, C. D.; Thompson, P. A.; Simon, J. D. J . Phys. Chem. 1991, 95, 8466. (17) Neerinck, D.; Lamberts, L. Bull SOC.Chem. Belg. 1966, 75, 473.
