17310
J. Phys. Chem. 1996, 100, 17310-17315
Aggregation Behavior of Water Soluble Bis(benzothiazolylidene)squaraine Derivatives in Aqueous Media† Suresh Das,* K. George Thomas,* K. J. Thomas, and V. Madhavan Photochemistry Research Unit, Regional Research Laboratory (CSIR), TriVandrum 695 019, India
D. Liu, Prashant V. Kamat,*,§ and M. V. George*,‡ Radiation Laboratory, UniVersity of Notre Dame, Notre Dame, Indiana 46556 ReceiVed: June 24, 1996; In Final Form: August 15, 1996X
Two new water soluble squaraine dyes, bis[3-(p-carboxybenzyl)benzothiazol-2-ylidene]squaraine (Sq1) and bis[3-(carboxymethyl)benzothiazol-2-ylidene]squaraine (Sq2), have been synthesized and their photophysical properties have been characterized. Sq1 and Sq2 form dimer aggregates in water that have absorption bands blue-shifted to those of the monomeric forms. Aggregate formation is more preferred in D2O than in H2O. In the presence of low concentrations (3 × 10-4 M) of PVP leads to the disruption of the aggregate and formation of a new species with absorption bands red-shifted to those of the corresponding monomers. The nature of these interactions has been investigated. It is proposed that hydrophobic interaction between the chromophoric units is the major driving force for the formation of the H-type (sandwich) aggregates in water. The red-shifted species are attributed to the monomeric forms, microencapsulated in a hydrophobic environment provided by PVP. Picosecond laser flash photolysis studies of the aggregates show clear evidence for the breakup of the aggregate from the excited state to yield an excited state-ground state monomer pair that rapidly recombines to reform the ground state dimer.
Introduction Squaraines are a class of visible and near-infrared absorbing dyes that have been extensively investigated in recent years for their sensitizing,1-3 conducting,4 and photoconducting5-7 properties. The nonlinear optical properties of symmetric8 and unsymmetric squaraines9 have also attracted significant attention. The photochemical and photophysical properties of squaraines can be conveniently modified by substitutional changes or by changing the nature of the medium.10-13 It has been shown that microencapsulation of some of these dyes by β-cyclodextrin11 or poly(vinylpyridine)12 brings about significant enhancement in their fluorescence yields by effectively controlling the nonradiative processes. An interesting feature of squaraine dyes is that their sharp and intense absorption observed in solution becomes broad and red-shifted in the solid state.13 These effects have been attributed to intermolecular charge transfer transitions. Ashwell et al. have recently reported that Langmuir-Blodgett films made of some centrosymmetric squaraine dyes are capable of second harmonic generation8 and have proposed that these effects arise owing to the formation of noncentrosymmetric aggregates. There have been several efforts to study the nature of the squaraine aggregates as well as the driving forces involved in the aggregation process.14-20 Some bis(hydroxyphenyl)squaraines were observed to form head-to-tail (J-type) aggregates due to intermolecular hydrogen bonding.14 Complexation with iodine14 and titanium dioxide15 brought about an enhancement in the aggregate formation. In a detailed study on the aggregation behavior of some surfactant squaraine dyes in Langmuir-Blodgett films and in DMSO-water mixtures, Liang et al. observed the formation of three types of aggregates † Dedicated to Professor Robert H. Schuler on the occasion of his 70th birthday. ‡ Also at Regional Research Laboratory, Trivandrum. § e-mail:
[email protected] and http://www.nd.edu:80/∼pkamat. X Abstract published in AdVance ACS Abstracts, October 1, 1996.
S0022-3654(96)01885-0 CCC: $12.00
with absorption bands either blue- or red-shifted from those of the monomer spectra.16 They observed that the red-shifted aggregates in the LB film could be converted partially to the blue-shifted aggregate by exposure to steam. Both hydrophobic interactions between the hydrocarbon chains and intermolecular charge transfer interactions between the donor-acceptor groups were proposed to be the driving forces behind the aggregation process. Buncel et al. have also reported the formation of blueshifted aggregates of some squaraines in DMSO-water mixtures when the percentage of water in the mixture was increased.19 Since water plays an important role in the aggregation behavior of squaraines, we have synthesized two new water soluble squaraine dyes Sq1 and Sq2 (Chart 1). Here, we report on the photophysical properties and aggregation behavior of Sq1 and Sq2 in purely aqueous media. The effect of adding poly(vinylpyrrolidone) and β-cyclodextrin on the photophysical and aggregation properties of Sq1 and Sq2 are also reported. Experimental Section 3-(p-Carboxybenzyl)-2-methylbenzothiazolium bromide and 3-(carboxymethyl)-2-methylbenzothiazolium bromide were synthesized by reacting 2-methylbenzothiazole with p-bromomethylbenzoic acid and bromoacetic acid, respectively, adopting methods similar to the reported procedures.21 Synthesis of Sq1 and Sq2. Sq1 and Sq2 were synthesized by refluxing the appropriate methylbenzothiazolium bromide (2 mmol) and squaric acid (1 mmol) in a solvent mixture containing 5 mL of benzene and 7 mL of n-butanol in the presence of quinoline (2 g, 15.5 mmol) accompanied by azeotropic distillation of water. Purification by repeated washings with boiling methanol gave a 10% yield of bis[3-(p-carboxybenzyl)benzothiazol-2-ylidene]squaraine, Sq1, mp 315 °C (decomp). Sq2 was purified by repeated washings with chloroform followed by methanol to give a 20% yield of a pure sample, mp 235 °C. © 1996 American Chemical Society
Aggregation in Aqueous Media
J. Phys. Chem., Vol. 100, No. 43, 1996 17311
CHART 1
Sq1. IR, νmax (KBr): 3600-3200 (CO2H), 1707 (CO), 1550, 1460, 1428, 1242 cm-1. UV, λmax (H2O, pH ) 8): 645 nm ( ) 190 000 M-1 cm-1). 1H NMR (DMSO-d6): δ 5.66 (4H, s, CH2), 5.76 (2H, s, CH), 7.23-7.96 (16H, m, aromatic). Anal. Calcd for C36H24N2S2O6: C, 67.07; H, 3.75; N, 4.34; S, 9.94. Found: C, 66.95; H, 3.93; N, 4.33; S, 9.69. MW calcd for C36H25N2S2O6 (M + H)+: 645.1154. Found: 645.1131 (highresolution mass spectrometry, FAB). Sq2. IR, νmax (KBr): 3600-3200 (CO2H), 1734, 1658, 1565, 1459, 1430, 1351, 1262 cm-1. UV, λmax (H2O, pH ) 8): 641 nm ( ) 170 000 M-1 cm-1). 1H NMR (DMSO-d6): δ 5.12 (4H, s, CH2), 5.67 (2H, s, CH), 7.23-7.87 (8H, m, aromatic). Anal. Calcd for C24H16N2S2O6: C, 58.53; H, 3.27; N, 5.69; S, 13.02. Found: C, 58.10; H, 3.66; N, 5.60; S, 12.77. MW Calcd for C24H17N2S2O6 (M + H)+: 493.0528. Found: 493.0522 (high-resolution mass spectrometry, FAB). Poly(vinylpyrrolidone) (MW ≈ 44 000), obtained from S. D. Fine Chemicals (Bombay, India), was further purified by dissolving it in chloroform and reprecipitating with hexane followed by repeated washings with hexane. All melting points are uncorrected and were determined on an Aldrich MEL-TEMP apparatus. IR spectra were recorded on a Perkin-Elmer Model 882 IR spectrometer and the UVvisible spectra on a Shimadzu 2100 spectrometer or on a GBC UV-visible 918 spectrophotometer. 1H NMR spectra were recorded on a Varian 300 MHz FT-NMR spectrometer. Mass spectra were recorded on a JEOL AX 505 HA mass spectrometer. Emission spectra were recorded on a SPEX fluorolog F112X spectrofluorometer. Quantum yields of fluorescence were measured by the relative method using optically dilute solutions with bis[4-(dimethylamino)-2-hydroxyphenyl]squaraine (Φf ) 0.84)22 as reference. The changes in absorptivity on addition of PVP have been taken into account for all quantum yield measurements, and the solutions were excited at 590 nm where these changes are minimal. Polymer concentrations are expressed in terms of monomer units. All the experiments in aqueous solutions were carried out at pH 8.0, which is obtained by adding the appropriate amount of KOH Picosecond laser flash photolysis experiments were performed with either 355 or 532 nm laser pulses from a mode-locked, Q-switched Quantel YG-501 DP Nd:YAG laser system (output 1.5-2.5 mJ/pulse, pulse width ∼18 ps). The white continuum picosecond probe pulse was generated by passing the fundamental output through a D2O/H2O solution. 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 IBMAT computer. Time zero in these experiments corresponds to the end of the excitation pulse. All the lifetimes and rate constants reported in this study have an experimental error of (5%. Because of the instability of the dye to laser excitation, the dye solution was continuously flowed through the sample cell. Results and Discussion Absorption and Emission Spectra. In water at very low concentrations, Sq1 shows a broad absorption in the range 500-
Figure 1. Effect of pH on the absorption spectrum of Sq1 (1.4 µM) in aqueous medium at 300 K at pH 5.93 (a), 6.11 (b), 6.18 (c), 6.37 (d), and 6.72 (e). Inset shows the plot of absorbance at 645 nm vs pH.
TABLE 1: Absorption and Emission Properties of Sq1 and Sq2 in Different Solvents Sq1
solvent
absorpt max nm
emiss max nm
watera methanola trifluoroethanola methoxyethanola DMSO DMF
645 656 642 663 679 678
659 673 657 679 702 693
a
Sq2
Φf
absorpt max nm
emiss max nm
Φf
0.027 0.17 0.19 0.38 0.32 0.28
641 650 640 659 667 676
658 667 657 675 687 691
0.03 0.12 0.10 0.20 0.14 0.16
In the presence of KOH (∼10-4 M).
750 nm with a maximum at ∼565 nm. A similar broad band is observed for Sq2, although the maximum is considerably redshifted to 670 nm. Addition of base to these solutions deprotonates the dye with the appearance of a sharp band with maxima at 645 and 641 for Sq1 and Sq2, respectively. The spectral changes are marked by the presence of clear isosbestic points (Figure 1). A plot of absorbance at 645 nm vs pH indicated a single acid-base equilibrium with a pKa of 6.4 for Sq1 (inset of Figure 1), whereas two distinct equilibria with pKa values of 4.4 and 6.7 were observed for Sq2. The pKa of 6.4 of Sq1 probably corresponds to deprotonation at the ring nitrogen, since pKa for deprotonation of the carboxylic groups would not be expected to be much different from 4.2 reported for benzoic acid. The pKa values of 4.4 and 6.7 observed for Sq2 correspond to the protonation equilibria at the carboxylic group and ring nitrogen, respectively. The absorption and emission maxima and quantum yield of fluorescence (Φf) of Sq1 and Sq2 in different solvents are listed in Table 1. In protic solvents, hypsochromic shifts in the absorption and emission maxima and a decrease in Φf with increasing hydrogen-bonding ability of the solvent were observed that are similar to those reported for some bis(benzothiazolylidene)squaraines.23 These effects have been attributed to hydrogen bonding of the O atoms of the central cyclobutane ring with solvent molecules. Aggregation of Sq1 and Sq2 in Water. Figure 2 shows the absorption spectra recorded at different concentrations of Sq1 and Sq2 in water. For Sq1, two absorption bands with maxima at 595 and 645 nm are observed. At low dye concentrations (
