J. Phys. Chem. B 2006, 110, 23783-23789
23783
PET Suppression of Acridinedione Dyes by Urea Derivatives in Water and Methanol R. Kumaran† and P. Ramamurthy*,†,‡ National Centre for Ultrafast Processes, UniVersity of Madras, Taramani, Campus, Chennai- 600 113, India, and Department of Inorganic Chemistry, UniVersity of Madras, Guindy Campus, Chennai- 600 025, India ReceiVed: May 9, 2006; In Final Form: September 12, 2006
Spectroscopic investigations involving the interaction of acridinedione dyes with urea and its derivatives in water and methanol were carried out by absorption, steady-state fluorescence, and time-resolved fluorescence measurements. The hydrogen-bonding properties of urea and derivatives in aqueous solutions are found to be distinctly different from those observed in methanol. Urea, which can serve both as a hydrogen bond donor as well as an acceptor and has a unique hydrogen-bonding feature, helps in studying urea interaction with fluorophores in aqueous solutions, micelles, and alcohol. In our studies, we have used acridinedione dyes as the probe. We report that the hydrophobic interaction of urea with dye predominates by weakening of the hydrogen-bonding interaction of the solvent and urea derivatives with increase in the hydrophobicity of urea derivatives. In methanol, the hydrogen bonding between solvent and urea derivatives predominating over the hydrophobicity of the urea derivatives is observed. The presence of alkyl group substitution in the N-H moiety with a function of increasing concentration resulting in the creation of a more favorable hydrophobic environment to the dye molecule to reside in the hydrophobic shell phase rather than in the bulk aqueous phase is illustrated. The hydrophobic interaction of dye with urea in aqueous solution predominates because of the weakening of the hydrogen bonding of the solvent and urea derivatives, and the photoinduced electron transfer (PET) process is used as a marker to identify the hydrophobic interaction illustrated in our studies.
Introduction The behavior of urea in solution is a very important topic in biological and environmental studies because of its involvement as a waste product in our daily life.1 The interactions of sparingly soluble solutes in aqueous solutions of urea are of contemporary interest at both the theoretical and practical level. Weak nonbonding interactions are important in many biological processes, and among these interactions occurring in aqueous solutions, the hydrophobic interactions are the most important driving force found in all biological processes.2 All biological processes and the formation of biological structures such as the folding of proteins by noncovalent interactions play a major role in the molecular recognition of biological molecules in water. Even though the properties of water and urea in aqueous solutions have been extensively studied, there exists a large variation in their behavior in physical and chemical properties of urea in liquid phase especially in aqueous solutions. There have been conflicting reports, considerable debate, and controversies which are still prevalent about the peculiar behavior of urea in aqueous solutions and its varying hydrogen-bonding properties exhibited in the solvents. Urea, which strongly interacts with the solvent, is still an area of considerable challenge for the chemists. Two such mechanisms were proposed to explain the role of urea in water: one mechanism depicts that urea acts as a structure breaker by breaking the water structure, whereas the other mechanism envisages that urea displaces some water molecules around a hydrophobic group and changes the solvation properties. Simulation studies, * Author to whom correspondence should be addressed. Tel: 091-4424925006. Fax: 091-44-24926709. E-mail:
[email protected]. † National Centre for Ultrafast Processes. ‡ Department of Inorganic Chemistry.
however, indicate that urea displaces water molecules in the neighborhood of the hydrophobic group, which favors the latter mechanism. Urea and related N-alkyl derivatives were considered as simple model compounds that exhibit strongly opposite features (hydrophilic and hydrophobic) and show the characteristic property of denaturing proteins in aqueous solutions to a varying extent.3 The interest on aqueous solutions of urea and its derivatives are mainly related to the denaturing properties of urea on proteins, and their involvement affects the hydrophobic and hydrophilic interactions determining the conformational stability of peptides and proteins which are dependent upon the concentration of urea and its derivatives. In our studies, the probe is largely hydrophobic in nature and the nature of interaction between the probe and urea derivatives in aqueous solution and methanol is revealed by fluorescence studies. Urea exists in planar and nonplanar geometries in solid and gaseous states, respectively.4 In aqueous solutions, it has been considered to exist in an intermediate state between these two phases. Carbonyl and N-H groups in urea act as hydrogen bond acceptor and donor, respectively, which provide different types of hydrogen-bonding arrangements such as urea-urea, ureawater, and water-water hydrogen-bonding interactions.5-7 The structures of various hydrogen-bonded urea-water complexes5 with the existence of hydrogen-bonded networks8,9 have been confirmed by experimental and molecular simulation methods.10-14 The high permittivity and viscosity of urea and its derivatives in aqueous solutions reveal that there exists strong hydrogen-bonding interactions,14 and urea can serve both as a hydrogen bond donor (through N-H) and as an acceptor (through CdO),5 which are very short-ranged and short-lived, and it changes on increasing or decreasing the concentration of urea derivatives in solution15 which is important in our studies. Apart from the solvent-urea hydrogen-bonding interactions, the
10.1021/jp0628378 CCC: $33.50 © 2006 American Chemical Society Published on Web 11/09/2006
23784 J. Phys. Chem. B, Vol. 110, No. 47, 2006 intermolecular hydrogen-bonding influences result in the variation in the chemical properties with increasing concentrations of urea and its derivatives and also in determining the relative stability of urea conformations.16,17 On the basis of Shellman,18 Kreschek and Scheraga,14 and Stokes19 (SKSS) model of urea in aqueous solutions, there exists a well-defined shell and bulk region in aqueous solution, supported by the molecular simulation studies by Kuharski and Rossky.11,12 Among the interactions involving urea in aqueous solutions, the urea-urea hydrogen bonding is stronger in the shell region compared to the bulk region and urea molecules are predominantly present in the shell region. The hydrogenbonding interaction of urea-urea in water plays an important role in formation of dimers16,20,21 and oligomers.17 As the concentration of urea increases, the framework and the dimension of the shell region increase with the displacement of water molecules from the shell region. The above-mentioned SKSS model is used in our present studies and provides us to explain the nature of interaction between acridinedione dyes and urea derivatives. Acridinedione dyes have biological importance because of their structural similarity with coenzyme NADH,22-24 and they have efficient lasing properties as compared to that of coumarin dyes. It has been observed that the spectral properties and the photophysics of the ADR dyes are largely influenced by the nature of substituents in the para position of the phenyl group. The following dyes (ADR1-4) were taken in our studies. We report the interaction of symmetrical and unsymmetrical alkyl urea derivatives with the ADR 1 dye in water and methanol by using steady-state and time-resolved fluorescence techniques. The interaction involving substituted 3H-indoles as the molecular probe25 with urea clearly reveals a hydrophobic interaction and also provides an insight into how the microviscosity and micropolarity vary in the absence and presence of urea. Likewise, the importance of hydrophobic interaction over the hydrogen-bonding interactions in the interaction of urea derivatives with acridinedione dye in aqueous solution is revealed in our studies.
Kumaran and Ramamurthy
Figure 1. Absorption spectra of ADR 1 dye-urea (U) in water. (1) 0.6 M U, (2) 1.2 M U, (3) dye alone, (4) dye + 0.6 M U, and (5) dye + 1.2 M U.
(EU), N,N-dimethylurea (DMU), and tetramethylurea (TMU) were purchased from Lancaster chemicals and were used as received. Methanol used in this investigation was of HPLC grade purchased from Qualigens India Ltd., and solutions of urea derivatives were prepared in triple distilled water. The concentration of urea and its derivatives involved in our studies was restricted up to 2.5 M because of the solubility, and also at very higher concentrations turbidity of the solutions poses problems in recording the absorption and emission spectra. The ADR dyes (1-4) were prepared by the procedure reported in the literature.23 Absorption spectra were recorded in Agilent 8453 diode array spectrophotometer. Fluorescence spectra were recorded in Perkin-Elmer MPF 44B Fluorescence spectrophotometer interfaced with PC through Rishcom-100 multimeter. Fluorescence decays were recorded in time-correlated single-photon-counting spectrometer with the following configuration. A diode-pumped millenia CW laser (Spectra Physics) 532 nm was used to pump the Ti:sapphire rod in Tsunami picosecond mode-locked laser system. (Spectra Physics). The 750 nm (82 MHz) was taken from the Ti:sapphire laser and was passed through pulse picker (Spectra Physics) to generate 4 MHz pulses. The flexible harmonic generator (FHG) generates the horizontally polarized 375-nm laser output, which was used to excite the sample. The fluorescence emission at the magic angle (54.7) was dispersed in a monochromator (f/3) aperture counted by an MCP-PMT (Hamamatsu R 3809) and proceeded through CFD-TAC and MCA. The instrument response function for this system is 52 ps. The fluorescence decay was analyzed by using the software provided by IBH (DASS-6) analysis software. Fluorescence quantum yield measurements were carried out using Perkin-Elmer MPF 44B fluorescence spectrophotometer interfaced with PC through Rishcom-100 multimeter. The quantum yield of the sample (φf) was obtained from the corrected fluorescence spectra using eq 1.
φf ) (Ar/As)(as/ar)(ns/nr)2 × 0.546
Experimental Methods Urea (U) (Molecular biology grade) was obtained from Merck chemicals and was used as such. Methyl urea (MU), ethyl urea
(1)
where As and Ar are absorption at the wavelength of excitation (366 nm), as and ar are the area under the corrected fluorescence spectrum, and ns and nr are the refractive indices of the solvent for the sample and the reference, respectively.The refractive index values of the solvents were taken from the literature.26-28 The absorbance value of the sample and the reference was adjusted to 0.02. The area under the spectra was obtained using
PET Suppression of Acridinedione Dyes by Urea
J. Phys. Chem. B, Vol. 110, No. 47, 2006 23785
Figure 3. Extent of fluorescence enhancement in the fluorescence intensity by steady-state measurements of ADR 1 dye with urea and alkyl urea derivatives in (a) water and (b) methanol. (0, U; ], DMU; 4, TMU; *, MU; +, EU.)
Figure 2. (a) Emission spectra of ADR 1 dye in different solvents. (1) Toluene, (2) dichloromethane, (3) chloroform, (4) acetonitrile, (5) ethanol, (6) methanol, and (7) water. (b) Emission spectra of ADR 1 dye in the absence and presence of urea in water. (1) 0.0 M, (2) 0.3 M, (3) 0.6 M, (4) 0.9 M, (5) 1.2 M, (6) 1.8 M, (7) 2.1 M, (8) 2.4 M, and (9) 3.0 M. (c) Emission spectra of ADR 1 dye in the absence and presence of TMU in water. (1) 0.0 M, (2) 0.6 M, (3) 1.2 M, (4) 1.8 M, and (5) 2.4 M.
Microsoft origin program. Quinine sulfate solution was prepared using the 0.1 N sulfuric acid and was used as the reference for quantum yield determination (φf of quinine sulfate ) 0.546). Results and Discussion Absorption Spectral Studies. The absorption spectrum of ADR 1 dye shows a maximum at 377 and 370 nm in water and methanol, respectively. This longest wavelength absorption has been assigned to the intramolecular charge transfer (ICT) from the nitrogen to ring carbonyl group.23 There is no change in absorbance at the longest wavelength absorption maximum of ADR 1 dye (which is the excitation wavelength) in water on the addition of urea and alkyl-substituted urea derivatives
(including symmetrical and unsymmetrical). We observe a change in the absorption spectra (in the spectral range of 200300 nm) on the gradual addition of increasing concentration of urea, which is attributed to the absorption of urea as shown in Figure 1. Steady-State Emission Spectral Studies. On excitation at the ICT, absorption results in an emission spectrum with the maximum at 436 and 430 nm in water and methanol, respectively. A red shift in the emission maximum on increasing the solvent polarity was observed as shown in Figure 2a. An enhancement in the fluorescence intensity is observed on addition of urea and its alkyl derivatives, and the position of emission maxima of ADR 1 dye remains the same even after the addition of high concentration of urea as in Figure 2b. The addition of TMU, which is more hydrophobic in nature as compared to all the other urea derivatives, results in a fluorescence enhancement, and the emission maxima remain unaltered as shown in Figure 2c. The enhancement in the fluorescence intensity involving ADR 1 dye and urea derivatives is due to the suppression of photoinduced electron transfer (PET) by urea derivatives only and is not due to the change in the polarity of the medium. This is confirmed by taking a nonPET dye ADR 2 where in the emission spectra on the addition of urea to dye shows no enhancement in the fluorescence intensity even at higher concentrations (3 M) which clearly rules out the possibility of polarity factor playing a role in the fluorescence enhancement. No fluorescence enhancement was observed on addition of TMU to ADR 2 dye which reveals that there is an excited-state interaction involving PET based ADR 1 dye with urea derivatives, which was further confirmed by time-resolved fluorescence studies. An interesting observation was revealed in water and in methanol where the extent of fluorescence enhancement was found to be different. The extent of fluorescence enhancement in water increases with increasing order of hydrophobicity of the urea derivatives
23786 J. Phys. Chem. B, Vol. 110, No. 47, 2006
Kumaran and Ramamurthy
Figure 4. Fluorescence decay properties of ADR 1 dye in the absence and presence of urea in water. (1) Laser profile, (2) 0.0 M U, (3) 1.2 M U, (4) 2.4M U, (5) 4.8 M U, (6) 9.6 M U, and (7) 10.9 M U.
Figure 7. Extent of fluorescence enhancement in the fluorescence lifetime of ADR 1 dye on the addition of urea and alkyl urea derivatives in (a) water and (b) methanol (0, U; ], DMU; 4, TMU; *, MU; +, EU.)
Figure 5. Fluorescence decay properties of ADR 1 dye in the absence and presence of symmetrical alkyl urea derivatives in water. (1) Laser profile, (2) ADR 1 dye, (3) 2.4 M U, (4) 2.4 M DMU, and (5) 2.4 M TMU.
Figure 6. Fluorescence decay properties of ADR 1 dye in the absence and presence of unsymmetrical alkyl urea derivatives in water. (1) Laser profile, (2) ADR 1 dye, (3) 2.4 M U, (4) 2.4 M MU, and (5) 2.4 M EU.
which is of the order (U < MU < DMU < EU < TMU), whereas in methanol the extent of enhancement is found to be quite similar for all the urea derivatives, which is clearly illustrated in Figure 3. PET and Fluorescence Quantum Yield. The experimentally determined fluorescence quantum yield (φf) is found to be 0.12,
0.90, 0.84, and 0.92, respectively, for ADR 1, ADR 2, ADR 3, and ADR 4 dyes in methanol. It is evident that the quantum yield of ADR 1 dye is very less compared to that of ADR 2, ADR 3, and ADR 4 dyes which is due to the photoinduced intramolecular electron transfer (PET) from the electrondeficient excited state of the acridinedione fluorophore in ADR 1 dye.23 The presence of donor moiety in the para position of the acridinedione structure of ADR 1 dye has resulted in the low fluorescence quantum yield in comparison with that of ADR 2 dye. Time-Resolved Fluorescence Lifetime Studies. ADR 1 dye shows the fluorescence lifetime as 500 ( 10 and 390 ( 10 ps in water and methanol, respectively. The fluorescence lifetime increases from 500 ps to 900 ps in aqueous solution and from 390 ps to 690 ps in methanol on the addition of urea as shown in Figure 4 and in Tables 1 and 2. In the presence of symmetrical and unsymmetrical alkyl urea derivatives, fluorescence lifetime decay pattern of ADR 1 dye shows biexponential behavior in both water and methanol. The addition of symmetrical (except TMU) and unsymmetrical alkyl urea derivatives to ADR 1 dye in aqueous solution and in methanol also results in an increase in the longer component fluorescence lifetime of the dye as shown in Figures 5 and 6. The residuals shown along with the decay (provided in the Supporting Information) are well within the error limits. The χ2 values are given in Tables 1 and 2. The extent of enhancement of fluorescence lifetime of the ADR 1 dye on addition of urea and its derivatives in aqueous solution is of the increasing order of hydrophobicity of the urea derivatives as shown in Figure 7 which is similar to the extent of fluorescence intensity enhancement. The fluorescence lifetime and the amplitude of the shorter component is more pronounced upon the addition of TMU both in water and methanol. The fluorescence lifetime of ADR 2-4 dyes, which do not have
PET Suppression of Acridinedione Dyes by Urea
J. Phys. Chem. B, Vol. 110, No. 47, 2006 23787
SCHEME 1: A Model for Mode of Interaction of ADR 1 Dye with Urea Derivatives in Watera
a (a) One urea molecule surrounded by 12-13 water molecules in the shell phase. (b) Increasing concentration of urea molecules displaces water from the shell region to the bulk phase. (c) The presence of more number of urea molecules in the shell phase paves way in creating a more hydrophobic environment in the shell phase than the bulk phase. (d) The ADR dye molecules are confined to the shell region in aqueous urea solution.
TABLE 1: Fluorescence Lifetime of ADR 1 Dye on the Addition of Urea and Its Derivatives in Water urea type U
DMU
TMU
MU
EU
conc (M)
τ1 (ns)
0.6 1.2 1.8 2.4 0.6 1.2 1.8 2.4 0.6 1.2 1.8 2.4 0.6 1.2 1.8 2.4 0.6 1.2 1.8 2.4
0.56 0.59 0.65 0.69 0.66 0.73 0.75 0.83 0.64 0.88 1.11 1.33 0.57 0.62 0.69 0.75 0.57 0.69 0.76 0.83
τ2 (ns)
B1
3.18 4.54 3.18 3.20 3.12 6.90 8.67 9.70 4.31 4.38 4.64 5.01 4.36 5.12 5.23 4.94
100 100 100 100 98.30 98.60 98.30 98.40 97.80 96.40 94.20 92.30 93.00 91.00 89.00 88.00 98.00 97.00 96.00 96.00
TABLE 2: Fluorescence Lifetime of ADR 1 Dye on the Addition of Urea and Its Derivatives in Methanol
B2
χ2
1.70 1.40 1.70 1.60 2.20 3.60 5.80 7.70 7.00 9.00 11.00 12.00 2.00 3.00 4.00 4.00
1.24 1.21 1.21 1.19 1.21 1.22 1.21 1.30 1.26 1.15 1.28 1.07 1.13 1.22 1.15 1.19 1.00 1.05 1.06 1.19
the PET, is not influenced by the addition of urea and alkyl urea derivatives. Alkyl Urea Behavior in Aqueous Solutions. Substitution by an alkyl group in urea changes the hydrogen-bonding character substantially, leading to an increase in the hydrophobic nature and a decrease in the hydrogen-bonding properties.29,30 The only possible hydrogen-bonding formation of TMU-water is through a carbonyl group. A well-defined hydrogen-bonding
urea type U
DMU
TMU
MU
EU
conc (M)
τ1 (ns)
0.6 1.2 1.8 2.4 0.6 1.2 1.8 2.4 0.6 1.2 1.8 2.4 0.6 1.2 1.8 2.4 0.6 1.2 1.8 2.4
0.48 0.62 0.68 0.72 0.42 0.50 0.54 0.62 0.46 0.49 0.53 0.59 0.52 0.60 0.68 0.75 0.43 0.51 0.64 0.74
τ2 (ns)
B1
3.70 3.80 3.30 3.90 5.35 5.20 5.65 5.85 4.20 4.30 5.00 5.30 3.40 3.40 3.30 3.40
100 100 100 100 97.50 97.50 97.50 98.00 95.00 89.00 87.00 85.00 94.50 92.00 88.50 87.50 97.00 96.00 96.00 96.00
B2
χ2
2.50 2.50 2.50 2.00 5.00 11.00 13.00 15.00 5.50 8.00 11.50 12.50 3.00 3.00 4.00 4.00
1.30 1.27 1.17 1.22 1.19 1.29 1.16 1.05 1.14 1.23 1.22 1.17 1.18 1.21 1.18 1.16 1.27 1.27 1.06 1.16
structure of TMU-water exists at lower concentrations,31,32 and further increase in the concentration of TMU results in the breaking up of the water-water hydrogen bonding in the shell region and the formation of TMU-water hydrogen bonding. This results in the creation of a more hydrophobic environment by TMU in the shell region by displacing several water molecules from the shell region to the bulk region11,12,33-35 than urea.
23788 J. Phys. Chem. B, Vol. 110, No. 47, 2006
Kumaran and Ramamurthy
SCHEME 2: A Model for Mode of Interaction of ADR 1 Dye with Urea and Its Derivatives in Methanola
a (a) Addition of urea molecule in methanol. (b) Uniform hydrogen-bonding interaction between urea-methanol throughout the phase. No distinct formation of bulk phase and shell phase as in water. (c) Hydrophobic environment in methanol by urea and its derivatives is suppressed by uniform urea-methanol hydrogen bonding. (d) The ADR dye molecules are confined throughout the phase as there is no preferred position for the dye molecule to reside.
Urea and Alkyl Derivatives Interaction in Methanol. In urea, CdO and N-H act as a hydrogen acceptor and donor, respectively, in methanol as water. In the case of TMU, both CdO and N-R2 act as hydrogen acceptors in methanol. It has been demonstrated that the alkyl groups on nitrogen hinder the hydrogen bond formation between TMU nitrogen and hydrogen in water36 and favor hydrogen bonding between TMU nitrogen and OH-hydrogen in methanol.37 This leads to a variation in the hydrogen-bonding properties of urea and urea alkyl derivatives in methanol and water. Because of the R2N‚‚‚HO-CH3 hydrogen bonding in TMU, methanol is not expelled as observed in the case of water, and hence no distinct shell and bulk region is established. Interaction of ADR 1 Dye with Urea Derivatives in Water and Methanol. We have recently reported that there is an interaction with urea dimers with acridinedione dyes in methanol,24 which results in the enhancement of fluorescence intensity and fluorescence lifetime. Subsequently, we have identified the presence of photoinduced electron transfer (PET) through space in this family of dyes.38 Discussion in the earlier paragraphs clearly illustrates the presence of a clear shell and bulk regions, which are predominant in urea and water as represented in Scheme 1. ADR dyes predominantly reside in the shell region because of its hydrophobic nature. ADR 1 dye alone shows fluorescence enhancement while the other dyes do not have any change in the fluorescence intensity as well as the fluorescence lifetime. It is due to the presence of the PET only in the case of ADR 1. We also recently39 observed that the inclusion of this dye in β-CD through hydrophobic interaction results in the suppression of PET, which enhances the fluorescence intensity
and fluorescence lifetime. The hydrophobic nature of the interaction between the urea derivatives and ADR 1 dye is further supported by the observation of higher fluorescence enhancement and fluorescence lifetime as the hydrophobicity of the urea derivative increases. In the case of methanol, no distinct shell region is possible and a uniform hydrogen bonding exists as given in Scheme 2. The hydrophobicity of the urea derivatives in methanol is not as pronounced as in water. ADR 1 dye interacts hydrophobically with urea derivatives as illustrated by the results observed in the fluorescence intensity and lifetime. The possibility of hydrogen-bonding interaction between the dye and urea derivatives is ruled out by the observation of enhancement even in the case of tetramethylurea, in which free amino hydrogen is absent. The enhancement in the fluorescence intensity and lifetime in methanol remains more or less the same as we increase the hydrophobicity of the urea derivative in contrast to that observed in water. This is due to the nonexistence of clear shell region in methanol. Conclusion The enhancement in fluorescence intensity by the suppression of PET of ADR 1 dye through space by urea and its derivatives was confirmed by enhancement in fluorescence intensity and the increase in the lifetime of the dye, whereas the interaction of ADR 2-4 dyes resulted in no change in the fluorescence intensity even after the addition of urea and its derivatives. The above observations confirm that the presence of electrondonating methoxy group has resulted in the fluorescence
PET Suppression of Acridinedione Dyes by Urea enhancement by suppression of the charge transfer from the donor to the acceptor moiety of the ADR 1 dye by urea derivatives in water and in methanol. The fluorescence enhancement on the addition of urea derivatives with the replacement of N-H by methyl groups in aqueous solution is attributed to the creation of hydrophobic environment resulting in displacement of water molecules in the shell region. The hydrophobic interaction of urea-dye in aqueous solution predominates because of the weakening of the hydrogen-bonding interactions of the solvent with urea molecules. In methanol, the hydrogen bonding of the solvent-urea molecules influences a larger role by lessening the hydrophobic nature of the urea derivatives and resulting in a less hydrophobic environment around the dye molecule. Acknowledgment. Financial support by DST-IRHPA and UGC-INNOVATIVE Program. R.K thanks the UGC-INNOVATIVE for providing financial assistance. Supporting Information Available: Residual plots for Figures 4-6. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Calvaruso, G.; Minore, A.; Turco Liveri, V. J. Colloid Interface Sci. 2001, 243, 227-232. (2) Castronuovo, G.; Dario, R. P.; Della Volpe, C.; Elia, V. Thermochim. Acta 1992, 206, 43-54. (3) Ferloni, P.; Gatta, G. D. Thermochim. Acta 1995, 266, 203-212. (4) Keuleers, R.; Desseyn, H. O.; Rousseau, B.; Van Alsenoy, C. J. Phys. Chem A 1999, 103, 4621-4630. (5) Bartkowiak, W.; Zalesny, R.; Kowal, M.; Leszcynski, J. Chem. Phys. Lett. 2002, 362, 224-228. (6) Hoccart, X.; Turrell, G. J. Chem. Phys. 1999, 99, 8498-8503. (7) Lee, C.; Stahlberg, E. A.; Fitzgerald, G. J. Phys. Chem. 1995, 99, 17737-17741. (8) Masunov, A.; Dannenburg, J. J. J. Phys. Chem. A 1999, 103, 178184. (9) Masunov, A.; Dannenburg, J. J. J. Phys. Chem. B 2000, 104, 806810. (10) Godfrey, P. D.; Brown, R. D. J. Mol. Spectrosc. 1997, 405, 413414. (11) Kuharski, R. A.; Rossky, P. J. J. Am. Chem. Soc. 1984, 106, 57865793.
J. Phys. Chem. B, Vol. 110, No. 47, 2006 23789 (12) Kuharski, R. A.; Rossky, P. J. J. Am. Chem. Soc. 1984, 106, 57945800. (13) Hoccart, X.; Turrell, G. J. Chem. Phys. 1999, 99, 8498-8503. (14) Kreschek, G. C.; Scherega, H. A. J. Phys. Chem. 1965, 69, 17041706. (15) Tanaka, H.; Touhara, H.; Nakanishi, K.; Watanaba, N. J. Chem. Phys. 1984, 80, 5170-5186. (16) Lee, C.; Stahlberg, E, A.; Fitzgerald, G. J. Phys. Chem. 1995, 99, 17737-17741. (17) Grdadolnik, J.; Marechal, Y. J. Mol. Struct. 2002, 615, 177-189. (18) Schellman, J. A. Comput. Rend. Trav Lab. Carlsberg, C. R. Ser. Chim. 1955, 29, 223-229. (19) Stokes, R. H. Aust. J. Chem. 1967, 20, 2067-2100. (20) Tanaka, H.; Nakanishi, K.; Touhara, H. J. Chem. Phys. 1985, 82, 5184-5191. (21) Astrand, P. O.; Wallqvist, A.; Karlstrom, G. J. Phys. Chem. 1994, 98, 8224-8233. (22) Singh, S.; Chinna, S.; Sharma, V. K.; Sachdev, S. S. J. Chem. Soc., Chem. Commun. 1982, 8, 453-459. (23) Srividya, N.; Ramamurthy, P.; Shanmugasundaram, P.; Ramakrishnan, V. T. J. Org. Chem. 1996, 61, 5083-5089. (24) Selvaraju, C.; Thiagarajan, V.; Ramamurthy, P. Chem. Phys. Lett. 2003, 379, 437-442. (25) Shen, X.; Belletele, M.; Durocher, G. J. Phys. Chem. B 1997, 101, 8212-8220. (26) Lakowicz, J. R. Principles of fluorescence spectroscopy, 2nd ed.; Plenum: New York, 1999. (27) Beardsley, K.; Cantor, C. R. Proc. Natl. Acad. Sci. U.S.A. 1970, 65, 39-46. (28) Handbook for Chemistry and Physics, 79th ed.; CRC Press LLC, 2000. (29) Fing, Y.; Zhi-Wu, Yu.; Quinn, P. J. Chem. Phys. Lipids 2002, 114, 149-157. (30) Rao, C. N. R.; Chaturvedi, G. C.; Gosavi, C. R. K. J. Mol Spectrosc. 1968, 28, 526-535. (31) Maharajah, D. M. J. Chem. Eng. Data 1980, 25, 384-386. (32) Okpala, C.; Maharajah, D. M. J. Chem. Eng. Data 1981, 26, 378381. (33) Ikeguchi M.; Nakamura, S.; Shimizu, K. J. Am. Chem. Soc. 2001, 123 677-682. (34) Ansell, S.; Cser, L.; Grosz, T.; Jansco, G.; Jovari, P.; Soper, A. K. Physica B 1997, 234-236, 347-348. (35) Sarkar, N.; Das, K; Nath, D.; Bhattacharya, K. Chem. Phys. Lett. 1992, 196, 491-496. (36) Belletato, P.; Freitas, L. C. G.; Areas, E. P. G.; Santos, P. S. Phys. Chem. Chem. Phys. 1999, 1, 4769-4776. (37) Harnaega, E. I.; Jagodzinski, P. W. Vib. Spectrosc. 1996, 10, 169175. (38) Thiagarajan, V.; Selvaraju, C.; Padma Malar, E. J.; Ramamurthy, P. Chem. Phys. Chem. 2004, 5, 1200-1209. (39) Thiagarajan, V.; Indirapriyadarshini, V. K.; Ramamurthy, P. J. Inclusion Phenom. Macrocyclic Chem., 2006, DOI: 10.1007/S10847-0059043-4.
