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J. Phys. Chem. B 2009, 113, 6826–6833
Photophysical Studies of a Hemicyanine Dye (LDS-698) in Dioxane-Water Mixture, in Different Alcohols, and in a Room Temperature Ionic Liquid Debabrata Seth,† Souravi Sarkar, Rajib Pramanik, Chiranjib Ghatak, Palash Setua, and Nilmoni Sarkar* Department of Chemistry, Indian Institute of Technology, Kharagpur 721 302, West Bengal, India ReceiVed: NoVember 14, 2008; ReVised Manuscript ReceiVed: March 17, 2009
In this paper, we have investigated the photophysics of 1-ethyl-2-(4-(p-dimethylaminophenyl)-1,3-butadienyl)pyridinium perchlorate (LDS-698) molecule in dioxane-water mixture and in a room temperature ionic liquid (RTIL) 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]). Both photoisomerization and twisted intramolecular charge transfer (TICT) are possible in this molecule. We have shown that TICT is favorable in this molecule in the dioxane-water mixture and in neat [bmim][BF4]. The change in absorption energy of LDS-698 molecule with mole fraction of water is nonlinear in these systems. The Stern-Volmer plot also deviates from linearity in these systems. This nonlinearity is due to the specific solvation of water molecules in the mixture. 1. Introduction
SCHEME 1: 1-Ethyl-2-(4-(p-dimethylaminophenyl)-1,3butadienyl)pyridinium Perchlorate [LDS-698]
Aromatic compounds containing electron donor and acceptor groups in the same system often exhibit solvent-dependent emission properties due to charge-transfer process from donor to acceptor in the excited state. Fluorescent amphiphilic hemicyanine dyes contain donor and acceptor groups in the same molecule linked by different number of methylene groups. Hemicyanine dyes are used as molecular probes to detect fast changing of membrane voltage in neurons.1-5 When these dyes are excited to the singlet excited state, charge transfer from the amine group to the pyridinium moiety has taken place.6 This results in a great change in dipole moment between ground and excited state. It is one of the most suitable molecules for the second harmonic generation (SHG).6-9 The zwitterionic hemicyanine dyes consist of a positively charged chromophore and a negative counterion. These dyes exhibit a symmetrical solvatochromic shift in polar solvent. With increase in polarity of the solvent absorption peak is blue-shifted and emission peak is shifted to the red due to displacement of positive charge from pyridinium to the aniline moiety.10 Room temperature ionic liquids (RTILs) are composed solely of ions and are liquids in room temperature. They are nonflammable and nonvolatile compared to volatile organic compounds (VOCs) and their properties can be tuned by selective combination of cations and anions to suit a particular application. For these reasons, they are used as green substitutes for VOCs in many organic reactions and also have many other innovative applications.11-15 Polarities of RTILs are close to those of shortchain alcohols.16 Several physical and photophysical studies were carried out in neat RTILs.17-22 There were several studies on solvation dynamics in neat RTILs19-22 after the first report of Samanta and co-workers.19a,b The dynamics of these photophyscial studies are widely different from those in other * To whom correspondence should be addressed. E-mail: nilmoni@ chem.iitkgp.ernet.in. Fax: 91-3222-255303. † Present address: Indian Institute of Technology Patna, Navin Government Polytechnic Campus, Patliputra Colony, Patna 800 013, Bihar, India.
conventional solvents due to some unique properties such as the presence of structural and dynamical heterogeneity in RTILs.23,24 We use 1-ethyl-2-(4-(p-dimethylaminophenyl)-1,3-butadienyl)pyridinium perchlorate [LDS-698] as a hemicyanine dye for our experiment. This molecule has the electron donor -N(Me)2 group on one end and electron acceptor ethylpyridinium on the other end which favor charge transfer in the excited state. Lippert first reported that p-(dimethylamino)benzonitrile (DMABN) exhibits dual emission band in polar solvent and single emission band in nonpolar solvent.25 The emission at red wavelength is due to twisted intramolecular charge transfer (TICT) state. The TICT process involves twisting of the dialkylamino part relative to the rest of the molecule along with charge transfer.25-31 Since TICT state is more polar than ground state, so with increase in polarity of the medium, the TICT state is more stabilized by solvation of the polar solvent molecule and we observed a red shift of the low-energy emission band. The role of specific hydrogen bonding between solvent and electron donor and acceptor species to stabilize TICT conformer was also investigated.32-35 The photophysical properties of different hemicyanine dyes were also studied.36-38 The structure of LDS-698 molecule is shown in Scheme 1. There is a certain probability of TICT occurring in this molecule. The rotation around the C-N(CH3)2 bond and pyridyl ring can form TICT state in this molecule. Cao et al.41 showed that energy barrier to the rotation of pyridyl ring is smaller in the first excited state than rotation of the -N(CH3)2 group. They used 4-[2-(4dimethylaminophenyl)ethenyl]-1-methylpyridinium iodide (HR) as a solvatochromic dye. The structure of HR is very similar to LDS-698 with a difference in the number of methylene groups joining the aniline and pyridyl ring. So it is expected that LDS-
10.1021/jp810045h CCC: $40.75 2009 American Chemical Society Published on Web 04/15/2009
Photophysical Studies of a Hemicyanine Dye
J. Phys. Chem. B, Vol. 113, No. 19, 2009 6827
Figure 1. (a) Absorption spectra of LDS-698 in dioxane-water mixture at (i) water (orange line), (ii) 11% dioxane (green line), (iii) dioxane (black line), (iv) 34.4% dioxane (blue line), and (v) 81.3% dioxane (red line). (b) Fluorescence emission spectra of LDS-698 in dioxane-water mixture at (i) dioxane (orange line), (ii) 81.3% dioxane (green line), (iii) 57.8% dioxane (black line), (iv) 43.2% dioxane (blue line), and (v) 11% dioxane (red line).
698 molecule exhibits TICT properties in the excited state. There is also a possibility of photoisomerisation in this molecule. The TICT and photoisomerization are competitive to each other. In this article we are going to establish which process is more important in LDS-698. To address it, we have studied excitedstate properties of LDS-698 in dioxane-water mixture and in different neat solvents. We have also studied the photophysics of LDS-698 in room temperature ionic liquid 1-butyl-3methylimidazolium tetrafluoroborate ([bmim][BF4]). 2. Experimental Section LDS-698 (laser grade, Exciton) was used as received. 1,4Dioxane (HPLC grade) was purchased from Spectrochem (India). Methanol, chloroform, dimethyl sulfoxide (DMSO), and 1-hexanol (spectroscopic grade) were purchased from SRL, India. Acetonitrile, 1-butanol, 1-propanol, 1-decanol, 1-octanol, and formamide were purchased from Spectrochem (India). 1-Butyl-3-methylimidazolium tetrafluoroborate([bmim][BF4]) was obtained from Merck. The RTIL was dried in vacuum for ∼24 h at 70-80 °C before use. The content of water in the RTIL is 50), we observed a linear relation. That is, in this region with increase in polarity of the medium, knr is gradually increasing and it is due to the polarity-dependent TICT process. That is, in the highpolarity region, TICT is the main nonradiative pathway of LDS698 molecule, whereas in the low-polarity region (ET(30)
