Journal of Photochemistry and Photobiology A: Chemistry 167 (2004) 23–30 Effect of hydrogen bonding on intramolecular charge transfer in aqueous and non-aqueous reverse micelles Partha Hazra, Debdeep Chakrabarty, Anjan Chakraborty, Nilmoni Sarkar ∗ Department of Chemistry, Indian Institute of Technology Kharagpur, Kharagpur 721 302, WB, India Received 5 February 2004; received in revised form 22 March 2004; accepted 13 April 2004 Available online 20 June 2004 Abstract In this paper, we have reported the behavior of intramolecular charge transfer (ICT) state of p-N,N-dimethylaminobenzoic acid (DMABA) and p-N,N-dimethylaminobenzonitrile (DMABN) in n-heptane/AOT/water, n-heptane/AOT/methanol and n-heptane/AOT/acetonitrile re- verse micelles. The different features of ICT emission of both the probes in water, methanol and acetonitrile reverse micelles are explained by the presence and absence of hydrogen bonded ICT state in the above-mentioned reverse micelles. Moreover, we have reported the decay characteristics of both the probes in locally excited (LE) and ICT state in these three reverse micelles. © 2004 Elsevier B.V. All rights reserved. Keywords: Hydrogen bonding; Intramolecular charge transfer; Aqueous and non-aqueous reverse micelles 1. Introduction The photoinduced intramolecular charge transfer (ICT) of various organic molecules containing electron donor and acceptor groups has been the burgeoning interest of recent investigation, because it is a possible mechanism for biolog- ical and chemical energy conversion [1–3]. Intramolecular charge transfer emission in which a dialkylamino group acts as an electron donor has been a subject of several recent investigations [4–19,21–30,33]. Among them, the most in- teresting are twisted intramolecular charge transfer (TICT) processes, which involve twisting of the dialkylamino part relative to the rest of the molecules along with the charge transfer [4–6]. The formation of excited TICT state is rec- ognized by a phenomena of dual fluorescence exhibiting a large Stokes’ shifted emission in addition to the normal emission from the local excited (LE) state. In addition to the TICT model, several other models were proposed to ex- plain dual fluorescence of DMABN and related compounds in polar solvents. The RICT model [7] involves a rehy- bridized (bent) cyano group. Recently, planar intramolecu- lar charge transfer (PICT) model [8] has been proposed as an alternative to explain the presence of dual fluorescence of DMABN and other related compounds. This model pos- tulates a planarized structure of the emissive CT state but ∗ Corresponding author. Tel.: +91-3222-283332; fax: +91-3222-255303. E-mail address: nilmoni@chem.iitkgp.ernet.in (N. Sarkar). does not involve state interaction. However, recent theoret- ical [9] and experimental results obtained from the studies of p-N,N-dimethylaminobenzonitrile (DMABN) and ethyl p-N,N-diethylaminobenzoate (DEAEB) in supercritical flu- ids and vapor phase [10–12] support the TICT mechanism in preference to the other model. Time resolved Raman studies of DMABN also support the TICT model [13]. The ICT state of DMABN has an extremely large dipole moment (23 D) [6] and hence its energy is expected to de- crease with an increase in solvent polarity. This has two con- sequences. Firstly, this lowering of the energy of the ICT state reduces the energy barrier between the Franck–Condon (FC) excited state and the ICT state [14,15]. Secondly, the stabilization of the ICT state decreases the energy gap be- tween the TICT state and the FC ground state [14,15]. Since lowering torsional barrier and increasing the non-radiative rates tend to have opposite effects on the ICT emission yield, as the solvent polarity increases the ICT emission yield should first increase and then after reaching a maximum ICT emission yield should decrease. The relative yield of ICT emission increases with solvent polarity up to a E T (30) value around 46 (acetonitrile) and decreases at a higher polarity [16]. Some authors have suggested that the specific hydro- gen bonding between the solvent and electron donor species also plays a major role to stabilize the twist conformer to facilitate the formation of ICT state [17,18]. Very recently, the role of hydrogen bonding of the electron acceptor with solvent in the formation of ICT state has attracted much attention [19]. Such a hydrogen bonding effect may be an 1010-6030/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jphotochem.2004.04.007