Res. Chem. Intermed., Vol. 31, No. 7–8, pp. 649–659 (2005)  VSP 2005. Also available online - www.vsppub.com Excitation energy transfer efficiency of dipole–dipole interaction in a dye pair in polymer medium UMAKANTA TRIPATHY 1,∗ , PREM B. BISHT 1 and KRISHNA K. PANDEY 2 1 Department of Physics, Indian Institute of Technology Madras, Chennai, 600036, India 2 Institute of Wood Science and Technology, Malleswaram, Bangalore, 560006, India Received 23 May 2004; accepted 9 June 2004 Abstract—Excitation energy transfer efficiency (η) of dipole–dipole interaction has been studied in the dye pair 3,3 ′ -dimethyloxacarbocyanine iodide (DMOCI) (donor) to o-(6-diethylamino-3- diethylimino-3H-xanthen-9-yl) benzoic acid (Rhodamine B, RB) (acceptor) in polyvinyl alcohol (PVA) thin films by steady-state and ps time-resolved fluorescence spectroscopy. In the presence of the acceptor the fluorescence intensity of the donor decreases, while that of the acceptor increases as a function of the added acceptor concentration. Time-resolved study of the donor at various acceptor concentrations suggest that the non-radiative energy transfer mechanism as proposed by Förster is responsible for the observed behaviour along with some modifications at very low acceptor concentrations. Modified η values have been simulated and compared with those obtained experimentally. It is found that the value of η increases with acceptor concentration. Keywords: Energy transfer efficiency; time-correlated single photon counting; 3,3 ′ -dimethyloxacarbo- cyanine iodide; Rhodamine B. INTRODUCTION The excitation energy transfer between a donor and an acceptor dye molecule has received a lot of attention in last four decades [1–14] after the pioneering works of Förster (dipole–dipole interactions) [1] and Dexter (higher multiple interactions) [2]. This is due to wide applications of energy transfer in the fields of physics, chemistry and biology [15 – 17]. The excitation energy transfer between donor–acceptor molecules is known as energy transfer, while that among donors is known as energy migration. In principle, the excitation energy transfer process can be modulated by the excitation energy migration and also by the material diffusion of the donor. However, the initial model proposed by Förster [1] does not take in to account the cases of mobile ∗ To whom correspondence should be addressed. E-mail: bisht@iitm.ac.in