Journal of Photochemistry and Photobiology A: Chemistry 191 (2007) 176–181 Intramolecular electronic energy transfer in rhodamine–azulene bichromophoric molecule Olga Kuznetz, Daly Davis, Husein Salman, Yoav Eichen, Shammai Speiser ∗ Schulich Faculty of Chemistry, Technion-Israel Institute of Technology, Haifa 32000, Israel Received 12 February 2007; received in revised form 15 April 2007; accepted 24 April 2007 Available online 3 May 2007 Abstract Electron transfer and electronic energy transfer (EET) processes are ways by which different molecules can interact, signalling their state, by EET from a donor (D) to an acceptor (A). This D–A transfer is often observed in an intermolecular process but it can also occur intramolecularly, that is between two bridged parts of a bichromophoric molecule. In this paper we present results pertaining to electronic energy transfer in a newly synthesized rhodamine–azulene (Rh–Az) a bichromophoric molecule, in which a full adder can be implemented. The intramolecular EET rate constant in the Rh–Az molecule was found to be 7.81 × 10 9 s -1 . © 2007 Elsevier B.V. All rights reserved. Keywords: Rhodamine; Azulene; Electronic energy transfer 1. Introduction Interaction between excited and ground states of two molecules involving electronic energy transfer (EET) has been the subject of considerable interest [1]. This process plays a key role in chemistry, biology and physics and is well documented and rather well understood. EET can be observed to occur in an intermolecular fashion between separate donor, D, and acceptor, A, molecules or intramolecularly in a bichromophoric molecule where D and A moieties are chemically connected by a molecu- lar bridge, B. The first observation of short range intramolecular EET (Intra-EET) was reported by Schnepp and Levy for the naphthalene-(CH 2 ) n -anthracene system [2]. Later, many other groups examined other bichromophoric systems [1]. The occur- rence of intra-EET could be readily evaluated from the excitation and emission spectra of each moiety alone and comparison with the corresponding spectra of the bichromophoric molecules. The basic intra-EET process can be described by Eq. (1): D ∗ –B–A k ET -→D–B–A∗, (1) Here the excitation energy is transferred from an excited donor D* chromophore to a ground state acceptor moiety A, result- ∗ Corresponding author. Tel.: +972 4 8293735; fax: +972 4 8295703. E-mail address: speiser@technion.ac.il (S. Speiser). ing in quenching of D* fluorescence and excitation of A. B may act as a mere molecular spacer connecting the two chro- mophores, or it may play a major role in promoting the transfer process [3]. In most cases the Intra-EET rate constant, k ET, is attributed to two possible contributions. The first is the long range Coulombic contribution which was formulated by F¨ orster in terms of dipole–dipole interaction [4]. Using the expressions for absorption coefficient of the acceptor ε A (λ) and the normal- ized emission spectral distribution of the donor F D (λ)theF¨ orster expression for dipole–dipole induced EET rate constant may be obtained, as it is shown in Eq. (2): k d–d ET = Q D κ 2 τ D R 6  9000(ln 10) 128π 5 Nn 4  ∞ 0 F D (λ)ε A (λ)λ 4 dλ (2) where F D (λ) is the normalized fluorescence intensity of the donor, ε A (λ) the extinction coefficient of the acceptor (in M -1 cm -1 ), Q D the fluorescence quantum yield of the donor in the absence of the acceptor, N denotes Avogadro number, n the refractive index of medium, τ D the lifetime of the donor in the absence of an acceptor and R is the distance between the donor and the acceptor. κ 2 is an orientation factor that can take values from 0 (for perpendicular to transition moments) to 4 (collinear to transition moments). When the transition dipole moments are parallel, κ 2 = 1. When the molecules are free to rotate at a rate that is much faster than the de-excitation rate of the donor, the 1010-6030/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jphotochem.2007.04.020