Singlet Energy Transfer in Porphyrin-Based Donor-Bridge-Acceptor Systems: Interaction between Bridge Length and Bridge Energy Karin Pettersson, ² Alexander Kyrychenko, ²,‡ Elin Ro 1 nnow, ² Thomas Ljungdahl, § Jerker Ma ˚ rtensson, § and Bo Albinsson* ,² Departments of Chemical and Biological Engineering, Physical Chemistry, and Organic Chemistry, Chalmers UniVersity of Technology, SE-412 96 Go ¨teborg, Sweden ReceiVed: July 12, 2005; In Final Form: October 14, 2005 Singlet excitation energy transfer is governed by two donor-acceptor interactions, the Coulombic and exchange interactions giving rise to the Fo ¨rster and Dexter mechanisms, respectively, for singlet energy transfer. In transfer between colliding molecules or between a donor (D) and acceptor (A) connected in donor-bridge- acceptor (D-B-A) system by an inert spacer (B), the distinction between these two mechanisms is quite clear. However, in D-B-A systems connected by a π-conjugated bridge, the exchange interaction between the donor and acceptor is mediated by the virtual low-lying excited states (unoccupied orbitals) of that bridge and, as a consequence, becomes much more long-range in character. Thus, the clear distinction to the Coulombic mechanism is lost. This so-called superexchange mechanism for singlet energy transfer has been shown to make a significant contribution to the energy transfer rates in several D-B-A systems, and its D-A distance as well as D-B energy gap dependencies have been studied. We here demonstrate that in a series of oligo- p-phenyleneethynylene (OPE) bridged porphyrin-based D-B-A systems with varying D-A distances the Fo ¨ rster and through-bond (superexchange) mechanisms both make considerable contributions to the observed singlet energy transfer rates. The donor is either a zinc porphyrin or a zinc porphyrin with a pyridine ligand, and the acceptor is a free base porphyrin. By comparison to a homologous series where only the D-B energy gaps varies, a separation between the two energy transfer mechanisms was possible and, moreover, an interplay between distance and energy gap dependencies was noted. The distance dependence was shown to be approximately exponential with an attenuation factor  ) 0.20 Å -1 . If the effect of the varying D-B energy gaps in the OPE series was taken into account, a slightly higher -value was obtained. Ground-state absorption, steady-state, and time-resolved emission spectroscopy were used. The experimental study is accompanied by time-dependent density functional theory (TD-DFT) calculations of the electronic coupling, and the experimental and theoretical results are in excellent qualitative agreement (same distance dependence). Introduction Excitation energy transfer has been studied for many years with the long time goal of developing molecular scale electronics 1-5 and construction of artificial photosynthesis systems. 6-9 To systematically study energy transfer molecular donor-bridge-acceptor (D-B-A) systems have been devel- oped and energy transfer rates as a function of bridge length, 10-18 conformation, 19 and electronic properties 20,21 have been inves- tigated. The electronic coupling for energy transfer can be ap- proximated as the sum of two terms, one that describes the Coulombic interaction and one that describes the electron exchange interaction. 22-25 The two terms are active at different length scales; the electron exchange interaction, which is described by Dexter, 26 requires an orbital overlap and is therefore active at distances less than 10 Å. The Coulombic interaction, which is described by Fo ¨rster, 27,28 is active at longer distances up to 100 Å. In both theories the bridge is considered as an inert spacer. In 1961 McConnell 29 derived the superex- change theory for electron transfer which also has been applied to energy transfer. According to the superexchange theory the electronic coupling between donor and acceptor can be ex- pressed as that is, both the electronic couplings between the bridge and the donor and the acceptor (V DB and V BA ) and the energy splitting between relevant states of the donor and bridge (ΔE DB ) are involved. Similar to other phenomena related to the exchange interaction, the superexchange coupling is believed to decay exponentially with distance: Here R DA is the distance between donor and acceptor and  is the so-called attenuation factor, which usually is considered to be bridge specific. More groups than can be mentioned here have studied the distance dependence 10-18 for either singlet or triplet energy transfer, but only a few have studied the dependence on the energy splitting between donor and bridge * Corresponding author. Phone: +46 31 772 30 44. Fax: +46 31 772 38 58. E-mail: balb@chalmers.se. ² Department of Physical Chemistry. ‡ Permanent address: Research Institute for Chemistry, V.N. Karazin Kharkov National University, 4 Svobody Square, 61077 Kharkov, Ukraine. § Department of Organic Chemistry. V DA ) V DB V BA ΔE DB (1) V DA ) A 0 exp ( -  2 R DA) (2) 310 J. Phys. Chem. A 2006, 110, 310-318 10.1021/jp053819d CCC: $33.50 © 2006 American Chemical Society Published on Web 12/08/2005