Volume 134, number 7 PHYSICS LETTERS A 23 January 1989 EFFICIENT ENERGY TRANSFER IN LANGMUIR-BLODGETr MONOLAYERS BY OPTIMIZED QUANTUM CAPTURE E.A. BARTNIK and K.J. BLINOWSKA Faculty of Physics, University of Warsaw, ul. Ho~a 69, 00681 Warsaw, Poland Received 31 May 1988; revised manuscript received 9 November 1988; accepted for publication 25 November 1988 Communicated by D. Bloch Energy transfer in Langmuir—Blodgett monolayers called f-aggregates is explained by direct exciton capture to bound states on acceptor molecules. The condition for maximal energy harvesting is found. Practically loss-free transfer of energy over dis- phonon contribution. The crucial point in our ap- tances of up to 100 nm was observed [1,2] in Lang- proach is the explicit introduction of the acceptor muir—Blodget (LB) monolayers called J-aggregates. potential which, under proper conditions, traps the A f-aggregate is a compact and regular arrangement exciton. of chromophores attached to hydrophobic carbon The usual intuition associated with a quantum tails. The interstices between them are filled by mol- mechanical bound state is that its extension is corn- ecules (e.g. octadecane), which make the layer rigid parable to that of a classical system of the same en- and compact. Kuhn [2] has prepared by LB tech- ergy. The exception to this rule is a system of very niques f-aggregates in which as little as one in 10000 small binding energy, whose wave function extends molecules of chromophores was an acceptor, the rest far into the classically forbidden region. Below we being donors. In such structures, irradiated with UV show that this is exactly the case for f-aggregates with or visible light, donor fluorescence was strongly efficient energy transfer. quenched and an acceptor fluorescence line of an Let us assume the tightest arrangement of mole- amplitude about equal to that of the primary donor cules in two dimensions — namely a hexagonal lat- spectral line, and slightly red shifted was observed tice. We modify the one-dimensional exciton ham- [21. It seems that the monolayer acts as a cooper- iltonian of Davydov [51, ative molecular array which after absorbing a pho- ton, channels the energy latterally over exceptionally H ~ [(C — D)Bn~m Bnm long distances to a single energy accepting molecule. 7ID+ _I_D+ LD+ \fl The effect is not observed in the aggregates, where ~n+l,m~-’n,m+I ~‘-~‘n—l,m+I1’-’n,m the molecular “filler” is absent and the structure is ~ 1? I? \ fl+ J~ n+l,mT~n,m+l U~n~l,,~+I) n,m less rigid and less ordered. The mechanism of energy transfer in one-dimen- where e is the exciton energy of an isolated donor sional, non-doped f-aggregates was considered by molecule, D the deformation energy of the donor Scherer and Fisher [31in terms of exciton—phonon molecule in the lattice, and J the resonance inter- interaction. Gutman et al. [4] postulate a mecha- action energy between neighbouring molecules. nism based on ring soliton propagation. We propose In the following we shall neglect the coupling to a fast, purely excitonic process based on quantum the phonons, since our process is fast (purely exci- mechanical transition. The narrow fluorescence line tonic). Let us introduce at the origin an acceptor suggests a short exciton hopping time (10—13 s ac- molecule with different e, D and J. The total ham- cording to ref. [21) and therefore we can neglect the iltonian is 448 0375-960 11891$ 03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)