Spontaneous emission and absorption in model aggregates of pi-conjugated oligomers

Spontaneous emission and absorption in an aggregate of pi-conjugated oligomers are studied theoretically. Each oligomer is taken as a ring of N carbon atoms with average bond length ct, and is treated using the Huckel Hamiltonian with bond alternation parameter delta. Coulombic interactions between rings are treated to first order using an effective Hamiltonian approach, For aggregates in which the oligomer planes are normal to the aggregate axis with in-phase bond alignment, intermolecular interactions cause a blue-shift in the absorption spectrum due to first order interchain excitation transfer. The average absorption frequency increases with N, peaks, and ultimately tends to a small value when the ring diameter Na/pi greatly exceeds d, the nearest neighbor separation. At this point the entire spectrum for an aggregate is indistinguishable from the isolated oligomer spectrum. The radiative decay rate gamma for an aggregate of M oligomers is superradiant, being M times faster than the decay rate of an isolated oligomer. When Na/pi much greater than d, the superradiant rate increases linearly with delta over most of the range 0 less than or equal to delta<1. Near delta=1, however, there is a sharp transition from a one-dimensional superradiant rate (gamma proportional to M) to a two-dimensional rate, with gamma scaling as the product MN when delta= 1. (C) 1997 American Institute of Physics.