Absorption and spontaneous emission in aggregates of conjugated polymers

Absorption and spontaneous emission from exciton states in ordered aggregates of pi-conjugated polymers are studied theoretically. Each polymer contains N carbon atoms with average bond length a and is treated using a Pariser-Parr-Pople Hamiltonian. Coulombic interactions between molecules are treated to first order using an effective Hamiltonian approach. The aggregate electronic excited states are Wannier-Frenkel (WF) excitons with intramolecular Wannier exciton character and intermolecular Frenkel exciton character. For aggregates with in-phase double bond alignment, interpolymer interactions cause a blue shift of the optically allowed (nu B-) WF excitons. The blue shift increases with N, peaks, and ultimately tends to a small value when the polymer length greatly exceeds d, the nearest neighbor separation. The radiative decay rate gamma(nu B-) of the optically allowed nu B- WF exciton is proportional to the number of polymers, M. Intermolecular interactions have little effect on the exciton sizes and energies when the polymer length is much greater than the exciton sizes where gamma(nu B-) scales linearly with N due to delocalization of the electron-hole (e-h) center of mass along each polymer. In this limit, the radiative decay rate is enhanced two dimensionally, gamma(nu B-)proportional to MN. However, when N is smaller, the exciton sizes and energies can be strongly affected by these interactions, leading to a markedly different N dependence for gamma(nu B-). For parameters which best describe realistic conjugated polymers, this system behaves like an H-aggregate, with a superradiant state at the top of the exciton band. The resulting emission will not be superradiant if the radiative decay rate is much smaller than the nonradiative population relaxation rate to lower energy, optically dark Frenkel exciton states. Applications of this analysis to recent experimental findings are discussed. (C) 1998 American Institute of Physics. [S0021-9606(98)50342-X]