Field induced charge-transfer exciton transitions

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Abstract
This paper reports the observation of a second zero‐phonon line, induced by a static electric field, lying 10 cm−1 above the first singlet charge‐transfer exciton state of crystalline anthracene‐PMDA. The splitting of these charge‐transfer exciton states is a quadratic function of the electric field strength for low fields and becomes linear for fields of order 104 V/cm. The origin of these features is discussed on the basis of a one‐dimensional model of the charge‐transfer exciton in the linear donor–acceptor stack. The effect of charge‐transfer admixture to the neutral donor–acceptor ground state and the effect of creating a local charge‐transfer (ionic) state by the absorption of light are considered by use of a second‐quantized Hamiltonian for the charge‐transfer exciton. The splitting of 10 cm−1 observed by use of the static electric field, which removes the inversion invariance of the one‐dimensional donor–acceptor chain along the stack axis arises from the mixing of symmetric and antisymmetric exciton eigenstates by the field. Numerical estimates of the splitting, based on experimental data, are in order of magnitude agreement with observation. The charge‐transfer exciton band structure is derived by considering both electron and hole hopping terms and tight binding (nearest unit cell) excitation transfer.