The low-lying excited states of neutral polyacenes and their radical cations: a quantum chemical study employing the algebraic diagrammatic construction scheme of second order

The vertical excited electronic states of linearly fused neutral polyacenes (PACs) and their radical cations have been investigated using the algebraic diagrammatic construction scheme of second order (ADC(2)). While strict ADC(2) (ADC(2)-s) correctly reproduces trends for mainly singly excited states, in extended ADC(2) (ADC(2)-x) the description of doubly excited states is critically improved. It is shown that a combined application of strict and extended ADC(2) nicely reveals the importance of doubly excited configurations in the description of the excited state spectrum of the neutral PACs. In contrast to general expectation, our calculations of the lowest excited states of the radical cations of the PACs employing unrestricted ADC(2)-s and ADC(2)-x identified one B1g electronic transition whose excitation energy increases with increasing molecular size. Thorough analysis of this electronic state and the involved molecular orbitals relates this unusual phenomenon to an increasing energy gap between the doubly occupied molecular orbital, out of which an electron is excited, and the singly occupied orbital, into which it is excited.