Donor-acceptor subunits overlap

It is possible to assume that the absorption intensity originates mainly from the locally excited zero-order state since the AB+ zero-order state should barely contribute at all owing to the small overlap density of donor and acceptor orbitals. Therefore, the S, or T( should be reached at planar geometries by vertical excitation into the dominantly locally excited states. Only for the best donor-acceptor subunit combinations and high-lying locally excited states, a motion from planar minimum toward twisted minimum in S, or T, will be barrierless. Otherwise, a thermal activation is needed to overcome the barrier caused by crossings of zero-order states. 

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