Absorption bandwidth formation

If the transition dipoles are aligned in a head-to-tail formation, then a red shift is expected. This is the reported explanation for the sharp bands at 573 and 578 (J bands). The narrow half-bandwidths of the split J aggregate absorption suggest that the exciton states are not strongly coupled with external perturbations. The two distinct electronic transitions were proposed to arise from two structural modifications of the aggregates. 

Ns laser photolysis measurements on (1-pyrenyl) - (CH2 ) (p-N,N-dimethylaminophenyl) (abbreviated as P-(CH2)n"D) were performed in some solvents (18). Their absorption spectra are reproduced by the superposition of bands of the donor cation and the acceptor anion. Only the bandwidth is dependent upon solvent polarity and the number of CH2 groups, which is reduced to the relative geometrical structure of the donor and the acceptor. Exciplex formation dynamics of these compounds has been established by ps transient absorption spectroscopy (19). The absorption spectrum of P-CH2-D in 2-propanol is almost independent of the delay time. On the other hand, P-(0112)3-0 gives a similar spectrum at about 1 ns after excitation, and its spectral shape broadens with a time-constant of 1.2 ns. 

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