S. Karabunarliev et al., Adiabatic one- and two-photon excited states in phenylene-based conjugatedoligomers: A quantum-chemical study, J PHYS CH A, 104(35), 2000, pp. 8236-8243
Four series of conjugated oligomers are studied by AMI-CAS-CI method with t
he purpose to describe the adiabatic lowest one- and two-photon excited sta
tes. Polyenes where the lowest singlet is of even parity (S-1 = 2A(g)) serv
e as test for the accuracy of the approach. Oligomers of p-phenylenevinylen
e (PVn), p-phenylene (PPn) and p-phenyleneethynylene (PEn) with up to n = 6
aromatic rings are taken as prototypes of luminescent conjugated systems w
here S-1 is dipole-allowed. One-photon excitation energies (E) are found to
follow closely the linear E vs 1/n rule, unlike polyenes where the slope i
s steeper. The lowest two-photon excited state in PV6, PP6 and PE6 is by at
least 0.5 eV higher in energy than the dipole-allowed one. The former bear
s relation to 2A(g) in polyenes, except for PE where a dipole-forbidden sin
gly excited state approaches 1B(u) faster. Relaxation energies are typicall
y smaller than those in polyenes, and decrease with it. For the one-photon
exciton they increase from PE to PV to PP, but fall too short to explain th
e Stokes' shift in the latter.