Adiabatic One- and Two-Photon Excited States in Phenylene-Based Conjugated Oligomers:  A Quantum-Chemical Study

Four series of conjugated oligomers are studied by AM1-CAS-CI method with the purpose to describe the adiabatic lowest one- and two-photon excited states. Polyenes where the lowest singlet is of even parity (S 1 = 2A g ) serve as test for the accuracy of the approach. Oligomers of p -phenylenevinylene (PV n ), p -phenylene (PP n ) and p -phenyleneethynylene (PE n ) with up to n = 6 aromatic rings are taken as prototypes of luminescent conjugated systems where 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 is steeper. The lowest two-photon excited state in PV 6, PP 6 and PE 6 is by at least 0.5 eV higher in energy than the dipole-allowed one. The former bears relation to 2A g in polyenes, except for PE where a dipole-forbidden singly excited state approaches 1B u faster. Relaxation energies are typically smaller than those in polyenes, and decrease with n . For the one-photon exciton they increase from PE to PV to PP, but fall too short to explain the Stokes' shift in the latter.