The lowest triplet electronic states of polyacenes and perfluoropolyacenes

Abstract

The optimized geometries, total energies, and vibrational frequencies of the polyacenes and perfluoropolyacenes in their closed-shell singlet and lowest triplet states have been studied using the B3LYP method in conjunction with double-ζ plus polarization (DZP) basis sets. The equilibrium structures of all the polyacenes and perfluoropolyacenes are planar, with D2h symmetry, and the singlet and triplet geometries display interesting patterns for each family of molecules. The largest singlet–triplet structural changes appear in the outermost C–C distances. The small perfluoropolyacene systems have ground state singlet states, and the singlet–triplet energy gaps decrease as the number of linearly fused benzene rings increases. With the number of rings greater than seven, the triplet state of the perfluorinated species falls below the closed-shell singlet. For the parent polyacenes, the same shift occurs between n = 8 and n = 9. Correspondingly, the LUMO–HOMO gap for the singlet state decreases with increasing number of linearly fused perfluoro benzene rings. The predicted singlet–triplet separations for the perfluorinated compounds range from 54 kcal mol−1 (perfluoronaphthalene) to −7 kcal mol−1 (n = 10).