Excited Triplet State of Heptacene Studied by Electron Spin Resonance.

Excited triplet states of acene (3n A*, n being the number of benzene rings) have recently attracted considerable attention because of their useful applications. However, 3n A* with n > 5 have not been experimentally explored, due to the insolubility and instability of the higher acenes. In this study, we succeeded in detecting spin-polarized 37A* by combining the photoprecursor method using a degassed amorphous matrix of o-terphenyl and time-resolved electron spin resonance. 37A* is the longest acene among the 3n A* observed to date. It has been clarified that 37A* is generated by preferential intersystem crossing to the in-plane sublevels and possesses zero-field splitting constants of D = 0.035 cm-1 and E = 0.000 cm-1. The D values for 3n A* (n = 2-7) were found to follow a linear 1/n-dependence, expressed as D(n)=0.180/n+0.011 [cm-1], which provides a quantitative basis for the spatial extent of excitation in even higher 3n A* states.