Photo-assisted intersystem crossing: The predominant triplet formation mechanism in some isolated polycyclic aromatic molecules excited with pulsed lasers

Abstract

Naphthalene, anthracene, and phenanthrene are shown to have very long-lived triplet lifetimes when the isolated molecules are excited with nanosecond pulsed lasers resonant with the lowest singlet state. For naphthalene, triplet state populations are created only during the laser pulse, excluding the possibility of normal intersystem crossing at the one photon level, and all molecules have triplet lifetimes greater than hundreds of microseconds, similar to the behavior previously reported for phenylacetylene. Although containing 7-12 thousand cm(-1) of vibrational energy, the triplet molecules have ionization thresholds appropriate to vibrationless T1 states. The laser power dependences (slopes of log-log power plots) of the excited singlet and triplet populations are about 0.7 for naphthalene and about 0.5 for anthracene. Kinetic modeling of the power dependences successfully reproduces the experimental results and suggests that the triplet formation mechanism involves an enhanced spin orbit coupling caused by sigma character in states at the 2-photon level. Symmetry adapted cluster-configuration interaction calculations produced excited state absorption spectra to provide guidance for estimating kinetic rates and the sigma character present in higher electronic states. It is concluded that higher excited state populations are significant when larger molecules are excited with pulsed lasers and need to be taken into account whenever discussing the molecular photodynamics.

In-situ nanospectroscopic imaging of plasmon-induced two-dimensional [4+4]-cycloaddition polymerization on Au(111)

“…This process requires less energy than that S 0 → S 1 transition via photon excitation (e.g., ca. 2.97 eV for anthracene) 65 . After intersystem crossing, one of the anthracene-based blades may undergo a non-radiative transition to the triplet excited state ( T 1 ) and change its spin multiplicity.…”

Vibrationally assisted below-threshold ionization

“…In [34] it is indicated that autoionization of anthracene and naphthalene with 1-2 eV of vibrational energy only takes place for energies up to 0.1 eV below the ionization threshold. Furthermore, the character of the state which dominates the absorption at 7.94 eV is that of a valence state (8 1 A(V 3 )).…”