Theoretical Insight Into the Ultralong Room-Temperature Phosphorescence of Nonplanar Aromatic Hydrocarbon

Purely aromatic hydrocarbon materials with ultralong room-temperature phosphorescence (RTP) were reported recently, but which is universally recognized as unobservable. To reveal the inherent luminescent mechanism, two compounds, i.e., PT with a faint RTP and HD with strong RTP featured by nonplanar geometry, were chosen as a prototype to study their excited-state electronic structures by using quantum mechanics/molecular mechanics (QM/MM) model. It is demonstrated that the nonplanar ethylene brides can offer σ-electron to strengthen spin-orbit coupling (SOC) between singlet and triplet excited states, which can not only promote intersystem crossing (ISC) of S1→Tn to increase the population of triplet excitons, but also accelerate the radiative decay rate of T1→S0, and thus improving RTP. Impressively, the nonradiative decay rate only has a small increase, owing to the synergistic effect between the increase of SOC and the reduction of reorganization energy of T1→S0 caused by the restricted torsional motions of aromatic rings. Therefore, a bright and long-lived RTP was obtained in aromatic hydrocarbon materials with twisted structure. This work provided a new insight into the ultralong RTP in pure organic materials.

Focused-ion-beam implantation in strained InGaAs-GaAs quantum wells

Photoluminescence spectroscopy (PL) is used to monitor the degree of focused ion beam induced interdiffusion in InGaAs–GaAs quantum wells using 100 keV Ga+ and Si+ ions. The observed transition energy shifts are consistent with the effects of channeling of the incident ions near normal incidence. PL decay times obtained from pulsed excitation experiments at higher dose rates indicate incomplete annealing of defects in the quantum wells resulting in an enhanced nonradiative decay rate. If Si+ ions are implanted instead of Ga+ ions we find a red shift of the PL peak positions at very low doses owing to a doping effect in the quantum wells.

Theory of nonradiative decay from the lowest singlet state of benzene: Excess energy dependence

A time-independent Green function formalism is developed to study the excess energy dependence on the nonradiative decay in the S1 state of benzene. Effects of purely electronic relaxation (internal conversion, IC) and intramolecular vibrational redistribution, IVR, are taken into account at the same time. Model calculations show that the drastic increase in the nonradiative decay rate at around 3 000 cm-1 excess energy is due to the same onset of both IC and IVR rates. Our theory can explain the difference in rate constant between IVR and IC observed by Moss and Parmenter.