Electrostatic Interaction-Induced Room-Temperature Phosphorescence in Pure Organic Molecules from QM/MM Calculations

Abstract

Room temperature phosphorescence (RTP) from pure organic material is rare due to the low phosphorescence quantum efficiency. That is why the recent discovery of crystallization induced RTP for several organic molecules aroused strong interests. Through a combined quantum and molecular mechanics CASPT2/AMBER scheme taking terephthalic acid (TPA) as example, we found that electrostatic interaction not only can induce an enhanced radiative decay T1 → S0 through the dipole-allowed S1 intermediate state, but also can hinder the nonradiative decay process upon crystallization. From gas phase to crystal, the nature of S1 state is converted to (1)(π,π*) from (1)(n,π*) character, enhancing transition dipole moment and serving as an efficient intermediate radiative pathway for T1 → S0 transition, and eventually leading to a boosted RTP. The intermolecular packing also blocks the nonradiative decay channel of the high-frequency C═O stretching vibration with large vibronic coupling, rather than the conventional low-frequency aromatic rotation in crystal. This mechanism also holds for other organic compounds that contain both ketones and aromatic rings.

Ternary Emission of Fluorescence and Dual Phosphorescence at Room Temperature: A Single‐Molecule White Light Emitter Based on Pure Organic Aza‐Aromatic Material

“…Thus, both S 1 and T 2 excited states of DPPZ should be attributed to n→π* transition. As a contrast, both S 2 and T 1 excited states should be assigned to π→π* transition . According to the El‐Sayed rule, the ISC processes along S 1 →T 1 and S 2 →T 2 are expected to occur efficiently in DPPZ (Figure a) due to the different transition patterns, which are in good agreement with the excitation‐wavelength‐dependent results.…”

Exploring Potential Energy Surfaces for Aggregation‐Induced Emission—From Solution to Crystal

“…Interestingly, in the context of the FGR‐RIM model, Ma et al. found that electrostatic interactions enhanced the rate constants for the T 1 –S 0 radiative decay in crystals of dicarboxylic acids, but they can also decrease the radiative decay . In another recent investigation, Presti et al.…”

“…Interestingly,i nt he contexto ft he FGR-RIM model, Ma et al found that electrostatic interactions enhanced the rate constants for the T 1 -S 0 radiative decay in crystals of dicarboxylic acids, but they can also decrease the radiative decay. [89] In another recent investigation, Presti et al considered the reasons behind AIE in crystals of 2,7-diphenylfluorenone. [90] In contrast with the previousi nterpretation of the emission mechanism based on the formationo fe xcimers, the authors found that the enhancement of the fluorescence was associated with the electrostatic interactions with the central molecule increasing the radiatived ecay rate.…”

Enhanced Room‐Temperature Phosphorescence through Intermolecular Halogen/Hydrogen Bonding

“…Additionally, according to the El‐Sayed rule, effective SOC occurs if S 1 →T 1 ISC is either the transition from S 1 (n, π*) to T 1 (π, π*) or the transition from S 1 (π, π*) to T 1 (n, π*). On the contrary, transitions from S 1 (π, π*) to T 1 (π, π*) and S 1 (n, π*) to T 1 (n, π*) are prohibited because of inefficient orbital overlap . Therefore, introducing groups with electron lone pairs (e.g., N, O, and S) into aromatics is also an efficient way to design and synthesize organic triplet‐producing molecules …”