Roles of Localized Electronic Structures Caused by π Degeneracy Due to Highly Symmetric Heavy Atom‐Free Conjugated Molecular Crystals Leading to Efficient Persistent Room‐Temperature Phosphorescence

Abstract

Conjugated molecular crystals with persistent room‐temperature phosphorescence (RTP) are promising materials for sensing, security, and bioimaging applications. However, the electronic structures that lead to efficient persistent RTP are still unclear. Here, the electronic structures of tetraphenylmethane (C(C 6 H 5 ) 4 ), tetraphenylsilane (Si(C 6 H 5 ) 4 ), and tetraphenylgermane (Ge(C 6 H 5 ) 4 ) showing blue‐green persistent RTP under ambient conditions are investigated. The persistent RTP of the crystals originates from minimization of triplet exciton quenching at room temperature not suppression of molecular vibrations. Localization of the highest occupied molecular orbitals (HOMOs) of the steric and highly symmetric conjugated crystal structures decreases the overlap of intermolecular HOMOs, minimizing triplet exciton migration, which accelerates defect quenching of triplet excitons. The localization of the HOMOs over the highly symmetric conjugated structures also induces moderate charge‐transfer characteristics between high‐order singlet excited states (S m ) and the ground state (S 0 ). The combination of the moderate charge‐transfer characteristics of the S m –S 0 transition and local‐excited state characteristics between the lowest excited triplet state and S 0 accelerates the phosphorescence rate independent of the vibration‐based nonradiative decay rate from the triplet state at room temperature. Thus, the decrease of triplet quenching and increase of phosphorescence rate caused by the HOMO localization contribute to the efficient persistent RTP of Ge(C 6 H 5 ) 4 crystals.

Vibrational Radiationless Transition from Triplet States of Chromophores at Room Temperature

“…Analysis from both experimental and theoretical viewpoints for k nr (RT) in a series of chromophores shows that the rate constant of the vibration-based T 1 –S 0 radiationless transition hardly increases from 77 K to RT and is small for many heavy-atom-free chromophores. Because chromophores where the T 1 –S 0 transition is not based on Franck–Condon process may be rare, a long-lived RT state for triplet energy harvesting is possible for many heavy-atom-free chromophores when appropriate conditions to largely reduce the intermolecular energy transfer and/or significant diffusion of triplet excitons that lead to surface and/or defect quenching are achieved. , Because such conditions are not always related to the rigidity of the host and aggregates, more flexible designs for p RTP are anticipated. The good correlation between k nr (RT) and the calculation using vibrational SOC will allow chemists to predict k nr (RT) for triplet energy harvesting before attempting synthesis, and machine learning based on this strong correlation may lead to high-throughput virtual screening of k nr (RT) .…”

Miracles of molecular uniting

“…In the last decade, the persistent RTP of pure organic molecules was an emerging phenomenon in the solid state as an important breakthrough ( Fig. 4a), since it is challenging for organic materials to overcome the spin-prohibition between single and triplet excited state, and achieve the stable triplet excited state [23][24][25]. Among various strategies for molecular design and engineering, the adjustment of molecular packing and intermolecular interactions is an efficient strategy to improve the RTP effect of organic materials (Fig.…”

Achieving Purely Organic Room-Temperature Phosphorescence Mediated by a Host–Guest Charge Transfer State

“…A singlet state having a transition forbidden character such as an n−π* transition can decrease the radiative rate while promoting the ISC process, leading to a significant triplet exciton population. − The phosphorescence lifetime (τ Phos ) depends on the transition character of the triplet states (π–π* character for long-lived emission) and multiple deactivation processes. Among them, exciton quenching induced by host–guest energy transfer and exciton diffusion contributes the most at room temperature, and its rate constant is usually a magnitude larger than that of phosphorescent emission. − Thus, some strategies had been adopted to “stabilize” the vulnerable triplet excitons, such as forming rigid single crystals, − introducing various intermolecular interactions, ,− and doping guests into rigid host matrices. − …”