Molecular physics of persistent room temperature phosphorescence and long-lived triplet excitons

Abstract

Persistent room temperature phosphorescence (pRTP) is important to high-resolution imaging independent of autofluorescence and the scattering of excitation light for security and imaging applications. Although efficient and bright pRTP is crucial to imaging applications, photophysical processes from the triple states of heavy-atom-free chromophores have been explained by making many assumptions that are potentially based on incorrect photophysical explanations. This often confuses researchers in their efforts to control and enhance the pRTP characteristics. This paper introduces recent advances in our understanding of photophysical processes from the lowest triplet excited state of heavy-atom-free chromophores based on statistical evidence from experimental and theoretical viewpoints. After the introduction of two photophysical processes showing persistent RT emissions and the characteristics of the persistent emissions, physical parameters relating to pRTP and appropriate techniques for measuring the parameters are explained. For molecularly dispersed heavy-metal-free chromophores in a solid state, recent understandings of the physical parameters verified by correlations from optically estimated and theoretical viewpoints are summarized. Using the photophysical insights obtained for the dispersed chromophores, uncertainties regarding the photophysical processes of aggregated chromophores are discussed. After highlighting recently developed materials showing efficient pRTP, the potential advantages of pRTP over previous persistent emissions are discussed considering recent demonstrations of persistent emitters. This review quantitatively summarizes the relationship between the molecular backbone and physical parameters of pRTP characteristics and guides the reader in their efforts to appropriately design materials with efficient pRTP and control long-lived triplet excitons for promising applications.

Manipulation of Triplet Excited States in Two‐Component Systems for High‐Performance Organic Afterglow Materials

“…Excellent review articles that summarized the recent progress in the corresponding aspects have been contributed by Huang, Tang, Tian, Ma, An, Chen, Hirata, Liu, Cariati and other research groups several years ago. [4][5][6][7][8][9][10][11][12][13][14][15][16][17] In recent years, RTP and organic afterglow materials have received increasingly attention; many research groups have entered this field. Tremendous development of novel chemical structures, new design strategies and intriguing applications have been achieved in the field of RTP and organic afterglow materials.…”

“…Manipulation of triplet excited states is of vital importance for devising high-performance luminescent materials such as phosphorescent transition-metal complexes, thermally activated delayed fluorescence emitters, and especially for room-temperature organic phosphorescence and afterglow materials. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] In organic systems, since phosphorescence decay of triplet excited states is spin-forbidden, organic molecules possess phosphorescence rate constants (k P ) on the order of 10 À 2 to 10 6 s À 1 . Because of their small k P , organic systems exhibit the potential to display long phosphorescence lifetimes (τ P ), whereas other photophysical processes, for example, nonradiative decay and oxygen quenching, can strongly compete with phosphorescence decay and quench triplet excited states.…”

“…[18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37] These advancements have been summarized in excellent review articles. [4][5][6][7][8][9][10][11][12][13][14][15][16][17] Among these design strategies, heavy atom effect (HAE) and n-π* transition have been mostly introduced into organic systems to enhance spin-orbit coupling, and thus facilitate intersystem crossing, increase phosphorescence decay rates, and consequently improve room-temperature phosphorescence efficiency (Φ P ) (Figure 1). However, HAE and n-π* transition also shorten phosphorescence lifetime (τ P ).…”

“…[25,26] Various elegant strategies such as Haggregation, crystal engineering, sensitization, and energy-gap narrowing have also been developed to enhance ISC and populate triplet excited states in organic systems. [4][5][6][7][8][9][10][11][12][13][14][15][16][17]20,23,38,39] Besides boosting ISC quantum efficiency and phosphorescence decay rates, suppression of nonradiative decay (k nr ) and oxygen quenching (k q ) of triplet excited states is essential for the construction of high-performance RTP and organic afterglow materials (Figure 1). Crystalline lattices can largely restrict intramolecular motion of triplet excited states, which have been frequently reported to reduce k nr + k q values of triplet excited states.…”

Necessary and sufficient condition for organic room-temperature phosphorescence from host-guest doped crystalline systems.

“…Metal-free organic materials involving triplet excited states in their luminescence process at ambient conditions have attracted widespread interest during the last decade, since they were behind the development of novel highly active research areas in organic electronics and photonics, namely: thermally activated delayed fluorescence (TADF) 1 and organic longlived luminescence, that includes organic room-temperature phosphorescence (RTP) [2][3] and organic long-persistent luminescence (LPL) 4 . Each of these distinct luminescence phenomena originates from complex emission mechanisms enabled by the crossover between various types of excited states with different electron spin multiplicities [5][6][7][8][9][10][11][12] . Despite manipulation of excited states energy levels is a difficult task, suitable emitting compounds have been engineered for each luminescence subtype [13][14][15][16][17][18][19][20][21] as well as for co-existing emissions (e.g.…”