Excited‐State Dynamics of Non‐Luminescent and Luminescent π‐Radicals

Abstract

Recently,t he potentialu se of organic p-radicals and relateds pin systems has been expanded to modern technological applications. The unique excited-state dynamics of organic p-radicals can be useful to improvet he stability of photochemically unstable organic compounds, make the polarization transfer applicable to information technology,a nd achieve effective up-conversion of interest for luminescence bioimaging, amongo thers. Furthermore, highly luminescent stable p-radicalsh ave been recently reported, which are especially interesting fora pplication in organic light-emitting devices owing to their potentialt op rovide an internal quantum efficiency of 100 %. Thus,t he excited-state nature of stable p-radicalsa sw ell as the control of their excited-state spin dynamics are emerging topics both in terms of fundamental science and related technological applications. In this minireview,w ef ocus on the excited-state dynamics of both photostable non(weakly)-luminescent and luminescent p-radicals, which are opposites of each other.I n particular, we cover the following topics:1 )effective generation of high-spin photoexcited states and control of the excited-state dynamics by using non-luminescent p-radicals, 2) unique excited-state dynamics of luminescent p-radicals and radical excimers, and 3) applicationsu tilizing excitedstate dynamics of p-radicals.[a] Prof.

Excitation Energy Transfer and Exchange‐Mediated Quartet State Formation in Porphyrin‐Trityl Systems

“…zero‐field splitting parameters D Q , E Q and the g value g Q ) and spin polarisation patterns of the quartet state, important information on the quartet state formation mechanism can be obtained. However, before we discuss the implications of the present experimental results, the proposed mechanism for quartet state formation, [3, 18, 19, 49] graphically summarised in Figure 6, shall briefly be outlined here.…”

“…An increased triplet yield can for instance serve to improve the efficiency of processes such as triplet‐triplet annihilation photon‐upconversion, [5–7] while other applications in organic solar cells or OLEDs make use of the photoluminescence (doublet emission) of a particular class of these π ‐radical systems [3, 8–10] …”

“…Motivated by the large number and potential impact of these applications, a multitude of recent studies have focused on increasing our understanding of the excited state dynamics and deactivation mechanisms in chromophore‐radical systems, with the long‐term goals of achieving photocontrol of the magnetic properties of materials via spin coupling and exploring the influence of molecular topology on the interaction efficiency [1–3, 15–23] …”

Radiative and Nonradiative Recombinations in Organic Radical Emitters: The Effect of Guest–Host Interactions

“…[ 21 ] Although the initial radical‐based OLEDs displayed only modest external quantum efficiencies (EQEs) around 2.4 %, follow‐up work led to new luminescent radicals that resulted in devices with higher efficiencies. [ 45–49 ] Recently, Li and co‐workers reported the fabrication of radical‐based OLEDs exploiting adducts between TTM and carbazole derivatives, with EQEs as large as ≈27%; such efficiencies point to ≈100% internal electroluminescence quantum yield. [ 24 ]…”

Radical‐Enhanced Intersystem Crossing in a Bay‐Substituted Perylene Bisimide−TEMPO Dyad and the Electron Spin Polarization Dynamics upon Photoexcitation**

“…When the paramagnetic moiety is fixed rigidly to the chromophore, their wave functions can't vary. However, the enhanced ISC (EISC) may occur [53,95,96] . Initially, the mixing of the D 2 and D 1 states by Δ J ‐mechanism has been proposed by Evans, [39] and later expanded by Hoytink, [40] and Ake and Gouterman [50] .…”