Electron Spin Dynamics of the Intersystem Crossing of Triplet Photosensitizers That Show Strong Absorption of Visible Light and Long-Lived Triplet States

Abstract

Intersystem crossing (ISC) is one of the fundamental photophysical processes. Achieving efficient ISC is critical in the design of triplet photosensitizers and also in the study of photochemistry. Most of the previous studies on ISC and the triplet state are focused on the kinetics of the ISC, the quantum yield, or the lifetime of the triplet states, with femtosecond or nanosecond transient absorption spectroscopic methods. However, another fundamental feature of ISC, i.e., the electron spin selectivity, or the electron spin polarization (ESP) of the triplet state, was much less studied, especially for the newly developed triplet photosensitizers. These features characterize the non-Boltzmann populated sublevels of the T1 state. Herein we summarized the recent progress on the study of the electron spin selectivity of ISC and the ESP of the T1 state of the triplet photosensitizers that show strong absorption of visible light and a long lifetime of the triplet state. The electron spin selectivity of the compact orthogonal electron donor–acceptor dyads (spin–orbit charge transfer ISC mechanism) and compounds with twisted π-conjugation molecular structures, studied with the time-resolved electron paramagnetic resonance (TREPR) spectroscopy, are discussed.

Application of time-resolved electron paramagnetic resonance spectroscopy in the mechanistic study of thermally activated delayed fluorescence (TADF) materials

“…TREPR spectra may provide information on the transient paramagnetic species involved in the TADF process, such as the 3 CT state, or the radical pair, and the precursor of the 3 LE states, the spatial localization of the triplet state wave function, and the ISC mechanisms. [37][38][39][40][41][42][43][44] Some methods based on magnetic field effect have been developed for studying organic semiconductors including magneto-electroluminescence (MEL), optically detected magnetic resonance (ODMR) and magneto-conductance (MC). However, these methods are mainly based on the change in the electroluminescence intensity or electrical conductance of the device with the magnetic field changes.…”

Origin of intersystem crossing in highly distorted organic molecules: a case study with red light-absorbingN,N,O,O-boron-chelated Bodipys

“…56 This result shows the rich electron spin selectivity of the ISC of the twisted Bodipy derivatives. 62,114 Simulation of the TR-EPR spectra gives ZFS D and E parameters for Ph-BO as −1470 MHz and 477 MHz, and almost the same for An-BO (Table 4). Similar D and E parameters indicate that the T 1 state of the molecules is conned on the BO chromophore.…”

“…53,55,61 Moreover, the electron spin selectivity of the ISC process, manifested by the electron spin polarization (ESP) phase pattern of the triplet state, is highly dependent on molecular structure. 55,58,61,62 Intriguingly, twisted Bodipy structures do not always give efficient ISC, in marked contrast to the helicenes, 57 and recent work by Vennapusa reports the same situation with PBI. 63 In seeking to expand the range of structurally distorted chromophores absorbing in the red region, our attention has returned to the helical N,N,O,O boron-chelated dipyrromethenes (BO).…”

“…Furthermore, no charge-transfer emission band could be observed for An-BO, the fluorescence emission intensity and wavelength do not change with solvent polarity, these properties are uncommon compared to previously reported anthryl-Bodipy dyads. 48,62,84 The singlet state energy (E00) of the compounds, determined as the crossover point of normalized absorption and fluorescence spectra, is 1.96 eV and 1.99 eV for An-BO and Ph-BO, respectively. The fluorescence lifetimes of An-BO and Ph-BO were measured by time-correlated, single photon counting (TCSPC), using picosecond pulsed laser excitation (Fig.…”

Predicting Kinetics and Dynamics of Spin-Dependent Processes

“…1 In addition to calculating transition rates between electronic states with different spin multiplicities, NAST can be used to obtain transition probabilities and rates between the individual M S components of the spin states. 29,30 To illustrate this NAST capability, it is beneficial to consider the Hamiltonian describing the crossing between electronic states of different spin multiplicities in the spin-diabatic representation {|S,M S ⟩}, which is used in nonrelativistic electronic structure calculations (Figure 2). The diagonal matrix elements are the spin-diabatic state energies.…”