Radical‐Enhanced Intersystem Crossing in Perylene‐Oxoverdazyl Radical Dyads

Abstract

Attaching stable radicals to organic chromophores is an effective method to enhance the intersystem crossing (ISC) of the chromophores. Herein we prepared perylene-oxoverdazyl dyads either by directly connecting the two units or using an intervening phenyl spacer. We investigated the effect of the radical on the photophysical properties of perylene and observed strong fluorescence quenching due to radical enhanced ISC (REISC). Compared with a previously reported perylene-fused nitroxide radical compound (triplet lifetime, τ T = 0.1 μs), these new adducts show a longer-lived triplet excited state (τ T = 9.5 μs). Based on the singlet oxygen quantum yield (Φ Δ = 7 %) and study of the triplet state, we propose that the radical enhanced internal conversion also plays a role in the relaxation of the excited state. Femtosecond fluorescence up-conversion indicates a fast decay of the excited state (< 1.0 ps), suggesting a strong spin-spin exchange interaction between the two units. Femtosecond transient absorption (fs-TA) spectra confirmed direct triplet state population (within 0.5 ps). Interestingly, by fs-TA spectra, we observed the interconversion of the two states (D 1 $ Q 1 ) at ~80 ps time scale. Time-resolved electron paramagnetic resonance (TREPR) spectral study confirmed the formation of the quartet sate. We observed triplet and quartet states simultaneously with weights of 0.7 and 0.3, respectively. This is attributed to two different conformations of the molecule at excited state. DFT computations showed that the interaction between the radical and the chromophore is ferromagnetic (J > 0, 0.05 ~0.10 eV).

Calculation of exchange couplings in the electronically excited state of molecular three-spin systems

“…In this series, perylene is covalently bound to seven different radicals, which are commonly used in experimental studies on molecular three-spin systems. [45][46][47][48][49][50][51][52][53][54] The goal was to determine the influence of the nature of the radical on the magnitude of the exchange interaction(s) and to verify whether systematic trends can be identified. Fig.…”

“…In this series, perylene is covalently bound to seven different radicals, which are commonly used in experimental studies on molecular three-spin systems. 45–54…”

Luminescent Radicals

“…As described in section in the previous section, radical groups often quench the luminescence of fluorophores, and many studies in the 20th century examined the quenching mechanism and dynamics. Much literature is concerned with such stable heteroatom-centered radicals whose emissions were attributed to closed-shell fluorophores: e.g., nitroxide, ,,,,− verdazyl, ,− and thiazyl radicals. ,, …”

Radical Spin Polarization and Magnetosensitivity from Reversible Energy Transfer

“…Radical–triplet pair eigenstates P j can undergo reverse transfer with rate constant γ − α true± 1 / 2 , j to re-form normalD 1 ( ± 1 2 ) + normalS 0 or dissociate into separate D 0 and T 1 states with rate constant γ D . Energy transfer rates for γ + , γ – , and γ D are assumed from previous studies of EISC , and triplet–triplet systems to range from 100 ps to 10 ns (section 1 of the Supporting Information). To focus our evaluation of the spin effects from doublet–triplet energy transfer, spin relaxation is ignored, the reversibility of energy transfer is set between D 1 + S 0 and the pair states, and coherence effects between P j states are ignored.…”