Assessing Intersystem Crossing Rates in Donor- and/Acceptor-Functionalized Corroles: A Computational Study

Abstract

Small singlet–triplet gap (ΔE S–T) and large spin–orbit coupling (SOC) between the low-lying excited spin singlet and triplet states greatly promote the intersystem crossing (ISC) and reverse intersystem crossing (RISC) processes that are keys to harvest the triplet population. The electronic structure of a molecule, which strongly depends on its geometry, governs the ISC/RISC. Herein, we have studied visible-light-absorbing freebase corrole and its electron donor/acceptor functional derivatives to explore and understand the effect of homo/hetero meso-substitution in the modulation of corrole photophysical characteristics using time-dependent density functional theory implementing optimally tuned range-separated hybrid. Dimethylaniline and pentafluorophenyl are considered as the representative donor and acceptor functional groups, respectively. Solvent effects are accounted for using a polarizable continuum model with the dichloromethane dielectric. Calculations reproduce the experimentally measured 0–0 energies for some of the functional corroles studied here. Importantly, results reveal that both the homo- and hetero-substituted corroles including the unsubstituted one show substantial ISC rates (∼108 s–1) that are comparable to the fluorescence rates (∼108 s–1). On the other hand, while homo-substituted corroles exhibit modest RISC rates (∼104 – 106 s–1), hetero-substituted ones show relatively lower RISC rates (∼103 – 104 s–1). These results together suggest that both homo- and hetero-substituted corroles could act as triplet photosensitizers, which is also evident from some experimental reports on modest singlet oxygen quantum yield. Calculated rates are analyzed with respect to the variation of ΔE S – T and SOC, and their dependence on the molecular electronic structure was evaluated in detail. The research findings reported in this study will add to understanding rich photophysical properties of functional corroles and also aid in devising molecular-level design strategies for developing heavy-atom free functional corroles or related macrocycles for applications in lighting, photocatalysis, photodynamic therapy, etc.

Does the Intersystem Crossing Rate of β-Iodinated Phosphorus Corrole Depend on Iodine Numbers and/or Positions?

“…However, since MLJ accounts for the nuclei quantum effects through a high-frequency effective vibrational mode, it is expected to provide a reliable description of the transfer process in the Marcus inverted regime, where Marcus theory grossly underestimates the rates. In fact, MLJ rate formalism implemented here successfully reproduced experimental ISC rates for several organic molecules in the solution phase. − , …”

“…The ground-state structure, FMOs and excited-state energy ordering of FBC are depicted in Figure S2. A buckled structure for the FBC is found as compared to planar FPC and is typical for freebase corroles. , The inherent geometrical twist and the reduced Δ E S–T for S 1 → T n ( n = 2,3) ISC channels of FBC produce the highest ISC rate of ∼10 8 s –1 , competing with fluorescence (∼10 8 s –1 ) as compared to the ISC rate of ∼10 9 s –1 found for the active channel in FPC (see data given in Supporting Information Table S3 and Figure S2). This may have resulted from the non-negligible contribution of central P and axial F atoms to FMOs.…”

“…η is the solvent refractive index (1.496 for toluene used here), and f is the dipole oscillator strength (OS) of the bright emissive S 1 state calculated at the solvated S 1 geometry. The above equation is much similar to Einstein’s rate coefficient for spontaneous emission, but explicitly accounts for solvent effects through η and is widely used for diverse molecular systems in the solvent phase to reliably calculate the k f . − ,, …”

“…Our previous work on light-element-based donor/acceptor functionalized freebase corroles showed competing ISC and fluorescence rates of the order ∼10 8 s –1 . One might expect that either presence of core-ion and or the incorporation of heavy elements will further enhance the ISC.…”

Molecular-Scale Design of Azulene-Based Triplet Photosensitizers: Insights from Time-Dependent Optimally Tuned Range-Separated Hybrid

“…This method was successfully adopted for reliably and accurately estimating k f for diverse classes of molecular systems in polar solvent medium. [28][29][30]45 Semiclassical Marcus 46 and Marcus−Levich−Jortner (MLJ) 47 formalisms are used to calculate and compare k ISC . We note that Marcus rate expression as given below can be derived from a full-quantum Fermi-golden rule 48 at the hightemperature and short-time limits.…”