Intersystem Crossing in Tetrapyrrolic Macrocycles. A First-Principles Analysis

We analyze reported trends of the photophysical properties in a series of tetrapyrrolic macrocycles with varied saturation level. We compare rates of intersystem crossing (ISC) and fluorescence upon photoabsorption in porphine (P), chlorin (CH), and bacteriochlorin (BC). CH and BC result from single hydrogenation and double hydrogenation of P, respectively. A first-principles time-dependent density functional theory based on a novel framework is used to implement a quantum-mechanical Fermi's golden rule (FGR) rate theory. We employ the recently developed screened range-separated hybrid (SRSH) functionals and a polarizable continuum model (PCM) achieving a polar- ization-consistent description of the embedded molecular electronic structure. We find, in agreement with the measurements, an increase of the ISC rate upon hydrogenation originating in an increase of the spin-orbit coupling (SOC). This trend is traced back to the overlap of attachment and detachment densities of the relevant singlet and triplet states. Simultaneously, we find an increase in the fluorescence rate competing with the ISC, which, overall, results in a lower ISC yield with increasing degree of hydrogenation despite the increased SOC. Crucially, both the quantum mechanical perspective in the FGR theory and the polarization consistent formulation achieved by the screened RSH used in the DFT calculations are required for achieving predictive quality in the calculated rates.

Automated summary

The photophysical properties of tetrapyrrolic macrocycles were analyzed using a novel framework of first-principles time-dependent density functional theory, revealing an increase in intersystem crossing (ISC) rate and competing fluorescence rate with hydrogenation, leading to a lower overall ISC yield. The increase in ISC rate was found to be due to enhanced spin-orbit coupling (SOC) resulting from hydrogenation. The quantum mechanical perspective in the FGR theory and the polarization-consistent formulation achieved by the screened RSH used in the DFT calculations were crucial for predictive quality of the calculated rates.