Intersystem Crossing in Tetrapyrrolic Macrocycles. A First-Principles Analysis

Abstract

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 firstprinciples 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 polarization-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.

Tailoring the Intersystem Crossing and Triplet Dynamics of Free-Base Octaalkyl-β-oxo-Substituted Porphyrins: Competing Effects of Spin–Vibronic and NH Tautomerism Relaxation Channels

“…Accordingly, the maximum SOCME and minimum energy gaps correlate well with the increased ISC rates monitored for the oxidized derivatives of OEP. It is interesting to note that the crossings occur between S 1 and higher energy triplet states and never directly with the low-lying triplet T 1 , as previously reported for chlorins and bacteriochlorins . The relatively large calculated spin-orbit couplings for these species are in line with the recently described effect of spin-orbit coupling amplification in hydrogenated porphyrin rings due to an increase of the superposition of the hole and electron densities between the S 1 and T n states in these molecules .…”

“…It is interesting to note that the crossings occur between S 1 and higher energy triplet states and never directly with the low-lying triplet T 1 , as previously reported for chlorins and bacteriochlorins. 45 The relatively large calculated spin-orbit couplings for these species are in line with the recently described effect of spin-orbit coupling amplification in hydrogenated porphyrin rings due to an increase of the superposition of the hole and electron densities between the S 1 and T n states in these molecules. 45 Thus, the systematic 2-fold increase in the ISC rate from porphyrin 1 to oxochlorin 2 and a further 2-fold increase to the doubly oxygenated porphyrinoids are interpreted as an increased value of the SOCME between S 1 and triplet states with the out-of-plane carbonyl distortion.…”

“… 45 The relatively large calculated spin-orbit couplings for these species are in line with the recently described effect of spin-orbit coupling amplification in hydrogenated porphyrin rings due to an increase of the superposition of the hole and electron densities between the S 1 and T n states in these molecules. 45 Thus, the systematic 2-fold increase in the ISC rate from porphyrin 1 to oxochlorin 2 and a further 2-fold increase to the doubly oxygenated porphyrinoids are interpreted as an increased value of the SOCME between S 1 and triplet states with the out-of-plane carbonyl distortion. We do not deny the possibility that other affected modes also play a role in controlling the ISC rate, but the C=O out-of-plane distortion mode is expected to be the most influential.…”

Understanding High Fluorescence Quantum Yield and Simultaneous Large Stokes Shift in Phenyl Bridged Donor−π–Acceptor Dyads with Varied Bridge Lengths in Polar Solvents

“…The k f is calculated using the following equation where OS is of the transition responsible for the radiative emission (i.e., fluorescence), υ abs and υ em refer to the absorption and emission energies (expressed in wavenumbers, i.e., in cm –1 ), respectively, and n is the medium refractive index. This method was widely used to reliably estimate the k f for a range of molecular systems in different solvents. , Calculations reveal almost similar but very high k f for all three dyads due to substantially large OS, which is significant for designing efficient light-emitting devices with these dyads (see k f data listed in Table ). − The k f values also match reasonably well with the experimentally reported radiative recombination rate of ∼10 9 s –1 …”

Origins of Molecular-Twist-Triggered Intersystem Crossing in Functional Perylenediimides: Singlet–Triplet Gap versus Spin–Orbit Coupling

“…Eco-friendly, heavy-atom-free small organic molecules are promising candidates because of their easily tunable physiochemical properties and flexibility. However, negligibly small SOC in these systems leads to inefficient ISC, restricting their practical use as triplet photosensitizers. − Conventionally, a large SOC is achieved by an internal heavy-atom effect. , In addition, strategies such as substitutional doping with light hetero-elements (for example, “N”), defects, dielectric screening, and also introducing geometrical strains were shown to improve the ISC/RISC rates. − Significantly enhanced ISC was reported for heavy-atom-free perylenediimide (PDI) bay-substituted with electron-donor units. , A considerable ISC rate was also reported for a geometrically twisted phenanthrene-fused PDI, whereas the corresponding planar analogue produced a negligibly small ISC rate . PDI functionalized with anthryl and carbazole at the bay positions exhibits long lived triplet with substantially high triplet quantum yield (Φ T ) .…”