Regio-Connectivity and Torsional Angle Effects on Singlet Fission and SOCT-ISC in Aza-BODIPY Dimers

TL;DR

This study explores how regioisomerism and torsional angles in aza-BODIPY dimers influence triplet-state generation mechanisms like singlet fission and SOCT-ISC, using advanced quantum-chemical calculations.

Contribution

It provides detailed mechanistic insights into how molecular geometry affects triplet formation pathways in aza-BODIPY dimers, highlighting the role of torsional angles and regio-connectivity.

Findings

Triplet formation is mainly governed by torsional angle {} between monomers.

Dimer D[1,1] and D[1,3] show favorable singlet fission energetics.

Dimer D[2,2] has low singlet fission rate but enhanced SOCT-ISC activity.

Abstract

Aza-BODIPY dimers represent promising molecular systems for efficient triplet-state generation through either intramolecular-singlet fission (iSF) or spin-orbit charge transfer intersystem crossing (SOCT-ISC). In this work, we investigate the role of molecular geometry in governing these mechanisms across four regioisomeric aza-BODIPY dimers (D[1,1], D[1,3], D[3,3], and D[2,2]) using multireference quantum-chemical calculations. Ground- and excited-state properties were analyzed at the MP2 and SA-XMCQDPT levels of theory, while diabatic couplings and spin-orbit matrix elements were evaluated to estimate iSF and SOCT-ISC rate constants, respectively. Our results reveal that triplet formation is strongly governed by the torsional angle ({\Phi}) between monomer units, with regio-connectivity exerting a secondary influence. Dimers D[1,1] and D[1,3] exhibit favorable iSF energetics and coupling magnitudes, whereas D[2,2] displays low iSF rate constant (kSF ) but enhanced SOCT-ISC activity. The D[3,3] dimer shows exothermic multiexciton formation but reduced iSF efficiency due to destructive coupling interactions. The dominant ISC channel proceeds through the S1-T3 transition with large spin-orbit coupling and a small energy gap. These findings provide critical mechanistic insights into geometry-dependent triplet generation in aza-BODIPY dimers.