Design Principles for Singlet Fission in Aza-BODIPY Dimers: Spacer-Controlled Electronic Structure and Energy Ordering

TL;DR

This study explores how spacer modifications in aza-BODIPY dimers influence singlet fission efficiency, revealing structure-property relationships that guide molecular design for improved energy conversion.

Contribution

It introduces a high-level electronic-structure framework to systematically analyze spacer effects on singlet fission in aza-BODIPY dimers, providing design principles.

Findings

Spacer reduction of state mixing enhances multiexcitonic states.

Substitution pattern modulates energy levels and couplings.

Energy ordering can be tuned over several orders of magnitude.

Abstract

In this work, we investigate the role of spacers in tuning singlet-fission (SF) activity in aza-BODIPY dimers using a diabatic electronic-structure framework. A series of nine spacer-linked dimers is analyzed employing high-level multireference methods to evaluate the energetics and couplings of the locally excited (LE), charge-transfer (CT), and multiexcitonic (ME) states. The introduction of spacers reduces excessive state mixing that was observed in C-C bonded aza-BODIPY dimers and promotes the emergence of more distinct ME states, while preserving the CT-mediated interactions required for efficient SF. Furthermore, the substitution pattern, degree of conjugation, and introduction of acetylene units strongly modulate the relative state energetics and effective couplings, thereby tuning the SF rate over several orders of magnitude. These findings establish clear structure--property relationships and provide molecular design guidelines for optimizing SF activity in aza-BODIPY-based chromophores.