Dual Role of a Conjugated Bridge in Intramolecular Singlet Fission: Light-Harvesting Antenna and Energy Funnel

Abstract

Herein, an 'antenna-mediated' singlet fission (SF) molecular platform is reported, in which a phenyldiketopyrrolopyrrole (PDPP) chromophore with strong visible-light absorption is strategically integrated into TIPS-pentacene (P) dimers to overcome the intrinsically weak absorption of conventional P-based SF systems. Owing to the complementary absorption and emission characteristics of PDPP and P, the resulting conjugates enable highly efficient intramolecular Förster resonance energy transfer (FRET) from PDPP to P, followed by intramolecular SF of the P dimer. Upon selective excitation of the PDPP antenna, ultrafast and nearly unity FRET occurs on a picosecond timescale, populating the singlet excited state of P and triggering SF with rates identical to those observed under direct P excitation. This demonstrates that antenna-mediated excitation fully preserves the intrinsic SF dynamics while extending excitation across the entire visible solar spectrum. The exceptional energytransfer efficiency is rationalized by a large orientation factor, substantial spectral overlap, and the high fluorescence quantum yield of PDPP. Moreover, solvent polarity provides an external handle to modulate the spectral overlap and thereby tune both the FRET and SF rate constant, without requiring chemical modification of the molecular framework. Overall, this work establishes an antenna-enabled strategy for broadband solar-energy harvesting coupled to efficient SF, offering a general molecular design concept for next-generation SF materials with enhanced solar utilization efficiency.