Positional Effects of Sila-Substitution on the Excited-State Energetics and Singlet Fission Propensity of Silyl-Substituted Pentacene: A Comparative DFT/TD-DFT Study

Abstract Singlet fission (SF) has emerged as a key photophysical process for generating multiple excitons through controlled excited-state energetics in π-conjugated systems. Herein, we employ density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations to systematically study the influence of positional silicon incorporation on the excited-state energetics of 6,13-bis(trihydrosilylethynyl)pentacene (THS-pentacene). A series of mono- and disila-substituted derivatives were analyzed to evaluate their singlet fission potential using the thermodynamic driving force (ΔESF), alongside the energetic alignment of the second triplet state (T2). This study reveals that sila-substitution serves as a regioselective strategy to simultaneously modulate ΔESF and the position of T2 relative to S1 and 2T1, providing favorable energetic prerequisites for identifying candidate chromophores. Core substitution results in excessive exoergicity, whereas terminal disila-substitution, particularly in 1,14-Si-THS-pentacene, achieves an optimal ΔESF (0.342 eV) along with a near-degenerate S1-T2 energy gap, thereby reducing the likelihood of energetically accessible competing relaxation pathways at the molecular level. Comprehensive electronic structure analyses, including density of states (DOS), NICS aromaticity, LOL-π, and transition density matrix (TDM) mapping, were performed for two representative systems (THS-Pn and 1,14-Si-THS-Pn), revealing that moderate π-electron redistribution and enhanced inter-ring π-electronic coupling occur without inducing long-range charge-transfer character. Overall, these results suggest that site-specific silicon incorporation is an effective strategy for tuning the electronic structure of promising molecular candidates, serving as a foundational first-tier screening prior to condensed-phase evaluation.