Controllable modulation of through-bond and through-space coupling channels in indene-derived photoresponsive molecules for single-molecule photoswitches

Abstract

Single-molecule photoswitches are indispensable building blocks for advanced photonic devices, smart materials and molecular machines. However, their performance is constrained by two critical challenges – the lack of precise control over through-bond (TBC) and through-space (TSC) coupling channels that govern photoresponsive dynamics and the difficulty of achieving reversible, mode-tunable charge transport at the single-molecule level – both have emerged as critical bottlenecks impeding the development of multi-functional molecular devices. Herein, we address these challenges by reporting a library of indene-derived photoresponsive molecules, including two model compounds (Ind-SM and IndM-SM), where TBC/TSC are controllably modulated via systematic structural engineering: substitution of electron-donating/withdrawing groups on the indene core, adjustment of π-conjugation length, modification of steric hindrance (e.g., methyl substituents) and regulation of aggregation behavior. Comprehensive characterization studies reveal three key findings: (1) UV-vis absorption and photoluminescence (PL) measurements confirm rapid, reversible photoisomerization (Ind-SM/IndM-SM: 365 nm for forward isomerization; 450/520 nm for reverse) and typical aggregation-induced emission (AIE) behavior; (2) scanning tunneling microscope breaking junction (STM-BJ) studies show that Ind-SM switches from the trans (dark) to the cis (light) state, while IndM-SM undergoes the opposite transformation due to methyl-induced steric hindrance, with 2D conductance analyses validating consistency between experimental features and theoretical molecular lengths; (3) increased steric bulk weakens TSC, enabling an excellent fatigue resistance (>5 cycles) at the single-molecular level. Flicker noise scaling analysis and DFT/TD-DFT calculations further clarify the transport mechanism: Ind-SM maintains TBC-dominated charge transport, whereas IndM-SM achieves light-driven TSC-to-TBC transport transition. Mechanistically, we propose a structure–property–transport relationship model that correlates molecular geometry/steric effects with TBC/TSC strengths, providing a clear guideline for tuning photoresponse and charge transport. This work establishes two complementary strategies – rational coupling channel modulation and substituent-space conjugation cooperation – addressing long-standing challenges in balancing TBC/TSC for high-performance photoresponse and single-molecule transport tunability and offers a universal approach to advance reversible, multi-functional single-molecule photoswitches for next-generation photonic and molecular electronic devices.