Diamino single benzene fluorophores by symmetry control and desymmetrization: Structural topology governing photophysical behavior

Diamino-functionalized single benzene fluorophores (SBFs) offer an unusually compact platform in which excited-state behavior is dictated less by π-extension than by how donors and acceptors are positioned around a single aromatic core. This review frames recent progress through a symmetry-centric design lens, contrasting electronically balanced architectures with intentionally desymmetrized frameworks that introduce controlled anisotropy. In symmetry-controlled diamino SBFs, predictable intramolecular charge-transfer (ICT) character and emission tuning emerge from balanced donor–acceptor topologies and steric/conformational locking, enabling rational control of emission energy and Stokes shift. By contrast, deliberate symmetry breaking—via mono-substitution, regioisomerism, or unsymmetrical protecting-group strategies—creates uneven dipoles and localized electronic frustration that amplifies solvatochromism, environmental responsiveness, and function-enabling interactions with lipid phases, metal surfaces, and nanoscale assemblies. Across both regimes, hydrogen-bond topology serves as a unifying structural variable that governs rigidity, pathway accessibility, and solid-state brightness, providing mechanistic continuity from simple aromatic derivatives to stimuli-responsive probes and mechanically adaptive optical crystals. By consolidating these symmetrical–unsymmetrical relationships into generalizable rules, this review highlights diamino SBFs as a minimal yet extensible model system for programmable photophysics and emerging luminescent applications without reliance on extended π-systems or complex molecular architectures.