Solvatochromic Behavior of 2,7-Disubstituted Sila- and Germafluorenes

Push-pull dyes exhibit intramolecular charge transfer behavior, which due to changes in the dipole moment upon excitation, is the origin of their sensitivity to the environment. Such compounds are of interest as probes for bioimaging and as biosensors to monitor cellular dynamics and molecular interactions. Desirable biological probes absorb in the visible region, have high extinction coefficients, high quantum yield and excellent photostability. Fluorophores with scaffolding that can be used to tune and optimize solvatochromic behavior are of particular interest. Here, we investigate the environmental sensitivity of a small library of highly fluorescent 2,7-disubstituted sila- and germafluorenes. Density functional theory (DFT) calculations show that charge transfer occurs from the alkyne core out to the 2,7-substitutents and 3,6-methoxy substituents, the hallmark of push-pull behavior. They exhibit HOMO-LUMO energy gaps of about 3 eV with desirable dipole moments ranging from 2 to 9 D. These compounds exhibit desirable Stokes shifts in various solvents (25 to 102 nm). Interestingly, silafluorene with a benzaldehyde substituent exhibits competitive solvatochromic behavior. With the ability to tune push-pull properties via the 2,7-substituent, these disubstituted sila- and germafluorenes have excellent potential as biological probes.