CONFORMATIONAL FLEXIBILITY OF 1,3-BIS(1-PYRENYL)PROPANE THROUGHOUT THE SOL-GEL TO XEROGEL PROCESS

We report on the conformational flexibility of 1,3-bis(1-pyrenyl)propa ne (BPP) doped within tetramethyl orthosilicate (TMOS)-derived sol-gel materials. In normal liquid solution, at low concentrations (similar to 10(-6) M), BPP molecules do not form any ground-state dimers; howev er, on photoexcitation, BPP reorients to form an intramolecular excite d-state dimer (excimer). We follow, using steady-state and time-resolv ed fluorescence spectroscopy, the excimer-like emission from BPP molec ules doped within a TMOS-derived sol-gel monolith throughout the entir e sol-gel to xerogel formation process. Our results indicate that ther e are no detectable ground-state dimers formed even after the xerogel has aged and dried for 3 months. In a fresh gel, there is substantial flexibility of the BPP molecules (like in solution) but the flexibilit y becomes restricted or slowed when the xerogel is formed. We also obs erve that the conformational flexibility of BPP molecules is reduced f urther if the solvent is allowed to escape at a faster rate from the s ol-gel matrix. As observed in dilute BPP solutions, the fluorescence i ntensity decay traces for BPP-doped sol-gel-derived glasses at various stages in the sol-gel to xerogel aging process are best described by a triple-exponential decay law. The time-resolved experiments clearly demonstrate that the BPP conformational dynamics are slowed once the x erogel is formed. Together these results provide information on the sc ale over which dopant dynamics can be controlled within sol-gel-derive d composite materials.