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)propane (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; however, on photoexcitation, BPP reorients to form an intramolecular excited-state dimer (excimer). We follow, using steady-state and time-resolved fluorescence spectroscopy, the excimer-like emission from BPP molecules doped within a TMOS-derived sol-gel monolith throughout the entire sol-gel to xerogel formation process. Our results indicate that there 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 flexibility becomes restricted or slowed when the xerogel is formed. We also observe that the conformational flexibility of BPP molecules is reduced further if the solvent is allowed to escape at a faster rate from the sol-gel matrix. As observed in dilute BPP solutions, the fluorescence intensity 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 xerogel is formed. Together these results provide information on the scale over which dopant dynamics can be controlled within sol-gel-derived composite materials.