Investigation of solute solvation within renewable solvents via pyrene fluorescence

Biorenewable solvents are emerging as suitable candidates to replace commonly used toxic organic solvents. Four such solvents, γ-valerolactone (GVL), dimethylisosorbide (DMI), 2-methyltetrahydrofuran (MeTHF), and cyclopentyl methyl ether (CPME), were investigated for their solute solvation behavior by utilizing the multi-dimensional fluorescence response of a well-established polycyclic aromatic hydrocarbon probe, pyrene, in the temperature range of 288 K to 348 K. Band 1-to-band 3 emission intensity ratios for these four solvents indicate widely varying cybotactic region dipolarities experienced by pyrene, following the trend GVL > DMI > MeTHF > CPME, which correlates well with the static dielectric constants of the solvents. Dipolarity decreases with increasing temperature, with the most polar solvent, GVL, exhibiting the highest sensitivity and the least polar solvent, CPME, showing the lowest sensitivity towards temperature change. Fluorescence lifetimes of pyrene were found to be considerably shorter in the two relatively nonpolar renewable solvents as compared to the two more polar ones, with lifetimes ranging from 15 ns to 150 ns. Fluorescence quenching of pyrene by nitromethane, an electron/charge acceptor during the quenching process, was found to obey the Stern-Volmer equation for both steady-state and lifetime data. The bimolecular quenching rate constants (k_q) imply effective quenching of pyrene via quencher nitromethane. The difference (k_diff - k_q), where k_diff is the rate constant for the diffusion-controlled process (k_diff) within the four renewable solvents estimated using the simplified Stokes-Einstein-Smoluchowski expression, reveals k_diff > k_q for low-viscosity systems (η less than ∼2.6 mPa s); however, within the more viscous solvent DMI, the simplistic Stokes-Einstein-Smoluchowski assumption appears to break down. For two different renewable solvents with similar dynamic viscosities (η), k_q are observed to be very different, hinting that the molecular architecture of the renewable solvent controls solvation and diffusion dynamics. Furthermore, while the linearity between k_q and 1/η is excellent for each of the renewable solvents investigated, this linearity does not hold when the four solvents are combined. These findings indicate that widely varying solute solvation and diffusion can be achieved based on the molecular structure of renewable solvents.