Molecular rotors and temperature dependency [abstract]
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When fluorescently excited, molecular rotors respond by either photon emission or non-radiative intramolecular rotation. In viscous solvents, intramolecular rotation is restricted, and the fluorescent molecule's quantum yield increases according to the equation logΦ=C+x*logη (where Φ = quantum yield, C is a constant, x = 0.6, and η is viscosity). Two different theories predict C to be temperature dependent; however, no model for C(T) exists. In order to utilize molecular rotors as a viscosity probe, an accurate model of the temperature-dependency of C must be obtained. In attempt to find this model, different solutions were made using five different rotors (CCVJ, DCVJ, CCVJ-TEG, CCVJ-NHS, and TB12 dual dye) each added to solvents of differing viscosities and polarities (ethylene glycol, glycerol, ethanol, and dimethyl sulfoxide). These solutions were mixed and then fluorescently excited at approximately 450nm (optimized for each solvent/rotor combination), and intensity measurements were recorded as temperature increased from 15ºC to 45ºC. Additionally, the viscosity of the solvents in the same temperature range was determined using a Brookfield DV-III+ rheometer, and temperature-dependent viscosity was found through nonlinear regression using the equation η(T)=exp(A+B/(C+Tc)). With known η(T), Equation 1 becomes C'=log I-x*logη(T), where C' reflects the proportionality constant between intensity and quantum yield. C' displayed very little variation with temperature; the maximum coefficient of variation for all solvent/rotor combinations was 0.931%. Additionally, the values for C' showed distinct curves for each solvent, none of which agree with theoretical models that suggest that C' decays exponentially with temperature. Our findings also lead us to believe that with a more accurate model for η(T), C' is actually a temperature-independent constant. To clarify this apparent disagreement, different viscosity-temperature models in conjunction with a more reliable determination of solvent viscosity is required, as well as a review of the mechanisms and theoretical models of intramolecular rotation.