1b) is likely due to small variations in dielectric constants of these mixtures at different temperatures (see the ESI for more details †). A slight spread of values obtained from solutions of 1 with viscosity of less than 30 cP (particularly for the quantum yields in Fig. Importantly, for both the lifetime and the quantum yield graphs, a good overlap of values measured at the same viscosity was observed, even though both the temperature and the solvent composition was varied. The corresponding fluorescence quantum yield increased from 0.02 to 0.77 in the same viscosity range. The fitted lifetimes ranged from 260 ps to 5700 ps for viscosities between 1. The fluorescence of 1 decayed monoexponentially in all solvents. Estimated measurement errors were 1% (lifetime), 1% (relative quantum yield) and 5% (absolute quantum yield). For each curve recorded at a fixed temperature increasing % of glycerol results in higher viscosity. The changes in fluorescence lifetime (a) and quantum yield (b) of 1 as a function of temperature recorded in methanol–glycerol mixtures of different viscosity, ranging from 30% to 100% glycerol content. Thus, it allowed us to (i) study the widest range of possible viscosities, and (ii) examine the effect of temperature on the fluorescence lifetime and the quantum yield of 1, Fig. Initially, we measured fluorescence quantum yields and lifetimes of 1 in methanol/glycerol mixtures of different viscosities at different temperatures in order to untangle the viscosity and temperature sensitivity. 9, 17 Temperature dependence of the photophysics of 3 has not been previously examined or utilized. 14 In contrast, it is well known that 2 can be employed as a temperature sensor, 15, 16 although it has been used as a viscosity probe on several occasions. bright excited states of this molecular rotor, and the effect of changing the temperature is that of changing viscosity alone, 7, 12 similar to the conclusions of Haidekker and co-workers for DCVJ, CCVJ and other anilino-based molecular rotors. 3, 4, 12, 13 For molecular rotors similar to 1 it was established that temperature does not change the population of dark vs. Molecular structures of Bodipy-C 10 ( 1), Kiton Red ( 2), and the porphyrin dimer ( 3).įluorescent molecules 1 and 3 have been used previously as viscosity sensors in cells and membranes. Despite the fact that changing temperature also causes changes in viscosity, we demonstrate how, for each of these fluorophores, the effect of temperature can be decoupled in order to provide independent measurements of either (or even both) of these parameters. Here we consider the effect of temperature on the photophysical behaviour of three molecular rotors: Bodipy-C 10 ( 1), sulforhodamine B ( 2) and a conjugated porphyrin dimer ( 3) ( Scheme 1). However, to the best of our knowledge, the effect of temperature on the photophysical behaviour of molecular rotors has not been systematically examined. Likewise, temperature can also affect the balance in the relative population of the bright and dark excited states of fluorophores by enabling or precluding the bright-to-dark state transitions. Thus both the fluorescence quantum yield and the lifetime of molecular rotors can be correlated to the viscosity of the surrounding environment, and can therefore be exploited for viscosity measurements in micro-heterogeneous systems such as biological cells, 3, 4, 7, 8 atmospheric aerosols, 9 and on the microscale of inhomogeneous materials. 6 In a viscous environment, the intramolecular rotation is slowed down, and the non-radiative decay of a molecular rotor is consequently suppressed. 1– 5 The sensitivity of such fluorophores is typically brought about by the interplay between ‘bright’ and ‘dark’ excited states, which is controlled by the rate of intramolecular rotation. Molecular rotors have demonstrated their usefulness as fluorescent molecules sensitive to the viscosity of their environment.
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