Ultrafast Twisting Dynamics of Photoexcited Auramine in Solution

Abstract

Subpicosecond fluorescence up-conversion and transient absorption spectroscopy is applied to study the excited-state dynamics of auramine, a diphenylmethane dye, in liquid solutions. The fluorescence decays, on a time scale of a few picoseconds to a few tens of picoseconds, are found to be nonexponential and solvent viscosity dependent. They can be fitted as a sum of two exponentials in ethanol and three exponentials in decanol with a larger average lifetime in the more viscous solvent. The decays exhibit wavelength-dependent time constants, whereas the fluorescence rise time is instrument limited (150 fs) at all wavelengths. The average decay time increases with the wavelength across the steady-state emission spectrum. The spectral reconstruction indicates a few hundred wavenumbers dynamic Stokes shift accompanied by a drop in the intensity in both solvents. From transient absorption experiments, the fluorescent state population is shown to decay to an intermediate dark state and then to the ground state, with a viscosity-dependent rate. A barrierless or quasi-barrierless photoreaction involving the rotational diffusion of the phenyl rings, with a change in the radiative transition rate along the reaction path, is proposed to explain the wavelength-dependent nonexponential fluorescence decays. Both fluorescence and transient absorption data are discussed in support of an adiabatic photoreaction involving internal twisting and charge shift.

Identifying the Bond Responsible for the Fluorescence Modulation in an Amyloid Fibril Sensor

“…Multi-exponential decay kinetics for the barrierless bond twisting process is well known in the literature. [40,41] The representative fluorescence decays of I a and I b in ethylene glycol are shown in Figure 3, whereas their estimated average fluorescence lifetimes in different solvents are presented in Table 2. It is evident from Figure 3 and also from Table 2 that the average lifetime of I a is always longer as compared to that of I b in all the solvents studied.…”

Ultrafast Photoisomerization of Photoactive Yellow Protein Chromophore Analogues in Solution: Influence of the Protonation State

“…Such behavior was reported for 1,1'-diethyl-4,4'-cyanine in hexanol and propanol, [64] bacteriorhodopsin, [65] trans-retinal in methanol, [66] and auramine in ethanol and decanol. [67] All these systems are known to evolve along a nearly barrierless reaction path. Maximal values for the barrier heights of pCK À , pCKH, pCE À , and pCEH can be estimated from the measured fluorescence lifetimes using Arrhenius' law.…”

A two-state model of twisted intramolecular charge-transfer in monomethine dyes

“…[20][21][22][23][24][25][26][27][28][29][30] Twisting of the methine bonds in the excited state is predicted to lower the potential energy of the excited state, and to lead to charge-localized (for neutral dyes, charge-separated) states from which internal conversion is possible due to a reduced adiabatic energy gap. 31,32 In a) seth.olsen@uq.edu.au.…”