Ultrafast excited state reaction dynamics in light-driven unidirectional rotary molecular motors and fluorescent protein chromophores

Excited state dynamics on an ultrafast timescale can provide insight into primary events in photochemical and photobiological processes. In this work, excited state dynamics of two important systems are characterized: unidirectional molecular rotary motors and HBDI derivatives (synthetic chromophores of the green fluorescent protein (GFP)). In both cases, the excited state is selectively probed by ultrafast fluorescence up-conversion with a time resolution better than 50 fs. Molecular motors have biphasic (sub-picosecond and picosecond) fluorescence decays and oscillations attributed to excitation of coherently excited vibrational modes. The fluorescence data were contrasted with excited state decay and ground state recovery kinetics recorded using ultrafast transient absorption. Combining these experimental data with substituent dependence and solvent dependence studies, as well as existing calculations, we proposed a coupled two-state model for dynamics on the excited state potential energy surface. These data have implications for the design and optimisation of optically driven molecular motors. A ‘molecular propeller’ was also studied and shown to be more sensitive to medium friction than the motor. The GFP experiments focused on determining the effect of alkyl substitution upon excited state dynamics of HBDI. HBDI in solution exhibits a very low quantum yield compared to the chromophore in its protein environment. Large alkyl substituents were found to shift the spectra but to exhibit only small retardation effects upon the excited state decay time, even in highly viscous solvents. This supports an assignment of a volume conserving isomerization mechanism promoting radiationless decay. Substituents which distort the planar structure of the chromophore lead to an enhanced radiationless decay. This provides further evidence for a link between radiationless decay of the excited state and twisting of HBDI.

Name dc:type.qualificationname

phd

Level dc:type.qualificationlevel

doctoral

Grantor dc:publisher.institution

University of East Anglia

Year dc:date.issued

2013

Author dc:creator

• Conyard, Jamie

Language dc:language

en