Ultrafast dynamics of a molecular rotor in chemical and biological nano-cavities

Abstract

Molecular rotors have become indispensable tools in monitoring several important processes in chemistry and biology owing to their sensitivity towards viscosity. Despite their importance, less attention has been paid to understanding the excited state properties of molecular rotors. Recently, Maroncelli and coworkers unraveled the excited state photochemistry of a julolidine based molecular rotor, 9-(2-carboxy-2-cyano)vinyl julolidine (CCVJ), and claimed that CCVJ is not a simple rotor probe. Unlike other molecular rotors, photoisomerization is believed to be the main non-radiative decay pathway for this molecule. Inspired by their report, herein, we tried to understand how the excited state dynamics of CCVJ is affected inside the nano-cavities of cyclodextrins (CDs) and human serum albumin (HSA) protein using steady-state and femtosecond fluorescence up-conversion techniques. We observed a pronounced enhancement in fluorescence quantum yield when CCVJ is encapsulated in CDs (β- and γ-CD) and HSA. Femtosecond up-conversion studies reveal that the ultrafast dynamics of CCVJ are drastically retarded inside the nano-cavities of CDs and protein. All these results suggest that photoisomerization, which is believed to be the major non-radiative decay pathway of CCVJ, is severely restricted inside the abovementioned bio-mimetic and biological nano-cavities. The molecular images of orientations of CCVJ inside the nano-cavities of CDs and protein have been discussed by theoretical and molecular modeling studies. We believe the present results might be helpful in exploiting this molecule more in biological and viscosity sensing applications.