Restriction of Molecular Rotation and Intramolecular Charge Distribution in the Photoexcited State of Coumarin Dyes on Gold Nanoparticle Surface

Effect of molecular structure on the excited state dynamics, molecular rotation, and intramolecular charge distribution in the excited states of two structurally similar coumarin dyes, namely, coumarin 343 (C-343) and 7-diethyl amino coumarin 3-carboxylic acid (D-1421), on the Au nanoparticle (NP) surface has been investigated using steady state and time-resolved emission spectroscopy. In the first excited state (S1), both the coumarin dyes exist as a locally excited (LE) state in nonpolar solvent; however, in polar solvent, C-343 exists as intramolecular charge transfer (ICT) state and D-1421 predominantly exists as a twisted intramolecular charge transfer (TICT) state. Photoexcited C-343 molecules transfer energy to Au NP in aqueous solution; however, photoexcited D-1421 molecules do not transfer energy to Au NP due to poor overlap between emission band of D-1421 and plasmon absorption band of Au NP. Interestingly, emission intensity of the S1 state of D-1421 drastically increases on the Au NP surface due to restriction of amino-group rotation which is responsible for the population of TICT states. Intramolecular charge distribution in the excited states for both C-343 and D-1421 dyes found to be restricted on Au NP surface.