Ultrafast Elementary Photochemical Processes of Organic Molecules in Liquid Solution

Abstract

Ultrafast photochemical reactions in liquids occur on similar or shorter time scales compared to the equilibration of the optically populated excited state. This equilibration involves the relaxation of intramolecular and/or solvent modes. As a consequence, the reaction dynamics are no longer exponential, cannot be quantified by rate constants, and may depend on the excitation wavelength contrary to slower photochemical processes occurring from equilibrated excited states. Such ultrafast photoinduced reactions do no longer obey the Kasha-Vavilov rule. Nonequilibrium effects are also observed in diffusion-controlled intermolecular processes directly after photoexcitation, and their proper description gives access to the intrinsic reaction dynamics that are normally hidden by diffusion. Here we discuss these topics in relation to ultrafast organic photochemical reactions in homogeneous liquids. Discussed reactions include intra- and intermolecular electron- and proton-transfer processes, as well as photochromic reactions occurring with and without bond breaking or bond formation, namely ring-opening reactions and cis-trans isomerizations, respectively.

Intramolecular photo‐driven electron transfer in the series of DMABN related compounds with para‐substituted acceptors. Study of the rate constants by Marcus theory

“…It is easy to notice that the contribution of the CT band to fluorescence is always (Table , Figure A) higher than the contribution of the LE band, which is often the case for DA fluorophores in polar media. However, it was found that the ratio of fluorescence quantum yields for the CT and LE bands can depend on not only the natures of the donor and acceptor and on the solvent polarity, viscosity, and temperature, but also on the excitation wavelength . Figure B shows these dependences for 1 to 4 and 5 to 8 .…”

Ultrafast Dynamics of a Fluorescent Tetrazolium Compound in Solution

“…[73] Although IVR for excited molecules in the liquid phase proceeds in far less than ap icosecond, [73] vibrational relaxation may exhibit ab iphasic behavior with a sub-ps component, reflecting the transfer of vibrationale nergy to the first solvation shell of the solute, followed by further thermalization within tens of picoseconds. These may comprise intramolecular vibrational redistribution( IVR),s tructurala nd vibrational relaxation, and solvation, that is, solvent molecules reorient after excitation of as olute molecule to minimize its overall free energy.…”

“…These may comprise intramolecular vibrational redistribution( IVR),s tructurala nd vibrational relaxation, and solvation, that is, solvent molecules reorient after excitation of as olute molecule to minimize its overall free energy. [73,76] Therefore, disentanglingt hese different contributions, which all add to the Stokes shift, is often not possible. [74,75] Solvation also gives rise to dynamics that are not describedm onoexponentially,b ut can be composed of ar apid decay followed by a slowero ne, which can last up to nanosecondsi ft he solvent viscosity is high.…”

Photochemistry — development and achievements

“…It should be pointed out that current techniques allow scientists also to determine the structure of electronically excited molecules and nanoparticles (Coppens et al, 2002;Coppens et al, 2014). It should be pointed out that outcomes of attosecond (photo)physics penetrate to chemistry providing valuable information on the electron dynamics connected to chemical processes (Nisoli et al, 2017;Kumpulainen et al, 2017).…”