Joonyoung F. Joung

Origin of the Reversible Thermochromic Properties of Polydiacetylenes Revealed by Ultrafast Spectroscopy

Polydiacetylenes (PDAs) with thermochromic properties undergo colorimetric transitions when the external temperature is varied. This capability has the potential to enable these materials to be used as temperature sensors. These thermochromic properties of PDAs stem from their temperature-dependent optical properties. In this work, we studied the temperature-dependent optical properties of Bis-PDA-Ph, which exhibits reversible thermochromic properties, and PCDA-PDA, which exhibits irreversible thermochromic properties, by UV-visible absorption and femtosecond transient absorption spectroscopy. Our results indicate that the electronic relaxation of PDAs occurs via an intermediate state in cases where the material exhibits reversible thermochromic properties, whereas the excited PDAs relax directly back to the ground state when irreversible thermochromic properties are observed. The existence of this intermediate state in the electronic relaxation of PDAs thus plays an important role in determining their thermochromic properties. These results are very important for both understanding and strategically modulating the thermochromic properties of PDAs.

Photoinduced reversible phase transition of azobenzene-containing polydiacetylene crystals

An azobenzene-containing supramolecular polydiacetylene (PDA) crystal undergoes a photoinduced reversible blue-to-red phase transition accompanied by crystal tearing.

Effect of NaCl Salts on the Activation Energy of Excited-State Proton Transfer Reaction of Coumarin 183

Coumarin 183 (C183) was used as a photoacid to study excited-state proton transfer (ESPT) reactions. Here, we studied the effect of ions on the ESPT of C183 in aqueous NaCl solutions using a steady-state fluorescence spectroscopy and time-correlated single photon counting (TCSPC) method. The acid dissociation equilibrium of excited-state C183 and the activation energy for the ESPT of C183 were determined as a function of NaCl concentration. The change in the equilibrium constant was found to be correlated with the solvation energy of deprotonated C183. Frequency-resolved TCSPC signals measured at several temperatures were analyzed by using a global fitting analysis method which enabled us to extract all the rate constants involving the ESPT reaction and the spectra of individual species. The activation energy for the ESPT reaction of C183 was highly dependent on NaCl concentration. Quantum chemical calculations were used to calculate the local hydrogen-bond (H-bond) configurations around C183 in aqueous NaCl solutions. It was found that the activation energy for the ESPT was determined by the local H-bond configurations around C183 which were significantly influenced by the dissolved ions.

Cationic Effect on the Equilibria and Kinetics of the Excited-State Proton Transfer Reaction of a Photoacid in Aqueous Solutions

Dissolved ions have a significant effect on the chemical equilibria and kinetics in aqueous solutions by changing the physical properties and hydrogen bond network of water. In this work, the ionic effects on the excited-state proton transfer (ESPT) reactions of Coumarin 183 (C183) in aqueous ionic solutions are comprehensively studied in terms of p Ka, p Ka*, activation energies, and kinetic isotope effect (KIE). The acid dissociation constants (p Ka and p Ka*) of C183 on the ground and excited states are determined by UV-visible absorption and steady-state fluorescence spectroscopy. The activation energies ( Ea) and KIE for the ESPT reaction of C183 are directly obtained by time-resolved fluorescence spectroscopy. The changes in p Ka, p Ka*, Ea, and KIE values of C183 are found to be dependent on the charge density of cations. The secondary KIE is more substantially influenced by the dissolved ions than the primary KIE. Furthermore, the ionic effects on the equilibrium (p Ka and p Ka*) and kinetic ( Ea and KIE) parameters of C183 are found to be well-explained by the free-energy reactivity relation. Our current results are very important in understanding the ionic effects on the equilibria and ESPT kinetics of photoacids in aqueous ionic solutions.