Quantum Dynamics Study of the Excited-State Double-Proton Transfer in 2,2′-Bipyridyl-3,3′-diol

• Published in: Chemphyschem 2004-09, Vol.5 (9), p.1372-1378
• Main Authors: Gelabert, Ricard, Moreno, Miquel, Lluch, José M.
• Format: Article
• Language: English
• Subjects: 2,2'-Dipyridyl - analogs & derivatives; 2,2'-Dipyridyl - chemistry; Atomic and molecular physics; Calculations and mathematical techniques in atomic and molecular physics (excluding electron correlation calculations); Chemistry; Computer Simulation; Density-functional theory; Electrochemistry; Electronic structure of atoms, molecules and their ions: theory; Exact sciences and technology; excited states; femtochemistry; General and physical chemistry; Molecular Structure; Photochemistry; Photochemistry and radiation chemistry; Physical chemistry of induced reactions (with radiations, particles and ultrasonics); Physics; proton transfer; Protons; quantum dynamics; Quantum Theory; reaction mechanisms; Thermodynamics; Vibration
• ISSN: 1439-4235; 1439-7641
• DOI: 10.1002/cphc.200400078

Density functional theory and quantum dynamics simulations have been used to study the double‐proton transfer reaction in 2,2′‐bipyridyl‐3,3′‐diol in the first singlet excited electronic state. This process is experimentally known to be branched: It consists of a fast, concerted reaction mechanism (τ≈100 fs) and a stepwise reaction mechanism [with a fast initial step (τ≈100 fs) and a slower final step (τ≈10 ps)]. Quantum dynamics simulations on a two‐dimensional model reveal that the concerted reaction occurs despite the nonexistence of a concerted reaction path, but they fail to explain the relative slowness of the stepwise mechanism. A qualitative simulation using a three‐dimensional model suggests that internal vibrational relaxation (IVR) might be the reason why the second stage of the stepwise mechanism is so slow. Slowing down an ultrafast process: The double‐proton transfer process in 2,2′‐bipyridyl‐3,3′‐diol in the first excited singlet state is a branched process with very different time scales for each mechanism (see Figure). A quantum dynamics study on a time‐dependent density functional theory (TDDFT) model surface is used to find an explanation to this behavior.