title: Excited-State Proton Transfer and Formation of the Excited Tautomer of 3-Hydroxypyridine-Dipicolinium Cyanine Dye.
abstract: Steady-state and time-resolved fluorescence techniques and theoretical calculations were employed to study the photoprotolytic properties of a newly synthesized photoacid 3-hydroxypyridine-dipicolinium cyanine (HPPC) dye. This dye is similar to quinone cyanine 9, which we have previously studied and is the strongest photoacid currently synthesized. In this compound, we found that several proton transfer phenomena occur after excitation. We found that the excited-state proton transfer (ESPT) rate in water is ultrafast with kPT ≈ 1.5 × 10(12) s(-1). In methanol and ethanol the rate is slower by about 5 and 6 times, respectively. The fluorescence spectrum of HPPC in water consists of three bands with maxima at 520, 600, and 665 nm, whereas in monols and other protic solvents the fluorescence spectrum consists only of two emission bands at 530 and ∼700 nm. We assign the emission bands of HPPC at 520 nm to the protonated form and the 700 nm band in monols and 665 nm in water to the deprotonated form. The 600 nm band that is the most intense band in the fluorescence spectrum of HPPC in water we assign to the tautomeric form in which the proton is attached to the pyridine's nitrogen atom. On the basis of density functional calculations, we suggest that in water the proton transfer process to the pyridine's nitrogen atom occurs in a stepwise manner via a two water molecule bridge.
authors.affiliation: Raymond and Beverly Sackler Faculty of Exact Sciences, School of Chemistry, Tel Aviv University , Tel Aviv 69978, Israel.
authors.forename: Ori
authors.initials: O
authors.lastname: Green
authors.affiliation: Raymond and Beverly Sackler Faculty of Exact Sciences, School of Chemistry, Tel Aviv University , Tel Aviv 69978, Israel.
authors.forename: Ron
authors.initials: R
authors.lastname: Simkovitch
authors.forename: Luís
authors.initials: L
authors.lastname: Pinto da Silva
authors.forename: Joaquim C G
authors.initials: JC
authors.lastname: Esteves da Silva
authors.affiliation: Raymond and Beverly Sackler Faculty of Exact Sciences, School of Chemistry, Tel Aviv University , Tel Aviv 69978, Israel.
authors.forename: Doron
authors.initials: D
authors.lastname: Shabat
authors.affiliation: Raymond and Beverly Sackler Faculty of Exact Sciences, School of Chemistry, Tel Aviv University , Tel Aviv 69978, Israel.
authors.forename: Dan
authors.initials: D
authors.lastname: Huppert