Study Of Excited-state Proton Transfer Research Articles

Analytic gradient and derivative couplings for the spin-flip extended configuration interaction singles method: Theory, implementation, and application to proton transfer.

We present the formalism of analytic gradients and derivative couplings for the spin-flip extended configuration interaction with single excitations (SF-XCIS) method. We report an efficient implementation of the SF-XCIS method in the framework of semiempirical quantum chemistry that allows fast excited-state calculations for large systems. The performance of the SF-XCIS method in combination with semiempirical orthogonalization-corrected models (OMx) is statistically evaluated for vertical singlet excitation energies. The SF-XCIS method treats the ground state and excited states in a fully balanced manner and properly describes conical intersections involving the ground state. It can thus be used in fewest switches surface hopping (FSSH) simulations of nonadiabatic dynamics processes. This is demonstrated in an OM2/SF-XCIS FSSH pilot study of excited-state proton transfer in 7-(2-pyridyl)indole.

Photoacidic and Photobasic Behavior of Transition Metal Compounds with Carboxylic Acid Group(s).

Excited state proton transfer studies of six Ru polypyridyl compounds with carboxylic acid/carboxylate group(s) revealed that some were photoacids and some were photobases. The compounds [Ru(II)(btfmb)2(LL)](2+), [Ru(II)(dtb)2(LL)](2+), and [Ru(II)(bpy)2(LL)](2+), where bpy is 2,2'-bipyridine, btfmb is 4,4'-(CF3)2-bpy, and dtb is 4,4'-((CH3)3C)2-bpy, and LL is either dcb = 4,4'-(CO2H)2-bpy or mcb = 4-(CO2H),4'-(CO2Et)-2,2'-bpy, were synthesized and characterized. The compounds exhibited intense metal-to-ligand charge-transfer (MLCT) absorption bands in the visible region and room temperature photoluminescence (PL) with long τ > 100 ns excited state lifetimes. The mcb compounds had very similar ground state pKa's of 2.31 ± 0.07, and their characterization enabled accurate determination of the two pKa values for the commonly utilized dcb ligand, pKa1 = 2.1 ± 0.1 and pKa2 = 3.0 ± 0.2. Compounds with the btfmb ligand were photoacidic, and the other compounds were photobasic. Transient absorption spectra indicated that btfmb compounds displayed a [Ru(III)(btfmb(-))L2](2+)* localized excited state and a [Ru(III)(dcb(-))L2](2+)* formulation for all the other excited states. Time dependent PL spectral shifts provided the first kinetic data for excited state proton transfer in a transition metal compound. PL titrations, thermochemical cycles, and kinetic analysis (for the mcb compounds) provided self-consistent pKa* values. The ability to make a single ionizable group photobasic or photoacidic through ligand design was unprecedented and was understood based on the orientation of the lowest-lying MLCT excited state dipole relative to the ligand that contained the carboxylic acid group(s).

The change in excited-state proton transfer kinetics of 1-naphthol in micelles upon the binding of polymers: The influence of hyaluronan hydration

The fast deprotonation of 1-naphthol was studied in aqueous solution and in polymer–cetyltrimethylammonium bromide (CTAB) mixtures using picosecond fluorescence spectroscopy to study the influence of hyaluronan hydration in polymer–surfactant interactions. The aqueous micelle solution showed the expected change of proton transfer rate around the reported critical micelle concentration (∼1mM). The proton transfer rate dependence on CTAB concentration in the hyaluronan–CTAB and polystyrenesulfonate–CTAB systems differed significantly from that in the aqueous micelle solution. The dynamic study of excited state proton transfer (ESPT) revealed the significant influence of hyaluronan hydration in CTAB micelles. When hyaluronan–CTAB aggregates were formed at the CTAB concentration of 0.5mM, a tenfold decrease in the rate of deprotonation was observed when compared to polystyrenesulfonate–CTAB aggregates due to hyaluronan hydration. In 2mM CTAB–hyaluronan aggregates, the rate of deprotonation was found to be almost two times faster than in the 2mM CTAB or polystyrenesulfonate–CTAB system. Furthermore, the study of excited-state proton transfer of 1-naphthol confirmed that hyaluronan hydration layer penetrates into the micelle and changes the emission characteristics of 1-naphthol.

Golden rule study of excited-state proton transfer in 2-(2-hydroxyphenyl)benzoxazole and 2-(2-hydroxy-4-methylphenyl)benzoxazole

Theoretical Golden Rule treatment of the dynamics of triplet and singlet excited-state intramolecular proton transfer (ESIPT) in 2-(2'-hydroxyphenyl)benzoxazole (HBO) and its derivative 2-(2'-hydroxy-4'-methylphenyl)benzoxazole (MHBO) is presented. These compounds express typical and very similar non-Arrhenius temperature behavior of the rate of triplet keto-enol equilibration θ 1 =k K-E +k E-K despite the experimental evidence of different reaction regimes and keto-enol energy gaps

Steady-state and time-resolved fluorescence study of excited-state proton transfer in 1-aminoalkyl-2-naphthols

Intramolecular excited-state proton transfer in 1-morpholinylmethyl- and 1-piperidinylmethyl-2-naphthol in various solvents and binary solvent mixtures is demonstrated by steady-state and time-resolved fluorescence spectroscopy. The excited-state proton-transfer equilibrium constant was calculated from a fluorescence bandshape analysis, and its large solvent dependence arises primarily from local solute–solvent interactions. Only monoexponential fluorescence decay, giving the same lifetime irrespective of the emission wavenumber, was resolved (time resolution ca. 200 ps), and this proved proton transfer to be considerably faster than fluorescence decay. Therefore, equilibrium solvation is also established very fast at low concentrations of weakly associating solvents (e.g. tetrahydrofuran and butyl chloride), and local solvation is characterized by multiple encounter and re-encounter reactions. A good correlation between the spectral shift of the zwitterionic form, its non-radiative deactivation rate and the excited-state equilibrium constant was established.