Excited State Equilibrium Constants Research Articles

Control of Excited-State Supramolecular Assembly Leading to Halide Photorelease.

Ground- and excited-state control of halide supramolecular assembly was achieved through the preparation of a series of ester- and amide-functionalized ruthenium polypyridyl complexes in CH2Cl2. Hydrogen-bonding amide and alcohol groups on the receptor ligand were found to direct interactions with halide, while halide association with the ethyl ester groups was not observed. The various functional groups on the receptor ligands tuned the ground-state equilibrium constants over 2 orders of magnitude (1 × 105 to 1 × 107 M-1), and the fractional contribution of each hydrogen-bond donor to the total equilibrium constant was determined. Pulsed-laser excitation of the complexes resulted in excited-state localization on the ester- or amide-functionalized ligands. In the case where the excited state was oriented toward an associated halide ion (the amide complexes), an 80 ± 10 meV Coulombic repulsion was induced that lowered the excited-state equilibrium constant ( K*eq) and resulted in halide photorelease. The rate constants for excited-state halide release ( k*21) were determined, and the values varied based on the functional groups present in the receptor ligand. Complexes with more hydrogen-bonding donors had smaller rate constants for halide photorelease. In a complex without a specific receptor ligand, the excited-state dipole was not oriented toward the associated halide, and the excited state was therefore found to have a larger equilibrium constant for halide association than the ground state.

Ligand Control of Supramolecular Chloride Photorelease.

Supramolecular assembly is shown to provide control over excited-state chloride release. Two dicationic chromophores were designed with a ligand that recognizes halide ions in CH2Cl2 and a luminescent excited state whose dipole was directed toward, 12+, or away, 22+, from an associated chloride ion. The dipole orientation had little influence on the ground-state equilibrium constant, Keq ∼ 4 × 106 M-1, but induced a profound change in the excited-state equilibrium. Light excitation of [12+,Cl-]+ resulted in time-dependent shifts in the photoluminescence spectra with the appearance of biexponential kinetics consistent with the photorelease of Cl-. Remarkably, the excited-state equilibrium constant was lowered by a factor of 20 and resulted in nearly 45% dissociation of chloride. In contrast, light excitation of [22+,Cl-]+ revealed a 45-fold increase in the excited-state equilibrium constant. The data show that rational design and supramolecular assembly enables the detection and photorelease of chloride ions with the potential for future applications in biology and chemistry.

Emission properties of oxyluciferin and its derivatives in water: revealing the nature of the emissive species in firefly bioluminescence.

The first systematic steady-state and time-resolved emission study of firefly oxyluciferin (emitter in firefly bioluminescence) and its analogues in aqueous buffers provided the individual emission spectra of all chemical forms of the emitter and the excited-state equilibrium constants in strongly polar environment with strong hydrogen bonding potential. The results confirmed the earlier hypothesis that excited-state proton transfer from the enol group is favored over proton transfer from the phenol group. In water, the phenol-keto form is the strongest photoacid among the isomers and its conjugate base (phenolate-keto) has the lowest emission energy (634 nm). Furthermore, for the first time we observed green emission (525 nm) from a neutral phenol-keto isomer constrained to the keto form by cyclopropyl substitution. The order of emission energies indicates that in aqueous solution a second deprotonation at the phenol group after the enol group had dissociated (that is, deprotonation of the phenol-enolate) does not occur in the first excited state. The pH-dependent emission spectra and the time-resolved fluorescence parameters revealed that the keto-enol tautomerism reaction, which can occur in a nonpolar environment (toluene) in the presence of a base, is not favored in water.

Distinguishing between keto-enol and acid-base forms of firefly oxyluciferin through calculation of excited-state equilibrium constants.

Although recent years have seen much progress in the elucidation of the mechanisms underlying the bioluminescence of fireflies, there is to date no consensus on the precise contributions to the light emission from the different possible forms of the chemiexcited oxyluciferin (OxyLH2) cofactor. Here, this problem is investigated by the calculation of excited-state equilibrium constants in aqueous solution for keto-enol and acid-base reactions connecting six neutral, monoanionic and dianionic forms of OxyLH2. Particularly, rather than relying on the standard Förster equation and the associated assumption that entropic effects are negligible, these equilibrium constants are for the first time calculated in terms of excited-state free energies of a Born-Haber cycle. Performing quantum chemical calculations with density functional theory methods and using a hybrid cluster-continuum approach to describe solvent effects, a suitable protocol for the modeling is first defined from benchmark calculations on phenol. Applying this protocol to the various OxyLH2 species and verifying that available experimental data (absorption shifts and ground-state equilibrium constants) are accurately reproduced, it is then found that the phenolate-keto-OxyLH(-) monoanion is intrinsically the preferred form of OxyLH2 in the excited state, which suggests a potential key role for this species in the bioluminescence of fireflies.

Experimental Evidence for a Highly Reversible Excited State Equilibrium between s-Cis and s-Trans Rotational Isomers of 2-Methoxynaphthalene in Solution

Detailed studies on the kinetics and the thermodynamics of the excited-state torsional isomerization of the title molecule (1) relative to exocyclic C2-O bond, when dissolved in 3-methylpentane, are reported by means of nontime- and time-resolved fluorescence spectroscopy. Over the broad temperature range studied, 1 exists in spectrally distinct, thermally equilibrated s-cis and s-trans conformations in the ground state (S(0)). In the lowest excited singlet state (S(1)) and above 260 K a pure adiabatic interconversion channel is activated that interconverts s-cis* and s-trans* conformers through a nearly fully reversible isomerization pathway with an activation energy of about 29 kJ/mol. The excited-state equilibrium constant is found to be remarkably temperature-independent just barely exceeding 1 above 260 K. Contrary to the predominantly irreversible photoisomerization mechanism generally observed in related compounds, this work provides insights into the high reversibility of an excited-state rotameric equilibration in solution.

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.

Effect of solvents and pH on the absorption and fluorescence spectra of 2-phenylbenzimidazole

Absorption and fluorescence spectra of 2-phenylbenzimidazole has indicated that the geometry of the molecule is the same in S 0 and S 1 electronic states. The fluorescence takes place from the L b electronic state. Excited-state equilibrium constants have been calculated for PBI + ⇌ PBI ⇌ PBI − equilibria.

Excited state prototropic equilibria of some quinolinols

The prototropic equilibrium constants for the first excited singlet states of the cation-zwitterion equilibria of a series of quinolinols and 5-isoquinolinol have been determined by fluorescence as well as absorption measurements. The results obtained by the two methods are not in good agreement and the reasons for this are discussed. Hückel MO calculations have been carried out on the molecules studied. The protonation energies and the one electron charge densities on the oxygen atoms can be qualitatively correlated with the excited state equilibrium constants.