Photoinduced Proton Transfer Reaction Research Articles

Evidence of coupled photoinduced proton transfer and intramolecular charge transfer reaction in para-N,N-dimethylamino orthohydroxy benzaldehyde: Spectroscopic and theoretical studies

Steady state and time resolved fluorescence spectroscopy and quantum chemical calculations have been used to study excited state properties of para-N,N-dimethylamino orthohydroxy benzaldehyde (PDOHBA). Spectral characteristics of PDOHBA support the existence of both donor–acceptor charge transfer (CT) and proton transfer (PT) reaction in the excited state. Structural calculations at Hartree Fock and Density Functional Theory (DFT) levels and theoretical potential energy surfaces (PESs) along the proton transfer and donor twisting coordinates using DFT and Time Dependent Density Functional Theory point towards the possibility of barrierless PT and CT reaction in the first excited state of PDOHBA.

Ultrafast Proton Transfer to Solvent: Molecularity and Intermediates from Solvation- and Diffusion-Controlled Regimes

Photoinduced proton transfer (PT) from cations 6-hydroxyquinolinium (6HQc) and 6-hydroxy-1-methylquinolinium (6MQc) to water and alcohols, and solvation of the zwitterionic conjugate base 1-methylquinolinium-6-olate (6MQz) were studied with stationary and transient absorption spectroscopy and by quantum chemical calculations. Transient emission spectra from 6MQz in acetonitrile and protic solvents shift dynamically to the red without changing their shape and intensity. The shift matches the solvation correlation function C(t) either measured with known solvatochromic probes coumarin 343 and coumarin 153 or derived from infrared/dielectric-loss data on neat solvents. This indicates that 6MQz monitors the solvation dynamics and that no intramolecular electron transfer occurs on a subpicosecond or longer time scale. The PT dynamics S(t) from 6HQc and 6MQc closely follows C(t), being initially 2-3 times slower. This allows for the conclusion that PT is controlled by solvation, with a barrier of 2 kJ/mol. In water, a pre-condition of this ultrafast reaction seems to be hydrogen-bonding between the negatively charged oxygen and two water molecules, resulting in a complex 6HQc:H2O:H2O. The complex is stable due to a high (47 kJ/mol) bonding energy between 6HQc and a water molecule. In acetonitrile, the reaction equilibrium is strongly shifted to the cation. There an intermediate PT state was detected, which may be ascribed to the cationic form 6HQc:H2O due to residual water impurities. In water-acetonitrile mixtures, the ultrafast solvent-controlled PT is followed by a diffusion-controlled reaction; the measured rate kD approximately 1010 s-1 M-1 is characteristic for simple bimolecular diffusion. The dependence of the short-time PT signal on water concentration can be fitted with a Poisson distribution of water molecules around the cation. Altogether, the short-time and long-time behaviors provide strong evidence that diffusion of only one water molecule is sufficient to detach the proton. Subsequent solvent stabilization of the products completes the PT reaction.

Studies on solid-state proton transfer along hydrogen bond of pyrazolone-ring

Evidence from the time-dependent UV–vis reflection spectra studies indicates the compound 1-phenyl-3-methyl-4-(4-methylbenzal)-5-pyrazolone 4-ethylthiosemicarbazone (PM4MBP-ETSC) undergoes a solid-state photochromism. The reaction rate constant was studied by the first-order kinetics curves. X-ray single crystal structural analysis shows that the pyrazolone-ring stabilizes in the keto form. The conclusion can be made that its photochromism in crystalline is associated with a photoinduced proton transfer reaction (inter- and intra-molecular hydrogen transfer) along hydrogen bond leading to a colored tautomer as the compound crystallizes in H-bonded supramolecular configuration.

New Insights on the Excited-State Proton-Transfer Reactions of Betacarbolines: Cationic Exciplex Formation

The photoinduced proton-transfer reactions of two betacarbolines, 9-methyl-9H-pyrido[3,4-b]indole, MBC, and 1,9-dimethyl-9H-pyrido[3,4-b]indole, MHN, in the presence of the proton donor 1,1,1,3,3,3...

Synthesis of 2‐Benzyl‐1,10‐phenanthrolines Substituted by Sulfoxide or Sulfone Groups as Potential Photochromic Compounds.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Sulfinyl- and Sulfonyl-Substituted2-Benzylbenzoxazoles and 2-Benzylbenzo­thiazoles as PotentialPhotochromic Compounds

Some novel 2-benzylbenzoxazoles and 2-benzylben-zothiazoles, substituted at the 2' position by electron-withdrawing groups, were synthesized via the lithiation of the benzoxazole or benzothiazole ring. The expected reversible photoinduced proton-transfer reaction of such compounds was studied.

Thermodynamic changes in the photoinduced proton-transfer reaction of the triplet state of safranine-T.

The enthalpy and volume changes occurring in the triplet excited state proton-transfer reactions of safranine-T (SH+) in aqueous solutions at pH 4.8, 8.3, and 10.4 were investigated using time-resolved photoacoustics (TRP). The transient triplet state species were also studied using laser-flash photolysis (LFP). The LFP experiments showed the prompt formation of 3SH+ with a triplet quantum yield phiT = 0.28 between pH 4.8 and 10.4. At pH 8.3 3SH+ decays directly to the ground state. However, at pH 4.8 and 10.4, 3SH+ reacts with protons or hydroxy ions to form the dication 3SH2(2+) or the neutral 3S species, with diffusion-controlled rate constants of kH+ = 1.6 x 10(10) M(-1) s(-1), and kHO- = 2.6 x 10(10) M(-1) s(-1), respectively. Under the same experimental conditions, the TRP measurements allowed the accurate determination of the energy content of the rapidly formed triplet state 3SH+ i.e. E(T) = 175 kJ mol(-1). The slow component (0.1-3 micros) of the TRP signal at pH 4.8 and 10.4 was attributed to the formation of the species 3SH2(2+) and 3S, respectively. The enthalpy changes associated with the proton-transfer reactions of 3SH+, calculated from the values of the heat released as obtained by TRP, were in remarkable agreement with the values estimated from the thermodynamic data of the acid-base equilibria of the triplet states of the dye. The formation of 3SH+ was accompanied by a volume expansion of 1.8 cm3 mol(-1), which was explained by changes in the hydrogen-bonding interaction of the dye with its solvation sphere. Instead, the volume changes observed upon the formation of 3SH2(2-) and 3S accounted for the electrostrictive effect produced by the change in the charge distribution on the dye after the proton-transfer reaction.

Dynamic study of excited state hydrogen-bonded complexes of harmane in cyclohexane-toluene mixtures.

Photoinduced proton transfer reactions of harmane or 1-methyl-9H-pyrido[3,4-b]indole (HN) in the presence of the proton donor hexafluoroisopropanol (HFIP) in cyclohexane-toluene mixtures (CY-TL; 10% vol/vol of TL) have been studied. Three excited state species have been identified: a 1:2 hydrogen-bonded proton transfer complex (PTC), between the pyridinic nitrogen of the substrate and the proton donor, a hydrogen-bonded cation-like exciplex (CL*) with a stoichiometry of at least 1:3 and a zwitterionic exciplex (Z*). Time-resolved fluorescence measurements evidence that upon excitation of ground state PTC, an excited state equilibrium is established between PTC* and the cationlike exciplex, CL*, lambdaem approximately/= 390 nm. This excited state reaction is assisted by another proton donor molecule. Further reaction of CL* with an additional HFIP molecule produces the zwitterionic species, Z*, lambda(em) approximately/= 500 nm. From the analysis of the multiexponential decays, measured at different emission wavelengths and as a function of HFIP concentration, the mechanism of these excited state reactions has been established. Thus, three rate constants and three reciprocal lifetimes have been determined. The simultaneous study of 1,9-dimethyl-9H-pyrido[3,4-b]indole (MHN) under the same experimental conditions has helped to understand the excited state kinetics of these processes.

Synthesis of 2-Benzyl-1,10-phenanthrolines Substituted by Sulfoxide or Sulfone Groups as Potential Photochromic Compounds

New 2-benzyl-1,10-phenanthrolines, substituted at the 2′ position by electron-withdrawing groups, were synthesized through nucleophilic substitution involving 2-chloro-1,10-phenanthroline and substituted benzyllithiums, respectively. The expected reversible photoinduced proton transfer reaction of such compounds were studied.

Multistep photoinduced proton transfer in crystalline 2-(2′,4′-dinitrobenzyl)pyridine

Photochromism in crystalline 2-(2′,4′-dinitrobenzyl)pyridine is associated with a photoinduced proton transfer reaction leading to a metastable colored tautomer of several hours lifetime at room temperature. Transient absorption experiments performed between 10 and 300 K show two competitive proton transfer routes: a fast direct and temperature-independent one, assigned to an excited state process, and a much slower one, thermally activated, involving a multistep proton transfer in the ground state via a precursor state, assigned to an additional tautomer. An energy level diagram, including energy barriers, is afforded by a temperature-dependent study of the photochromic reaction.

Experimental observation of light-induced solitary waves of analyte bands in capillary electrophoresis.

The phenomenon of electrophoresis in free solution has been studied theoretically down to the molecular level for decades. In addition, intermolecular photo-induced proton transfer reactions, which occur in a wide class of molecules (phenols and aminoarenes) as well as proteins (green fluorescent protein), were also studied extensively. However, the study of the effect of light-induced electrophoretic mobility changes of the analytes in electrophoresis was begun only recently. In the present work, capillary zone electrophoresis was chosen as the environment to measure the magnitude of these electrophoretic mobility shifts induced by light. Background electrolytes (running electrolytes) with high refractive indices were developed, allowing the capillary to work like an optical fiber. The experimental conditions for obtaining stable coupling and guided laser light along the liquid core are discussed. Experimental evidence of band compression is observed, leading to a solitary wave behavior of the analyte band (2-naphthol). These solitary waves result from competition between thermal diffusion (dispersion mechanism) and a nonlinear (band compression) effect due to the combined electrophoresis phenomenon and absorption of guided light by the molecules of the band (which are subjected to a "reversible intermolecular proton transfer reaction" as one of their decay routes). The possibilities of applying this effect to different methods and techniques are also discussed.

Deuterium isotope effects in collision-induced dissociation and photodissociation of the (N2O,H2O)+ cluster ion

We present low energy collision-induced dissociation (CID) and visible photodissociation (PD) data of jet-cooled (N2O,H2O/D2O)+ cluster ions. The same three fragment ions are observed in both CID and PD, namely, H2O+/D2O+, N2O+, and N2OH+/N2OD+. The H2O+ and N2O+ product channels exhibit a small deuterium isotope effect in both CID (10%) and PD (15%). The N2OH+ product channel, on the other hand, exhibits a much larger deuterium isotope effect for CID (30%) and PD (230%). The large difference in deuterium isotope effects observed in the N2OH+ product channel for CID and PD suggests that this product is formed via collision-induced and photoinduced proton-transfer reactions. These results are consistent with the interpretation that a single isomeric form, namely a (N2O⋅H2O)+ association complex, predominates. No evidence is found supporting the existence of a proton-transfer complex, i.e., N2OH+⋅OH. Evidence for the formation of minor amounts of a weakly bound isomeric form with the charge localized on the H2O moiety, i.e., N2O⋅H2O+, is found.

Excited state proton transfer of methyl- and cyano- substituted alloxazines in the presence of acetic acid

The photoinduced excited state double proton transfer reaction of methyl- and cyano-substituted alloxazines was examined by steady state and time-resolved methods. The fluorescence decay times of the alloxazinic forms in 1,2-dichloroethane and 1,2-dichloroethane in the presence of acetic acid are similar and of the order of hundreds of picoseconds. The fluorescence decay times of the isoalloxazinic forms, created by excited state proton transfer, are of the order of nanoseconds. On the basis of the function describing the emission decay of the bands corresponding to the isoalloxazinic and alloxazinic forms and the lifetimes obtained, it was found that the excited alloxazinic form is a precursor of the excited isoalloxazinic form, and the rate constant of the proton transfer process is lower than 10 9 s −1. These results and those of stationary studies, indicating that the isoalloxazinic and alloxazinic species derive from different forms in the ground state, have enabled a model of the proton transfer reaction of alloxazines to be proposed. In this model, the possible formation of alloxazine-acetic acid complexes in a conformation permitting proton transfer, in both the ground and excited states, was assumed.

Photochromism and Thermochromism Driven by Intramolecular Proton Transfer in Dinitrobenzylpyridine Compounds

The photoinduced proton transfer reaction taking place in 2-(2‘,4‘-dinitrobenzyl)pyridine (α-DNBP) and in some of its derivatives is characterized by IR, visible, and NMR spectroscopy. The enamine “NH” structure of the blue phototautomer is confirmed by the analysis of the IR spectra of α-DNBP and its deuterated analogue. 2D NOESY 1H NMR data indicate that this tautomer is predominantly in the cis configuration. In the 2-(2‘,4‘-dinitrobenzyl)phenanthroline derivative, the stabilization of the phototautomer is sufficient to make it thermally accessible. A quantitative analysis of the resulting thermochromism indicates that in toluene solutions the ground state energy of the “NH” form is lowered to 2.9 kcal mol-1 above the thermodynamically stable “CH” tautomer compared to more than 8 kcal mol-1 in the parent α-DNBP compound.

Quantum Chemical Investigations of the Thermal and Photoinduced Proton-Transfer Reactions of 2-(2‘,4‘-Dinitrobenzyl)pyridine

Quantum chemical calculations of the proton-transfer system 2-(2‘,4‘-dinitrobenzyl)pyridine (DNBP) and of derivatives with larger aromatic bases are presented. These systems undergo photochemical isomerizations from a stable form to two different photoisomers. Ab initio methods are employed to investigate the isomerization of methylimine to vinylamine as a model system for the proton-transfer reaction of DNBP. The ground and excited states of both isomers and of the transition state for the isomerization are studied with Hartree−Fock and MRD-CI (multireference singles and doubles CI) methods. The isomerizations of the larger system DNBP and its derivatives are investigated with semiempirical PM3-SCF and PM3-MRD-CI methods. The intrinsic reaction coordinate formalism is used for calculating the reaction pathways from the optimized transition states. The relative stabilities of the various isomers and the mechanism of the intramolecular proton-transfer reaction are discussed. The ortho-nitro group assists t...

Ab initio study of the potential energy functions relevant for hydrogen transfer in formamide, its dimer and its complex with water

The potential energy (PE) functions of the electronic ground state and the lowest nπ ∗ and ππ ∗ excited singlet states of formamide, its dimer and its complex with water have been characterized theoretically along the proton transfer (PT) reaction coordinate. The calculations were performed using the ab initio complete-active-space self-consistent-field (CASSCF) method and second-order perturbation theory employing the CASSCF wavefunction as the reference state. The results suggest that the photoinduced PT reaction in an isolated molecule occurs via successive dissociation and association of the hydrogen atom, whereas in hydrogen-bonded complexes, the PE functions along the PT reaction coordinate are qualitatively the same as for the PT reaction along the intramolecular hydrogen bond. Some implications concerning biological molecules containing peptide linkages are discussed.

Photoinduced Proton Transfers in Methyl Salicylate and Methyl 2-Hydroxy-3-Naphthoate

The effects of solvent and temperature on the dual fluorescence emission of methyl salicylate (MSA) have been reinvestigated, and new insight regarding the photoinduced proton-transfer reactions is reported. The steady-state spectral data obtained in this work are found to be consistent with the spectral assignments proposed by previous investigators. Specifically, the dual emission bands in alcohols are shown to occur from excited states derived from two ground-state rotamers, a and b. The emission band from excited a (the normal band) is in mirror-image relationship with the absorption band and the Stokes shift of this band is ∼5000 cm -1

A novel optical transistor device based on photo-induced proton transfer reactions

An optical transistor device is fabricated with thin-film lumichrome (7,8-dimethylalloxazine) on conductive SnO2 glass; the cathodic current recorded with the emitter electrode is substantially enhanced when the electrode is illuminated (365 nm).

Evidence for the need of a non-Born—Oppenheimer description of excited-state hydrogen transfer

The HOCHCHCHNH (enol from) to OCHCHCHNH 2 (keto form) excited-state isomerization of 1-amino-3-propenal is considered as a model system for photo-induced intramolecular hydrogen (or proton) transfer. The potential energy surfaces of the electronic ground state as well as of the lowest nπ* and ππ* excited singlet states have been characterized by ab initio compelte-active-space self-consistent-field and multireference configuration-interaction calculations. It is found that the nπ* potential energy function exhibits a significant barrier along the proton-transfer reaction coordinate, while the ππ* potential energy function is almost barrierless and intersects the nπ* surface, providing a pathway for ultrafast photo-induced proton transfer. It is argued that consideration of nπ*ππ* vibronic coupling is essential in understanding the photo-induced proton-transfer reaction.

Steady-state and time-resolved studies on intramolecular proton transfer in benzotriazole and benzothiazole crystals

2-(2′-hydroxy-5′-methylphenyl) benzotriazole (TIN) and 2-(2′-hydroxyphenyl) benzothiazole (HBT) are compared in this work with respect to their photoinduced intramolecular proton transfer reaction cycle in single crystal environment. Stationary and ps time-resolved optical spectroscopy is applied between 5 and 300 K. The enol emission and the keto O-O origin are detected for the first time. For both compounds complete energy level schemes are deduced. Temperature dependent reaction dynamics are discussed.