NEW FEATURES IN THE PHOTOPHYSICS and PHOTOCHEMISTRY OF 2-(2''-HYDROXY-PHENYL)BENZOTHIAZOLES INTRODUCED BY AMINE SUBSTITUTION

The photophysics and photochemistry of the 4'-diethylamino derivative of both 2-phenyl-benzothiazole and 2-(2'-hydroxyphenyl)benzothiazole have been studied by nanosecond and microsecond laser flash photolysis and picosecond emission spectroscopy. For the non-hydroxy substituted molecule, the singlet excited state was shown to relax primarily via fluorescence emission, and a very weak triplet transient was observed after laser flash excitation. The 2-(2'-hydroxy-4'-diethylaminophenyl)benzothiazole (AHBT) was shown to undergo excited state intramolecular proton transfer (ESIPT) in the picosecond timescale (k greater than 3 x 10(10) s-1) to form a colored zwitter-ion/keto form in solution at room temperature while the ground state back proton transfer was slower by a factor of approximately 10(5). However, in marked contrast with other derivatives of 2-(2'-hydroxyphenyl)benzothiazole and related molecules, the ESIPT was not the only deactivation process of the lowest singlet excited state of the enol form. Under steady-state excitation at room temperature (and low temperature), the fluorescence emission of the enol form was observed. The T-T absorption of the enol form was also observed and furthermore, the ESIPT was shown to have an activation energy which was estimated to be approximately 4 kJ. None of the foregoing, fluorescence and T-T absorption of the enol nor activation energy for proton transfer have been observed for the parent or derivatives of 2-(2'-hydroxyphenyl)benzothiazoles. The striking new features for the ESIPT photochemistry and photophysics for the 4'-diethylamino derivative of 2-(2'-hydroxyphenyl)benzothiazole are discussed and MO calculations are used to aid in the interpretation of some of the experimental results.     

Intramolecular Proton Transfer in 2‐(2′‐hydroxyphenyl)oxazolo[4,5‐b]pyridine: Evidence for Tautomer in the Ground State

The intramolecular proton transfer in a newly synthesized molecule, 2‐(2′‐hydroxyphenyl)oxazolo[4,5‐b]pyridine (HPOP) is studied using UV‐visible absorption, fluorescence emission, fluorescence excitation and time‐resolved fluorescence spectroscopy. In the ground state, the molecule exists as cis‐ and trans‐enol in all the solvents. However, in dioxane, alcohols, acetonitrile, dimethylformamide and dimethylsulfoxide the keto tautomer is also observed in the ground state. Dual fluorescence is observed in HPOP where the large Stoke shifted emission is due to emission from the excited‐state intramolecular proton transfer product, whereas the other emission is the normal emission from enol form. The fluorescence (both normal and tautomer emission) of HPOP is less than those of corresponding benzoxazole and imidazopyridine derivatives. This reveals that the nonradiative decay becomes more efficient upon substitution of electronegative atom on the charge acceptor group. The pH studies substantiate the conclusion that (unlike in its imidazole analog) the third ground state species is the keto tautomer and not the monoanion. The effect of temperature on cis‐enol‐trans‐enol‐keto equilibrium and the nonradiative deactivation from the excited state are also investigated.     

Two competitive routes in the lactim-lactam phototautomerization of a hydroxypyridine derivative cation in water: dissociative mechanism versus water-assisted proton transfer

Ground-state tautomerism and excited-state proton-transfer processes of 2-(6'-hydroxy-2'-pyridyl)benzimidazolium in H2O and D2O have been studied by means of UV-vis absorption and fluorescence spectroscopy in both steady-state and time-resolved modes. In the ground state, this compound shows a tautomeric equilibrium between the lactim cation, protonated at the benzimidazole N3, and its lactam tautomer, obtained by proton translocation from the hydroxyl group to the pyridine nitrogen. Direct excitation of the lactam tautomer leads to its own fluorescence emission, while as a result of the increase of acidity of the OH group and basicity at the pyridine N upon excitation, the lactim species undergoes a proton translocation from the hydroxyl group to the nitrogen, favoring the lactam structure in the excited state. No fluorescence emission from the initially excited lactim species was detected due to the ultrafast rate of the excited-state proton-transfer processes. The lactim-lactam phototaumerization process takes place via two competitive excited-state proton-transfer routes: a one-step water-assisted proton translocation (probably a double proton transfer) and a two-step pathway which involves first the dissociation of the lactim cation to form an emissive intermediate zwitterionic species and then the acid-catalyzed protonation at the pyridine nitrogen to give rise to the lactam tautomer.     

Characteristic changes in the fluorescence spectra of 1-naphthol at the gelation point during the sol-gel-xerogel transitions

The fluorescence spectra of 1-naphthol were observed during the sol-gel-xerogel transitions of two different systems as a function of time; one is in the silicon and titanium (Si:Ti = 4500:1) binary systems involving no catalysts and the silicon and lithium (Si:Li = 99:1) binary systems involving HC1 as the catalyst. During the first stage of the sol-gel reaction of the 1-naphthol system, the fluorescence spectra mainly originated from the broad ’L₂ state. The fluorescence spectrum originating from the anionic species at around 470 nm increased as the reaction proceeded. It was found that the fluorescence spectra originating from the anionic species of 1-naphthol drastically decreased in both systems just after gelation. These findings indicate that it becomes difficult for the dissociation of the excited state of 1-naphthol to give a dissociative proton to the surrounding matrix. The fluorescence-excitation spectra for the Si/Ti system indicated that the main route for the excited state of 1-naphthol to form a dissociative proton is through the excited state of the contact ion pair, while the main route in the Si/Li system is via the direct excitation of the neutral 1-naphthol and its dissociation. The observed changes in the fluorescence spectra of 1-naphthol in these sol-gel systems provide a sensitive means to monitor changes during the sol-gel transition process.     

Proton transfer reaction of a new orthohydroxy Schiff base in protic solvents at room temperature

Ground and excited state inter- and intramolecular proton transfer reactions of a new o-hydroxy Schiff base, 7-ethylsalicylidenebenzylamine (ESBA) have been investigated by means of absorption, emission and nanosecond spectroscopy in different protic solvents at room temperature and 77 K. The excited state intramolecular proton transfer (ESIPT) is evidenced by a large Stokes shifted emission (approximately 11000 cm(-1)) at a selected excited energy in alcoholic solvents. Spectral characteristics obtained reveal that ESBA exists in more than one structural form in most of the protic solvents, both in the ground and excited states. From the nanosecond measurements and quantum yield of fluorescence we have estimated the decay rate constants, which are mainly represented by nonradiative decay rates. At 77 K the fluorescence spectra are found to be contaminated with phosphorescence spectra in glycerol and ethylene glycol. It is shown that the fluorescence intensity and nature of the species present are dependent upon the excitation energy.     

TD-DFT study on fluoride-sensing mechanism of 2-(2'-phenylureaphenyl)benzoxazole: the way to inhibit the ESIPT process

The fluoride-sensing mechanism of the sensor 2-(2'-phenylurea-phenyl)benzoxazole (PUBO) has been investigated by means of the TD-DFT method. The present theoretical study indicates that there is an excited-state intramolecular proton transfer (ESIPT) process in the sensor PUBO. The added fluoride anion could capture the proton in the free N-H moiety instead of the hydrogen-bonding one. The experimental UV/Vis and fluorescence spectra (J. Org. Chem. 2007, 72, 62) are well reproduced by the calculated vertical excitation energies in the ground state and the first singlet excited state. For example, the calculated emission wavelength of PUBO at 534 nm is very close to the fluorescence band at 554 nm. Furthermore, we theoretically confirmed that the added fluoride anions could inhibit the ESIPT process in PUBO. But different from the classical ESIPT-inhibition mechanism, the ESIPT process in the sensor PUBO is inhibited by the high energy barrier of its deprotonated form rather than by the absence of the transferred proton.     

An experimental and theoretical investigation of the photophysics of 1-hydroxy-2-naphthoic acid

Photophysical and photochemical properties of 1-hydroxy-2-naphthoic acid (1,2-HNA) have been investigated experimentally by steady state and time domain fluorescence measurements and theoretically by Hartree-Fock (HF), configuration interaction at the single excitation (CIS) level, density functional theoretic (DFT), and semiempirical (AM1) methods. 1,2-HNA exhibits normal fluorescence that depends on its concentration, nature of the solvent, pH, temperature, and wavelength of excitation. It seems to form different emitting species in different media, akin to 3-hydroxy-2-naphthoic acid (3,2-HNA). The large Stokes shifted emission observed at pH 13 is attributed to species undergoing excited-state intramolecular proton transfer. Nonradiative transition seems to increase on protonation and decrease on deprotonation. AM1(PECI=8) calculations predict the absorption maximum (lambda(max) = 335.9 nm) in reasonable agreement with experiment (lambda(max) = 352 nm) for the neutral 1,2-HNA. They also predict a red shift in absorption on protonation and a blue shift on deprotonation as observed experimentally. CIS calculations tend to overestimate the energy gap and hence underestimate the absorption maxima between the ground and the excited electronic states of 1,2-HNA and its protonated and deprotonated forms. However, they do predict correctly that the excited state intramolecular proton transfer is likely to occur in the deprotonated form of 1,2-HNA and not in the neutral and the protonated forms. A single minimum is found in the potential energy profile for the ground state as well as the first excited state of 1,2-HNA and its protonated species. In contrast, a double minimum with a nominal barrier in between is predicted for the ground state and also the first three excited states of the deprotonated species. The keto form of the deprotonated species is found to be slightly less stable than the enol form in all the states investigated.     

Proton translocation in monomolecular Langmuir-Blodgett films including 2-naphthol and 1,4-anthraquinone derivatives

The scope of the present work is the investigation of proton transport through monomolecular Langmuir-Blodgett (LB) films. The films were formed from amphiphilic molecules: 2-naphtholo-6-sulfonamide of dodecylamine (N) and 1,4-anthraquinono-2 sulfonamide of dodecylamine (A). The 2-naphthol derivative can act as a proton donor due to excited state proton transfer (ESPT) and the 1,4-anthraquinone group can play the role of proton acceptor because of protonation of the reduced form if it is present. Absorption and emission spectra of LB films containing N and A were registered and separated into component bands. Individual absorption and emission peaks observed were assigned to given forms of chromophores. The behavior of different component bands reflects the state of anthraquinone dependent on proton concentration. A correlation of rate and efficiency of ESPT, with changes of the spectra of A, may be expected to yield information concerning the transport of protons from N to A. The influence of the donor-acceptor distance, sample humidity, film arrangement and the presence of protonophores (Gramicidin A) on proton transfer is studied. Our results indicate that the proton can be transported through the film but its concentration vanishes at the distance greater than 30 A. The efficiency of proton transfer depends strongly on water content, film structure and the presence of ion channels.     

Excitation wavelength dependence of the proton-transfer reaction of the green fluorescent protein

Picosecond time-correlated single-photon counting was used to measure the proton-transfer rate of green fluorescent protein (GFP) excited by several wavelengths between 266 and 405 nm. When samples of GFP in water and D2O are excited at short wavelengths, lambda(ex) < 295 nm, the fluorescence properties are largely modified with respect to excitation at a wavelength around 400 nm, the peak of the absorption band of the S0 --> S1 transition of the ROH form of the chromophore. The shorter the excitation wavelength, the longer the decay time of the ROH emission band at 450 nm and the longer the rise time of the RO- emission band at 512 nm. The proton transfer is slower by an order of magnitude and about a factor of 3 when GFP in water and D2O are excited by 266 nm, respectively.     

Inter- and intramolecular excited state proton transfer in 2-hydroxy-9H-carzole-1-carboxylic acid

Absorption, fluorescence and fluorescence excitation spectroscopy and single photon counting time dependence spectrofluorimetry have been used to study the inter- and intramolecular excited state proton transfer (ESIPT) reactions in 2-hydroxy-9H-carbazole-1-carboxylic acid (2-HCA). Except in cyclohexane and water (pH 5) dual fluorescence is observed in rest of the solvents used. Normal Stokes shifted band seems to originate from 2-HCA-1-c and tautomer emission band from the tautomer formed by ESIPT in 2-HCA-1-c followed by structural reorganization. Both these emission band systems originate from the same ground state species. AM1 and CNDO/S-CI calculations have been carried out to establish the identity of the species. Different prototropic equilibria have been determined and discussed.     

Modulation of Excited‐State Proton Transfer of 2‐(2′‐Hydroxyphenyl)benzimidazole in a Macrocyclic Cucurbit[7]uril Host Cavity: Dual Emission Behavior and pKa Shift

The effect of the macrocyclic host, cucurbit[7]uril (CB7), on the photophysical properties of the 2‐(2′‐hydroxyphenyl)benzimidazole (HPBI) dye have been investigated in aqueous solution by using ground‐state absorption and steady‐state and time‐resolved fluorescence measurements. All three prototropic forms of the dye (cationic, neutral, and anionic) form inclusion complexes with CB7, with the largest binding constant found for the cationic form (K≈2.4×10⁶ M ^?1). At pH≈4, the appearance of a blue emission band upon excitation of the HPBI cation in the presence of CB7 indicates that encapsulation into the CB7 cavity retards the deprotonation process of the excited cation, and hence reduces its subsequent conversion to the keto form. Excitation of the neutral form (pH≈8.5), however, leads to an increase in the keto form fluorescence, indicating an enhanced excited‐state intramolecular proton‐transfer process for the encapsulated dye. In both the ground and excited states, the two pK_a values of the HPBI dye show upward shifts in the presence of CB7. The prototropic equilibrium of the CB7‐complexed dye is represented by a six‐state model, and the pH‐dependent changes in the binding constants have been analyzed accordingly. It has been observed that the calculated pK_a values using this six‐state model match well with the values obtained experimentally. The changes in the pK_a values in the presence of CB7 have been corroborated with the modulation of the proton‐transfer process of the dye within the host cavity.     

Barrier for excited-state intramolecular proton transfer and proton tunneling in bis-2,5-(2-benzoxazolyl)-hydroquinone

The excitation spectra of dual fluorescence for isolated bis-2,5-(2-benzoxazolyl)-hydroquinone at low temperatures in a supersonic jet is reported. The vibronic structure near the electronic origin for the 410 nm band is attributed to proton transfer. Proton transfer was observed for the vibrationally cold excited state. From the relative fluorescence quantum yields in organic glasses below 100 K, a barrier for the excited-state proton transfer or 121 ± 17 cm^?1 is obtained. It is concluded that proton tunneling occurs. The relative yield of the usual Stokes fluorescence in an organic glass, as a function of temperature. is compared with the relative yield in the supersonic jet as a function of excitation energy. This leads to estimates of the temperature of the isolated molecule in the excited state.     

Solvatochromic study of 1,2-dihydroxyanthraquinone in neat and binary solvent mixtures

Preferential solvation of a solvatochromic probe has been studied in binary mixtures comprising of a non-protic and a protic solvent. The non-protic solvents employed are carbon tetrachloride (CCl(4)), acetonitrile (AcN) and N,N-dimethyl formamide (DMF) and the protic solvents are methanol (MeOH) and ethanol (EtOH). The probe molecule exhibits different spectroscopic characteristics depending upon the properties of the solubilizing media. The observed spectral features provide an indication of the microenvironment immediately surrounding the probe. Solvatochromic shifts of the ground and excited states of the probe were analysed by monitoring the charge transfer absorption band and the fluorescence emission spectra in terms of the solute-solvent and solvent-solvent interactions. Fluorescence emission spectra show the dual emission due to excited state proton transfer nature of the probe molecule. The effect of solvent and the excitation energy on dual emission are also studied. The observed magnitude of the Stokes shift in the above solvents has been used to deduce experimentally the dipole moment ratio of the probe molecule for the excited state to the ground state. The dipole moment of excited state is higher than the ground state.     

Laser induced fluorescence and resonant two-photon ionization spectroscopy of jet-cooled 1-hydroxy-9,10-anthraquinone

We carried out laser induced fluorescence and resonance enhanced two-color two-photon ionization spectroscopy of jet-cooled 1-hydroxy-9,10-anthraquinone (1-HAQ). The 0-0 band transition to the lowest electronically excited state was found to be at 461.98 nm (21,646 cm(-1)). A well-resolved vibronic structure was observed up to 1100 cm(-1) above the 0-0 band, followed by a rather broad absorption band in the higher frequency region. Dispersed fluorescence spectra were also obtained. Single vibronic level emissions from the 0-0 band showed Stokes-shifted emission spectra. The peak at 2940 cm(-1) to the red of the origin in the emission spectra was assigned as the OH stretching vibration in the ground state, whose combination bands with the C=O bending and stretching vibrations were also seen in the emission spectra. In contrast to the excitation spectrum, no significant vibronic activity was found for low frequency fundamental vibrations of the ground state in the emission spectrum. The spectral features of the fluorescence excitation and emission spectra indicate that a significant change takes place in the intramolecular hydrogen bonding structure upon transition to the excited state, such as often seen in the excited state proton (or hydrogen) transfer. We suggest that the electronically excited state of interest has a double minimum potential of the 9,10-quinone and the 1,10-quinone forms, the latter of which, the proton-transferred form of 1-HAQ, is lower in energy. On the other hand, ab initio calculations at the B3LYP/6-31G(d,p) level predicted that the electronic ground state has a single minimum potential distorted along the reaction coordinate of tautomerization. The 9,10-quinone form of 1-HAQ is the lowest energy structure in the ground state, with the 1,10-quinone form lying approximately 5000 cm(-1) above it. The intramolecular hydrogen bond of the 9,10-quinone was found to be unusually strong, with an estimated bond energy of approximately 13 kcal/mol (approximately 4500 cm(-1)), probably due to the resonance-assisted nature of the hydrogen bonding involved.     

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.     

Excited-state proton transfer in 2-(2'-hydroxyphenyl)benzothiazole: formation of the anion in polar solvents

Excited-state proton transfer in 2-(2'-hydroxyphenyl)benzothiazole (HBT) dissolved in pyridine is investigated. New absorptions and fluorescence bands are detected after addition of water or NaOH to the solution. The spectra are identical to the HBT anion absorption and fluorescence. Picosecond spectroscopy is used to determine the kinetics of electronically excited states. The excited-state lifetime of the anion is 3.5 ns. The tautomeric fluorescence of HBT after proton transfer builds up within 4 ps after excitation and decays with a time constant of 20 ps.     

Excited state proton transfer reaction of two new intramolecularly hydrogen bonded Schiff bases at room temperature and 77K.

Two new orthohydroxy Schiff bases, 7-phenylsalicylidene benzylamine (PSBA) and 7-ethylsalicylideneaniline (ESA) have been synthesized. The excited state intramolecular proton transfer (ESIPT) and the structure of PSBA and ESA in its crystalline form and in the solvents n-hexane, n-heptane and 1,4-dioxane have been investigated by means of absorption, emission and nanosecond spectroscopy at room temperature and 77K. One ground state species has been detected both in neutral and basic solutions of both PSBA and ESA: the cis-enol form with an intramolecular hydrogen bond. The ESIPT and formation of keto tautomer are evidenced by a large Stokes shifted emission (approximately 12000 cm(-1)) at room temperature only in the case of ESA. On the other hand the keto tautomer is the predominant species at 77K in a solid matrix and as a solid sample at room temperature both in the case of ESA and PSBA. In the case of both ESA and PSBA the more intense, higher energy emission is due to the species which has not undergone ESIPT and attributed mainly due to cis-enol form. The trans-enol form is also observed by changing the excitation wavelength. Both the compounds are found to undergo a structural change to a zwitterionic and intermolecular hydrogen bonded form in the presence of a strong base like triethylamine. From the nanosecond measurements and quantum yield of fluorescence we have estimated the decay rates of proton transfer reaction in the case of PSBA. Our theoretical calculation at the AM1 level of approximation shows that the ground singlet state has a rather large activation barrier both in the case of PSBA and ESA. The barrier height is much lower on the corresponding excited singlet surface only in the case of ESA. The process is predicted to be endothermic in the ground state and exotherrmic in the excited singlet state.     

Dramatic pressure-dependent quenching effects in supercritical CO2 assessed by the fluorescence of 4'-dimethylamino-3-hydroxyflavone. Thermodynamic versus kinetics control of excited-state intramolecular proton transfer

Steady-state fluorescence of 4'-dimethylamino-3-hydroxyflavone (DMA3HF) was observed in supercritical carbon dioxide (scCO(2)). Excited-state intramolecular proton transfer (ESIPT) occurs resulting in two well-separated emission bands corresponding to the normal and tautomer forms. As the scCO(2) density exceeds 0.7 g/mL, the relative intensity of the two bands tends to a constant value, comparable to that observed for organic solvents with ET(30) = 33.0 +/- 0.5 kcal/mol, such as toluene and di-n-butyl ether. At lower densities, the substantial decrease of the total fluorescence intensity (a 600-fold decrease as the pressure decreases from 100 to 80 bar) is accompanied by an even more accentuated decrease of the tautomer fluorescence. This can be explained by a shift in the equilibrium between normal and tautomer forms, concomitant with a more efficient quenching of the less solvated fluorophore, that may change the thermodynamic control of the relative population of the two emissive species to a kinetic control.     

TD-DFT Investigation of 2,5-Bis(2-benzothiazolyl)hydroquinone and 2,5-Bis(benzo[d]thiazol-2-yl)-4-methoxyphenol

Density functional theory (DFT) and time dependent density functional theory (TD-DFT) calculations of two excited state intramolecular proton transfer (ESIPT) molecules [2,5-bis(2-benzothiazolyl)hydroquinone and 2,5-bis(benzo[d]thiazol-2-yl)-4-methoxyphenol] were performed to study their structural and photo-physical behavior upon excitation. The most stable structure was established by optimizing all possible rotamers. The vertical excitation and emission wavelengths obtained by using TD-DFT show very good correlation with the experimental values. A correlation has been established based on the absorption values to determine the contribution of stable rotamers.     

Solvent‐ and concentration‐sensitive fluorescence of 2‐(3,4,5,6‐tetrafluoro‐2‐hydroxyphenyl)benzoxazole

2‐(3,4,5,6‐Tetrafluoro‐2‐hydroxyphenyl)benzoxazole ( 2 ) emits the long wavelength fluorescence around 500 nm in nonpolar solvent via the intramolecular proton transfer process in the excited state of 2 (enol‐form) and also emits the intermediate wavelength fluorescence around 440 nm in polar solvent, which is assumed to originate from the excited state of 2 (anion). The ease of formation of 2 (anion), compared to 2‐(2‐hydroxyphenyl)benzoxazole ( 1 ), is explained by the strongly inductive fluorine atoms. In a solvent with the intermediate polarity, 2 emits both fluorescences and their relative intensity is dependent on the concentration of 2 , which is supposed to be caused by the high sensitivity of the intermediate wavelength emission to the concentration quenching.