Excited Keto Form Research Articles

Impact of Polymer Matrix on Polymer Mechanochromism from Excited State Intramolecular Proton Transfer†

Comprehensive SummaryMechanochromic polymers based on non‐covalent changes have attracted much attention recently. Herein, we report the impact of inter/intramolecular hydrogen bonds on polymer mechanochromism from the excited state intramolecular proton transfer (ESIPT) process. PhMz‐NH2‐OH and PhMz=2A are designed and obtained by simple and high‐yield synthesis, and are connected into polyurethane and poly(methyl acrylate‐co‐2‐ethylhexyl acrylate), respectively. In the initial state, the PhMz‐NH2‐OH@PU sample shows blue fluorescence from the excited enol form (E*) excitons, owing to intermolecular hydrogen bonds that interrupt the ESIPT reactions but the PhMz=2A@PMA‐2‐EA sample expresses cyan fluorescence belonging to the excited keto form (K*) emission, implying that the intramolecular hydrogen bonds matter. Furthermore, under stretching, external force can tune the emission of the PhMz=2A@PMA‐2‐EA sample from K* to E* state. Though external force can putatively still promote a bond rotation, ESIPT reactions remain equivalently interrupted in both the relaxed and stressed states in a hydrogen‐bond donating environment. DFT calculation confirms the force‐induced increase in dihedral angle for the transition of ESIPT‐on/off. Thus, PhMz‐NH2‐OH@PU and PhMz=2A@PMA‐2‐EA showed disparate initial ESIPT states and further different responses/sensitivity to force. This study reports a novel and efficient strategy for enriching mechanochromic investigation and extending the applications of ESIPT reactions.

Excited-State Intramolecular Proton-Transfer Process of Crystalline 6-Cyano-2-(2′-hydroxyphenyl)imidazo[1,2a]pyridine, as Revealed by Femtosecond Pump–Probe Microspectroscopy

We have investigated the excited-state dynamics of 6-cyano-2-(2′-hydroxyphenyl)imidazo[1,]pyridine (6-CN HPIP) in tetrahydrofuran (THF) solution and crystalline phases by means of femtosecond transient absorption spectroscopy. In the THF solution, the excited-state intramolecular proton transfer (ESIPT) of the hydrogen-bonded enol form (E form) occurred with a time constant of 0.6 ps, followed by the formation of the excited state of a twisted keto form. On the other hand, in the crystalline phase, heteroexcimer formation between the excited keto form with coplanar conformation and the ground-state E form was observed after ESIPT of the E form in a 1 ps time scale. 6-CN HPIP has three crystalline polymorphs: 6-CN(O), 6-CN(Y), and 6-CN(R), which emit fluorescence having different spectral shapes and lifetimes. We compared the ESIPT and heteroexcimer formation dynamics for three polymorphs. It was found that the time constant of the ESIPT process was in the order 6-CN(Y) (0.30 ± 0.1 ps) < 6-CN(R) (0.50 ± 0....

ESIPT-Modulated Emission of Lanthanide Complexes: Different Energy-Transfer Pathways and Multiple Responses.

Two series of isostructural lanthanide coordination complexes, namely, LIFM-42(Ln) (Ln=Eu, Tb, Gd, in which LIFM stands for the Lehn Institute of Functional Materials) and LIFM-43(Ln) (Ln=Er, Yb), were synthesized through the self-assembly of an excited-state intramolecular proton transfer (ESIPT) ligand, 5-[2-(5-fluoro-2-hydroxyphenyl)-4,5-bis(4-fluorophenyl)-1H-imidazol-1-yl]isophthalic acid (H2 hpi2cf), with different lanthanide ions. In the coordination structures linked by the ligands and oxo-bridged LnIII 2 clusters (for LIFM-42(Ln) series) or isolated LnIII ions (for LIFM-43(Ln) series), the ESIPT ligand can serve as both the host and antenna for protecting and sensitizing the photoluminescence (PL) of LnIII ions. Meanwhile, the -OH⋅⋅⋅N active sites on the ligands are vacant, which provides availability to systematically explore the PL behavior of Ln complexes with ESIPT interference. Based on the accepting levels of different lanthanide ions, energy transfer can occur from the T1 (K*) or T1 (E*) (K*=excited keto form, E*=excited enol form) excited states of the ligand. Furthermore, the sensitized lanthanide luminescence in both visible and near-infrared regions, as well as the remaining K* emission of the ligand, can be modulated by the ESIPT responsiveness to different solvents, anions, and temperature.

Structural and Spectral Features of 4'-Substituted 2'-Hydroxychalcones in Solutions and Crystals: Spectroscopic and Theoretical Investigations.

The article describes investigations of 2'-hydroxychalcone and its three derivatives bearing differently sized alkyloxy groups at position 4'. The compounds are investigated from the point of view of crystal structure, electronic absorption, fluorescence features in solutions and crystals using X-ray diffraction and electronic spectroscopy methods, and quantum chemistry calculations. In general, both in solutions and in the crystal phase, the influence of substituents on absorption spectra of chalcones was found to be insignificant. Exclusively in the case of 4'-(4-methoxybenzyloxy)-2'-hydroxychalcone, molecular packing influences the absorption features, which is because of the intermolecular interactions of substituent's phenyl ring and chromophore fragment of the neighboring molecules. The lack of fluorescence of the excited enol form of chalcones in solutions and crystals is mainly due to intersystem crossing and excited-state intramolecular proton transfer. Fluorescent properties of the phototautomer keto species formed by the proton transfer depend on molecular conformation. In solutions, the excited keto form is twisted and effectively deactivates nonradiatively due to conical intersection. In the crystal phase, the fixed planar geometry disables the conical intersection and the fluorescence of the keto form becomes detectable.

A zwitterionic pH responsive ESIPT-Based fluorescence “Turn-On” Al3+ ion sensing probe and its bioimaging applications

A zwitterionic compound (L) as a specific detector for aluminum ion in methanol/HEPES buffer (5mM, pH 7.2; 1:5v/v) by fluorescence “turn on” was synthesized by the condensation of propargylated 2,4-dihydroxybenzophenone with ethanolamine. The compound L was characterized by various spectral methods and also crystalographically. L was found to be in equilibrium with the deprotonated form with a pKa value of 8.63. During photophysical process, L exhibits weak fluorescence with low quantum yield (Φ=0.069) at 440nm due to the quick conversion of excited keto-form into the excited enol-form by transfer of a proton (ESIPT). The interaction of L with Al3+ inhibits the ESIPT process and the emission enhanced band (20 fold) appears at shorter wavelength (427nm) with increased quantum yield (Φ=0.217). The binding of L with Al3+ and stoichiometry (1:1) has been established by UV–vis, NMR, emission and ESI–MS spectral methods L.Al3+ is reversible with EDTA and can be reused. The effect of pH, time, temperature and solvent concentrations on fluorescence intensity were studied. Fascinatingly, the fluorescence microscopic studies established that L could be used as an efficient probe for detection of intracellular Al3+ ions in HeLa cells without any toxicity.

Low‐Threshold Wavelength‐Switchable Organic Nanowire Lasers Based on Excited‐State Intramolecular Proton Transfer

AbstractCoherent light signals generated at the nanoscale are crucial to the realization of photonic integrated circuits. Self‐assembled nanowires from organic dyes can provide both a gain medium and an effective resonant cavity, which have been utilized for fulfilling miniaturized lasers. Excited‐state intramolecular proton transfer (ESIPT), a classical molecular photoisomerization process, can be used to build a typical four‐level system, which is more favorable for population inversion. Low‐power driven lasing in proton‐transfer molecular nanowires with an optimized ESIPT energy‐level process has been achieved. With high gain and low loss from the ESIPT, the wires can be applied as effective FP‐type resonators, which generated single‐mode lasing with a very low threshold. The lasing wavelength can be reversibly switched based on a conformation conversion of the excited keto form in the ESIPT process.

Low-threshold wavelength-switchable organic nanowire lasers based on excited-state intramolecular proton transfer.

Coherent light signals generated at the nanoscale are crucial to the realization of photonic integrated circuits. Self-assembled nanowires from organic dyes can provide both a gain medium and an effective resonant cavity, which have been utilized for fulfilling miniaturized lasers. Excited-state intramolecular proton transfer (ESIPT), a classical molecular photoisomerization process, can be used to build a typical four-level system, which is more favorable for population inversion. Low-power driven lasing in proton-transfer molecular nanowires with an optimized ESIPT energy-level process has been achieved. With high gain and low loss from the ESIPT, the wires can be applied as effective FP-type resonators, which generated single-mode lasing with a very low threshold. The lasing wavelength can be reversibly switched based on a conformation conversion of the excited keto form in the ESIPT process.

Simulation of a Liquid State Photoinduced Enol-Keto Tautomerization Involving Long-Range Proton Transfer

Photoexcited 7-hydroxy-8-(N-morpholinomethyl)quinoline (HMMQ) has previously been shown to transform from the excited enol form E* into the excited keto form K* in a number of consecutive elementary processes involving formation of an excited zwitterionic form Z* and rotational Brownian motion of the morpholino group which acts as a proton carrier and gets into motion only if its initial intramolecular H-bond is broken thermally. The latter motion is not restricted to cases in which HMMQ is dissolved in polar solvents, but may proceed also in nonpolar media which contain proton accepting solvents such as 1,4-dioxane. Molecular dynamics simulations of the motion of the morpholino group in the forms E and Z* in liquid cyclohexane and in liquid 1,4-dioxane are presented. The simulation of Z* in cyclohexane reveals that an intramolecular hydrogen bond between the protonated N atom in the morpholino group and the deprotonated O atom inhibits the rotational motion of the proton carrier during the whole period o...

Conformational effects on the photoinduced proton-transfer processes in 1-methyl-2-(3′-hydroxy-2′-pyridyl)benzimidazole

The ground- and excited-state behaviour of 1-methyl-2-(3′-hydroxy-2′-pyridyl)benzimidazole (MeHPyBI) in various solvents has been studied by UV–Vis absorption spectroscopy and by time-resolved and steady-state fluorescence spectroscopy. Three species were detected in the ground state in aqueous solutions of pH near to neutrality: a non-planar enol form, a planar cis-enol form and a planar keto tautomer. Electronic excitation of both the cis-enol form and the keto tautomer yields the excited keto form, the former through an ultrafast excited-state intramolecular proton-transfer (ESIPT). The non-planar enol form cannot undergo ESIPT and partly deprotonates at the hydroxy group to afford the anion. In non-aqueous solvents only the cis-enol form was detected in the ground state, yielding the keto tautomer in the excited state. The first protonation of MeHPyBI takes place at the benzimidazole N(3). Whereas the photoexcited cation shows fluorescence in acidified acetonitrile, it deprotonates at the hydroxy group in acidified ethanol, yielding the keto tautomer. In acidic aqueous solution, the excited cation undergoes a two-step tautomerization by two different routes to afford a keto cation, protonated at the pyridyl N. All the protonated and deprotonated forms of MeHPyBI are non-planar in the ground state both in water and ethanol, but they adopt a planar structure in the excited state. Comparison of the behaviour of MeHPyBI with related molecules reveals that the existence of planar forms and the neutral keto species in the ground state is favoured by the presence of the pyridyl nitrogen. The conformational equilibria in the ground state are crucial for the excited-state behaviour of these types of molecules.

Intramolecular proton-transfer cycle of 2,4-dimethoxy-6-(1-hydroxy-2-naphthyl)- s-triazine studied by laser photolysis

The intramolecular proton-transfer cycle of 2,4-dimethoxy-6-(1-hydroxy-2-naphthyl)- s-triazine (NTR) was studied by laser photolysis and fluorescence measurements with the aid of MO calculations. A moderately large Stokes-shifted fluorescence ( Δ ν ̄ ≈6400 cm −1) was observed for NTR in methylcyclohexane, which was attributable to the fluorescence from proton-transferred keto tautomer. Transient absorption measurements of NTR in various solvents showed the presence of a long-lived transient which was not quenched by dissolved oxygen in the solution. Remarkable viscosity effects were found for the relative formation yield of the long-lived transient and both the fluorescence lifetime and quantum yield of the excited keto form, indicating the involvement of cis–trans isomerization after the excited-state intramolecular proton transfer. The isomerization was also supported by the MO calculations. The kinetics of the ground-state reverse proton transfer of the trans-keto form was strongly affected by the presence of protic solvents, showing solvent-catalyzed recovery processes through mutual hydrogen exchange.

Electrically Generated Intramolecular Proton Transfer: Electroluminescence and Stimulated Emission from Polymers

Abstract : Application of a voltage across a thin film of a rod coil copolymer containing 2-(2'- hydroxyphenyl)benzothiazole in the backbone induces a reversible intramolecular proton transfer. The observed electroluminescence occurs from the excited keto form, produced electrically by a chemical reaction termed electrically generated intramolecular proton transfer, EGIPT. Stimulated emission between 540-640 nm was observed by Ps transient absorption spectroscopy. EGIPT and related proton transfer electroluminescence have implications for understanding proton transfer reactions and organic light emitting devices.

Photoinduced Inter- and Intramolecular Proton Transfer in Aqueous and Ethanolic Solutions of 2-(2‘-Hydroxyphenyl)benzimidazole: Evidence for Tautomeric and Conformational Equilibria in the Ground State

Excited-state proton transfer in aqueous and ethanolic solutions of 2-(2‘-hydroxyphenyl)benzimidazole (HBI) was investigated by means of UV−vis absorption and fluorescence spectroscopy. The behavior of HBI in water differed from its behavior in ethanol, and in both solvents fluorescence behavior depended on acidity. In both neutral water and neutral ethanol, ground-state HBI exhibits conformational equilibrium between a cis-enol form with an intramolecular hydrogen bond and a trans-enol form that is hydrogen-bonded to the solvent; the ground-state keto tautomer is also present in water but was not detected in ethanol. The excited cis-enol conformer always undergoes ultrafast intramolecular proton transfer to afford the excited keto tautomer. The excited trans-enol form fluoresces in both solvents, and in water it also loses its hydroxyl proton to the solvent, leaving the excited anion. In both acidic aqueous solution and acidic ethanol, excited protonated HBI loses its hydroxyl proton to give the excited keto form, but this process is faster in water than in ethanol, in which fluorescence by the cation is also observed.

State-selectivity of excited-state intramolecular proton transfer in a ‘‘double’’ benzoxazole: Jet spectroscopy and semiempirical calculations

The fluorescence excitation spectra of free 2,5-bis(2′-benzoxazolyl)-hydroquinone (BBXHQ) cooled in a supersonic jet are examined. By using double-resonance saturation spectroscopy, we show that adjacent vibronic transitions, leading to either blue or red fluorescence, belong to a single molecular species in the electronic ground state. The molecular geometries of the enol and keto forms which are relevant for intramolecular transfer of a single hydrogen atom in the S1 state are obtained by the MNDO/H method. The translocation distance is calculated to be 0.43 Å. The complexity of the vibronic spectrum, near the electronic origin for the S0(1Ag) → S1(1Bu) transition of the enol form at 4178.1 Å, is attributed to vibronic coupling with the nonplanar excited keto form due to H tunneling.

Singlet excited-state intramolecular proton tranfer in 2-(2t'-hydroxyphenyl) benzoxazole: spectroscopy at low temperatures, femtosecond transient absorption, and MNDO calculations

We examine intramolecular proton transfer in 2-(2′-hydroxyphenyl)benzoxazole in its electronic S 1 state. The experimental methods are fluorescence spectroscopy of the compound isolated in an argon matrix at 11 K and in a supersonically cooled jet, as well as femtosecond pump-and-probe spectroscopy in cyclohexane at 298 K. From the spectroscopic measurements we conclude that the vibronic bands near the electronic origin have a homogeneous width ⩾ 150 cm −1, corresponding to a vibronic lifetime of less than 35 fs. Residual structure reveals a short progression in a vibrational mode of 147 cm −1. The time-resolved measurements indicate a short-lived initial Franck-Condon distribution which evolves, with a time constant of 60±30 fs, into a distribution of vibrational levels which partly belong to the excited keto form of the molecule. The molecular geometries of the excited enol and keto forms are calculated using MNDO methods. For these geometries, the positions of the mobile hydrogen atom are separated by 0.41 Å. We construct a model potential for excited-state intramolecular proton transfer which is consistent with the observations.

Prospects for using photoinduced intramolecular proton transfer to study the dynamics of conformational changes in flexible molecular chains

Photoinduced excited-state intramolecular proton transfer in 2-hydroxy-1-(N-morpholinomethyl)naphthalene (HMMN) and in 7-hydroxy-8-(N-morpholinomethyl)quinoline (HMMQ) has been studied by means of stationary and time-resolved fluorescence. Both molecular systems have an intramolecular hydrogen bond. This causes a very fast ESIPT process to occur directly upon excitation with a rate constant >2 × 1011 s–1. HMMN is known to show dual fluorescence; however, HMMQ in neat polar solvents is found to exhibit three fluorescence bands. The bands at the short- and long-wavelength side of the spectrum are assigned, respectively, to emission from the excited initial tautomeric form (enol form) and the adiabatically produced excited keto form. This agrees with the fact that these two bands are also observed in the fluorescence spectrum of solutions of 7-hydroxyquinoline in neat alkanols, in which the excited keto form is known to be formed adiabatically as a result of active solvent participation in the ESIPT process. The third band originates from an excited zwitterionic species in which the proton is located on the N atom in the morpholino group. The side group acts as a molecular crane, picking up the proton from the initial binding-site and depositing it at the final proton binding-site. The rate constant for the transport of the proton from the initial to the final location is determined by the rate constant for the required motion of the free end of the side group. This rate constant has been studied as a function of viscosity η of the solvent and is found to be proportional to η–c, with c= 0.6 and 0.4 in the case of n-alkanols and alkanenitriles as solvents, respectively. A process consisting of two sequential ESIPT processes, as in HMMQ, offers precise control of initial and final conformation in the study of the dynamics of flexible molecular chains. The prospects for using such a double ESIPT process as a tool to study chain dynamics are discussed.

Photostability and molecular structure

Photostability, defined as the inverse of the quantum yield of a photochemical reaction, is assumed to be proportional to the rate of internal conversion. The rate of internal conversion is inversely proportional to the energy difference between the first excited state and the ground state. In intramolecularly hydrogen-bonded π systems, the photoexcited enol form rearranges itself to an excited keto form. SCF LCAO MO calculations indicate that the energy difference between the first excited state and the ground state is smaller in the keto form than in the enol form. Experiments are reported in support of these theoretical considerations. Weak, strongly red-shifted fluorescence is observed at 77°K for several photostable quinazolines, pyrimidines, quinolines, pyrazines, benzotriazoles, and benzophenones with an o-hydroxyphenyl group in ortho position to a ring nitrogen or a carbonyl group. For some compounds a moderately red-shifted fluorescence is observed as well. No phosphorescence is detected. None of the compounds fluoresce at room temperature. When intramolecular hydrogen bonding is destroyed by, e.g., methylation, the compound shows phosphorescence and moderately red-shifted fluorescence. With data from the literature, the increase in photostability due to enol-keto tautomerism in the excited state is estimated. The increase agrees well with the experimental results.