Combined effect of hydrogen bonding interactions and freezing of rotameric equilibrium on the enhancement of photostability.

The photophysics and photostability of 12,13-dihydro-5H-indolo[3,2-c]acridine (IA), a rigid bifunctional indole derivative with proton donor/acceptor functionalities, can be drastically changed by the environment. The formation of hydrogen bonds with alcohols leads to a significant decrease of the triplet formation efficiency and an increase of photostability. The photodegradation yield was found to be about two hundred times lower in methanol and 1-propanol than in n-hexane or acetonitrile. A similar effect has been reported for two indole-naphthyridines, molecules that can exist in syn and anti rotameric forms. We demonstrate that IA, which can exist only in the syn form, is more photostable in alcohols than similar, but non-rigid molecules. This additional photostability enhancement is due to the elimination of a slower channel of excited state deactivation in alcohol complexes, S0 ← S1 internal conversion. The dominant, faster channel of S1 depopulation is the excited state double proton transfer, manifested by the presence of low energy tautomeric fluorescence.

Non-typical fluorescence studies of excited and ground state proton and hydrogen transfer.

Fluorescence studies of tautomerization have been carried out for various systems that exhibit single and double proton or hydrogen translocation in various environments, such as liquid and solid condensed phases, ultracold supersonic jets, and finally, polymer matrices with single emitters. We focus on less explored areas of application of fluorescence for tautomerization studies, using porphycene, a porphyrin isomer, as an example. Fluorescence anisotropy techniques allow investigations of self-exchange reactions, where the reactant and product are formally identical. Excitation with polarized light makes it possible to monitor tautomerization in single molecules and to detect their three-dimensional orientation. Analysis of fluorescence from single vibronic levels of jet-isolated porphycene not only demonstrates coherent tunneling of two internal protons, but also indicates that the process is vibrational mode-specific. Next, we present bifunctional proton donor-acceptor systems, molecules that are able, depending on the environment, to undergo excited state single intramolecular or double intermolecular proton transfer. For molecules that have donor and acceptor groups located in separate moieties linked by a single bond, excited state tautomerization can be coupled to mutual twisting of the two subunits.

Solvent-Induced Changes in Photophysics and Photostability of Indole-Naphthyridines.

Molecules that can simultaneously act as hydrogen bond donors and acceptors often exhibit completely different photophysical behavior in protic and aprotic solvents. Formation of multiple hydrogen bonds with, for example, water or alcohols, may lead to enhanced internal conversion; as a result, triplet formation efficiency can be reduced. These changes in photophysical characteristics may influence the photostability. In order to check this hypothesis, we have investigated spectroscopy, photophysics, and changes in photostability caused by interaction with aprotic and protic solvents for 2-(1'H-indol-2'-yl)-[1,5]naphthyridine and 2-(1'H-indol-2'-yl)-[1,8]naphthyridine, molecules with hydrogen bond accepting and donating functionalities. The photostability of these compounds in n-hexane, acetonitrile, and alcohols was studied in the regime of 365 nm irradiation. The photodegradation yield was found to be significantly lower in alcohols. In polar and protic solvents, the presence of two species was detected and attributed to syn and anti rotameric forms; the former are dominant in all environments.

A simple route to alloyed quaternary nanocrystals Ag-In-Zn-S with shape and size control.

A convenient method of the preparation of alloyed quaternary Ag-In-Zn-S nanocrystals is elaborated, in which a multicomponent mixture of simple and commercially available precursors, namely, silver nitrate, indium(III) chloride, zinc stearate, 1-dodecanethiol, and sulfur, is used with 1-octadecene as a solvent. The formation of quaternary nanocrystals necessitates the use of an auxiliary sulfur precursor, namely, elemental sulfur dissolved in oleylamine, in addition to 1-dodecanethiol. Without this additional precursor binary ZnS nanocrystals are formed. The optimum reaction temperature of 180 °C was also established. In these conditions shape, size, and composition of the resulting nanocrystals can be adjusted in a controlled manner by changing the molar ratio of the precursors in the reaction mixture. For low zinc stearate contents anisotropic rodlike (ca.3 nm x 10 nm) and In-rich nanocrystals are obtained. This is caused by a significantly higher reactivity of the indium precursor as compared to the zinc one. With increasing zinc precursor content the reactivities of both precursors become more balanced, and the resulting nanocrystals are smaller (1.5-4.0 nm) and become Zn-rich as evidenced by transmission electron microscopy, X-ray diffraction, and energy-dispersive spectrometry investigations. Simultaneous increases in the zinc and sulfur precursor content result in an enlargement of nanocrystals (2.5 to 5.0 nm) and further increase in the molar ZnS content (up to 0.76). The prepared nanoparticles show stable photoluminescence with the quantum yield up to 37% for In and Zn-rich nanocrystals. Their hydrodynamic diameter in toluene dispersion, determined by dynamic light scattering, is roughly twice larger than the diameter of their inorganic core.

7-Hydroxyquinoline-8-carbaldehydes. 1. Ground- and excited-state long-range prototropic tautomerization.

Ground- and excited-state long-range prototropic tautomerization were studied for a series of 7-hydroxyquinoline-8-carbaldehydes (7-HQCs) by (1)H and (13)C NMR spectroscopy, photostationary and time-resolved UV-vis spectroscopic methods, and quantum chemical computations. These molecules represent trifunctional proton-donating/accepting systems that have been proposed to serve as models of a reversible optically driven molecular switch composed of two moieties: a molecular "frame" (7-hydroquinolines, 7-HQs) and a proton "crane" (carbaldehyde group). The NMR and electronic absorption spectra indicate a solvent-dependent equilibrium between two tautomeric forms, OH (7-quinolinol)) and NH (7(1H)-quinolinone), already in the ground state of all the compounds under study (7-hydroxy-2-methoxy-4-methylquinoline-8-carbaldehyde, HMMQC, shows only a trace of the NH form in highly polar and/or protic media). Electronic absorption and fluorescence of 7-HQCs are rationalized in terms of the ground- and excited-state hydrogen atom transfer (HAT). This process was identified by comparing the UV-vis spectroscopic properties of 7-HQCs with those of 7-HQs, synthetic precursors of the former, as well as with the characteristics of corresponding protonated cations and deprotonated anions (part 2). The experimental results are corroborated by the density functional theory (DFT) and ab initio computations, which shed some light on the differences in photophysics between variously substituted 7-HQCs.

7-Hydroxyquinoline-8-carbaldehydes. 2. Prototropic equilibria.

Prototropic equilibria were studied for a series of 7-hydroxyquinoline-8-carbaldehydes (7-HQCs) by (1)H NMR spectroscopy, photostationary and time-resolved UV-vis spectroscopic methods, and quantum chemical computations. These molecules represent trifunctional proton-donating/accepting systems that in aqueous solutions may assume four main neutral and ionic structures: 7-quinolinol (OH), 7(1H)-quinolinone (NH), deprotonated anion (A), and protonated cation (C). Electronic absorption and fluorescence of 7-HQCs are rationalized in terms of the ground and excited-state long-range tautomerization (part 1) as well as protonation and deprotonation processes. The photophysical properties of neutral and ionic forms of 7-HQCs are compared with those of 7-hydroxyquinolines (7-HQs), synthetic precursors of the former. The experimental results are corroborated by ab initio computations.

Spectroscopy and photophysics of bifunctional proton donor-acceptor indole derivatives.

We report spectroscopic and photophysical studies of a series of selected indole derivatives in solutions and under supersonic jet isolation conditions. All the compounds can assume two rotameric forms, syn and anti. The bifunctional molecules containing both the hydrogen bond donor (indole NH group) and acceptor centers (oxygen, nitrogen, or sulfur atoms) located in separate moieties covalently linked by a single bond are compared with the compound that does not have any acceptor center, 2-(1H-pyrrol-2'-yl)-1H-indole. The former compounds (containing furan, thiazole, or thiophene moieties) were anticipated to show solvent-dependent photophysics. Contrary to the expectations, all the compounds reveal very efficient fluorescence, independent of solvent polarity and hydrogen bond donor and acceptor abilities. Laser spectroscopic studies combined with supersonic jet techniques and quantum chemical computations have been performed in order to identify the rotameric forms and to gain insight into the changes in the molecular structure accompanying electronic excitation.

1H-pyrrolo[3,2-h]quinoline: a benchmark molecule for reliable calculations of vibrational frequencies, IR intensities, and Raman activities.

Reliable assignment of 55 out of 57 vibrational modes has been achieved for 1H-pyrrolo[3,2-h]quinoline using IR, Raman, and fluorescence spectroscopy combined with quantum chemical calculations. The experiments provided a data set for assessing the performance of different models/basis sets for predicting the vibrational frequencies, as well as IR and Raman intensities for a molecule with 13 heavy atoms. Among six different tested DFT functionals, the hybrid B3LYP used with Pople's split-valence basis sets is suggested as the best choice for accurate and cost-effective IR/Raman spectral simulations. Neither HF nor MP2 methods can satisfactorily describe the vibrational structure. Increasing the basis set size from double to triple-ζ and by adding polarization and diffuse functions does not necessarily improve the results, especially regarding the predictions of vibrational frequencies. With respect to the intensities, extending the basis set helps, with the accuracy increasing systematically for the Raman spectra, and in a less regular fashion for the IR. A large difference in accuracy is observed while comparing the spectral parameters predicted for in-plane and out-of-plane normal modes. The former are reliably computed with modest basis sets, whereas for the out-of-plane vibrations, larger basis sets are necessary, but even in this case, the out-of-plane vibrations are reproduced with much less accuracy than in-plane modes. This effect is general, as it has been observed using different functionals and basis sets.

Three modes of proton transfer in one chromophore: photoinduced tautomerization in 2-(1H-pyrazol-5-yl)pyridines, their dimers and alcohol complexes.

Studies of 2-(1H-pyrazol-5-yl)pyridine (PPP) and its derivatives 2-(4-methyl-1H-pyrazol-5-yl)pyridine (MPP) and 2-(3-bromo-1H-pyrazol-5-yl)pyridine (BPP) by stationary and time-resolved UV/Vis spectroscopic methods, and quantum chemical computations show that this class of compounds provides a rare example of molecules that exhibit three types of photoreactions: 1) excited-state intramolecular proton transfer (ESIPT) in the syn form of MPP, 2) excited-state intermolecular double-proton transfer (ESDPT) in the dimers of PPP in nonpolar media, as well as 3) solvent-assisted double-proton transfer in hydrogen-bonded 1:1 complexes of PPP and MPP with alcoholic partners. The excited-state processes are manifested by the appearance of a dual luminescence and a bimodal irreversible kinetic coupling of the two fluorescence bands. Ground-state syn-anti equilibria are detected and discussed. The fraction of the higher-energy anti form varies for different derivatives and is strongly dependent on the solvent polarity and hydrogen-bond donor or acceptor abilities.

On the origin of fluorescence quenching of pyridylindoles by hydroxylic solvents.

Three isomeric 4'-pyridyl-substituted indoles, with the substituent in positions 2, 3 and 7, reveal strong fluorescence in aprotic solvents, both polar and nonpolar, whereas the emission is strongly quenched in water and alcohol solutions. Both viscosity and alcohol acidity play a role in efficient excited state deactivation. The process becomes faster for more acidic alcohols. It can be slowed down by increasing viscosity, which indicates that the proton movement is accompanied by large amplitude motions in the hydrogen-bonded complex. Quenching is not observed upon formation of solvates in which pyridylindoles act as hydrogen bond donors. The experimental results, combined with calculations can be explained by a model which assumes excited state protonation of the pyridine nitrogen atom, followed by twisting of the pyridyl group, leading to a low-energy structure. An alternative mechanism is also considered, in which the excited state proton transfer is accompanied by electron transfer from water or alcohol into a half-filled orbital of the chromophore, which leads to a conical intersection of the S(1) and S(0) energy surfaces.

Separation of different hydrogen-bonded clusters by femtosecond UV-ionization-detected infrared spectroscopy: 1H-pyrrolo[3,2-h]quinoline.(H2O)(n=1,2) complexes.

Experimental and theoretical studies are presented for complexes of water with 1H-pyrrolo[3,2-h]quinoline (PQ), a bifunctional compound acting simultaneously as a hydrogen-bond donor and acceptor. A 1:1 complex, which is not fluorescent and only very short-lived in the electronically excited state, was analyzed by isolating the complex under supersonic jet conditions and characterizing its structure by infrared-induced ion depletion spectroscopy utilizing multiphoton ionization by femtosecond UV pulses (IR/fsMPI spectroscopy). On the other hand, a long-lived 1:2 complex was identified as the smallest microhydrate of PQ contributing to the laser-induced fluorescence excitation spectrum. Its structure was assigned by fluorescence-detected IR spectra and analyzed using density functional theory. The structures of the 1:1 and 1:2 clusters are assigned to species in which the water molecule(s) form a hydrogen-bonded solvent bridge between the two functional groups. In accord with calculations, both 1:1 and 1:2 PQ/water complexes reveal weaker hydrogen bonding than the analogous clusters of PQ with methanol.

Supersonic jet studies of solvation effects on the spectroscopy and photophysics of 4-diethylaminopyridine.

We present the results of spectroscopic and photophysical investigations of 4-diethylaminopyridine (DEAP) and its 1 : 1 complexes with a number of protic solvents such as water and various alcohols of different acidity isolated under supersonic jet conditions. While a double resonance vibrational spectroscopic method was employed to investigate the size and geometrical structure of jet-cooled clusters, laser-induced fluorescence spectroscopy was used to examine the changes of photophysics induced by complexation of DEAP with solvent molecule(s). The results obtained from ab initio calculations enable the assignment of geometries and of the vibrational spectra of the clusters in the OH-stretch region. The comparison of the experimental and calculated vibrational spectra indicates that the solvent molecule is hydrogen-bonded to the pyridine nitrogen atom. Dual luminescence is observed only for the complexes with alcohols of relatively strong acidity.

Fluorescence quenching in cyclic hydrogen-bonded complexes of 1H-pyrrolo[3,2-h]quinoline with methanol: cluster size effect.

Laser induced fluorescence (LIF) excitation spectra of molecular complexes of 1H-pyrrolo[3,2-h]quinoline (PQ) with methanol (n= 1, 2, 3) as well as vibrational spectra of their electronic ground state are reported. The latter have been recorded in the mid-infrared region by fluorescence depletion (FDIR). The only PQ.methanol(n) complex clearly identified in the LIF spectrum is the triply hydrogen-bonded cyclic 1 ratio 2 aggregate. Its stoichiometry has been proven by the femtosecond multiphoton ionization detected infrared measurements [J. Am. Chem. Soc., 2006, 128, 10 000]. The structure of the 1 ratio 2 cluster is determined by means of FDIR spectroscopy in combination with ab initio and DFT calculations. No fluorescence was detected that could be attributed to the 1 ratio 1 cluster. This behaviour of the 1 ratio 1 complex is explained in terms of rapid excited state double proton transfer followed by a non-radiative relaxation. The n= 3 and heavier clusters are fluorescent. Their electronic spectra overlap, preventing the selective measurement of the FDIR spectra of individual complexes.

Detection and structural characterization of clusters with ultrashort-lived electronically excited states: IR absorption detected by femtosecond multiphoton ionization.

The vibrational fingerprint of the electronically excited short-lived complex of 1-H-pyrrolo[3,2-h]quinoline:methanol was measured using femtosecond multiphoton ionization detected infrared (IR/fsMPI) spectroscopy under supersonic jet conditions. A cyclic doubly hydrogen-bonded structure of the cluster has been proven from the comparison of the measured vibrational spectrum with that calculated with density functional theory. The employed nsIR-fsUV double resonance scheme is shown to be an effective tool for structural analysis of precursors that undergo fast deactivation and/or photoreactions.

Picosecond kinetics of excited-state charge separation in 4-(dimethylamino)pyridine.

4-(Dimethylamino)pyridine (DMAP) shows solvent-dependent dual fluorescence from the initially excited state B* and a highly polar TICT state A*. Room-temperature time-resolved picosecond fluorescence investigations prove the bimodal kinetics of the excited-state electron transfer reaction B*-->A* in polar aprotic media. In medium polarity solvents (such as ethyl acetate) two emitting states of DMAP are shown to reach equilibrium within 50 ps. Both emitting states originate from the same ground state. The rate of excited-state charge separation depends on polarity and proton donating ability of the surrounding medium. The effects of temperature on the quantum yields of both fluorescences of DMAP in polar aprotic media indicate the transition from the kinetic regime (at low temperatures) to the equilibrium regime (at high temperatures). The kinetic behaviour of the dual luminescence of DMAP in protic solvents is more complex than in aprotic ones. In alcohols an efficient nonradiative channel competes with excited-state charge separation.

Excited states of 4-dimethylaminopyridines: magnetic circular dichroism and computational studies.

The ultraviolet transitions of 4-(dimethylamino)pyridine and its derivatives: 3-methyl-4-(dimethylamino)pyridine and 3,5-dimethyl-4-(dimethylamino)pyridine were investigated by electronic absorption and magnetic circular dichroism (MCD) spectroscopy, and by quantum chemical calculations. The ortho-methylation in the pyridine ring creates a steric hindrance to coplanarity. The resulting changes in the strength of the dimethylamino substitutent are reflected in the MCD spectra. Near-UV experimental data indicate a presence of three low-energy transitions, assigned to 1Lb, 1La and 1(n,pi*) excited states. These results are corroborated by TD-DFT and INDO/S calculations.