Michał Kijak

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.

Tautomerism in porphycenes: analysis of rate-affecting factors.

Double hydrogen transfer occurring in both ground and the lowest electronically excited singlet states was studied for a series of 19 differently substituted porphycenes. The rates of tautomerization have been determined using femtosecond pump-probe spectroscopy with polarized light. The values vary by over 3 orders of magnitude, suggesting the importance of tunneling. Good correlation exists between the values of the rates and the parameters characterizing the strength of two intramolecular hydrogen bonds: proton NMR shift, distance between the hydrogen-bonded nitrogen atoms, and the NH stretching frequency. While hydrogen-bond strength is the main factor determining the rate of double hydrogen transfer, other factors, such as static and dynamic symmetry breaking and the population of low-frequency vibrations also have to be taken into account.

Photoinduced double proton transfer in water complexes of 1H-pyrrolo[3,2-h]quinoline and dipyrido[2,3-a:3', 2'-i]carbazole

Abstract Dual fluorescence is observed in water and water-containing solutions of 1H-pyrrolo[3,2- h ]quinoline and dipyrido[2,3- a :3 ′ ,2 ′ - i ]carbazole. The low energy band is assigned to a product of excited state double proton transfer reaction, occurring in cyclic 1:1 complexes with water. Such complexes constitute only a small fraction of the ground state population. The majority of water complexes are not prone to phototautomerization, but are deactivated via a process that is not efficient in a non-hydrogen-bonded chromophore.

Spectroscopic Study of Jet-Cooled Deuterated Porphycenes: Unusual Isotopic Effects on Proton Tunneling

Porphycene (Pc) is a well-known model for studying double hydrogen transfer, which shows vibrational-mode-specific tunneling splitting when isolated in supersonic jets or helium nanodroplets. The effect of deuteration on tunneling splitting is reported for jet-cooled heterogeneous, deuterated Pc samples (Pc-d(mix)) with the prevailing contribution of Pc-d12 isotopologue. The sample introduced into the gas phase using laser desorption is studied by means of laser-induced fluorescence (LIF) and single vibronic level fluorescence (SVLF) measurements, in combination with quantum chemical calculations. The influence of molecular symmetry is studied by comparing Pc, Pc-d12, and Pc-d11. The spectra of Pc-d12 show strong similarity to those of the parent undeuterated porphycene (Pc). Comparable tunneling splitting is observed in the two isotopologues, both for the 0-0 transition and the most efficient promoting 2Ag mode. In contrast, an unusual isotopic effect is observed for the totally symmetrical 4Ag mode. While this vibration behaves as a neutral mode in Pc, neither enhancing nor decreasing the tunneling efficiency, it strongly promotes hydrogen transfer in Pc-d12. This observation is explained in terms of modification of the displacement vectors of the 4Ag mode upon deuteration. It demonstrates that isotope substitution affects hydrogen transfer even when the weak structural modifications are far from the reaction center, emphasizing the strongly multidimensional nature of the tunneling process.

Excited-state intramolecular proton transfer reaction modulated by low-frequency vibrations: An effect of an electron-donating substituent on the dually fluorescent bis-benzoxazole

Excited-state intramolecular proton transfer (ESIPT) reaction has been studied in a molecule showing dual fluorescence, the 2,5-bis(2-benzoxazolyl)-4-methoxyphenol (BBMP), and its isotopomers, where the methoxy, and alternatively, the OH group has been deuterated. Attention is focused on the influence of electron donating OCH(3) substituent on fast excited state reaction. Comparison between the resonance-enhanced multiphoton ionization spectrum and the laser-induced excitation of the primary and phototautomeric emissions has been done. The geometry, electron density distribution, vibrational structure as well as the potential energy profiles in the S(0) and S(1) states of four possible rotameric forms of BBMP were calculated with application of the density functional theory (DFT). It allowed identifying the most probable conformer and assessing the role of low-frequency motions for the ESIPT efficiency.

In search for phototautomerization in solid dipyrido[2,3-a:3', 2'-i]carbazole

Abstract Dipyrido[2,3-a:3′,2′-i]carbazole (DPC) is known to undergo excited state double proton transfer in cyclic complexes with alcohols. This process is detected by the appearance of low-energy fluorescence, red-shifted with respect to the “normal” emission. X-ray studies show that DPC forms cyclic, but strongly nonplanar doubly hydrogen-bonded dimers in the crystalline phase. No tautomeric fluorescence could be detected for DPC crystals. However, this emission becomes readily observed for solid DPC samples that have been exposed to water vapor, or prepared on hydroxylic supports, such as wool, filtration paper, or mica. These results suggest possible use of DPC as a probe for detection of humidity and hydroxyl groups. Tautomeric emission was also observed for DPC embedded into a hydroxyl-group-containing polymer, poly(vinyl butyral-co-vinyl-alcohol-co-vinyl acetate). The ability to form cyclic, doubly hydrogen-bonded dimers and complexes with hydroxyl-group-containing partners is compared in a series of structurally related molecules: DPC, 1H-pyrrolo[3,2-h]quinoline, 7-azaindole, 1-azacarbazole. Molecular geometry dictates that the systems with the strongest propensity to form flat dimers should have the weakest tendency for the formation of cyclic solvates, and vice versa.

Excited state substituent constants: to Hammett or not?

In this study, an answer has been sought to several questions: Can substituents’ impact on the electronic properties of molecules in the excited singlet states be summed up by a set of substituent constants? Are the well known, ground state Hammett σ constants practical for this purpose? To answer these questions, the potentials and charges on atoms of the functional groups in two classes of compounds, p-substituted benzoic acids and p-substituted nitrosobenzenes, were regressed against σp. It appeared that all correlations found in the ground state of the molecules also hold in the excited state, with lower correlation coefficients in the latter. An attempt has also been made to find a molecular characteristic which could be useful in a comparison of the acidic properties of benzoic acids (in gas phase) in the first excited and ground states.

Parent, Unsubstituted Hemiporphycene: Synthesis and Properties

Among seven possible nitrogen-in constitutional isomers of porphyrin only one, porphycene, has been obtained so far in the free, unsubstituted form. Herein, the synthesis of another isomer, parent hemiporphycene (HPc), and its thorough structural, spectral, photophysical, electrochemical, and theoretical characterization are reported. Most of the properties of HPc are intermediate between those of porphyrin and porphycene, as evidenced by the values of inner-cavity dimensions, orbital-energy splittings, absorption coefficients, magnetic circular dichroism parameters, NH-stretching frequencies, fluorescence quantum yields, tautomerization rates, and redox potentials. The largest differences arise with respect to tautomerism, due to the low symmetry of HPc and inequivalence of the four nitrogen atoms that define the inner cavity. Two trans tautomers are observed, separated in energy by about 1 kcal mol-1 . Tautomerization from the higher- to the lower-energy form is detected in the lowest-excited singlet state and occurs at a rate that is about four orders of magnitude lower than that observed for porphycene. Hemiporphycene is a very good model for the investigation of inequivalent intramolecular H-bonds present in one molecule; two such bonds in HPc reveal unusual characteristics, and the bond strength results from the interplay between the N⋅⋅⋅N distance and the N-H-N angle.

Supersonic Jet Spectroscopy and Density Functional Theory Study of Isomeric Diazines: 1,4- and 1,8-Diazatriphenylene. Why Do They Differ So Deeply?

We report on laser-induced fluorescence excitation and dispersed fluorescence spectra of two isomeric compounds: 1,4- and 1,8-diazatriphenylene (1,4- and 1,8-DAT) isolated in supersonic molecular jets, and their 1:1 complexes with protic solvents. We found that the ground and excited state vibronic patterns of bare 1,4-DAT differ significantly from those of 1,8-DAT, and those of the complexes of both isomers. A marked activity of several out-of-plane vibrations in 1,4-DAT and the symptoms of the distortion of the S1 excited molecule were diagnosed from the vibronic spectra, whereas planar structures were predicted for 1,8-DAT in S0 and S1 states. An anharmonic double-minimum potential along an out-of-plane coordinate has been derived and used to predict higher overtones of the S1 state vibration at 113 cm-1. Large enhancement of fluorescence was observed upon formation of 1:1 complexes of 1,4-DAT with water or methanol, which is explained in terms of an increased separation of interacting (n,π*) and (π,π*) electronic states in the H-bonded complexes, and/or a suppression of the intersystem crossing process.

Supersonic jet spectroscopy of parent hemiporphycene: Structural assignment and vibrational analysis for S0 and S1 electronic states

Hemiporphycene (HPc), a constitutional isomer of porphyrin, is studied under supersonic expansion conditions by means of laser-induced fluorescence, visible-visible hole-burning experiments, single vibronic level fluorescence techniques, and quantum chemical calculations. Only one trans form of jet-cooled HPc is observed, in contrast to solution studies that evidence a mixture of two trans tautomeric forms separated in energy by ∼1 kcal/mol. Reliable structural assignment is provided by simulating absorption and emission patterns at the density functional theory and time-dependent density functional theory levels of theory. The vibronic spectra are nicely reproduced for both electronic ground and lowest excited singlet states for the most stable trans form. In contrast to another porphyrin isomer, porphycene (Pc), no tunneling or photo-induced hydrogen transfer is detected. The lower symmetry of HPc compared with Pc and the concomitant non-equivalent positions of the inner-cavity nitrogen atoms result in a non-symmetric double minimum potential for tautomerization, larger energy barrier, and a longer tunneling distance, with the average intramolecular hydrogen bond length larger in HPc than in Pc. HPc readily forms hydrates that show red-shifted absorption relative to the bare molecule.