Vibrationally Resolved Absorption and Luminescence Spectra of Mass-Selected Free-Base and Zinc Phthalocyanine Radical Cations Isolated in Solid Ne.

We report the first vibrationally well-resolved absorption and laser-induced fluorescence spectra of the radical cations of free-base phthalocyanine (H2Pc+) and zinc phthalocyanine (ZnPc+) isolated in 5 K neon matrices and compare them to the spectral properties of the corresponding neutrals. The samples were generated by low-energy deposition of the mass-selected ions. The spectra are also discussed in terms of time-dependent density functional theory calculations and compared with recently reported scanning tunneling microscopy-induced single-molecule luminescence of the same species adsorbed on NaCl-covered Au(111) or Ag(111) single crystal supports.

Triggering near-infrared luminescence of vanadyl phthalocyanine by charging.

Probing electrofluorochromism (EFC) at the molecular level remains challenging. Here we study the strongly charge state-dependent photoluminescence of vanadyl phthalocyanine. We report vibrationally resolved absorption and laser-induced fluorescence (LIF) spectra of samples comprising both the mass-selected neutral molecule (VOPc⋅, a stable radical) and its cation produced upon electron ionization (EI) isolated in 5 K neon matrices. Ionization of the essentially non-emissive VOPc⋅ forms a high-spin diradical cation (VOPc+.. ) which shows profound photoluminescence (PL) in the NIR range. This unique phenomenon is potentially of interest for NIR-emitting electro-optic devices.

Hydrogen Abstraction/Addition Tunneling Reactions Elucidate the Interstellar H2NCHO/HNCO Ratio and H2 Formation.

Formamide (H2NCHO) is the smallest molecule possessing the biologically important amide bond. Recent interstellar observations have shown a strong correlation between the abundance of formamide and isocyanic acid (HNCO), indicating that they are likely to be chemically related, but no experiment or theory explains this correlation satisfactorily. We performed H + H2NCHO reactions in a para-hydrogen quantum-solid matrix host and identified production of H2NCO and HNCO from hydrogen-abstraction reactions. We identified also D2NCO, DNCO, HDNCO, and HDNCHO from the reaction H + D2NCHO, indicating the presence of hydrogen-addition reactions of DNCO and HDNCO. From the observed temporal profiles of H2NCHO, H2NCO, HNCO, and their deuterium isotopologues, we showed that a dual-cycle consisting of hydrogen abstraction and hydrogen addition can satisfactorily explain the quasi-equilibrium between H2NCHO and HNCO and explain other previous experimental results. Furthermore, this mechanism also indicates that the catalytic formation of H2 from H atoms might occur in interstellar ice grains.

Hydrogen-atom tunneling reactions with methyl formate in solid para-hydrogen: Infrared spectra of the methoxy carbonyl [•C(O)OCH3] and formyloxy methyl [HC(O)OCH2•] radicals.

We investigated the reaction of methyl formate, HC(O)OCH3, with hydrogen atoms in solid para-hydrogen (p-H2) at 1.74 and 3.3 K with infrared absorption spectroscopy. Hydrogen atoms were produced either upon direct photolysis of HC(O)OCH3 at 193 nm or upon irradiation of Cl2, codeposited with HC(O)OCH3 in p-H2, with light at 365 nm to produce Cl atoms that react with the p-H2 host via the reaction Cl + H2 (ν = 1) → HCl + H induced by subsequent IR irradiation of the p-H2 matrix. We assigned lines at 1785.2, 1170.6, 1104.6, and 879.4/880.5 cm-1 and five additional weak lines to the methoxy carbonyl radical, •C(O)OCH3, and three weak lines at 1751.3, 1152.9, and 994.4/996.8 cm-1 to the formyloxy methyl radical, HC(O)OCH2•, according to the consideration of possible reactions, correlated variations in intensities after each experimental step, and comparison of observed vibrational wavenumbers and IR intensities with values predicted with the B3LYP/aug-cc-pVTZ method. Unlike most reported H-atom diffusion tunneling reactions, the reaction H + HC(O)OCH3 in solid p-H2 at 3.3 K was found to diminish rapidly after IR irradiation, but, similar to reactions of H + N2O and H + HC(O)OH, this reaction was reinitiated when the matrix temperature was decreased from 4.0 K to 1.5 K. We confirmed that the method used to generate the mobile H atoms does not affect the subsequent chemistry. A possible reaction mechanism and the role of this reaction in dark interstellar clouds are discussed.

Infrared absorption spectra of partially deuterated methoxy radicals CH2DO and CHD2O isolated in solid para-hydrogen.

The investigation of partially deuterated methoxy radicals is important because the symmetry lowering from C3v to Cs provides new insights into the couplings between rovibronic states via Jahn-Teller and spin-orbit interactions. The vibrational spectrum of the partially deuterated methoxy radical CH2DO in a matrix of p-H2 has been recorded. This species was prepared by irradiating a p-H2 matrix containing deuterated d1-nitritomethane (CH2DONO) at 3.3 K with laser light at 355 nm. The identification of the radical is based on the photochemical behavior of the precursor and comparison of observed vibrational wavenumbers and infrared (IR) intensities with those predicted from a refined quartic, curvilinear, internal coordinate force field calculated with the coupled-cluster singles and doubles with perturbative triples/cc-pVTZ method. CH2DO reacts with H2 with a rate coefficient (3.5 ± 1.0) × 10-3 s-1. Predominantly c-CHDOH and a negligibly small amount of t-CHDOH were produced. This stereoselectivity results from the reaction H + Cs-CH2DOH, which was demonstrated by an additional experiment on irradiation of a CH2DOH/Cl2/p-H2 matrix with ultraviolet and IR light to induce the H + CH2DOH reaction; only c-CHDOH was observed from this experiment. Even though the energies of transition states and products for the formation of c-CHDOH and t-CHDOH differ by only ∼10 cm-1, the selective formation of c-CHDOH can be explained by tunneling of the hydrogen atom via an optimal tunneling path. Similarly, the vibronic spectrum for the partially deuterated specie d2-methoxy radical (CHD2O) was obtained upon irradiation of d2-nitritomethane (CHD2ONO) at 355 nm. Lines associated with the fundamental vibrational modes were observed and assigned; line positions agree with theoretically predicted vibrational wavenumbers. CHD2O reacts with H2 with a rate coefficient (6.0 ± 1.4) × 10-3 s-1; CD2OH was produced as a major product because the barrier for the formation of CHDOH from H + CHD2OH is greater by ∼400 cm-1. Rate coefficients of the decays of CH3O, CH2DO, CHD2O, and CD3O and their corresponding potential energy surfaces are compared.

Vibrationally Resolved Absorption, Fluorescence, and Preresonance Raman Spectroscopy of Isolated Pyronin Y Cation at 5 K.

Exploring how charge-changing affects the photoluminescence of small organic dyes presents challenges. Here, helium tagging photodissociation (PD) action spectroscopy in the gas phase and dispersed laser-induced fluorescence (DF) spectroscopy in the solid Ne matrix are used to compare the intrinsic photophysical properties of pyronin Y cation [PY]+ and its one electron-reduced neutral radical [PY]• at 5 K. Whereas the cation shows efficient visible photoluminescence, no emission from the neutral, in line with theoretical predictions, was detected. B3LYP/aug-cc-pVDZ calculations based on the TD-DFT/FCHT method allow for unambiguous assignment of recorded vibrationally resolved absorption and emission spectra. Surprisingly, our experimental sensitivity was high enough to also observe electronic preresonance Raman (ePR-Raman) spectra of [PY]+, with a significant efficiency factor (EF). These characteristics of the [PY]•/[PY]+ pair suggest that appropriately functionalized derivatives may open new perspectives in the area of in vivo bioimagining microscopy and find applications in various sophisticated stimulated-Raman spectroscopies.

Photoactivated Refractive Index Anisotropy in Fluorescent Thiophene Derivatives.

The optical control of anisotropy in materials is highly advantageous for many technological applications, including the real-time modulation of another light signal in photonic switches and sensors. Here, we introduce three thiophene derivatives with a donor-acceptor structure, which feature different positions of an electron-acceptor nitrile group, and both photoalignment and luminescence properties. Quantum chemical calculations highlight the presence of trans-forms stable at room temperature and metastable cis-isomers. Besides photoluminescence peaked at 440-460 nm and 0.4 ns lifetime, the three nonlinear optical chromophores exhibit photoinduced anisotropy of the refractive index closely depending on the specific molecular structure, with higher values of birefringence at lower driving signal being obtained for ortho substitution of the nitrile group. All-optical modulation of an external light beam at rates of hundreds of hertz is demonstrated in the fluorescent systems. This finding opens an interesting route to multispectral photonic switches embedded in the active layers of light-emitting devices.