Magnetic Circular Dichroism of meso-Phenyl-Substituted Pd-Octaethylporphyrins.

Absorption and magnetic circular dichroism (MCD) spectra have been measured and theoretically simulated for a series of palladium octaethylporphyrins substituted at the meso positions with phenyl groups (n = 0-4). Analysis of the spectra included the perimeter model and time-dependent density functional theory (TDDFT) calculations. With the increasing number of phenyl substituents, the molecule is transformed from a positive hard (ΔHOMO > ΔLUMO) to a soft (ΔHOMO ≈ ΔLUMO) chromophore. This is manifested by a drastic decrease of the absorption intensity in the 0-0 region of the Q-band and by the strongly altered ratio of MCD intensities in the Q and Soret regions. Such behavior can be readily predicted using perimeter model, by analyzing frontier orbital shifts caused by various perturbations: alkyl and aryl substitution, insertion of a metal, and deviations from planarity. TDDFT calculations confirm the trends predicted by the perimeter model, but they fail in cases of less symmetrical derivatives to properly reproduce the MCD spectra in the Soret region. Our results confirm the power of the perimeter model in predicting absorption and MCD spectra of large organic molecules, porphyrins in particular. We also postulate, contrary to previous works, that the isolated porphyrin dianion is not a soft chromophore, but rather a strongly positive-hard one.

Anharmonicity in a double hydrogen transfer reaction studied in a single porphycene molecule on a Cu(110) surface.

Anharmonicity plays a crucial role in hydrogen transfer reactions in hydrogen-bonding systems, which leads to a peculiar spectral line shape of the hydrogen stretching mode as well as highly complex intra/intermolecular vibrational energy relaxation. Single-molecule study with a well-defined model is necessary to elucidate a fundamental mechanism. Recent low-temperature scanning tunnelling microscopy (STM) experiments revealed that the cis↔cis tautomerization in a single porphycene molecule on Cu(110) at 5 K can be induced by vibrational excitation via an inelastic electron tunnelling process and the N-H(D) stretching mode couples with the tautomerization coordinate [Kumagai et al. Phys. Rev. Lett. 2013, 111, 246101]. Here we discuss a pronounced anharmonicity of the N-H stretching mode observed in the STM action spectra and the conductance spectra. Density functional theory calculations find a strong intermode coupling of the N-H stretching with an in-plane bending mode within porphycene on Cu(110).

Spectroscopic and microscopic investigations of tautomerization in porphycenes: condensed phases, supersonic jets, and single molecule studies.

We describe various experimental approaches that have been used to obtain a detailed understanding of double hydrogen transfer in porphycene, a model system for intramolecular hydrogen bonding and tautomerism. The emerging picture is that of a multidimensional tautomerization coordinate, with several vibrational modes acting as reaction-promoters or inhibitors through anharmonic intermode coupling. Tunnelling processes, coherent in the case of isolated molecules and incoherent in condensed phases, are found to play a major role even at elevated temperatures. Single-molecule spectroscopy studies reveal large fluctuations in hydrogen transfer rates observed over time for the same chromophore. Scanning probe microscopy is employed to directly observe the structure and tautomerization dynamics of single molecules adsorbed on metal surfaces and demonstrates how the interactions of the molecules with atoms of the supporting surface affect their static and dynamic properties: different tautomeric forms are stabilized for molecules depending on the surface structure and the reaction mechanism can also change, from a concerted to a stepwise transfer. The scanning probe microscopy studies prove that tautomerization in single molecules can be induced by different stimuli: heat, electron attachment, light, and force exerted by the microscope's tip. Possible applications utilizing tautomerism are discussed in combination with molecular architectures on surfaces, which could pave the way for the development of single-molecule electronics.

Direct Observation of Photoinduced Tautomerization in Single Molecules at a Metal Surface.

Molecular switches are of fundamental importance in nature, and light is an important stimulus to selectively drive the switching process. However, the local dynamics of a conformational change in these molecules remain far from being completely understood at the single-molecule level. Here, we report the direct observation of photoinduced tautomerization in single porphycene molecules on a Cu(111) surface by using a combination of low-temperature scanning tunneling microscopy and laser excitation in the near-infrared to ultraviolet regime. It is found that the thermodynamically stable trans configuration of porphycene can be converted to the metastable cis configuration in a unidirectional fashion by photoirradiation. The wavelength dependence of the tautomerization cross section exhibits a steep increase around 2 eV and demonstrates that excitation of the Cu d-band electrons and the resulting hot carriers play a dominant role in the photochemical process. Additionally, a pronounced isotope effect in the cross section (∼100) is observed when the transferred hydrogen atoms are substituted with deuterium, indicating a significant contribution of zero-point energy in the reaction. Combined with the study of inelastic tunneling electron-induced tautomerization with the STM, we propose that tautomerization occurs via excitation of molecular vibrations after photoexcitation. Interestingly, the observed cross section of ∼10(-19) cm(2) in the visible-ultraviolet region is much higher than that of previously studied molecular switches on a metal surface, for example, azobenzene derivatives (10(-23)-10(-22) cm(2)). Furthermore, we examined a local environmental impact on the photoinduced tautomerization by varying molecular density on the surface and find substantial changes in the cross section and quenching of the process due to the intermolecular interaction at high density.

Electronic states of cyclophanes with small bridges.

Electronic absorption and magnetic circular dichroism were recorded for five cyclophanes with ethano bridges: [2.2]paracyclophane, (1,2,4)[2.2.2]cyclophane, (1,2,4;1,2,5)[2.2.2]cyclophane, (1,2,3,4,5,6)(1,2,3,4,5,6)cyclophane, and trans-[2.2]metacyclophane. Spectral and structural analyses were based on geometry optimization and calculations of transition energies, carried out using density functional theory methods. The assignments have been proposed for several electronic transitions observed in the region below 52,000 cm(-1). The observation of transitions which should be forbidden in the high D(2h) symmetry [2.2]paracyclophane suggests a twisted ground state structure of D(2) symmetry, although the former structure with large amplitude vibrations at room temperature cannot be excluded. The PBE0 functional turned out to appropriately reproduce the inter-ring distances and electronic transition energies.

Spectral and photophysical modifications of porphyrins attached to core-shell nanoparticles. Theory and experiment.

Plasmonic nanostructures, of which gold nanoparticles are the most elementary example, owe their unique properties to localized surface plasmons (LSP), the modes of free electron oscillation. LSP alter significantly electromagnetic field in the nanostructure neighborhood (i.e., near-field), which can modify the electric dipole transition rates in organic emitters. This study aims at investigating the influence of Au@SiO2core-shell nanoparticles on the photophysics of porphyrins covalently attached to the nanoparticles surface. Guided by theoretical predictions, three sets of gold nanoparticles of different sizes were coated with a silica layer of similar thickness. The outer silica surface was functionalized with either free-basemeso-tetraphenylporphyrin or its zinc complex. Absorption and emission bands of porphyrin overlap in energy with a gold nanoparticle LSP resonance that provides the field enhancement. Silica separates the emitters from the gold surface, while the gold core size tunes the energy of the LSP resonance. The signatures of weak-coupling regime have been observed. Apart from modified emission profiles and shortened S1lifetimes, Q band part intensity of the excitation spectra significantly increased with respect to the Soret band. The results were explained using classical transfer matrix simulations and electronic states kinetics, taking into account the photophysical properties of each chromophore. The calculations could reasonably well predict and explain the experimental outcomes. The discrepancies between the two were discussed.

Ground- and excited-state tautomerization rates in porphycenes.

The rates of double hydrogen transfer in the ground and excited electronic states have been measured for porphycene and its derivatives by using a new method based on pump-probe polarization spectroscopy. Changing the strength of two intramolecular hydrogen bonds by altering the NHN distance leads to differences in the tautomerization rate exceeding three orders of magnitude. The reaction is considerably slower in the lowest electronically excited state. A correlation was found between the tautomerization rates and (1)H chemical shifts of the internal protons.

Resonance Raman spectroscopy study of protonated porphyrin.

Resonance Raman microscopy was used to study the resonance Raman scattering of the diacid (diprotonated form) of free-base porphyrin (21H,23H-porphine) in a crystal powder and KBr pellets. Intensive lines in the spectral range between 100÷1000cm-1 have been detected and assigned as spectral manifestation of out-of-plane modes. The Raman spectra were simulated by means of DFT methods and compared with the experimental data. It is evident from experimental and theoretical results that the activation of out-of-plane modes arises from saddle distortion of the porphyrin macrocycle upon formation of its diprotonated form.

Solvent-Controlled Excited State Relaxation Path of 4-Acetyl-4'-(dimethylamino)biphenyl.

Stationary and picosecond time-resolved fluorescence (TRF) and absorption spectra were compared in different aprotic solvents at various temperatures for 4-acetyl-4'-(dimethylamino)biphenyl (ADAB). A large value of the excited state dipole moment, 18-25 D, was estimated from the plot of solvatochromic shift. TRF spectra of ADAB recorded as a function of solvent polarity and temperature show unusual temporal evolution (shift and decay) of the fluorescence bands. In some cases, the dynamic Stokes shifts occur on a time scale much shorter than expected on the basis of literature data on solvent relaxation. In order to investigate variations in the energy of electronic transitions, oscillator strengths, and dipole moments upon changing the molecular geometry, quantum chemical modeling (DFT, TD-DFT, CIS) was performed for ADAB and its ground-state pretwisted derivative, 4-acetyl-4'-dimethylamino-2,2'-dimethylbiphenyl (ADAB-Me). Combination of spectroscopic data and computational results leads to the model of excited state relaxation which involves dynamic solvent-dependent interaction between two close-lying (1)nπ* and (1)ππ* excited electronic states.

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 proton transfer through water bridges and structure of hydrogen-bonded complexes in 1H-pyrrolo[3,2-h]quinoline: adiabatic time-dependent density functional theory study.

Proton transfer reaction is studied for 1H-pyrrolo[3,2-h]quinoline-water complexes (PQ-(H(2)O)(n), n = 0-2) in the ground and the lowest excited singlet states at the density functional theory (DFT) level. Cyclic hydrogen-bonded complexes are considered, in which water molecules form a bridge connecting the proton donor (pyrrole NH group) and acceptor (quinoline nitrogen) atoms. To understand the effect of the structure and length of water bridges on the excited-state tautomerization in PQ, the potential energy profile of the lowest excited singlet state is calculated adiabatically by the time-dependent DFT (TDDFT) method. The S(0) --> S(1) excitation of PQ is accompanied by significant intramolecular transfer of electron density from the pyrrole ring to the quinoline fragment, so that the acidity of the N-H group and the basicity of the nitrogen atom of the quinoline moiety are increased. These excited-state acid-base changes introduce a driving force for the proton transfer reaction. The adiabatic TDDFT calculations demonstrate, however, that the phototautomerization requires a large activation energy in the isolated PQ molecule due to a high energy barrier separating the normal form and the tautomer. In the 1:1 cyclic PQ-H(2)O complex, the energy barrier is dramatically reduced, so that upon excitation of this complex the tautomerization can occur rapidly in one step as concerted asynchronous movements of the two protons assisted by the water molecule. In the PQ-(H(2)O)(2) solvate two water molecules form a cyclic bridge with sterically strained and unfavorable hydrogen bonds. As a result, some extra activation energy is needed for initiating the proton dislocation along the longer hydrogen-bond network. The full tautomerization in this complex is still possible; however, the cooperative proton transfer is found to be highly asynchronous. Large relaxation and reorganization of the hydrogen-bonded water bridge in PQ-(H(2)O)(2) are required during the proton translocation from the pyrrole NH group to the quinoline nitrogen; this may block the complete tautomerization in this type of solvate.

Vibrations of the phenoxyl radical.

Phenoxyl radical (C(6)H(5)O) was prepared photochemically in low-temperature argon matrices. The infrared absorption spectra were obtained for C(6)H(5)O and for the isotopically labeled species C(6)D(5)O and 1-(13)C(12)C(5)H(5)O. All but one IR-active fundamental vibrations were detected, most of them not previously observed. Combination of results from IR linear dichroism measurements on photooriented samples, determination of absolute IR intensities with the help of internal standards, analysis of isotopic shifts, and quantum chemical predictions (B3LYP/cc-pVTZ) led to a detailed assignment of phenoxyl radical vibrations. Significant frequency shifts are observed with respect to previously reported data based on resonance Raman studies in polar solutions. For some vibrations, these shifts reflect environment-induced structural changes, such as increase of the quinoid character of the phenoxyl radical in polar media. In particular, the frequency of the CO stretching vibration, readily observable in both IR and Raman experiments, is extremely sensitive to the environment and can thus be used to probe its polarity.