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Abstract: Geometry and vibrational structure of dibenzo[b,i][1,4,8,11]tetraaza[14]annulene (TAA) have been studied using infrared and Raman spectroscopy combined with quantum-chemical calculations. The assignments were proposed for 106 out of the total of 108 TAA vibrations, based on comparison of the theoretical predictions with the experimental data obtained for the parent molecule and its isotopomer in which the NH protons were replaced by deuterons. Reassignments were suggesteded for the NH stretching and out-of-plane vibrations. The values of the parameters of the intramolecular NH⋯N hydrogen bonds were analysed in comparison with the corresponding data for porphyrin and porphycene, molecules with the same structural motif, a cavity composed of four nitrogen atoms and two inner protons. Both experiment and calculations suggest that the molecule of TAA is not planar and is present in a trans tautomeric form, with the protons located on the opposite nitrogen atoms. [Copyright &y& Elsevier]

Abstract: Dynamics of relaxation from higher electronic excited states was studied for 9,10,19,20-tetraalkylporphycenes with methyl and propyl substitutents. Depopulation of S3/S4 Soret states leading into S2/S1 Q levels occurs in about 50fs. This time is similar, or even shorter than that recently determined for porphyrin, even though the Q-Soret energy separation in the latter is about twice smaller than in porphycenes. The time constant for H or D transfers are shorter than 50fs indicating very low energy barriers to their transfer. The non-radiative rate constants of both derivatives are related to the motion of the alkyl chain. [Copyright &y& Elsevier]

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 D2h symmetry [2.2]paracyclophane suggests a twisted ground state structure of D2 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. [ABSTRACT FROM AUTHOR]

Resonance Raman microscopy was used to study the resonance Raman scattering of the diacid (diprotonated form) of free-base porphyrin (21 H ,23 H –porphine) in a crystal powder and KBr pellets. Intensive lines in the spectral range between 100 ÷ 1000 cm − 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. [ABSTRACT FROM AUTHOR]

Porphycene(Pc) is a well-known model for studying double hydrogentransfer, which shows vibrational-mode-specific tunneling splittingwhen isolated in supersonic jets or helium nanodroplets. The effectof deuteration on tunneling splitting is reported for jet-cooled heterogeneous,deuterated Pc samples (Pc-dmix) with the prevailing contributionof Pc-d12isotopologue. The sample introduced into thegas phase using laser desorption is studied by means of laser-inducedfluorescence (LIF) and single vibronic level fluorescence (SVLF) measurements,in combination with quantum chemical calculations. The influence ofmolecular symmetry is studied by comparing Pc, Pc-d12,and Pc-d11. The spectra of Pc-d12show strongsimilarity to those of the parent undeuterated porphycene (Pc). Comparabletunneling splitting is observed in the two isotopologues, both forthe 0–0 transition and the most efficient promoting 2Agmode. In contrast, an unusual isotopic effect is observedfor the totally symmetrical 4Agmode. While this vibrationbehaves as a neutral mode in Pc, neither enhancing nor decreasingthe tunneling efficiency, it strongly promotes hydrogen transfer inPc-d12. This observation is explained in terms of modificationof the displacement vectors of the 4Agmode upon deuteration.It demonstrates that isotope substitution affects hydrogen transfereven when the weak structural modifications are far from the reactioncenter, emphasizing the strongly multidimensional nature of the tunnelingprocess. [ABSTRACT FROM AUTHOR]

Highlights: [•] Resonance Raman spectra of porphyrazine derivatives were recorded. [•] Spectral assignments of the vibrational modes have been proposed. [•] Noticeable changes in the spectral features due to aza-substitution were established. [•] Special features due to the presence of the aryl rings have been discussed. [ABSTRACT FROM AUTHOR]

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 cm² 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 cm²). 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. [ABSTRACT FROM AUTHOR]