Spectroscopic investigation of photophysics and tautomerism of amino- and nitroporphycenes.

Parent, unsubstituted porphycene and its two derivatives: 2,7,12,17-tetra--propylporphycene and 2,7,12,17-tetra--butylporphycene were substituted at the position with amino and nitro groups. These two families of porphycenes were characterized in detail with respect to their spectral, photophysical, and tautomeric properties. Two tautomers of similar energies coexist in the ground electronic state, but only one form dominates in the lowest excited singlet state.

Hydrogen-Bonded Cyclic Dimers at Large Compression: The Case of 1-pyrrolo[3,2-]quinoline and 2-(2'-pyridyl)pyrrole.

1-pyrrolo[3,2-]qinoline (PQ) and 2-(2'-pyridyl)pyrrole (PP) are important systems in the study of proton-transfer reactions. These molecules possess hydrogen bond donor (pyrrole) and acceptor (pyridine) groups, which leads to the formation of cyclic dimers in their crystals. Herein, we present a joint experimental (Raman scattering) and computational (DFT modelling) study on the high-pressure behaviour of PQ and PP molecular crystals.

Substituent screening effect on single-molecule photostability: comparison of three differently substituted porphycenes.

Photobleaching of single molecules has been studied using confocal fluorescence microscopy for porphycene, a porphyrin isomer, and its two derivatives. Fourfold substitution of porphycene with bulky-butyl groups leads to the enhancement of photostability, even though the spectral, photophysical, and redox parameters remain similar. We attribute this effect to the increase of the efficiency of physical quenching of the chromophore triplet state by oxygen, compared with the yield of chemical reaction that leads to photobleaching.

Controlling Emissive Properties by Intramolecular Hydrogen Bonds: Alkyl and Aryl meso-Substituted Porphycenes.

Porphycene, a porphyrin isomer, is an efficient fluorophore. However, four-fold meso substitution with alkyl groups decreases the fluorescence quantum yield by orders of magnitude. For aryl substituents, this effect is small.

Fluorinated Porphycenes: Synthesis, Spectroscopy, Photophysics, and Tautomerism.

Six porphycenes have been synthesized, bearing one, two, or three fluorine atoms attached directly to the 18-π-electron system at the meso positions. These novel compounds have been characterized by structural, electrochemical, and spectral techniques, combined with quantum chemical calculations. In three fluoroporphycenes, the unsymmetric substitution pattern leads to the presence of two nonequivalent trans tautomeric forms.

Influence of local microenvironment on the double hydrogen transfer in porphycene.

We performed time-resolved transient absorption and fluorescence anisotropy measurements in order to study tautomerization of porphycene in rigid polymer matrices at cryogenic temperatures. Studies were carried out in poly(methyl methacrylate) (PMMA), poly(vinyl butyral) (PVB), and poly(vinyl alcohol) (PVA). The results prove that in all studied media hydrogen tunnelling plays a significant role in the double hydrogen transfer which becomes very sensitive to properties of the environment below approx.

Towards More Photostable, Brighter, and Less Phototoxic Chromophores: Synthesis and Properties of Porphyrins Functionalized with Cyclooctatetraene.

Free base and zinc porphyrins functionalized with cyclooctatetraene (COT), a molecule known as a good triplet-state quencher, have been obtained and characterized in detail by structural, spectral, and photophysical techniques. Substitution with COT leads to a dramatic decrease of the intrinsic lifetime of the porphyrin triplet. As a result, photostability in oxygen-free solution increases by two to three orders of magnitude.

2 + 2 Can Make Nearly a Thousand! Comparison of Di- and Tetra--Alkyl-Substituted Porphycenes.

Two porphycenes, substituted at the positions with two and four methyl groups, respectively, reveal similar absorption spectra, but their photophysical properties are completely different. 9,20-dimethylporphycene emits fluorescence with about 20% quantum yield, independent of the solvent. In contrast, fluorescence of 9,10,19,20-tetramethylporphycene is extremely weak in nonviscous solvents, but it can be recovered by placing the chromophore in a rigid environment.

Antiaromatic or Nonaromatic? 2 H,6 H-2,6(2,5)-Dipyrrola-1,5(2,6)-dipyridinacyclooctaphane-3,7-diene: a Porphycene Derivative with 4 N π Electrons.

A porphycene-derived compound with a 20 π-electron skeleton has been obtained by replacing two pyrrolene units of porphycene by pyridine rings. NMR, electronic absorption and MCD spectra, and the lack of fluorescence are typical for 4 N cyclic π electron systems. The electronic structure and the differences with respect to porphycene can be rationalized by treating these compounds as perturbed, doubly positively charged [22]annulene and [20]annulene perimeters, respectively.

Evidence for Dominant Role of Tunneling in Condensed Phases and at High Temperatures: Double Hydrogen Transfer in Porphycenes.

Investigation of the double hydrogen transfer in porphycene, its 2,7,12,17-tetra-tert-butyl derivative, and their N-deuterated isotopologues revealed the dominant role of tunneling, even at room temperature in condensed phase. Ultrafast optical spectroscopy with polarized light employed in a wide range of temperatures allowed the identification and evaluation of contributions of two tunneling modes: vibrational ground-state tunneling, occurring from the zero vibrational level, and vibrationally activated, via a large amplitude, low-frequency mode. Good correspondence was found between the rates of incoherent tunneling occurring in condensed phase and the values estimated on the basis of tunneling splittings observed in molecules isolated in supersonic jets or helium nanodroplets.

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.