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. To explain this difference, we have synthesized and studied a mixed aryl‐alkyl‐substituted compound, 9,20‐diphenyl‐10,19‐dimethylporphycene, as well as the 9,20‐diphenyl and 9,20‐dimethyl derivatives. Analysis of the structural, spectroscopic, and photophysical data of the six porphycenes, combined with quantum chemical calculations, shows a clear correlation between the strength of the intramolecular NH⋅⋅⋅N hydrogen bonds and the efficiency of the radiationless depopulation of the lowest‐excited singlet state. This result led us to propose a model in which the delocalization of the inner protons in the cavity of the macrocycle is responsible for the nonradiative deactivation channel. The applicability of the model is confirmed by the literature data for other alkyl‐ or aryl‐substituted porphycenes. The finding of a correlation between structural and emissive characteristics enables a rational design of porphycenes with desired photophysical properties.

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A series of co-crystals of trans-1,2-bis(4-pyridyl) ethylene (bpe) with aliphatic saturated or unsaturated dicarboxylic acids, of the type [(bpe)(L)] (L = dicarboxylic acid), were prepared and characterized by single crystal X-ray crystallography. In all cases chains of the type acid•••bpe•••acid were formed via strong O−H•••N hydrogen bonds. The chains were further linked between them by weak C−H•••O hydrogen bonds, π−π and herringbone interactions to form three-dimensional structures. Thermal analysis showed considerable elevation of the melting/decomposition point of the co-crystals with respect either to both components or to bpe, ascribed to the formation of multiple complementary C−H•••O bonds as well as to π−π interactions between the two conformer molecules. When L = fumaric acid, the two components stack separately with their CC bonds strictly parallel between them at 3.83 Å from one another. However, photodimerization reaction takes place only between the olefin bonds of bpe. Photodimerization reaction between one CC bond of the acid and that of bpe was also induced when L = trans,trans-muconic acid.