Controlling Emissive Properties by Intramolecular Hydrogen Bonds: Alkyl and Aryl meso‐Substituted Porphycenes. Issue 20 (12th March 2021)

• Record Type: Journal Article
• Authors: Listkowski, Arkadiusz; Masiera, Natalia; Kijak, Michał; Luboradzki, Roman; Leśniewska, Barbara; Waluk, Jacek
• Is Part Of: Chemistry. Volume 27:Issue 20(2021)
• Page Start: 6324
• Page End: 6333
• Publication Date: 2021-03-12
• Subjects: density functional calculations -- fluorescence -- hydrogen bonds -- porphyrinoids -- substituent effects; Chemistry -- Periodicals; 540
• DOI: 10.1002/chem.202005440
• Languages: English
• ISSNs: 0947-6539

Abstract

Abstract: 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. Abstract : Hydrogen bonds are key : The fluorescence intensity of meso ‐substituted porphycenes drops sharply when the distance between the hydrogen‐bonded nitrogen atoms decreases below a certain value and the intramolecular hydrogen bonds get stronger. This is attributed to the delocalization of the inner protons, which opens an efficient radiationless deactivation channel.