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

Abstract

Two porphycenes, substituted at the meso 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. We propose a model that explains these differences, based on calculations and structural analogies with other extremely weakly emitting derivatives, dibenzo[ cde , mno ]porphycenes. The efficient S 1 deactivation involves delocalization of two inner cavity protons coupled with proton translocation toward a high-energy cis tautomer. The latter process leads to distortion from planarity. The probability of deactivation increases with the strength of the intramolecular NH···N hydrogen bonds. The model also explains the observation of biexponential fluorescence decay in weakly emitting porphycenes. It can be extended to other derivatives, in particular, the asymmetrically substituted ones. We also point to the possibility of using specific porphycenes as viscosity sensors, in particular, when working in single molecule regime.

Fluorinated Porphycenes: Synthesis, Spectroscopy, Photophysics, and Tautomerism

“…Another direction of future studies will be focused on elucidation of the mechanisms responsible for efficient radiationless depopulation of the S 1 state in some porphycenes. We recently postulated that the nonradiative channel is related to the strength of the H‐bonds and that deviations from planarity may be responsible for ultrafast excited state decay . Fluoroporphycenes provide the possibility of controlling the H‐bond strength in a controlled fashion, at the same time avoiding steric effects that could lead to nonplanarity.…”

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

“…However, our recent work suggests another mechanism: S 1 deactivation not occurring from cis 2, but along the path leading to it [7] . Calculations indicate that a distortion from planarity already occurs in 5 in the electronic ground state once a single proton starts moving from the trans towards the cis 2 form.…”

“…However,o ur recent work suggests another mechanism:S 1 deactivation not occurring from cis2, but along the path leading to it. [7] Calculationsi ndicate that ad istortion from planarity already occurs in 5 in the electronic ground state once as ingle proton starts movingf rom the trans towards the cis2f orm. Such an effect is not observed in 2.W ep ostulated that the efficient S 1 depopulationt akes place when the inner protons become delocalized, such that the trans and cis1t automeric forms can no longer be distinguished.…”

“…The above model has recently been extended with the proposal that the rapid excited‐state deactivation is a result of the delocalization of the two inner protons along the path leading from the trans to the high‐energy cis tautomer [7] . According to this postulate, the rate of deactivation should increase with the strength of the intramolecular hydrogen bonds.…”

“…[6] The above model has recently been extended with the proposalt hat the rapid excited-state deactivationi saresult of the delocalization of the two inner protons along the path leading from the trans to the high-energy cis tautomer. [7] According to this postulate, the rate of deactivation should increase with the strength of the intramolecular hydrogenb onds. We now test this hypothesis by comparing the photophysical properties of as eries of meso-alkyl-, meso-aryl-, and mixed meso-aryl,meso-alkyl-substitutedp orphycenes (Scheme 1).…”