Fluorinated Porphycenes: Synthesis, Spectroscopy, Photophysics, and Tautomerism

Abstract

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. They have been identified using electronic absorption, emission, and magnetic circular dichroism spectroscopies. Their relative energies have been estimated for the ground and lowest excited electronic states. Tautomerization potential is quasi‐symmetric in S0, but becomes strongly nonsymmetric in S1. Femtosecond transient absorption studies allowed determination of tautomerization rates, larger and similar for both directions of the double hydrogen transfer in S0, lower and disparate in S1. Fluoroporphycenes emerge as good candidates for detailed studies of mechanisms of double hydrogen transfer, as well as processes responsible for rapid radiationless excited state depopulation.

Spectroscopic investigation of photophysics and tautomerism of amino- and nitroporphycenes

“…A similar effect has been recently reported for 9-fluoroporphycene. 22 Our assignments agree with the results of a theoretical paper in which the absorption of 9-amino-2,6,12,17-tetraphenylporphycene has been modelled using the nuclear ensemble method. 49 The authors concluded that the first absorption band originates from trans1, but the second band is dominated by transition from trans2.…”

“…Photophysical studies demonstrate that both, amino and nitroporphycenes behave similarly to other unsymmetrically substituted porphycenes, such as 9-acetoxy 18,50 or 9-fluoro 22 derivatives. In the ground electronic state, two trans tautomers of similar energy coexist, whereas in S 1 the equilibrium is shifted towards the structure which was already more stable in S 0 ( trans 1 in amino-, trans 2 in nitroporphycenes).…”

“…Under such conditions, tautomerization in solution is very fast (femtoand picoseconds); 13 moreover, it is governed by tunneling, both ''deep'' (occurring from the vibrational ground state) and thermally activated after excitation of specific vibrational modes. 14 Tautomerization in porphycenes has been studied in various experimental regimes: ensemble studies in condensed phases, [13][14][15][16][17][18][19][20][21][22][23][24] investigations of ultracold molecules isolated in supersonic jets 25,26 or helium nanodroplets, 27 and, finally, single molecule techniques involving fluorescence, [28][29][30][31] Raman, 32,33 and scanning probe microscopy. [34][35][36][37][38][39][40] It has been demonstrated that photophysics of porphycenes and their tautomeric properties are strongly related.…”

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

“…Thus, parent porphycene (1) and its 2,7,12,17-tetra-tert-butyl derivative (2) have very similar spectral and photophysical characteristics [21], but differ by the absence/presence of the substituent 'shield'. In contrast, the same shield is present in 2 and 3 (9-fluoro-2,7,12,17-tetra-tert-butyl-porphycene), but now the spectral parameters differ somewhat [26]. Finally, the three porphycenes selected for this study have similar redox properties, which is crucial when photodestruction efficiencies are to be compared.…”

Latest trends on photodynamic disinfection of Gram-negative bacteria: photosensitizer’s structure and delivery systems

“…Another kind of PS is porphycenes, which are isomers of porphyrins. Their unique aromatic structure provides them with different photophysical properties than those observed for the porphyrins, as a stronger absorption at 600-700 nm and a strong fluorescence [65] . Although the less, there are some examples of cationic porphycenes being used against bacteria, as Ruiz-González et al research, whom synthesized two porphycenes (57 and 58, Fig.…”