Conformational and Electronic Effects of Phenyl-Ring Fluorination on the Photophysical Properties of Nonplanar Dodecaarylporphyrins

Abstract

Static and time-resolved optical measurements have been performed on a number of free-base and zinc dodecaarylporphyrins with varying degrees of fluorination of the peripheral aryl rings. These studies were performed in a variety of solvents of differing polarity and metal-ligating ability and at room and low temperatures. All of the compounds are deduced to be nonplanar based on their perturbed photophysical properties relative to planar analogues and on the X-ray data available for these molecules. The dodecaarylporphyrins studied generally separate into two classes based on their photophysical properties and the presence or absence of meso-pentafluorophenyl rings. The photophysical properties are clearly affected by the electron-withdrawing characteristics of these fluorinated phenyl rings, but structural effects derived from the interaction of the phenyl rings with each other, the macrocycle, and the solvent are also apparent. Many of the differences in properties among the molecules studied and the perturbed photophysical properties of nonplanar porphyrins in general are associated with the ability of these molecules to undergo photoinduced conformational changes. For the dodecaarylporphyrins, the perturbed properties are also a result of the ability of these compounds to access multiple conformations in the ground-and excited-electronic states. The studies demonstrate the strong linkage that nonplanar porphyrins have between their electronic properties, structural characteristics, conformational dynamics, porphyrin-solvent interactions, and photophysical behavior. These connections are far stronger than those exhibited by planar porphyrins and lead to detailed differences in properties among the compounds studied here as well as the photophysical properties of nonplanar porphyrins as a whole.

Structural Dynamics and Relaxation Processes with Participation of Excited Singlet and Triplet States in Sterically Hindered Porphyrins and Their Chemical Dimers

“…The presented argumentation is in line from basic photophysical background with ideas discussed in [39,40,59] earlier.…”

“…The overall interpretation of the above findings is that according to is well-documented results, [14,[34][35][36][37][38][39][40][41][55][56][57][58][59] the presence of a large number of mesophenyl rings with flanking bulky β-alkyl substituents in porphyrin molecules leads to steric interactions between these molecular fragments and is accompanied by out-ofplane distortions of the π-conjugated tetrapyrrole macrocycle in the ground state. In addition, our data show that these compounds also exhibit a gradual increase (from 170 to 880 cm -1 ) of Stokes shift Δν S , which is indicative of the dynamic structural relaxation of the tetrapyrrole macrocycle at 293 K already in the excited S 1 state.…”

“…For this rearranged conformation of the porphyrin macrocycle one should expect an increase in the rate constants of internal conversion S 1 ∼> S 0 (channel d, Figure 7) and intersystem crossing S 1 ∼> T 1 (channel r, Figure 7), [34,57,59] due to the energy gap rule and the increase of Franck-Condon factor for non-radiative transitions. [96] In this case, upon a sequential addition of a number of meso-phenyl substituents in octaalkylporphyrins, there occurs a gradual enhancement of the non-radiative channels S 1 (F)∼>S 0 and S 1 (F) ∼>T 1 , thus manifesting itself in a fluorescence noticeable quenching (decay shortening and quantum yield decrease).…”

A Surface Chemical Reaction in Organic–Inorganic Materials Using a New Chemical Evaporation System

“…There are two interesting features that arise as the DEZ exposure time to the H 2 TPP thin layer is increased: the two Q-bands at 528 and 660 nm disappear, and the other Q-band peaks at 554 and 594 nm become significantly blue-shifted. 34 This is indicative of the formation of ZnTPP hybrid thin films. AFM measurements on 5×5 µm 2 areas were also performed in order to obtain information regarding thin film topology (Figure 4).…”

Novel cycloketo tetraphenylporphyrins: spectroscopic study of structure-properties relationships

“…Moreover, the probabilities of radiative and non-radiative transitions from excited states can be affected by metal insertion and/or creation of a distorted π-system. [12][13][14][15] Free-base porphyrins are known to show tautomerism of the inner N-bound hydrogen atoms, and in the most stable tautomeric structures they are bound to opposite nitrogens. [16][17][18] The migration of these hydrogen atoms was found to be relatively quick at room temperature.…”