Variations and Temperature Dependence of the Excited State Properties of Conformationally and Electronically Perturbed Zinc and Free Base Porphyrins

Abstract

The photophysical properties and their temperature dependence are reported for the sterically encumbered nonplanar zinc and free base 2,3,5,7,8,10,12,13,15,17,18,20-dodecaphenylporphyrins (ZnDPP and H2DPP), and 2,3,7,8,12,13,17,18-octaethyl-5,10,15,20-tetraphenylporphyrins (ZnOETPP and H2OETPP), and the zinc complex of 5,10,15,20-tetra-tert-butylporphyrin (ZnT(t-Bu)P). Compared to planar 5,10,15,20-tetraphenylporphyrins (ZnTPP and H2TPP), the above compounds exhibit reduced lifetimes of the lowest excited singlet state, reduced fluorescence yields, and large shifts between their absorption and emission maxima at room temperature. ZnT(t-Bu)P, which is known to adopt a ruffled conformation, displays dramatically altered photophysical properties including a 7 ps 1(π,π*) lifetime compared to one of ∼2 ns for ZnTPP at 296 K. Equally noteworthy is the return of the ZnT(t-Bu)P singlet lifetime to a “normal” value of 2.5 ns at 78 K. An analogous temperature dependence has been observed previously for the free base analog H2T(t-Bu)P. The other porphyrins investigated, with different modes of nonplanarity, display smaller temperature variations but also tend toward more normal properties at low temperatures. A more extreme case of perturbation to the tetrapyrrole electronic structure is found in zinc 2,3,5,5‘,7,8,12,18-octamethyl-13,17-bis(3-methoxy-3-oxopropyl)isoporphyrin perchlorate, a porphyrin tautomer with an interrupted π system. This zinc isoporphyrin also exhibits a short excited state lifetime of 130 ps at 296 K, which again increases to 0.8 ns at 78 K. The results for the various nonplanar porphyrins and for the isoporphyrin in several solvents indicate that the principal cause of the altered excited state lifetimes is the ability of the molecules to traverse multiple conformational surfaces in the excited state. These surfaces appear to be separated by only small energy barriers that vary with the types of conformational distortions and their concomitant perturbations of the electronic structures of the chromophores.

Femtosecond time‐resolved spectroscopy on biological photoreceptor chromophores

“…The reduced porphyrin macrocycle occurs in the pheophytins and, further complexed with magnesium, in the chlorophylls both of which are involved in light harvesting and the transformation of trapped sun light into stored chemical energy in the course of photosynthesis [74]. Because of this significance as biological reaction centers, but also because of their function as photosensitizers in photodynamic therapy and as catalysts in precision macromolecular synthesis [75,76], porphyrins have been the focus of a series of time-resolved spectroscopic investigations over the past years in order to gain a better insight into the internal excited-state dynamics [77][78][79][80][81][82][83][84][85]. For these studies a wide variety of time-resolved techniques has been used including transient absorption, nonlinear absorption, fluorescence and also four-wave-mixing spectroscopy.…”

Manifestation of nonplanarity effects and charge-transfer interactions in spectral and kinetic properties of triplet states of sterically strained octaethylporphyrins

“…We found earlier [20] that the energy of the charge-transfer (CT) state for OEP-Ph(o-NO 2 ) in toluene exceeds the energy of the locally excited T 1 state by 0.24 eV. Therefore, from energy considerations, the new bands observed cannot be assigned to the T 1 CT absorption transitions. A manifestation of the intrinsic absorption in a system of the states of the ion-radical pair is also unlikely because of a fast transition from the 3 [P + … ] CT state to the lower-lying T 1 state (E CT > E T1 [20]).…”

“…Recent spectral, kinetic, and physicochemical studies [1][2][3][4] showed that a number of "hybrid" porphyrins with the steric hindrance (free bases and complexes with metals Ni, Cu, and Fe) have the nonplanar porphyrin macrocycle in the initial S 0 state, which is manifested in a considerable change in the absorption spectrum (strong bathochromic shift and broadening of the bands) and in redox properties compared to planar tetrapyrrole macrocycles. Upon excitation of such molecules, the dynamic conformational mobility of the tetrapyrrole macrocycle in the S 1 state is manifested in strong deformations of the fluorescence spectra (bathochromic shift and broadening of the bands, an increase in the Stokes shift) and in a strong decrease in the quantum yield ϕ F and shortening of the fluorescence lifetime τ S .…”

“…However, at present there is no detailed information on spectral and kinetic parameters of the T 1 states of hybrid porphyrins under the conditions of dynamic conformational lability of the porphyrin macrocycle in liquid solutions at 293 K. The data available are scarce and are often indirect. Thus, in experiments on bleaching of the absorption bands of porphyrins with the steric hindrance (saddle and ruffle conformations), the quantum yields of the S 1 T 1 intersystem crossing were estimated to be γ = 0.2-0.7, the lowest values being typical for the ruffle structures [5][6][7][8][9]. In some cases, differential spectra of the induced absorption have been obtained in the region from 600 to 900 nm upon picosecond excitation [5,8,9].…”

“…Thus, in experiments on bleaching of the absorption bands of porphyrins with the steric hindrance (saddle and ruffle conformations), the quantum yields of the S 1 T 1 intersystem crossing were estimated to be γ = 0.2-0.7, the lowest values being typical for the ruffle structures [5][6][7][8][9]. In some cases, differential spectra of the induced absorption have been obtained in the region from 600 to 900 nm upon picosecond excitation [5,8,9]. Direct experimental data on the decay of the triplet states for two types of nonplanar tetra-substituted porphyrins are presented in the only paper [9] known to us, where it was shown that, along with a decrease in the quantum yield of intersystem crossing, the lifetime τ T of the T 1 state greatly shortened at room temperature in deoxygenated solutions and the quantum yield γ ∆ of the singlet oxygen generation decreased.…”