Abstract
UV–vis absorption and magnetic circular dichroism (MCD) spectra of octakis thioethyl “free base” porphyrazine H2OESPz and its metal complexes MOESPz (M = Mg, Zn, Ni, Pd, Cu), as well as of [MnOESPz(SH)] were recorded. In the last case, MCD proved to have quite good sensitivity to the coordination of this complex with 1-methylimidazole (1-mim) in benzene. Time-dependent density functional theory (TDDFT) calculations were carried out for the considered porphyrazine complexes and showed good performance on comparing with MCD and UV–vis experimental spectra, even in the open-shell Cu and Mn cases. Calculations accounted for the red shift observed in the thioalkyl compounds and allowed us to reveal the role of sulfur atoms in spectroscopically relevant molecular orbitals and to highlight the importance of the conformations of the thioethyl external groups. Calculated MCD spectra of [MnOESPz(SH)] confirm the Mn(III) → Mn(II) redox process, which leads to the [Mn(OESPz)(1-mim)2] species, and the relevance of the spin state for MCD is revealed.
Highlights
The highly delocalized π-electron system of tetrapyrrole macrocycles makes these systems ideal substrates for optoelectronics
Porphyrazines MOESPz were prepared as already described in refs 11 and 55−58
All UV−vis spectra exhibit the typical features of nonaggregate thioalkyl porphyrazines; for the “free base”, the two Qx and Qy bands can be distinguished at 1.77 eV (704 nm) and at 1.94 eV (635 nm), respectively; the Soret band is observed at about 3.53 eV (357 nm), and its shape suggests contributions from two bands at about 3.43 (361 nm) and 3.58 eV (346 nm)
Summary
The highly delocalized π-electron system of tetrapyrrole macrocycles makes these systems ideal substrates for optoelectronics. Porphyrins and phthalocyanines, the two most important subgroups of this family, find applications as dyes in organic photovoltaics (OPV)[1,2] and for the development of materials for nonlinear optics (NLO).[3−6] The structurally related porphyrazine macrocycles[7−10] have, been much less studied in this field, they display interesting structural and optical properties They allow ample and facile synthetic modularity, display a wider UV−vis absorption range, promising the development of panchromatic photovoltaic materials and, under some circumstances, can give rise to columnar liquid crystal mesophases.[11−15] Very few examples of porphyrazine applications in NLO16−24 and OPV25,26 have been reported, and only very recently some of us have described the potentiality of nonsymmetrically substituted thioalkyl porphyrazines in OPV27,28 and NLO.[29,30] For the development of new optoelectronic materials based on the porphyrazine framework, a detailed knowledge of their electronic structure is of utmost importance. The basis of the MCD theory was developed in the 1960−1970s.38,39 A further key development of this technique has been provided by the recent application of density functional theory (DFT) computations for MCD spectra simulation;[40−47] correspondence between simulated and experimental spectra provides confidence in the picture obtained by simple time-dependent DFT (TDDFT) calculations, with due attention to possible TDDFT limitations, in the case of open-shell systems.[48,49]
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