The spectroscopic impact of interactions with the four Gouterman orbitals from peripheral decoration of porphyrins with simple electron withdrawing and donating groups

Abstract

Tetrapyrroles are of great interest for solar cell and photodynamic therapy applications due to their structural analogy with chlorophyll, a natural photosensitizer. Unsubstituted symmetric porphyrins exhibit weak absorption in the red region which makes them unsuitable for these applications. The push-pull peripheral decoration modifies the energies of the frontier molecular orbitals, which in turn influences the tetrapyrrole's spectroscopic properties. The absorption, magnetic circular dichroism, and emission spectra were measured for four zinc tetratolylporphyrin compounds substituted peripherally with a fused dimethoxybenzo group as an electron withdrawing group (EWG) on one pyrrole and on the opposite pyrrole, a single acetamido (1), a nitro (2), a proton (3), or a benzoylamino (4) substituent. Unusually, the magnetic circular dichroism spectrum of 2 exhibited a negative A term for the lowest energy absorption band (the Q band) and its emission spectrum was also unlike those of 1, 3, and 4. A complete computational analysis was carried out to obtain the energies and electron distribution, shown by electron density surfaces, of the four Gouterman MOs. TD-DFT calculations showed that for 2, ΔLUMO was greater than ΔHOMO, which accounted for the observed negative A term. The trend in the estimated MCD A term magnitudes, normalized to the absorbance as [A/(dipole strength) BM], provides experimental confirmation of the computationally determined ratio of ΔLUMO/ΔHOMO data. The value of ΔHOMO was confirmed by the trend in oscillator strengths. A series of fictive porphyrins (F1-F5) incorporating simple push-pull substituents were designed and their electronic structures were investigated using TD-DFT calculations. The substituents in the five fictive molecules illustrate the differential effect of the donor and acceptor groups in the β-position of the pyrroles on the relative stabilities of the four Gouterman orbitals. NO groups result in the greatest splitting of the LUMO pair. We show that on using strong EWGs, opposite electron donating groups result in a ΔLUMO > 0, which red-shifts the Q band and introduces a strong dipole. With the nitro and formyl EWGs, ΔLUMO becomes greater than ΔHOMO, resulting in a complex electronic structure of the Q band, recognizable by a negative A term suggesting a design objective for future photosensitizers.

Discovery of a Nitric Oxide-Responsive Protein in Arabidopsis thaliana

“…In more detail, the porphyrin structure contains an aromatic system of π electrons consisting of molecular orbitals which are occupied and unoccupied (π and π*). UV/Vis spectra show characteristic Soret peaks which stem from π → π* electron transitions [76,77].…”

Switching the Photoluminescence and Electrochemiluminescence of Liposoluble Porphyrin in Aqueous Phase by Molecular Regulation

“…(solid blue,F igure 1A)d isplays as trong Soret band ( % 410 nm) and four less strong Qb ands (470-700 nm), which corresponds to the transition of the electrons from the ground state to the second excited state (S 0 !S 2 )and the first excited state (S 0 !S 1 ), respectively. [26] While ab road absorption band (250-380 nm) attributed to tetraphenylvinyl group can be seen from the TPE-PA( solid black, Figure 1A). Strikingly,the absorption spectrum of ATPP-TPE (solid red, Figure 1A)contains absorption features from both ATPP and TPE-PA, confirming the successful formation of the expected adduct.…”

Benchmarking computational methods and influence of guest conformation on chirogenesis in zinc porphyrin complexes

“…It is assumed that the homologues alkyl substituents of similar electronic and steric influences have a negligible effect on the chiroptical properties. [48][49][50][51] Host A, with two hydroxynaphtyl groups at the 5-,15-meso positions, is able to form hydrogen bonds with the guests. With this host, four supramolecules were modelled using alanine, leucine, and phenylalanine methyl esters (Ala-OMe, Leu-OMe, and Phe-OMe, respectively), as well as leucinol as the corresponding guests.…”