A Computational Study of the Ground and Excited State Structure and Absorption Spectra of Free-Base N-Confused Porphine and Free-Base N-Confused Tetraphenylporphyrin

Abstract

Computational investigations into the ground and singlet excited-state structures and the experimental ground-state absorption spectra of N-confused tetraphenylporphyrin tautomers 1e and 1i and N-confused porphines (NCP) 2e and 2i have been performed. Structural data for the ground state, performed at the B3LYP/6-31G(d), B3LYP/6-31+G(d)//B3LYP/6-31G(d), and B3LYP/6-311+G(d)//B3LYP/6-31G(d) levels, are consistent with those performed at lower levels of theory. Calculations of the gas-phase, ground-state absorption spectrum are qualitatively consistent with condensed phase experiments for predicting the relative intensities of the Q(0,0) and Soret bands. Inclusion of implicit solvation in the calculations substantially improves the correlation of the energy of the Soret band with experiment for both tautomers (1e, 435 nm predicted, 442 nm observed in DMAc; 1i, 435 nm predicted, 437 nm observed in CH2Cl2). The x- and y-polarized Q-band transitions were qualitatively reproduced for 1e in both the gas phase and with solvation, although the low-energy absorption band in 1i was predicted at substantially higher energy (646 nm in the gas phase and 655 nm with solvation) than observed experimentally (724 nm in CH2Cl2). Franck-Condon state and equilibrated singlet excited-state geometries were calculated for unsubstituted NCP tautomers 2e and 2i at the TD-B3LYP/SVP and TD-B3LYP/TZVP//TD-B3LYP/SVP levels. Electronic difference density plots were calculated from these geometries, thereby indicating the change of electron density in the singlet excited states. Adiabatic S1 and S2 geometries of these compounds were also calculated at the TD-B3LYP/SVP level, and the results indicate that while 2i is a more stable ground-state molecule by approximately 7.0 kcal mol-1, the energy difference for the S1 excited states is only approximately 1.0 kcal mol-1 and is 6.1 kcal mol-1 for the S2 excited states.

An ultrafast time‐resolved infrared and UV–vis spectroscopic and computational study of the photochemistry of acyl azides

“…To overcome this problem, one can compute the electronic density distribution in both the ground state and the Frank–Condon excited state followed by subtraction of the ground state electronic density from the Frank–Condon excited state electronic density, thereby giving an electron density difference plot. In the recent past, we have utilized this strategy to analyze the photochemical behavior of many different chemical systems . For these plots, a red or green contour indicates depletion or accumulation of electronic density in the excited state relative to the ground state, respectively.…”

Solvent Effects on the UV–Vis Absorption Properties and Tautomerism of N-Confused Tetraphenylporphyrin

“…[22][23][26][27][28] Recently, the construction of self-assembled supramolecular systems was investigated as a function of NCTPP tautomer, and a proposed mechanism of tautomeric structures in a combination of four protic and aprotic solvents using SEM and AFM analysis was described. 29 Computational work by Marchand and Jacquemin, 30 and separately by us, 21,31 using the polarization continuum model in Gaussian09 and density functional theory (DFT), confirmed energetic differences as a function of solvent polarity, but also showed a switch in tautomer preference did not occur under these conditions. We previously examined the solvent dependent nature of the tautomerization by UV-Vis spectroscopy, confirming Furuta's observations that both NCTPP tautomers can exist simultaneously in certain solvents, presumably in equilibrium with one another.…”

“…This conjugated π-system is the electronic "heart" of the macrocycle and is responsible for the intensity, color, and optical properties of porphyrins. Changes to the tetrapyrrolic macrocycle upon reduction of the pyrrole rings, [7][8] fusing different rings across the pyrrole rings of the macrocycle, [9][10][11] or exchanging, adding or removing atoms or 6 We have previously investigated the photophysical properties of unsubstituted and substituted NCTPPs 1i and 1e in solution using a combination of computations, 21 steady-state fluorescence, time-resolved fluorescence and ultrafast absorption spectroscopies. [22][23] These properties are of great interest because of the potential for incorporation of NCTPPs into applications for which porphyrins are normally considered.…”

Catalysis of Oxygen Reduction by Metal-Free Porphyrins in One- and Two-Phase Liquid Systems

“…The visible and near-UV region of a porphyrin free base spectrum is characterized by an intense Soret band around 400 nm and a weak Q band around 550 nm. These transitions were qualitatively assigned by the four orbital model proposed by Gouterman,72,73 and quantum chemical calculations later confirmed and refined this assignment either by ab initio, [74][75][76] time dependent density functional theory (TDDFT), 65,[77][78][79][80][81] or a combined DFT/multireference configuration interaction (MRCI) approach. 82 DFT calculations reveal changes in the electron density distribution after excitation of the substituted porphyrins.…”