DFT Studies of the Resonance Raman Spectra of Ground and Excited Triplet State Free Base meso-Tetraphenylporphyrin (H₂TPP)

Abstract

DFT calculations at the B3LYP/6-31G(d) level were carried out for the ground and excited triplet states of free base meso-tetraphenylporphyrin, H2TPP, and its d 2, d 8, d 10, d 20, d 22, and 13C4 isotopomers. The agreement between experimental and calculated (scaled with a single factor of 0.973) band positions for the ground state was acceptable (rms = 9.9 cm-1). In addition, although the shifts on isotopic substitution were frequently of the same order as this rms error in the absolute positions, it was found that the cancellation of errors in the calculations meant that the direction and magnitude of even small isotope shifts < 10 cm-1 were also calculated correctly. In the D 2 symmetry of the calculations the lowest lying triplet state corresponded to a one-electron transition from the b1 HOMO to a b3 LUMO. It was found that the calculated changes in cm-1 of the vibrational modes on excitation to this triplet (ΔS - T) reproduced the ΔS - T values of the seven bands found in the experimental spectra. Three of these bands (ν2, ν1 2, and ν15) moved to lower cm-1 on excitation, two were essentially unchanged (φ4 and ν1), and two moved to higher cm-1 ( ν4 and ν6). Since the ΔS - T values are typically small (≤10's of cm-1), the correct prediction of the pattern of small shifts associated with population of an excited state with a particular electronic configuration is impressive. As before, this improved accuracy presumably arises because errors in the calculations of absolute positions cancel when values for the same modes in different electronic states are subtracted to give shifts on excitation. This level of accuracy is necessary if ΔS - T values are to be used to assign orbital parentage. Surprisingly, for several of the modes the calculated ΔS - T shifts differed dramatically between isotopomers. For example, for ν2: ΔS - T(calc), d 0 = −5 cm-1, d 8 = −30 cm-1; Δ S - T(obs), d 0 = −15 cm-1, d 8 = −26 cm-1. These differences were found to reflect not only the changes in force constants due to promotion to the triplet state (which are the same irrespective of which isotopomer is under consideration) but also changes in the mode composition on excitation which alters the isotope sensitivity. In the case of H2TPP, the mode compositions change because excitation from b1 → b3 orbitals accentuates the difference in bonding between the pairs of protonated and unprotonated pyrrole rings. In effect, excitation increases the rectangular distortion that is already present in the ground state of free base porphyrins and which distinguishes them from more regular D 4 h (square) metalloporphyrins. The further distortion causes modes that in the more regular systems involve all four of the pyrrole rings to become increasingly localized on just the protonated or unprotonated pyrroles. More generally, the success of the DFT calculations at this level of theory in predicting frequency shifts on excitation for an extensive series of isotopomers clearly validates the approach. Interpretation of the resonance Raman spectra of t...

Oligo‐ and Polyselenophenes: A Theoretical Study

“…At least the first six lowest transitions were calculated for sexiselenophene. Vibrational frequencies were calculated and a scaling factor of 0.97 [34] was used for IR and Raman frequencies. Density of states (DOS) diagrams were obtained by convoluting the calculated crystal orbital energies (at B3LYP/6-31G(d)) using the GaussSum program for oligomers [35] and by using Gaussian curves (s = 0.1 eV) and the MatLab program for polymers.…”

Photodecay dynamics of octaethylporphine in the condensed phase explored via resonance Raman spectroscopy and density functional theory calculation

“…Among techniques for studying porphyrins in solution, resonant Raman techniques have been widely used [3][4][5][6][7][8][9][10][11]. Resonance Raman Spectroscopy provides overwhelming superiorities over the other spectroscopy techniques with the vibrational mode-specific dynamic information and electronic state-specific information.…”

“…Resonance Raman Spectroscopy provides overwhelming superiorities over the other spectroscopy techniques with the vibrational mode-specific dynamic information and electronic state-specific information. In addition to Raman spectroscopic techniques, density functional theory (DFT) calculations can also be employed to obtain valuable information about the structural and vibrational properties of porphyrins [7,[12][13][14]. The focus of this work lies in the DFT calculations using the hybrid B3LYP functional, the interpretation and vibrational mode assignments of the Resonance Raman spectroscopy of OEP.…”

Resonance Raman Studies of β‐Substituted Porphyrin Systems with Unusual Electronic Absorption Properties

“…[23] The B3LYP functional and 6-31G(d) basis set have been used to study the structure and vibrational frequencies of a number of porphyrin species. [24][25][26][27] Time-dependent DFT (TDDFT) calculations may be used to study the electronic properties of porphyrin systems. [12,[28][29][30] Herein, we study two metalloporphyrins (1a and 2a, Figure 1) and a substituent model (3, Figure 1) using resonance Raman spectroscopy.…”