Abstract
Density functional theory (DFT and time-dependent-DFT (TD-DFT) were employed to investigate the vibroelectronic structural properties of porphyrin and some derivatives: unsubstituted porphyrin (TPyr), meso-tetraphenylporphyrin (TPP), meso-tetrakis(p-sulfonatophenyl)porphyrin (TSPP), protonated-TPyr (H2TPyr), deuterated-H2TPyr (D4TPyr), protonated-TPP (H2TPP) and deuterated-H2TPP (D4TPP), protonated TSPP (H2TSPP), deuterated-H2TSPP (D4TSPP), dicationic TSPP (H6TSPP) and deuterated-H6TSPP (D8TSPP). The possible internal conversion (IC) and intersystem crossing (ISC) processes of these compounds were investigated. Finally, the relaxed ground state potential energy surface (PES) (S0), and singlet (Sn, n = 1–24) and triplet (Tn) excited state PESs of the TSPP molecule were calculated as function of the dihedral angle (Cα-Cm-Cϕ-C(ph)) rotation. The results of the calculations indicated that while the meso-substitutions caused a significant shift in frequencies when the meso-carbons within the parent-porphine (or TPyr) are involved in the vibrational motion of molecules; the protonation of the N atoms at the porphine/porphyrin core causes a significant blue shift when the H on the N atoms within the pyrroline are dominantly involved in the vibrational motions. The deuteration of N atoms not only caused a red-shift in the frequencies of the corresponding peaks below 1600 cm−1, but also produced new vibrational modes of frequencies in the 2565–2595 cm−1 range caused by the N-D bond stretching. Similarly, the deuteration of O atoms within the sulfonato groups (-SO3−) exhibited a new peak at around 2642 cm−1 due to O-D bond stretching. The measured Raman spectrum of the H2TSPP is assigned based on the predicted Raman spectra of the compounds studied here and measured Raman spectrum of the TPP (from our previous work). The IR spectrum is assigned based on our calculations and measured IR spectra obtained from the literature. The results of the TD-DFT calculations did not only indicate that the meso-substitution and protonation of the porphyrin bring about a significant read shift in the electronic transitions; but also provided a strong evidence for the IC from the Soret band to Q-bands beside possibility of the ISC process; its existence depend on the other excited state process such as much faster vibrational relaxation; the IC and etc. The ground state PES curve (S0) of the ionic TSPP exhibited two minima at the dihedral angle (Cα-Cm-Cϕ-C) of about 66° (corresponds to the lowest ground state) and 110° (corresponds to next energetically stable state or the local minima). The energy deference between these two minima is 0.0132 eV (or 106 cm−1) and the highest potential energy barrier when undergoing from the lowest ground state to this local state is only 0.0219 eV (177 cm−1; which is compatible with the thermal energy (kT) at 298 K is 207.2 cm−1.
Highlights
Porphyrin and its derivatives have attracted considerable attention from both experimentalists and theoreticians for the last few decades and still remain an attractive research subject due to their broad applications in industrial and biomedical fields
With respect to the average plane of porphryin to macrocycle as seen in Figure 1 and Table 1, the peripheral phenyl and sulfonatophenyl substituents are oriented at a tilt angle of about 72° for the TPP and TSPP and about 48° for the H2TPyr (D4TPyr); (F) protonated-TPP (H2TPP), H2TPP (D4TPP); (H) protonated TSPP (H2TSPP) and H6TSPP
The results of the calculated Raman and IR spectra of porphyrin and its derivatives in conjunction with the animation of the vibrational motions of the molecule showed if the peak(s) are only or mainly due to the vibrational motions of the meso-substitutions, H atoms covalently bound to the porphyrin core (and sulfonato groups (-SO3−)), the meso-substitutions, protonation and deuteration lead to a significant frequency shifts in these peaks positions
Summary
Porphyrin and its derivatives have attracted considerable attention from both experimentalists and theoreticians for the last few decades and still remain an attractive research subject due to their broad applications in industrial and biomedical fields. Density functional theory (DFT) is one of the most important techniques used by theoreticians to provide deep insight into spectroscopic and structural properties, even for complex molecular systems, especially those of large sizes such as the porphyrinoids [23,24,25,26,27,28]. In order to investigate the effect of meso-substitution groups on the geometric and vibroelectronic properties of the parent-porphine or porphyrin macrocycle, density functional theory (DFT) and time-dependent-DFT (TD-DFT) were employed to investigate the vibroelectronic structural properties of porphyrin and derivatives: unsubstituted porphyrin (TPyr), meso-tetraphenylporphyrin (TPP), meso-tetrakis(p-sulfonatophenyl)porphyrin (TSPP), protonated-TPyr (H2TPyr), deuterated-H2TPyr (D4TPyr), protonated-TPP (H2TPP or dicationic TPP) and deuterated-H2TPP (D4TPP), protonated. IR spectra of the H2TSPP were assigned by comparison with the calculated IR spectra of other porphyrin derivatives studied here and the experimentally measured IR spectra obtained from the literature
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