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We have performed quantum chemical calculations for the optimized structure and Raman vibrational frequenciessbased on density functional theory at the B3LYP level and using the 6-31G(dp) basis setsfor the cyanine dye cation 1,1′-diethyl-2,2′-quinocyanine, also referred to as pseudoisocyanine (PIC). We have ascertained that the equilibrium structure of ground-state PIC has near C 2 symmetry, with a 46°twist between the planes of the two quinoline moieties that are positioned about the central methine carbon. Vibrational mode analysis of the calculated Raman spectrum suggests that many of the experimentally observed Raman bands between 500 and 1800 cm -1 for monomeric PIC are associated with totally symmetric in-plane deformations of phenyl and/or pyridyl rings, while several weak bands below 500 cm -1 are attributed to out-of-plane doming and ruffling of quinoline macrocycles. We further have noted that (1) upon comparison of the Raman spectrum of PIC monomer with the Raman spectrum measured for aggregated PIC, under nonresonant condition, changes reveal that aggregation results in enhanced scattering for specific vibrational modes that contain principal contributions from in-plane deformation of the phenyl ring in the quinoline moiety; (2) with resonant excitation for the aggregate, observed vibrational modes associated with out-ofplane distortions of the quinoline macrocycle are found to exhibit even greater enhancement. Analysis of our Raman measurements for monomeric and aggregated PIC provides details about the structure of the molecular aggregate. Additionally, calculation of charge distribution, utilizing the Mulliken population analysis approach, indicates that positive and negative charges are alternately and symmetrically distributed over a conjugated ring system, and the positive charges among the peripheral hydrogens and those in the N-ethyl side chains are asymmetrically distributed.

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), mesotetrakis(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 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 OPEN ACCESSMolecules 2014, 19 20989 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 .

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