**3.4 Solvent dependence of the H4TSPP**

**Singlet H4TSPP:** The results from calculations shows that the solvent effect on the dipole allowed vertical electronic transition energies of the H4TSPP is similar to

#### **Figure 6.**

*(A) The calculated solvent-dependence of the triplet-triplet electronic absorption spectrum of the TSPP in the thirty-nine different environments, where the dielectric constant of the molecular environment increases from bottom (*ε *= 1.00) to top (*ε *= 181.560); (B): The shift in the energy level of the Tn state reference to their corresponding value in gas-phase spectrum as function of dielectric constant of the solvent (*ε*). The shift in the energy level of the T1 state is obtained using the equation:* Δ*E(T1,*ε*) = E(T1,*ε*)-E(S0,*ε*).*

*Density Functional Theory Study of the Solvent Effects on Electronic Transition Energies… DOI: http://dx.doi.org/10.5772/intechopen.99613*

these in the singlet-TSPP spectrum, but exhibit more stronger blue-shift. As seen in **Figure 7A** and **B**, with increasing value of ε from 1.00 to about 20.493, the nearly degenerated Q-bands predicted at 10692/10737 cm−1 (935.3/932.4 nm) in gas-phase exhibit extremely blue-shift with the increase of ε such as are respectively blue-shift to 15173/15188 cm−1 (659.1/658.4 nm) in acetone as solvent with ε = 20.493 and to 155304/15317 cm−1 (or 653.4/652.9 nm) in water (ε = 78.355).

Likewise, the nearly degenerated B-bands at 12720/12796 cm−1 (or 786.1/ 781.5 nm) in gas phase spectrum are respectively 9839/9755 cm−1 blue shifted to 22559/22551 cm−1 (443.3/443.4 nm) in acetone (ε =20.493) and to 22755/22751 cm−1 (439.5/439.5 nm) in water (ε =78.355). Relatively very weak bands with relatively weak intensity (labeled as L-bans) at 14211/14189 cm−1 (703.7/704.8 nm) are substantially blue-shifted (9139/9108 cm−1) to 23350/23297 cm−1 (428.3/429.2 nm) in acetone and to 23907/23861 cm−1 (528.2/528.2 nm) in water, respectively.

In literature, we could not find an experimentally observed electronic spectrum of H4TSPP in the gas-phase or in a less polar solvent environment to compare with our calculated results. In high polar or acidic solutions, the observed B- and Q-bands in the electronic spectra of H4TSPP have been published by several researchers as discussed in the TSPP section, which are in agreement with the results presented here. Furthermore, results also shows that the stability of the electronic spectrum of the diprotonated TSPP (H4TSPP) rapidly increases with increasing polarity or dielectric constant of the solvent (**Figure 7A** and **B**).

**Triplet H4TSPP:** The calculations predicted the lowest triplet state (T1) at 5046 cm−1 (1982 nm) in gas phase shifts toward shorter wavelength region with increasing value of ε from 1 to 28.29, and stay almost unchanged around 880 cm−1 (1136 nm) with the further increase in ε. The calculated triplet-triplet electronic transition energies of the H4TSPP exhibit strong solvent-dependence in the range of solvent dielectric constant ε from 1.00 to 28.29. As seen in **Figure 8A** and **B**, the dipole allowed vertical electronic transition energies from the T1 state to T4, T9, T11 and T12 triplet states are substantially blue shifted to 12047, 13554, 14359, and 15959 cm−1 (or 830.1, 737.8, 696.5 and 626.6 nm) from 6332, 8707, 12784 and 11947 cm−1 (or 1579, 1149, 782 and 837 nm) in the gas phase, respectively.

#### **Figure 7.**

*(A) The calculated solvent-dependence of the singlet-singlet electronic absorption spectrum of the H4TSPP in the thirty-nine different environments, where the dielectric constant of the molecular environment increases from bottom (*ε *= 1.00) to top (*ε *= 181.560); (B): The red-shifts in the absorption band positions as function of dielectric constant of the solvent (*ε*), reference to their corresponding values in gas-phase spectrum.*

#### **Figure 8.**

*(A) The calculated solvent-dependence of the triplet-triplet electronic absorption spectrum of the H4TSPP in the thirty-nine different environments, where the dielectric constant of the molecular environment increases from bottom (*ε *= 1.00) to top (*ε *= 181.560); (B): The shift in the energy level of the Tn state reference to their corresponding value in gas-phase spectrum as function of dielectric constant of the solvent (*ε*). The shift in the energy level of the T1 state is obtained using the equation:* Δ*E(T1,*ε*) = E(T1,*ε*)-E(S0,*ε*).*

The solvent-dependency of the smallest energy gap between the singlet and SCF corrected-triplet states, ΔE(singlet-triplet, ε), are estimated to be around 2300 cm−1 (T4-Q states) and about 300 ± 150 cm−1 (T9-B) and (T11-L).

### **4. Conclusion**

Results from calculated electronic spectra of the TPP, TSPP and their diprotonated structures in gas-phase and thirty eight different solvent showed that solvent gives rise to a blue/red shifts in the singlet-singlet and triplet-triplet dipole allowed vertical electronic transition energies as a function of solvent dielectric constant only in the region of ε = 1 to about 20.493 (acetone), but no significant spectral shifts found for the solvent dielectric constant ε ≥ about 20.493.

In the low energy region (Q-band region), the Q-bands of the TPP first exhibit a red-shift (of about 70 cm−1) with increasing solvent dielectric constant up to ε = 5.32 (diiodomethane) and then to a blue shift with increasing of the ε (from 5.32 to 20.493), However, the Q-bands of the TSPP display only a blue shift (as much as 295 cm−1) with increasing of solvent dielectric constant up to 20.493. The Q-bands in both H4TPP and H4TSPP spectra exhibits a rapid blue-shift with increasing of solvent dielectric constant, but the shifts in the H4TSPP is much stronger than that in the H4TPP spectrum; such as 4480 cm−1 in the H4TSPP and 490 cm−1 in the H4TPP in acetone as solvent.

In the high energy region, the solvent effect on the Soret band (B-band) of the TPP is stronger than on that of the TSPP. The B-band of both molecules first display a red-shift in range from 1 to 5.32 and then turn to blue shift up to acetone (ε = 20.493). In both H4TPP and H4TSPP compounds, the B-band exhibit a strong blue-shift with increase of ε, but the shift in the B-band position of H4TSPP is much stronger than that of the H4TPP. With the further increase of the solvent dielectric constant (ε), spectral position of the Q- and B-bands become almost stabile within a few-tens wavenumber ranges. Furthermore, the intensity of the B-band slightly increases with increase of solvent dielectric constant.

#### *Density Functional Theory Study of the Solvent Effects on Electronic Transition Energies… DOI: http://dx.doi.org/10.5772/intechopen.99613*

The solvent results in both red- and blue-shifts in the triplet-triplet transition energies in the TPP, H4TPP and TSPP spectra in the calculated spectral region, but only blue-shift takes place in the H4TSPP. Furthermore, the calculations also showed that the ISC process many not only take place between the Q-bands and the nearest triplet states, but also it is possible between the B- and L-bands and the nearest triplet states because these singlet states almost overlapping with the higher triplet states. Therefore, the ISC may occurs in the singlet state B (or B-band) via surface touching, based on the competition between the IC (from the B- to Q-bands) and the ISC process. This energy gaps between Q/B-bands and the nearest triplet states is also solvent dependent for ε < 20.

The results of calculations also indicate that solvent effect on the nonplanar macrocycle conformation of porphyrin (H4TPP/H4TSPP) is more significant than that on the planar macrocycle conformation of porphyrin (TPP/TSPP). This observation suggest that the nonplanar macrocycle conformation increase the electrostatic/electronic interaction between the four hydrogen atoms at the macrocycle core and molecular environment.

The results from calculations suggest that the polarized porphyrin molecule may interact strongly with increasing solvent polarity and lead to modification of dipole allowed vertical electronic transition energies with increasing solvent polarity arising from partial charge transfer between solvent and the porphyrin macrocycle and/or the intramolecular charge transfer. Moreover, in terms of the electronic configuration of the molecular orbitals of the porphyrin, the additional charge partially may occupy a nonbonding and/or antibonding orbitals that brings about the weakening of the chemical bonding in porphyrin. Also, the partial charge transfer from an occupied nonbonding (atomic orbital of the N atom) and/or antibonding orbital results in the strengthening of the chemical bonding in porphyrin. The change in occupancy of the bonding/nonbonding orbitals affect the electronic excited state, which gives rise to shift in the electronic absorption spectrum of the porphyrin molecule.

As a result, the solvent-dependence of the spectroscopic features of porphyrin can be used to monitor micro environmental changes of porphyrin-like compounds incorporated in biological systems and nanoparticles, which also may be appropriate for study and monitoring changes of the chemical environment in different solutions and interactions in biological systems, as well as deal with nonspecific *adsorption on nanomaterials* and their orientations on the surface, which is very important for the *Surface-Enhanced Resonance Raman Scattering* (SERRS) and Aggregation-Enhanced Raman Scattering (AERS) in different solvent. Furthermore, such strongly solvent-dependent electronic bands can be used as a marker of the environmental dielectric constant.
