# Electronic Properties of Electron-Deficient Zn(II) Porphyrins for HBr Splitting
## 3 Results and Discussion
### Computational Results  
Ground state optimized structures of Zn(II) porphyrins 1 and 2 are shown in Figure 4, along with with the orbitals involved in the main visible transitions, reported in Figure 5 together with the Electron Density Difference Maps (EDDMs) associated to the main optical transitions originating in the Q and B bands. The Zn(II) porphyrinates have a planar tetrapyrroling ring whith pentafluorophenyl groups oriented at right angles with respect to the ring plane. In the \(\beta\)-substituted moiety 2, the BTD group form a dihedral angle of 36.5\({}^{\circ}\) with respect to the macrocyclic ring. The benzene ring bridging BTD with the cyano acrylic group displays a tilt of 34.5\({}^{\circ}\) with respect to the former. In principle, these angles are the consequence of a certain degree of electronic delocalization between the porphyrin core and the acceptor arm, tasked with binding the metal oxide and providing coupling between the excited state and the conduction band of SnO\({}_{2}\). This is confirmed by the spatial distribution of the frontier orbitals, which, consistent with the strong electron acceptor character of BTD, show extended delocalization of the HOMO-2, HOMO, LUMO, and LUMO+2. By contrast in porphyrin 2, the fluorinated phenyl ring bearing the triazole bridge is oriented at 90 degrees with respect to the plane, interrupting electronic conjugation. As a consequence, the frontier orbitals of 2 show two pairs of nearly degenerate orbitals all centred on the porphyrin core, a situation recalling the classical \"Gouterman 4 orbital model\" [32] typical of symmetric _D4h_ porphyrins. No electronic distribution on the cyano-acrylic group is present at the level of HOMO-1, HOMO, LUMO, and LUMO+1, whereas HOMO-2 and LUMO+2 are entirely localized on the planar arm constituted by the triazole connected to the cyanoacrylic group via a phenyl spacer. Figure 3: HOMO–LUMO levels for porphyrins **1** and **2**. TDDFT usually describes the electronic transitions with a mixing of single KSO contributions, therefore the spatial redistribution of the electronic density following optical excitation is conveniently described by EDDMs, showing the transitions in term of holes (decreased electronic density) and electrons. First, we observe that in both cases, despite the difficulty of TDDFT to describe long range charge transfer states (CT), the computation reproduces two distinct regions with stronger (B bands) and weaker oscillator strengths (Q bands). The energy of the computed transitions is generally higher with respect to the experimental bands, but the errors are not large, varying between 0.22 (1) and 0.13 eV (2). Correction by these systematic amounts results in a reasonable match of the computed vertical transitions with the experimentally observed bands (Figure 5). EDDMs show essentially no involvement of the linker arm in the case of **2**, consistent with the localization of the frontier orbitals on the macrocycle. On the other hand, porphyrin **1** shows a larger electronic delocalization on the electron accepting arm, at least in the case of the most intense transitions between 2 and 2.2 eV, corresponding to the Q band region. However, the lowest singlet state has no electron density on the cyano-acrylic group, and electron delocalization does not extend beyond the  
Figure 4: **Left**: optimized ground state geometries of the meso-porphyrin 1 and the \(\beta\)-substituted porphyrin 2. **Right**: frontier energy levels associated to their respective Kohn-Sham Orbitals (KSO) (B3 LYP-6311G d,p). Figure 5: Computed vertical transitions at the TDDFT 6311 Gd,p level (black vertical bars) overlapped with the experimental spectrum. Vertical transitions were red shifted by 0.22 eV (**A**; **1**) and 0.13 eV (**B**; **2**) to match the Soret band maximum with the most intense vertical transition. EDDMs (isovalue \(10^{-3}\)) associated with the most intense transitions in Q and B band region are shown as insets.