# Electronic Properties of Electron-Deficient Zn(II) Porphyrins for HBr Splitting
## 3 Results and Discussion
### Transient Absorption Spectroscopy  
In order to specifically address charge separation dynamics, we performed nanosecond transient absorption spectroscopy (TAS) experiments on both the porphyrin dyes in solution and after their anchoring onto SnO\({}_{2}\) electrodes. The photophysical behaviour of both porphyrins in 1:1 ethanol/toluene (5 \(\times\) 10\({}^{-4}\) M) was recorded after the ns-laser excitation at 532 nm and it tracks the characteristics of the dyes' excited states. Figure 10 reports the TAS obtained for **1** and **2** at different time delays with respect to the laser pulse. Signals sampled at 0 delay are convolved with laser pulse, which in the presence of the amplifier impedance, has an instrumental response of approximately 300 ns. TAS taken at 0 delay, thus, still show in both cases features of the lowest singlet excited states (black line plus black squares) associated with the spontaneous emission, appearing as bleaching in the wavelength interval 600/700 nm. Such feature is quickly substituted by a weak and flat absorption extending from 600 to 800 nm and assigned to the lowest excited triplet (T\({}_{1}\)) population, whose decay, being spin forbidden, occurs in a few microseconds (a weak \(\Delta\)OD signal is still observable after 3 \(\upmu\)s from the excitation (orange line)). The most prominent spectral feature of the triplet state is the presence of a strong absorption band centered around 500 nm (**1**) and 480 nm (**2**). For both dyes, the TAS spectra are in-line with previous findings, regarding other \(\beta\)-substituted porphyrins functionalized with donor/acceptor groups [37; 38], as well as more symmetric structures [39], for which the lowest electronic states are mainly localized on the tetrapyrrolic ring, consistent with a small influence of the electron withdrawing branch in either the _meso_ or \(\beta\) position of of the macrocycle on the electronic properties of the lowest excited states. When these dyes are adsorbed on SnO\({}_{2}\) in contact with 0.1 M HClO\({}_{4}\), the excited state leads  
Figure 9: IPCE spectra of SnO\({}_{2}\)**-**1** and SnO\({}_{2}\)**-**2** photoanodes recorded in 0.1 M HBr (pH 1) under 0.6 V versus SCE applied bias. Illumination was through the ohmic contact (FTO). to the formation of a charge-separated (CS) state (SnO\({}_{2}\)(e\({}^{-}\))-dye(h\({}^{+}\))). The CS state of porphyrin **1** (Figure 11A) is characterized, along with the ground state (Soret and Q-bands) bleaching, by the presence of a broad absorption band peaking at 730 nm and by the absence of the intense excited state absorption band at 500 nm, at least for a certain delay after the laser pulse. Indeed, spectra taken at 0 delay (Figure S6) are substantially convolved with the instrumental response, but still show features of the excited state, lacking the diagnostic 730 nm absorption band, which is offset by spontaneous emission. This indicates, at a qualitative level, incomplete quenching of the excited state by charge injection as a consequence of slow electron transfer rate from the excited dye to the semiconductor. Similar spectral features and dynamics are found with **2**, which compared to **1**, presents a narrower dye\({}^{+}\) absorption band, having its maximum at approximately 700 nm. In both cases, the CS state evolves slowly due to recapture of the photoinjected electrons with the oxidized dye and the transient signals do not recover completely within the timeframe of the experiment (3 ms), showing, in principle, a great potential for the exploitation of the long-lived CS state to drive redox reactions. Recombination kinetics of the CS state (SnO\({}_{2}\)(e\({}^{-}\))-dye(h\({}^{+}\))) were monitored at 690 and 700 nm corresponding to the diagnostic CS absorption manifold of porphyrins **1** and **2**, respectively. In general,  
Figure 10: Transient absorption spectra of **1** (**A**) and **2** (**B**) at different delays from the 532 nm ns-laser excitation recorded in 1:1 EtOH/toluene; 350 \(\Omega\) oscilloscope impedance.