Optical properties

The UV-Visible peak positions of Soret band and Q band, and molar absorption coefficient (ε) of oxasmaragdyrins Sm-BR2 in THF are listed in Table 4-1. The absorption spectra of the studied oxasmaragdyrins Sm-BR² display two well separated split Soret bands in 400-500 nm and Q bands in 550-750 nm region as shown in Figure 4-2 (a). Markedly, the split Soret band covers a broader range of absorption wavelengths than regular porphyrins. The most interesting feature of BF2 complexes is the strong absorption band at ~706 nm.

Table 4-1. Photophysical and Electrochemical data for oxasmaragdyrin dyes

a In THF. ^bEmission maximum measured in THF by exciting at Soret band. ^cFirst oxidation potentials vs. NHE in THF calibrated by Fc/Fc⁺ couple.^[11]d Estimated from the intersection of the absorption and emission spectra. ^e approximated from Eox and E(0,0).

This Q band is three times more intense than the absorption band of free-base oxasmaragdyrin present in the same region. The alkoxy chain substitution in Sm-B(OR)2 on boron atom slightly red shifted the absorption spectra compared to the BF2 chelated oxasmaragdyrin Sm-BF2 which is supposed to be caused by the inductive effect of the alkoxy chains. Additionally, the Q bands, which are more intense than typical zinc or free base porphyrins and are mainly contributed from the HOMO to LUMO transition (Table 4-2) give the highest extinction coefficient (ε) at 710 nm, a shift of almost 100 nm into the NIR region compared to Zn(TPP). Figure 4-2 (b) displays absorption spectra of oxasmaragdyrins Sm-BR2 adsorbed on TiO2 film, illustrating the adsorption behavior of oxasmaragdyrins on TiO2.

400 500 600 700 800

Figure 4-2. Absorption spectra of oxasmaragdyrins: (a) in THF, (inset: expansion of Q bands) and (b) adsorbed on TiO2.

After adsorption on TiO2 the absorption onset of the oxasmaragdyrins reached 800 nm, indicating that these dyes can act as NIR sensitizers. It is obvious that the Soret as well as Q bands are broadened after adsorption on TiO² which will ensure the collection of more photons when exposed to sunlight and consequently might result in higher efficiencies. We used the multi-gaussian functions to fit the absorption spectra in the Q band region (1300-1800 cm^-1).

The absorption bands of oxasmaragdyrins on TiO2 films are red-shifted as compared to the absorption in THF as shown in Figure 4-3. It also shows the packing modes of oxasmaragdyrins on TiO² are J-type aggregation. However, the alkyl chains attached on boron atom can protect the dye molecules from intermolecular aggregation. The energies of red-shifts at lowest band are 125.8, 56.0, and 27.9 cm-1 for Sm-BF2, Sm-B(OEt)2, and Sm-B(ODec)2, respectively. The red-shift energies of Sm-B(OBu)2, Sm-B(OHep)2 and Sm-BF2 are similar.

0.0

12000 13000 14000 15000 16000 17000 18000 0.0

0.5

1.0 Sm-B(ODec)2

Wavenumber / cm^-1

Figure 4-3. Absorption spectra of oxasmaragdyrins Sm-BR² in THF and on TiO² films.

The steady-state fluorescence spectra of all oxasmaragdyrins were measured in THF by excitation at Soret band and displayed in Figure 4-4. It revealed a similar pattern to the UV-Visible spectra, with slight red-shift due to alkoxy chain substitution but with considerably decreased intensity. The chain length has no effect on the fluorescence properties of these oxasmaragdyrins.

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Figure 4-4. Fluorescence spectra of oxasmaragdyrins Sm-BR2. 4.2.3 Cyclic Voltammetry Studies

The cyclic voltammetry (CV) measurements of all oxasmaragdyrins Sm-BR² observed two reversible oxidations and one reversible reduction couples as depicted in Figure 4-5. The static potentials and currents under multiple scans suggest high stability and reversibility of these compounds. The data in Table 4-1 show that the oxidation potentials of alkoxy substituted oxasmaragdyrins, shifted towards less positive by 140 to 170 mV resulting in elevated HOMO levels compared to BF² chelated oxasmaragdyrin. The reduction potentials of Sm-B(OR)2

shifted toward more negative by approximately 120 mV than Sm-BF2 complex.

-8

Figure 4-5. CV spectra of Sm-BR2 oxasmaragdyrins.

From the intersection of the normalized absorption and emission spectra at Q(0,0) band, the zero-zero excitation energies E(0,0) for Sm-BR2 were determined and listed in Table 4-1. Based on E^(0,0) and the first oxidation potential obtained from CV, the ground state oxidation potentials (Eox) and excited state oxidation potentials (Eox*) were calculated as listed in Table 4-1. The ground state oxidation potentials (Eox) and excited state oxidation potentials (Eox*) are plotted along with TiO2 conduction band and iodide/triiodide redox potential to gain an insight into the electron injection and dye regeneration dynamics (Figure 4-6). For an efficient electron injection from the excited state of the dye into the conduction band of TiO² and a faster regeneration of the oxidized dyes, appropriate tuning of HOMO with iodide/triiodide couple and the LUMO with TiO2 conduction band is necessary. Although the red-shifting in absorption band of oxasmaragdyrins stands for smaller band gap between HOMO and LUMO levels, the electrochemical data confirm the high driving force for electron injection from the oxidized dye to the conduction band of TiO2.

Figure 4-6. Energy level diagram of oxasmaragdyrins Sm-BR2.

For all of the oxasmaragdyrin boryl complexes, Eox* are more negative than the conduction band edge of the TiO² electrode, indicating suitable driving force for efficient electron injection.

The difference between the iodine/triiodide redox potential and the ground state oxidation potentials of the oxasmaragdyrin Sm-BF2 is 0.42V while it is 0.25 V for Sm-B(OEt)2, 0.27 V for Sm-B(OBu)2, 0.28 for Sm-B(OHep)2 and 0.27 for Sm-B(ODec)2. The potential difference for oxasmaragdyrin Sm-BF2 is sufficient for effectual dye regeneration while it is insufficient for Sm-B(OR)2 (< 0.3 eV), which can also be seen from the high efficiency of the dye Sm-BF2. It can be suggested from this difference that the dye regeneration for the alkoxide

substituted dyes Sm-B(OR)2 is not as efficient as Sm-BF2. This might be one of the reason for the inferior efficiencies of former dyes compared to the later.