La pobreza en las cartas de la reina

The synthesis of the new ethynyl modified porphyrins provided us the opportunity to investigate their spectroscopic and photovoltaic properties in DSSCs. Comparisons could be made to the previously synthesised and studied alkane and alkene porphyrin derivatives.

3.4.1 UV-Vis Absorption Spectroscopy

UV-Vis spectroscopy gave us an insight into the relative energy levels of the new ethynyl compounds. Figure 3.13 showed that both the Soret and the two Q bands of the ethynyl benzoic acid 82 were red shifted 1.5, 2.5 and 5.5 nm, respectively, relative to the equivalent ethene compound 87. Shifts of 13.0, 12.5 and 14.5 nm were observed compared to ethane compound 86.88 This suggested that there was increased conjugation, or more precisely an increased electron withdrawing effect,129 between the aromatic ring and the porphyrin core in the ethynyl derivative. This is consistent with observations in other β-alkyne porphyrins.109, 130

N N N N Ph Ph Ph Ph COOH Zn N N N N Ph Ph Ph Ph Zn N N N N Ph Ph Ph Ph Zn COOH COOH 82 86 87 0 100 200 300 400 400 450 500 550 600 650 λ/nm ε (c m 2 m m o l -1 )/ 1 0 0 0 0 5 10 15 20 500 550 600 650

Figure 3.13 UV-Vis spectra of the ethynyl, alkene and alkane porphyrinic acids in DMF at 25 ˚C.

Ethynyl 82 (thinner line), alkene 87 (thicker line) and alkane 86 (dotted line). Inserted is an expansion of

the Q-band region.

3.4.2 X-ray Crystallography

It was not possible to obtain a single crystal of the acid 82, but a crystal of aldehyde 80 suitable for X-ray diffraction was obtained by slow diffusion of methanol into a solution

of compound 80 in DCM (Figure 3.14). This could be compared to the crystal structure of the previously published CuII ethene derivative 88 (Figure 3.15).78, 131

Figure 3.14 Crystal structure of 2-(4ʹ-formyl)phenylethynyl-5,10,15,20-tetraphenylporphyrinato zinc (II)

80 with methanol coordinated to the zinc. The angle between the benzene ring and the plane of best fit

made by the porphyrin ring is 31.98(16) degrees (phenyl rings omitted for clarity). The thermal ellipsoids were set at 50% probability level. The hydrogen atoms are drawn as spheres of arbitrary radii.

Figure 3.15 Crystal structure of 4-{trans-2′-[2′′-(5′′,10′′,15′′,20′′-tetraphenylporphyrinato copper (II) yl)

ethen-1′-yl]}-1-benzaldehyde 88 taken from Bonfantini et al.78

It has been shown that the extent of conjugation between the porphyrin and the β-

pyrrolic substituent increases as the dihedral angle between the two moieties becomes more planar.132 Thus, it would be expected from the UV-Vis spectroscopy that a more planar structure would exist in the ethynyl linkage compared to the alkene linkage. Against expectations, the dihedral angle between the plane of the benzene ring and the

plane of best fit made by the porphyrin core was 31.98(16) degrees, and almost twice that of the corresponding angle in 88 of 17(2) degrees. Interestingly, the increase in planarity that is suggested in the UV-Vis data is not supported by the crystal structure of compound 80. Thus, it is difficult to draw conclusions relating to the relevant extent of the electronic communication in ethene and ethynyl derivatives. It is possible that this angular deformity may be a result of crystal packing forces.

3.4.3 Dye Sensitised Solar Cells (DSSCs)

The synthesised benzoic (82), cyanoacetic (83), and malonic acids (84/85) were tested under conditions as described by Campbell3 and compared to the previously synthesised alkene and alkane acids 86, 87, and 89 (Figures 3.13 and 3.16).

N N N

N N

N N

N

HOOC COOH HOOC H

Zn Zn 70% _30% N N N N HOOC CN Zn N N N N HOOC CN Zn 83 ₈₉ 84/85

Figure 3.16 Malonic acid and cyanoacetic acid dyes tested in DSSCs.

Solutions of these porphyrin acids were prepared in THF (0.2 mM, BHT stabilised). Porphyrins were then absorbed onto sintered TiO2 glass electrodes by soaking overnight

at RT in darkness. The TiO2 electrodes with bound porphyrins were removed from the

dye solutions, rinsed, dried under high vacuum and tested immediately. The cells were assembled and tested in an open unsealed cell holder shown in Figure 3.17. Introduction of an I-_/I

3- redox electrolyte via capillary action between the working electrode and the

counter electrode completed the working cell. Four identical cells for each acid were irradiated under the equal intensity of 1 sun and the average values calculated (Table 3.3). The efficiencies (η) presented using an unsealed cell have been shown to be

approximately half of those expected in a sealed optimal cell.121

Figure 3.17 DSSC cell holder containing a TiO2 cell with bound porphyrin. Insert is a photograph of

TiO2 cells with bound acids 86, 87 and 82 respectively.

Table 3.3 Results of DSSC testing.

Compound ηηηη (%) FF Voc (V) Jsc (mAcm-2) 86 1.29 (1) 0.55 0.555 4.20 87 2.12 (2) 0.52 0.583 6.97 82 1.86 (1) 0.50 0.577 6.39 89 2.65 (3) 0.52 0.611 8.41 83 2.80 (1) 0.53 0.624 8.58 84/85 2.64 (4) 0.57 0.665 7.09

η = cell efficiency (1 standard deviation), FF = fill factor, Voc = open circuit voltage, Jsc = short circuit

The DSSC results can be separated into porphyrins possessing benzoic acids (86, 87 and 82) and those containing cyanoacetic/malonic acids (89, 83 and 84/85). The alkene linkage in compound 87 was found to be the best performing benzoic acid dye followed by the alkyne 82 and alkane 86 respectively, as shown by the higher cell efficiency (η). As can be seen in Table 3.3, the results show that dyes with cyanoacetic and malonic acid binding groups have higher efficiencies than benzoic acids. This is consistent with previously published porphyrinic acids using these binding groups.89 The ethynyl cyanoacetic porphyrin 83 showed higher efficiencies than the alkene variant 89. The malonic acid showed similar efficiency to that of the double bond cyanoacetic acid but it is difficult to compare these as the malonic acid was not pure.

It is plausible that variations in cell efficiencies could be the result of differences in dye loading on the TiO2 surface, rather than due to the real efficiency of the chromophore.

To investigate this we determined the dye loading of the three benzoic acids dyes (86, 87 and 82) on the TiO2 surface using UV-Vis spectroscopy (see experimental methods

section for details). Results showed that varying the linker did not substantially affect the surface coverage of the dye on TiO2 (Table 3.4). It can therefore be assumed that the

trends in DSSC efficiency are not a direct result of different dye loading on the surface.

Table 3.4 Dye loading on TiO2

Compound Moles absorbed Mass absorbed (g) Dye loading (mol/g of TiO2)

86 2.2 × 10-7 1.8 × 10-4 5.3 × 10-5

87 2.3 × 10-7 1.9 × 10-4 5.5 × 10-5

82 2.3 × 10-7 1.9 × 10-4 5.6 × 10-5

To provide further insight into the acids used in DSSCs a DFT study is being undertaken in collaboration with Otago University (not discussed in the current thesis).11, 127