Syntheses of SM dyes

Scheme 5-1. Molecular structures of oxasmaragdyrin dyes.

5.2 Results and Discussion

5.2.1 Syntheses of SM dyes

SM dyes were synthesized in least possible steps with minimum application of tedious coupling reactions, starting with the key precursors dipyrromethane and 16-oxatripyrrane. The assembly of the oxasmaragdyrins SM1-SM3 and its boron chelation is displayed in Scheme 5-2. One equivalent of methyl-4-formylbenzoate was treated with excess pyrrole in presence of TFA as acid catalyst to obtain the meso-(4-methoxyphenyl)dipyrromethane in good yields. For oxatripyrrane, furan was treated with corresponding aldehydes in presence of n-BuLi to get a symmetrical furan diols which were further reacted with excess pyrrole in presence boron trifluoride-diethyl etherate as catalyst to yield meso-aryl-16-oxatripyrranes. The oxasmaragdyrins were prepared by the 3+2 MacDonald-type condensation of meso-(4-methoxyphenyl)dipyrromethane with meso-aryl-16-oxatripyrrane under mild acidic condition followed by subsequent oxidation with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) to afford the meso substituted oxasmaragdyrins as major product. These free base oxasmaragdyrins were further treated with excess triethylamine (NEt3) followed by boron trifluoride-diethyl etherate (BF3•OEt2) at room temperature to give  boron chelated

oxasmaragdyrins. After basic workup the oxasmaragdyrins were chromatographed to the purest possible form at this stage so as to avoid the purification in the last step. These mono ester oxasmaragdyrins were hydrolyzed with aqueous solution of KOH to yield analytically pure oxasmaragdyrins as green solids in decent yields.

Scheme 5-2. Synthesis of oxasmaragdyrin dyes SM1-SM3. Reagents and Conditions: i) TFA;

ii) n-BuLi, TMEDA, Hexane/THF; iii) Pyrrole, BF³•OEt²; iv) TFA, DDQ, DCM; v) NEt³, BF3•OEt2, DCM; vi) KOH (aq.), THF.

To prepare SM4 (Scheme 5-3), one equivalent of paraformaldehyde was treated with excess pyrrole in presence of TFA to obtain the unsubstituted dipyrromethane 14. The meso-tolyl-16-oxatripyrrane was prepared following the synthetic path for oxatripyrrane 16. The meso-unsubtituted oxasmaragdyrin is assembled under acidic conditions followed by subsequent oxidation with DDQ which was further treated with excess NEt3 and BF3•OEt2 at room temperature to give boron chelated oxasmaragdyrin 17 in 3% yield as shown in Scheme 5-3. After standard workup and chromatographic purification, the oxasmaragdyrin was isolated as green solid. This meso-unsubstituted oxasmaragdyrin 17, is then brominated with

N-bromosuccinamide (NBS) at low temperature to get the oxasmaragdyrin 18. This meso-bromooxasmaragdyrin was reacted with 4-ethynylbenzoic acid at 60 ^oC under Sonogashira coupling protocol to obtain the desired SM4 in good yield.

Scheme 5-3. Synthesis of oxasmaragdyrin dyes SM4. Reagents and Conditions: i) TFA; ii) n-BuLi, TMEDA, Hexane/THF; iii) Pyrrole, BF3•OEt2; iv) a. TFA, DDQ, DCM; b. NEt3, BF³•OEt², DCM; v) NBS, THF; vi) 4-ethynylbenzoic acid, Pd²(dba)³, AsPh³, Toulene/NEt³. The boryl oxasmaragdyrins SM5-SM8 were prepared following the same synthetic path used for SM1-SM3 and is presented in Scheme 5-4. The meso-aryldipyrromethanes were prepared in decent yields by treating one equivalent of corresponding aldehydes with excess pyrrole in presence of TFA as catalyst. Furan was treated with methyl-4-formylbenzoate in presence of n-BuLi to get a symmetrical diol which was further reacted with excess pyrrole in presence of BF3•OEt2 to yield meso-(4-methoxyphenyl)-16-oxatripyrrane. The oxasmaragdyrins were prepared by following the 3+2 MacDonald-type condensation protocol of meso-aryldipyrromethanes with the 16-oxatripyrrane under mild acidic conditions followed by succeeding oxidation with DDQ to afford the dicarboxylate substituted oxasmaragdyrins. These free-base oxasmaragdyrins were further treated with excess NEt³ and BF³•OEt² at room temperature to give  boron chelated oxasmaragdyrins. After regular workup the oxasmaragdyrins were purified by column chromatography to afford the pure products. These diester oxasmaragdyrins were hydrolyzed with aqueous solution of KOH to yield analytically pure oxasmaragdyrins as green solids in decent yields. All these boryl oxasmaragdyrins along

with their intermediates were thoroughly characterized with the help of common analytical techniques such as ¹H, ¹³C, ¹¹B, ¹⁹F NMR spectroscopy, UV-Visible spectroscopy, cyclic voltammetry, and HRMS spectroscopy.

Scheme 5-4. Synthesis of oxasmaragdyrin dyes SM5-SM8. Reagents and Conditions: i) TFA;

ii) n-BuLi, TMEDA, Hexane/THF; iii) Pyrrole, BF3•OEt2; iv) TFA, DDQ, DCM; v) NEt3, BF³•OEt², DCM; vi) EtOH, AlCl³, DCM; vii) KOH (aq.), THF.

5.2.2 Optical properties

The UV-Visible peak positions of the Soret and Q bands and the molar extinction coefficients (ε) of the oxasmaragdyrins in THF are summarized in Table 5-1. The absorption spectra of these oxasmaragdyrins display split Soret bands in the 445-500 nm region and Q bands in the 550–

750 nm region as shown in Figure 5-1. Evidently, the split Soret band covers a broader range of absorption wavelengths than regular porphyrins and BODIPY dyes. Additionally, the Q bands, which are more intense than those for typical porphyrins are mainly contributed from the HOMO to LUMO transition. The absorption wavelengths of these oxasmaragdyrins depend largely on the nature of the substituents. In monocarboxylic acid substituted oxasmaragdyrins, the addition of electron donating hexylthiophene group in SM2 and triphenylamine group in SM3 red-shifted the absorption wavelengths by 3-6 nm compared to SM1, while the coupling of ethynylphenyl linker in SM4 observed the highest bathochromic shift of 15 nm for the Soret band compared to SM1.

Table 5-1. Photophysical and electrochemical data for SM dyes.

aIn THF. ^bEmission maximum measured in THF by exciting at Soret band. ^cFirst oxidation potentials vs. NHE in THF calibrated with Fc/Fc⁺ couple. ^dEstimated from the intersection of the absorption and emission spectra. ^eApproximated from Eox and E(0,0).

The absorption onset in dye SM4 reached beyond 750 nm, indicating the light harvesting for this dye might be higher in visible as well as in near infra-red (NIR) region. In dicarboxylic acid substituted oxasmaragdyrins, the substitution of electron donating thiophene carbazole unit on the oxasmaragdyrin ring in SM5, bathochromically shifted the absorption in Soret as well as Q band region, extending the absorption onset beyond 750 nm. The broadened absorption is beneficial as increased light harvesting can improve the photocurrent density. Changing the number and position of the hexyloxyphenyl and carboxylic acid in SM6 compared to SM1 did not have any effect on the absorption spectrum. Ethoxy substitution on the boron atom in case of dye SM8 as compared to dye SM7 slightly red-shifted the absorption spectrum.

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Figure 5-1. UV-Visible spectra of SM dyes in THF.

This suggests that the electron donating nature of ethoxy group has negligible influence on the optical properties of this dye. The higher molar extinction coefficients (1.65-4.38 × 10^-5 M^-1 cm^-1)for SM1-SM8 also supports the proposition of a good light harvesting nature of these dyes. To better comprehend the adsorption behavior of the dyes on TiO2, absorption spectra of these dyes as thin films were studied and the results are displayed in Figure 5-2.

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Figure 5-2. UV-visible spectra of SM dyes in adsorbed on TiO2.

To obtain the absorption spectra on TiO² films, the films with ~3 μm thickness were immersed in 0.1 mM THF solution of the SM dyes at room temperature. The adsorption spectra were recorded by reflectance measurements using an integrated sphere. The absorption spectra of dyes show significant broadening and slight red-shifts upon adsorption on the TiO2 surface compared to their UV-Visible spectra in THF with threshold of absorption around 800 nm.

This broadening and significant red-shifts in the UV-Visible spectra after adsorption on TiO2

films suggests that more charge collection is possible in Soret and Q band region which in turn may result in higher efficiency in DSSCs.

650 700 750 800 850 900

Figure 5-3. Fluorescence spectra of SM dyes.

The steady-state fluorescence spectra of these dyes were measured in THF by excitation at Soret band. As displayed in Figure 5-3, it revealed a similar trend to the UV-Visible spectra, with slight red-shift in the wavelengths due to the increased π-conjugation through donor and linker variations.