Substitution Effects of Phenothiazine and Porphyrin Dyes in Dye-sensitized Solar Cells Page: 24
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Figure 2.1b shows the emission spectrum of the investigated compounds for
concentrations normalized to the visible band intensity (Figure 2.1a) in DCB. The Type-1
phenothiazine derivatives revealed a broad emission in the range of 580-620 nm. The relative
emission quantum yield was found to depend upon the type of substituent on the N-atom being 1
(methyl) > 2 (2-hydroxyethane) > 3 (triethylene glycol monomethyl ether). Interestingly, Type-2
compound, 4 and the control phenothiazine had a small emission peak around 450 nm (<2%
intensity compared to Type-1 compounds). The better spectral coverage and emission properties
covering most of the visible portion of the spectrum of Type-1 derivatives suggest them being
better DSSC dyes compared to Type-2 dye.
2.3.3. Computational and Electrochemical Studies. Molecular orbital calculations using
B3LYP/6-31G* basis set42 were performed to arrive at the geometry and electronic structure of
Type-1 and Type-2 compounds. The structures were fully optimized on Born-Oppenheimer
potential energy surface, and the frequency calculations revealed absence of negative
frequencies. Figure 2.2a and 2.2d show the optimized structures of 1 and 4 representing Type-1
and Type-2 compounds, respectively. The non-planar structure of phenothiazine macrocycles
was clear for both types of compounds which would result in reduced dye-aggregation. That is,
the angle between two terminal phenyl rings was 1490 from the sulfur side, and 1440 from the
nitrogen side of the macrocycle. The cyanoacrylic acid functionality was in-plane with the
substituted phenyl ring of phenothiazine in case of 1 while for 4, the phenyl cyanoacrylic acid
functionality was perpendicular the phenothiazine ring. The frontier HOMO was found to be
largely located on the phenothiazine macrocycle (Figures 2.2b and e) while the LUMO was on
the cyanoacrylic acid group and part of the macrocycle in case of 1 (Figure 2.2c) while for 4 it
was fully on the phenyl cyanoacrylic acid functionality (Figure 2.2f). The location of the24
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Hart, Aaron S. Substitution Effects of Phenothiazine and Porphyrin Dyes in Dye-sensitized Solar Cells, thesis, December 2013; Denton, Texas. (https://digital.library.unt.edu/ark:/67531/metadc407837/m1/30/: accessed April 19, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; .