3.1. Structural Properties
The dyes studied in this work are of (3)D-π-A type, where donor
mentioned as D is electron rich specie, the acceptor mentioned as A is
electron deficient specie and spacer connecting them is in form of
π-bridge. The dyes D1 and D1A contain carbazole dendritic as oxidizing
donor specie, cyanoacrilic acid as accepter reduction species connected
via variable length π-bridge of thiophene rings. In D1, there are 3
carbazole donors with each comprising of three rings in which 2 rings
are 6-membered be nzene ring attached from both side to nitrogen
containing 5-member ring. The 1st and
2nd carbazole members are attached to
3rd carbazole member via their nitrogen atoms. The
Donor are then attached to π-bridge consisting of three oligothiophene
units named as I, II and III as shown in figures 2 and 3. The π-bridge
acts as a charge transfer channel which receives electrons from the
donorto pass on to the acceptor specie which finally transports it to
the semiconductor. Oligothiophene have property of self-assembly and it
enhances the transfer of charge from donor to acceptor.
D1A have similar structure as D1 will an additional oligothiophene unit
at IV position as shown in figure 3. The respective values of length of
π-spacer in D1 and D1A after optimization are 10.32 Ǻ and 14.23 Ǻ
whereas angle between the donor and π-spacer is 125.7ofor both the dyes.
The D2 contains an oxadiazole ring between two thiophene rings. The
addition of electrophilic heterocyclic molecules with thiophene in
π-spacer can produce such changes in energy levels which are favorable
for photosensitizers
[55-57].Oxadiazole is an efficient
electron acceptor with elevated thermal stability and photo-luminescence
quantum yield [58].The oxadiazole has
high compatibility with thiophene because of their structural
similarities [59]. Hence, the
efficient electron accepting ability of oxadiazole and its compatibility
with thiophene can lead to tune the properties of molecules base on
thiophene. The structure of D2A is similar as D2 except an extra
thiophene ring at IV position in theπ-spacer. The respective values of
length of π-spacer in optimized structures of D2 and D2A are 9.75 Ǻ and
13.6 Ǻ whereas there is an angle of 125ᵒ between donor and π-spacer for
both the dyes.
The D3 contains an oxadiazole ring at 4th position of
thiophene bridge of D1 whereas D3A has an additional oxadiazole at V
position in π-spacer. The values of length of π-spacer in optimized
structures ofD3 and D3A are10.2Ǻ, 17.4Ǻ whereas there is an angle of
125ᵒ between donor and π-spacer for both dyes.