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.