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Hammett plot reveals that there is a significant electronic effect on the rate of resonance assisted hydrogen bond (RAHB) directed diaza-Cope rearrangement reaction with a rho value of 1.6. DFT computation shows that the rearrangement reaction becomes thermodynamically more favorable for the substrates with electron withdrawing substituents. A substrate with the nitro substituent (1a) reacts about 50-fold faster than that with the methoxy substituent (1g).

This study aimed to develop low-cost D-π-A structured porphyrin and organic dyes with easily synthesizable donor units instead of the conventional complex multistep synthetic donor unit of Hexyloxy-BPFA [bis(7-(2,4-bis(hexyloxy)phenyl)-9,9-dimethyl-9H-fluoren-2-yl)amine] used in SGT-021 and SGT-149 as well-known record cosensitizers with an extremely high power conversion efficiency (PCE). The design strategy concerned the easier synthesis of low-cost donor units with inversion structures in donor groups via donor structural engineering, particularly by changing the position of the fluorene and phenylene units in the donor moiety while keeping the π-bridge and acceptor unit unchanged, leading to the synthesis of two D-π-A structured porphyrins [SGT-021(D0) and SGT-021(D)] and one D-π-A structured organic sensitizer [SGT-149(D)] for dye-sensitized solar cells (DSSCs). Specifically, porphyrin SGT-021(D0) incorporated two hexyl chains into the 9-position of each fluorene, while SGT-021(D) and SGT-149(D) substituted two hexyloxy chain units to the terminal position of each fluorene in the donor groups of porphyrin dyes. The effect of the position of the fluorene and phenylene units in the donor moiety on the photochemical and electrochemical properties, as well as the photovoltaic performance, was compared with the reference dyes of SGT-021 and SGT-149, previously reported by the research group. After optimizing the DSSC devices, SGT-021(D) and SGT-021(D0) achieved a high PCE of 11.6 and 10.5%, respectively, while SGT-149(D) exhibited a little lower PCE of 10.3% under the standard AM 1.5G light intensity. The cell performance of DSSC devices based on SGT-021(D) and SGT-149(D) was inferior to the corresponding reference dyes of SGT-021 and SGT-149 due to their lower donating ability of Hexyloxy-BPFA than Hexyloxy-BFPA.