Near-infrared squaraine sensitizers bearing benzo[c,d]indolenine as an acceptor moiety

• A benzo[c,d]indolenium salt with a carboxy group was synthesized as an anchor component for DSSCs.
• Squaraine sensitizers exhibit NIR absorption due to the benzo[c,d]indolenine unit.
• Asymmetrical benzo[c,d]indolenine based squaraines showed aggregation tendencies.
• Benzo[c,d]indolenine-based squaraines contribute to the NIR response of DSSCs.

Near-infrared absorbing unsymmetrical squaraine dyes bearing benzo[c,d]indolenine as an acceptor moiety have been synthesized and their light-absorbing properties and performance as sensitizers for dye-sensitized solar cells were studied. A molecular-orbital calculation indicated that the electron distribution moved from the cyclobutene skeleton to the benzo[c,d]indolenine components with a carboxy group through photoexcitation. The squaraines exhibited prominent absorption properties in the range 600–900 nm, whereas these dyes show a tendency to aggregate in solution as well as on a titania due to an effect of the π-extended benzo[c,d]indolenine. Their absorption and electrochemical properties revealed that these dyes have deep lowest unoccupied molecular orbital levels which are close to the potential of the titania conduction band. Dye-sensitized solar cell using the present dyes achieved a spectral response in the range from 940 nm to 400 nm. These results indicated that benzo[c,d]indolenine with a carboxy group can be applied to the components for near-infrared-absorbing sensitizers in photovoltaic cells.

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