Development of asymmetrical near infrared squaraines with large Stokes shiftElectronic supplementary information (ESI) available. See DOI: 10.1039/c5ra18998e

A new strategy of obtaining large Stokes shift squaraine dyes is reported. Archetypal near infrared squaraines typically have very sharp absorption peaks and small Stokes shifts due to their very rigid ground and excited state molecular structures. TDDFT calculations revealed that large Stokes shift in squaraines can be reached by structural relaxation of the excited state. We achieved Stokes shifts of 90 nm by introducing a dibutyl-aniline side group and an electron withdrawing dicyano group to the squarate core. Wavefunction analysis indicates that that steric interactions and mesomeric effects in the ground and excited states of squaraines are crucial in determining the Stokes shift of the dye. New asymmetrical squaraines with large Stokes shifts of up to 90 nm were synthesized. TDDFT calculations indicate that steric effects from N , N -dibutylaniline and squaraine contribute to the large geometric change, resulting in the large Stokes shift.