Excited state absorption and decay kinetics of near IR polymethine dyes

We performed a detailed experimental investigation and quantum-chemical analysis of linear, excited state absorption (ESA) spectra, and lifetime dynamics of a new series of near IR symmetrical cationic polymethine dyes, with 5-butyl-7,8-dihydrobenzo[cd]furo[2,3-f]indolium terminal groups. Additionally, two neutral dyes, squaraine and tetraone, were synthesized with the same terminal groups. We compare the nonlinear absorption properties of these three dyes with an analogous set of three “visible” dyes with simpler benzo[e]indolium terminal groups. Measurements are performed using laser systems with femtosecond and picosecond pulsewidths. We find that the dyes with dihydrobenzo[cd]furo[2,3-f]indolium terminal groups are characterized by a remarkably large red shift (200-300 nm) of their linear and excited state absorption bands. The absorption bands for these dyes can now be shifted systematically to the near IR region by a contribution from their terminal group that strongly increases the total length of conjugation. ESA spectra in the “near IR” set of molecules have strong absorption bands with excited state absorption cross sections increasing from 2 × 10−16 cm2 for the shortest polymethine chain (trimethine cyanine) to 9 × 10−16 cm2 for the longer chain (heptamethine cyanine). The peak ESA magnitude for heptamethine cyanine is approximately twice as large compared to its peak ground state absorption, but the oscillator strengths of the two bands are almost equal. This observation indicates that the ESA oscillator strengths can be as large as those of ground state transitions. The results of these experiments combined with agreement of the quantum-chemical calculations move us closer to a predictive capability for molecular structure-nonlinear properties relations of cyanine-like molecules.