Speaker
Description
Squaraine dyes are a unique class of organic semiconductors, featuring properties such as strong absorption in the visible and near infrared, high photostability and low biotoxicity, resulting in a range of applications reaching from photovoltaics to biomedical applications. Here, we focus on 2,4-bis[4-(N,N-diisobutylamino)-2,6-dihydroxyphenyl]squaraine (SQIB), which exhibits an orthorhombic crystal structure, containing four molecules per unit cell. Davydov splitting into four sublevels is induced by Coulombic interactions, with three of them being optically active while the fourth is a dark state, due to a vanishing transition dipole moment from the ground state. The population transfer dynamics between the Davydov components, including the dark state, determine the energy- and charge-transfer dynamics of SQIB but have evaded observation so far.[1]
To go past the steady-state optical properties we performed transient absorption microscopy with a temporal resolution on the femtosecond scale.[2] We observed several excited state absorption (ESA) features as well as the subsequent population transfer and vibrational cooling. Moreover, the temporal evolution of some of the spectral features indicates the involvement of the dark state in the population dynamics. Additional calculations based on the Bethe-Salpeter equation (BSE), provided by our cooperation partners, as well as recent publications using the Frenkel-Holstein Hamiltonian, are taken into account for the interpretation of our measurements.
References
[1] D. Giavazzi, R. Schwarzl, M. Koch, M. Schiek, A. Painelli, F. Spano, J. Phys. Chem. Lett. 16, 41, 10763–10770 (2025)
[2] R. Schwarzl, P. Heim, M. Schiek, D. Grimaldi, A. Hohenau, J. Krenn, M. Koch, Opt. Express 30, 34385-34395 (2022)