Near-infrared (NIR) absorbing/emitting systems nowadays attract considerable attention of the researchers due to their promising applications in secure displays, night-vision goggles, tissue welding, telecommunications, biological imaging and dye-sensitized solar cells (DSCs). Significant results in solar light to electricity power conversion efficiencies (PCEs) up to >12% for organic molecules have been reached in this direction. Pure organic molecules possess lowered cost in contrast to the metal-complexes analogs and broad variety for functionalizations allowing fine-tuning of their photophysical properties. As a rule the systems possess D-π-A architecture and mostly utilize di-/triphenylamine (D/TPA) or indoline fragments as D motifs. However these systems have restricted donor strength because of the non-planar twisted structure of the nitrogen lone pair relative to the π-bridges resulting in reducing electron donation capability into acceptor moiety upon excitation. Recently indolizine-based donors have been found as more efficient donating units [1, 2]. Moreover indolizine-based systems demonstrated intensive electronic absorption/emission up to 700-900 nm revealing themselves as perspective candidates to the role of active layers in DCSs devices or NIR emitting materials [3, 4].

Herein we report on the study of excited state (ES) properties in series of novel recently synthesized indolizine-based systems bearing D-π-A architecture. Using the theoretical approaches based on combinations of density functional theory (DFT), time-dependent DFT (TD-DFT), Møller-Plesset second-order perturbation theory (MP2) and algebraic diagrammatic construction through second order (ADC2) methods, their ground and excited states features have been rationalized. Employing independent mode displaced harmonic oscillator (IMDHO) approximation the spectral features of experimental electronic absorption/emission spectra have been described and interpreted. It was found that the shape of the lowest-energy electronic absorption bands is strongly influenced by vibronic progressions. In particular, it was shown that the most intensive absorption is located nearby 0-0 adiabatic transition. IMDHO could be utilized for the prediction of electronic absorption properties in a wide range of indolizine derivatives toward light-harvesting applications.