Aggregates of quadrupolar DAD-type chromophores are investigated theoretically using the essential states model (ESM) enhanced to include vibronic coupling. On the basis of vibronic signatures in the absorption and photoluminescence spectra, important connections are made to the Frenkel exciton model, which is the basis for defining conventional H-and J-aggregates. In contrast to the exciton theory, the ESM allows for an unusual type of the red-shif ted Haggregate, driven by the large quadrupole−quadrupole interaction. Except for the red shift of the main absorption peak, such aggregates display all of the characteristics of conventional Haggregates, including suppressed radiative decay rates and all of the vibronic signatures in the absorption and photoluminescence spectra consistent with Frenkel exciton models. Red-shifted Hdimers occur for a range of slip angles, which separate conventional H-and J-aggregate behavior, with the slip range diminishing with increasing intermolecular separation. Davydov splitting and vibronic signatures for dimers with nonparallel transition dipole moments are also investigated. The theoretical insight provided herein may be exploited for optimizing optoelectronic device applications for growing families of quadrupolar molecules.

A highly sensitive short‐wave infrared (SWIR, λ > 1000 nm) organic photodiode (OPD) is described based on a well‐organized nanocrystalline bulk‐heterojunction (BHJ) active layer composed of a dicyanovinyl‐functionalized squaraine dye (SQ‐H) donor material in combination with PC61BM. Through thermal annealing, dipolar SQ‐H chromophores self‐assemble in a nanoscale structure with intermolecular charge transfer mediated coupling, resulting in a redshifted and narrow absorption band at 1040 nm as well as enhanced charge carrier mobility. The optimized OPD exhibits an external quantum efficiency (EQE) of 12.3% and a full‐width at half‐maximum of only 85 nm (815 cm−1) at 1050 nm under 0 V, which is the first efficient SWIR OPD based on J‐type aggregates. Photoplethysmography application for heart‐rate monitoring is successfully demonstrated on flexible substrates without applying reverse bias, indicating the potential of OPDs based on short‐range coupled dye aggregates for low‐power operating wearable applications.

The essential states model (ESM) for donor− acceptor−donor (D−A−D) chromophores is used to explore absorption and photoluminescence (PL) in molecular dimers composed of centrosymmetric and non-centrosymmetric squaraine molecules. The spectral line shapes and shifts relative to the monomer spectrum are due to an interesting interplay between three-center charge distributions responsible for ground-and excited-state (permanent) dipole and quadrupole moments and two-center charge distributions responsible for transition dipole moments. The Davydov splitting is sensitive only to the interactions between the (extended) transition dipoles , whereas the permanent dipole-dipole and quadruple-quadrupole interactions impact the midpoint frequency of the two Davydov components, leading to a generally asymmetric splitting relative to the peak monomer transition frequency. The theory accurately reproduces the steady-state absorption and PL line shapes recently obtained for covalently bound squaraine dimers. The ESM also predicts an extreme type of non-Kasha behavior, where both Davydov components are blue-shifted above the monomer transition frequency.