Davydov Splitting in Squaraine Dimers

Abstract

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.

Polymorphism in Squaraine Dye Aggregates by Self‐Assembly Pathway Differentiation: Panchromatic Tubular Dye Nanorods versus J‐Aggregate Nanosheets

“…Thereby, each essential state is described by two harmonic oscillators, that is, one for each donor‐acceptor side arm. In doing so, the UV/Vis/NIR spectrum of reference dye SQ1 is well reproduced by our simulation (Figure S15, blue solid line) using the parameters shown in Table S2, which are in line with previous studies on squaraine dyes [20b] . The zwitterionic character ρ in the ground state is given by equation 2 [36] …”

“…Transitions to both the highest and lowest excited states can be allowed in case of a rotational displacement of the chromophores resulting in the appearance of a weaker J‐band in the absorption spectrum (Davydov splitting) [33] . However, this would require a large rotational angle in our case since both bands show significant intensity (Figure 4 a, blue solid line) [20b] . For such a situation, however, only small shifts are expected which is obviously not the case for the BisSQ2‐AggI nanorods.…”

Solvent Induced Helix Folding of Defined Indolenine Squaraine Oligomers

“…This emphasized the correctness of our structural model. The large pitch of our structural model also indicates that the blue shift of absorption maxima (H‐type) is not caused by parallel arrangement of transition dipole moments in π‐stacked squaraines but is because of the gauche conformation [38] of the banana‐bent squaraines around the connecting biaryl axis. This can easily be seen when comparing the TD‐DFT computed absorption spectra of linear SQ 3 vs. the closed form, see Figure S59 in the Supporting Information.…”

Polymorphic chiral squaraine crystallites in textured thin films

“…Polymorphism and anisotropic thin film texture, as well as the formation of molecular H‐ and J‐aggregates, are typical features of squaraine dyes. Often two or more translationally invariant molecules are present per unit cell enabling a spectrally well‐resolved Davydov splitting of the coupled molecular resonance …”