Ultrafast Energy Transfer Dynamics in a Squaraine Heterotriad.

A squaraine heterotriad consisting of three different covalently linked squaraine chromophores was synthesized, and its absorption spectra were interpreted in terms of Kasha's exciton coupling theory. Using the exciton couplings derived from model dyads (ca. 700 cm-1) as the input, we were able to predict the exciton state energies of the heterotriad. Transient absorption spectroscopy with femtosecond time resolution showed that excitation of the highest exciton state populates a state mainly localized at one terminal squaraine chromophore, and energy transfer to the lowest exciton state localized at the other terminal squaraine occurs within 30 fs. Field-induced surface hopping dynamics simulations support the assumption of ultrafast energy transfer. Moreover, they show the close relationship between internal conversion and energy transfer in the intermediate to weak coupling regime. The latter is a consequence of excitation localization caused by molecular vibrations.

Exciton coupling effects on the two-photon absorption of squaraine homodimers with varying bridge units.

We explored a series of squaraine homodimers with varying π-bridging centres to probe the relationship between the chemical structure and the two-photon absorption (2PA) characteristics. To this end, we designed and synthesised six linear homodimers based on two indolenine squaraine dyes with transoid configuration (SQA) which are connected by diverse bridges. In this regard, we investigated the effect of exciton coupling in these dimeric systems where the variation of the bridging units affects the magnitude of exciton coupling and leads to an alteration of their linear optical properties. Using two-photon absorption induced fluorescence measurements we determined the two-photon absorption cross section in this series of homodimers and found sizable values up to 5700 GM at ca. 11 000 cm-1 and 12 000 GM at 12 500 cm-1. The 2PA strength roughly follows the exciton coupling interaction between the squaraine chromophores which therefore may be used as design criteria to achieve high 2PA cross sections. The results were substantiated by polarization dependent linear and nonlinear optical measurements and by density functional theory calculations based on time dependent and quadratic response theory.

Coherently and fluorescence-detected two-dimensional electronic spectroscopy: direct comparison on squaraine dimers.

Optical two-dimensional electronic spectroscopy (2DES) is now widely utilized to study excitonic structure and dynamics of a broad range of systems, from molecules to solid state. Besides the traditional experimental implementation using phase matching and coherent signal field detection, action-based approaches that detect incoherent signals such as fluorescence have been gaining popularity in recent years. While incoherent detection extends the range of applicability of 2DES, the observed spectra are not equivalent to the coherently detected ones. This raises questions about their interpretation and the sensitivity of the technique. Here we directly compare, both experimentally and theoretically, four-wave mixing coherently and fluorescence-detected 2DES of a series of squaraine dimers of increasing electronic coupling. All experiments are qualitatively well reproduced by a Frenkel exciton model with secular Redfield theory description of excitation dynamics. We contrast the spectral features and the sensitivities of both techniques with respect to exciton energies, delocalization, coherent and dissipative dynamics, and exciton-exciton annihilation. Discussing the fundamental and practical differences, we demonstrate the degree of complementarity of the techniques.

A Self-Assembled Unit Comprising 12 Squaraine Dyes Built Up from Two Star-Shaped Hexasquarainyl-Benzene Molecules.

We describe the aggregate formation and optical properties of a star-shaped hexaarylbenzene with six squaraine chromophores (=hexasquarainyl benzene). Comprehensive concentration-dependence studies in acetone/CHCl3 mixtures reveal a strong propensity to form discrete dimeric aggregates with a high binding constant in excess of 106 m-1 . In this context, a large hypsochromic shift of almost 2700 cm-1 was found in the absorption spectrum, indicating H-type exciton coupling. The aggregate band is characterised by a very small band width of only 560 cm-1 , probably caused by exchange narrowing. Both experimental and computational methods were used to elucidate the supramolecular aggregate structure, which is assumed to consist of two stacked hexasquarainyl benzene monomers.

fs-ps Exciton dynamics in a stretched tetraphenylsquaraine polymer.

A tetraphenylsquaraine was synthesized whose structure was elucidated by single crystal X-ray structure analysis. Unlike all known indolenine squaraines, the tetraphenylsquaraine shows an unusual nonplanar structure with the four phenyl groups pointing away from the squaric acid core in order to avoid steric congestion. This tetraphenylsquaraine was polymerized by a Yamamoto coupling to form a conjugated polymer with Xn = 38. The absorption spectra of this polymer are red-shifted compared to that of the monomer and show a J-type absorption band due to exciton coupling. Transient absorption spectra with fs-time resolution display a strong ground state bleaching signal with a peak on the red side rising concurrently with the decay of a peak on the blue side of an isosbestic point at 12 000 cm-1. This behavior is caused by energy transfer between two slightly different sections of the polymer with time constants of 0.3 and 2.6 ps. According to semiempirical calculations these different sections are stretched and slightly bent conformations of the polymer strand. Power dependent transient absorption measurements indicate exciton annihilation which also proves the excitons to be very mobile.

Green-to-Red Electrochromic Fe(II) Metallo-Supramolecular Polyelectrolytes Self-Assembled from Fluorescent 2,6-Bis(2-pyridyl)pyrimidine Bithiophene.

The structure and properties of metallo-supramolecular polyelectrolytes (MEPEs) self-assembled from rigid 2,6-bis(2-pyridyl)pyrimidine and the metal ions FeII and CoII are presented. While FeL1-MEPE (L1 = 1,4-bis[2,6-bis(2-pyridyl)pyrimidin-4-yl]benzene) is deep blue, FeL2- and CoL2-MEPE (L2 = 5,5'-bis[2,6-bis(2-pyridyl)pyrimidin-4-yl]-2,2'-bithiophene) are intense green and red in color, respectively. These novel MEPEs display a high extinction coefficient and solvatochromism. Ligand L2 shows a high absolute fluorescence quantum yield (Φf = 82%). Viscosity and static light-scattering measurements reveal that the molar masses of these MEPEs are in the range of 1 × 108 g/mol under the current experimental conditions. In water, FeL1-MEPE forms a viscous gel at 20 °C (c = 8 mM). Thin films of high optical quality are fabricated by dip coating on transparent conducting indium tin oxide (ITO) glass substrate. Optical, electrochemical, and electrochromic properties of the obtained MEPEs are presented. Green to red and blue to colorless electrochromism is observed for FeL2-MEPE and FeL1-MEPE, respectively. The results show that the electrochromic properties are affected by the ligand topology. The Fe-MEPEs show a reversible redox behavior of the FeII/FeIII couple at 0.86 and 0.82 V versus Fc+/Fc and display an excellent redox cycle stability under switching conditions. FeL2-MEPE in its oxidized state exhibits a broad absorption band covering the near-IR region (ca. 1500 nm) due to the ligand-to-metal charge transfer transition originating due to charge delocalization in the bithiophene spacer.

Cooperative enhancement versus additivity of two-photon-absorption cross sections in linear and branched squaraine superchromophores.

The linear and nonlinear optical properties of a series of oligomeric squaraine dyes were investigated by one-photon absorption spectroscopy (1PA) and two-photon absorption (2PA) induced fluorescence spectroscopy. The superchromophores are based on two indolenine squaraine dyes with transoid (SQA) and cisoid configuration (SQB). Using these monomers, linear dimers and trimers as well as star-shaped trimers and hexamers with benzene or triphenylamine cores were synthesised and investigated. The red-shifted and intensified 1PA spectra of all superchromophores could well be explained by exciton coupling theory. In the linear chromophore arrangements we also found superradiance of fluorescence but not in the branched systems. Furthermore, the 2PA showed enhanced cross sections for the linear oligomers but only additivity for the branched systems. This emphasizes that the enhancement of the 2PA cross section in the linear arrangements is probably caused by orbital interactions of higher excited configurations.