Cationic Polyelectrolyte for Anionic Cyanines: An Efficient Way To Translate Molecular Properties into Material Properties.

In this work, a new phosphonium-containing cationic polyelectrolyte (PE1) has been rationally designed and developed via a facile click-chemistry type postfunctionalization, which can form complexes with highly polarizable anionic cyanines to significantly reduce the strong and random cyanine-cyanine interactions (i.e., aggregation) in the solid-state. This material design strategy enables an efficient translation of the favorable molecular properties of cyanines into macroscopic material properties. One of such complexes exhibits a very large third-order susceptibility over 10-10 esu with low nonlinear optical loss suitable for all optical signal processing.

Ultrafast energy transfer in biomimetic multistrand nanorings.

We report the synthesis of LH2-like supramolecular double- and triple-stranded complexes based upon porphyrin nanorings. Energy transfer from the antenna dimers to the π-conjugated nanoring occurs on a subpicosecond time scale, rivaling transfer rates in natural light-harvesting systems. The presence of a second nanoring acceptor doubles the transfer rate, providing strong evidence for multidirectional energy funneling. The behavior of these systems is particularly intriguing because the local nature of the interaction may allow energy transfer into states that are, for cyclic nanorings, symmetry-forbidden in the far field. These complexes are versatile synthetic models for natural light-harvesting systems.

Dioxaborine- and indole-terminated polymethines: effects of bridge substitution on absorption spectra and third-order polarizabilities.

Cyanine-like dyes are promising candidates for third-order nonlinear optical (NLO) applications such as all-optical switching. Here, we examine the consequences for linear and nonlinear optical properties of varying substituents on the central methine unit of bis(dioxaborine)-terminated anionic pentamethines and bis(indole)-terminated cationic heptamethines. The variation in absorption maxima and electrochemical potentials with structure can generally be rationalized using the Dewar-Knott rules, providing that mesomeric and inductive electron-withdrawal and donation are explicitly considered. In the case of nitro- and (dioxaborinyl)vinyl-substituted bis(dioxaborine) pentamethines, the low-energy transitions are significantly broadened, consistent with (1)H NMR spectra indicating deviation from cyanine-like geometry. Real and imaginary parts of the third-order polarizabilities, Re(γ) and Im(γ), were measured at 1.3 µm. The values of Im(γ) indicate that the values of Re(γ) are significantly resonantly enhanced, while the positive value of Re(γ) found for a nitro-substituted dioxaborine example is atypical for a symmetrical polymethine and suggests that a two-state treatment is inadequate. The relevance of these results to chromophore design for third-order NLO applications is discussed.

Using end groups to tune the linear and nonlinear optical properties of bis(dioxaborine)-terminated polymethine dyes.

Six anionic pentamethine dyes with different 2,2-difluoro-4-aryl-1,3,2(2 H)-dioxaborin-6-yl termini were synthesized and isolated as tetra-n-octylammonium salts with a variety of aryl groups appended to increase conjugation beyond the dioxaborine termini. The increased conjugation was expected to decrease the energy of the lowest-lying excited state, and increase the transition dipole moment linking this state to the ground state, which would be anticipated to result in an increase in the real part of the third-order polarizability, Re(gamma). UV/Vis-NIR absorption spectroscopy indicates that the absorption maxima in DMSO vary from 691 to 761 nm, with the longest wavelength transitions observed for a derivative where the aryl group is 4-nitrophenyl. Closed-aperture Z-scan measurements at 1.3 microm in DMSO indicate that Re(gamma) varies from -2.9x10(-33) to -5.4x10(-33) esu in these systems. The largest magnitude of Re(gamma) was observed for a dye with E-4-styrylphenyl aryl groups. This result can be rationalized using a two-state expression which relates Re(gamma) to the energy and transition dipole moment of the transition from the ground state to the lowest-lying excited state. A nonamethine analogue of this compound was also synthesized and exhibits a slightly larger Re(gamma) with respect to a previously reported bis(dioxaborine)-terminated nonamethine. The extension of conjugation beyond the dioxaborine termini seems to result in an overall increase in Re(gamma). However, the effects are smaller than those found by increasing conjugation in the polymethine bridge due to reduced participation of terminal groups in the HOMO.

Synthesis and two-photon spectrum of a bis(porphyrin)-substituted squaraine.

A chromophore in which zinc porphyrin donors are linked through their meso positions by ethynyl bridges to a bis(indolinylidenemethyl) squaraine core has been synthesized using Sonogashira coupling. The chromophore exhibits a two-photon absorption spectrum characterized by a peak cross section of 11,000 GM and, more unusually, also exhibits a large cross section of >780 GM over a photon-wavelength window 750 nm in width.