Diameter-dependent single- and double-file stacking of squaraine dye molecules inside chirality-sorted single-wall carbon nanotubes.

The filling of single-wall carbon nanotubes (SWCNTs) with dye molecules has become a novel path to add new functionalities through the mutual interaction of confined dyes and host SWCNTs. In particular cases, the encapsulated dye molecules form strongly interacting molecular arrays and these result in severely altered optical properties of the dye molecules. Here, we present the encapsulation of a squaraine dye inside semiconducting chirality-sorted SWCNTs with diameters ranging from ∼1.15 nm, in which the dye molecules can only be encapsulated in a single-file molecular arrangement, up to ∼1.5 nm, in which two or three molecular files can fit side-by-side. Through the chirality-selective observation of energy transfer from the dye molecules to the surrounding SWCNTs, we find that the absorption wavelength of the dye follows a peculiar SWCNT diameter dependence, originating from the specific stacking of the dye inside the host SWCNTs. Corroborated by a theoretical model, we find that for each SWCNT diameter, the dye molecules adopt a close packing geometry, resulting in tunable optical properties of the hybrid when selecting a specific SWCNT chirality.

Diameter-Dependent Optical Absorption and Excitation Energy Transfer from Encapsulated Dye Molecules toward Single-Walled Carbon Nanotubes.

The hollow cores and well-defined diameters of single-walled carbon nanotubes (SWCNTs) allow for creation of one-dimensional hybrid structures by encapsulation of various molecules. Absorption and near-infrared photoluminescence-excitation (PLE) spectroscopy reveal that the absorption spectrum of encapsulated 1,3-bis[4-(dimethylamino)phenyl]-squaraine dye molecules inside SWCNTs is modulated by the SWCNT diameter, as observed through excitation energy transfer (EET) from the encapsulated molecules to the SWCNTs, implying a strongly diameter-dependent stacking of the molecules inside the SWCNTs. Transient absorption spectroscopy, simultaneously probing the encapsulated dyes and the host SWCNTs, demonstrates this EET, which can be used as a route to diameter-dependent photosensitization, to be fast (sub-picosecond). A wide series of SWCNT samples is systematically characterized by absorption, PLE, and resonant Raman scattering (RRS), also identifying the critical diameter for squaraine filling. In addition, we find that SWCNT filling does not limit the selectivity of subsequent separation protocols (including polyfluorene polymers for isolating only semiconducting SWCNTs and aqueous two-phase separation for enrichment of specific SWCNT chiralities). The design of these functional hybrid systems, with tunable dye absorption, fast and efficient EET, and the ability to remove all metallic SWCNTs by subsequent separation, demonstrates potential for implementation in photoconversion devices.

Three-stage pH-switchable organic chromophores with large nonlinear optical responses and switching contrasts.

Three-stage pH-switchable organic second-order nonlinear optical (SO NLO) chromophores are synthesized and characterized by wavelength-dependent linear and nonlinear spectroscopy. The chromophores exhibit huge SO NLO responses in their "on" stages, and large switching contrasts between adjacent stages in both SO NLO response and fluorescence quantum yield, with moreover different "on/off" sequences for closely related compounds.