Asymmetric dyes align inside carbon nanotubes to yield a large nonlinear optical response

Abstract

Asymmetric dye molecules have unusual optical and electronic properties. For instance, they show a strong second-order nonlinear optical (NLO) response that has attracted great interest for potential applications in electro-optic modulators for optical telecommunications and in wavelength conversion of lasers. However, the strong Coulombic interaction between the large dipole moments of these molecules favours a pairwise antiparallel alignment that cancels out the NLO response when incorporated into bulk materials. Here, we show that by including an elongated dipolar dye (p,p'-dimethylaminonitrostilbene, DANS, a prototypical asymmetric dye with a strong NLO response) inside single-walled carbon nanotubes (SWCNTs), an ideal head-to-tail alignment in which all electric dipoles point in the same sense is naturally created. We have applied this concept to synthesize solution-processible DANS-filled SWCNTs that show an extremely large total dipole moment and static hyperpolarizability (β0 = 9,800 × 10(-30) e.s.u.), resulting from the coherent alignment of arrays of ∼70 DANS molecules.

Quasiphase Transition in a Single File of Water Molecules Encapsulated in (6,5) Carbon Nanotubes Observed by Temperature-Dependent Photoluminescence Spectroscopy

“…[43,44] The resulting chirality-sorted nanotube suspension was first dialyzed to 1%wt/V DOC, using a stirred ultrafiltration cell, and subsequently density gradient ultracentrifugation was used to separate the empty and water-filled (6,5) SWCNTs. [48,49] Similar to previous work [13][14][15][16]48, 49] a combination of optical absorption, resonant Raman and PL-excitation spectroscopy was used to asign the composition of each of the fractions, see SM [46] indicating that the amount of filled SWCNTs are negligible in the empty fraction (< 3.7%), while in the PL spectra of the filled fraction a small contribution of empty SWCNTs can be observed, with an amplitude of ∼ 20% of the total PL intensity. For temperature-dependent optical experiments, SWCNTs were drop-cast on quartz substrates, dried and mounted in a Microstat He2 cryostat (Oxford Instruments).…”

“…Molecular confinement inside SWCNTs moreover results in new functionalities such as amphoteric doping [18] or photosensitization [19,20] of the SWCNTs, enhanced stabilities of the encapsulated molecules, [21] and unique molecular order which is only achievable in such quasi-1D nanocavities. [16] Moreover, recent studies [5,22,23] have revealed nearly zero friction for confined water transportation and superior water permeability in CNTs, indicating their potential applciations in efficient desalination and water purifcation devices. [24,25] Both theoretical calculations [4,26] and experiments [13-15, 27, 28] have demonstrated that water molecules can spontaneously enter the smooth, hydrophobic channels of SWCNTs.…”

“…Recent advances in chiral sorting of carbon nanotubes, [42][43][44] combined with the extreme sensitivity (and chiral selectivity [45]) of optical spectroscopy for the filling of SWCNTs with various molecules, [13][14][15][16] provides an ideal platform for studying the thermodynamic properties of single files of water molecules confined in SWCNTs with a well-defined, small diameter. In this Letter, we report temperature-dependent photoluminescence (PL) spectroscopy of empty and water-filled sorted, single-chirality (6,5) SWCNTs.…”

“…1). Theoretical calculations [59] and experimental studies [16,59] have suggested that optical transition energies of SWCNTs are extremely sensitive to both the effective size and orientation of the molecular dipoles encapsulated in or adsorbed on SWCNTs (larger effect for dipoles oriented perpendicular to the tube axis [59]). Therefore, a significant change in the orientation of the water dipoles most likely lies at the origin of the observed spectral shift.…”

Fitting Single-Walled Carbon Nanotube Optical Spectra

“…Despite this, advances in the physical information that can be obtained with this technique continue to be made and the level of finesse with which PL spectra can be individualized into single ( n , m ) contributions and analyzed is improving. 24−27 Raman spectral mapping may offer an alternative solution in the future but currently suffers from the need for elaborate tunable excitation sources. In addition to that, highly structure-dependent and unknown sensitivity factors exacerbate the interpretation of the measurement data.…”