Alignment and Relaxation Dynamics of Dye Molecules in Host−Guest Inclusion Compounds As Probed by Dielectric Spectroscopy

Abstract

The alignment and relaxation dynamics of a polar dye molecule, N,N-dimethyl-4(4-nitrophenylazo)aniline (DNAA), in zeolite L and perhydrotriphenylene (PHTP) channels were investigated by means of a combination of optical, dielectric, and quantum-chemical methods. Both the zeolite L and PHTP channels enable the dye molecules to align along the channel axis. An amplified net dipole moment of DNAA in PHTP is observed and attributed to enhanced 1D close alignment of dye molecules. In zeolite L channels, a concentration gradient is found with aggregation at the channel entrances. The dynamics of the dye in zeolite L channels reveals localized conical rotational fluctuation modes following Arrhenius-type activation with energy of 0.31 eV, which we assign to small noninteracting fluctuating polar units of the dyes being loosely aligned or isolated. Unlike zeolite L, relaxations in PHTP are characterized by cooperative wobbling motions interpreted as increased intermolecular dipole interaction due to a closely packed one-dimensional array. Temperature-dependent activation energies of 0.25 eV below 0 degrees C and 0.37 eV at ambient temperature reflect the role of the soft channel walls in the activation process.

Supramolecular Organization in Confined Nanospaces

“…Chromophores bearing a positive charge – from the simplest ones like thionine,,,,, xanthene dyes,,,,,,„ acridine (and derivatives),,,,, to more complex molecules like methylviologen or BODIPY‐dyes,,, – are naturally prone to be incorporated by a zeolite via straightforward ion exchange, a practice which is also used for dye incorporation into anionic MOF nanochannels ,. As a consequence, cationic dyes are very popular as luminescent guests, and their encapsulation has been widely documented by experimental, theoretical, or combined investigations.…”

Directional exciton transport in supramolecular nanostructured assemblies

“…44 PHTP provides a tight environment due to the narrow channels; on the other hand, the channel walls should be rather soft, since the crystal is held together by van der Waals forces only. To monitor the guest dynamics, a number of different spectroscopic techniques were investigated, including fluorescence excitation, 21 Raman, 9 2 H-NMR, 21 and dielectric investigations, 22 accompanied by quantum-chemical 9,21,22 and molecular mechanics/dynamics (MM/MD) studies, 21 see Fig. 4.…”

“…21 Finally, the dielectric study on terminal donor-acceptor substituted stilbene guest molecules in PHTP showed that intramolecular rotational mobility is suppressed in the tight channels, but instead collective intermolecular mobility is enhanced through dipole amplification and weak hydrogen bonds. 22…”

“…51 However, while zeolite L shares the hexagonal arrangement of the channels with similar channel diameters and inter-channel distances compared to PHTP (Fig. 2), the zeolite channel walls are not homogeneous; instead, the cavity sizes are large enough to allow for different chromophore orientations which can access high-amplitude rotational intramolecular (noncooperative) modes; 22 furthermore, the charged channel walls influence sensitively the optical properties of the guest molecules. 45 Indications for small aggregate formation were given by steadystate and time-resolved fluorescence spectroscopy and microscopy.…”

“…Applied research benefits from the control of the geometry by the supramolecular organization in a variety of cavity sizes and shapes, as well as from high fluorescence efficiencies and exciton diffusion by the suppression of emission quenching (as often induced by aggregation) despite the high chromophore concentration in the HGCs, 4,6,8,12,[17][18][19][20] allowing for bright color-tuned materials. 4,12 Inorganic hosts demonstrated superior chemical and mechanical stability, and tunability of crystal size and aspect ratio, as well as the possibility of optically or electrically addressing such materials via the 'stopcock principle' introduced by G. Calzaferri et al 4,6 Basic research on the other hand finds in such materials an ideal system to investigate the impact of spatial confinement on chromophore properties, 9,14,[21][22][23][24] but especially to study the 'ingredients' of three-dimensional exciton transport through a radical reduction of parameters, that is the use of weakly coupled, well-defined oligomers in a highly ordered system. 8,18,19 From such studies also traditional active layers of (polymeric) conjugated materials might benefit by learning how to achieve high exciton diffusion lengths.…”