Cyclodextrin Cavity Size Effect on the Complexation and Rotational Dynamics of the Laser Dye 2,5-Diphenyl-1,3,4-oxadiazole:  From Singly Occupied Complexes to Their Nanotubular Self-Assemblies

Abstract

The general complexation scheme as well as the dynamic features of the supramolecular structures resulting from the interaction of the laser dye 2,5-diphenyl-1,3,4-oxadiazole (PPD) with the naturally occurring alpha-, beta-, and gamma-cyclodextrins in water are studied by means of fluorescence spectroscopy, both steady-state (SS) and time-resolved (TR). PPD interacts weakly, from a thermodynamic point of view, with alpha-cyclodextrin (alpha-CD), forming 1:1 complexes with an association constant of K(11) = 85 +/- 4 M(-1). However, the local motion of the substrate (PPD) with respect to the ligand (CD) in the complexed form is hindered; namely, dynamically, they are strongly coupled and only a global tumbling motion, = 370 +/- 30 ps, of the whole adduct is observed. The next homologue beta-CD also forms 1:1 entities with PPD, but although the binding strength of reactants (K(11) = 682 +/- 60 M(-1)) is almost an order of magnitude greater than the former case with the alpha-CD, these are dynamically weakly coupled. In fact, two independent motions are detected: one is that of the whole nanostructure motion (1:1, PPD/beta-CD) with a global rotational relaxation time of = 480 +/- 30 ps, and the other is an internal librational motion of the dye inside the host cavity with an average angular displacement of theta approximately 27 degrees . Finally, the interaction of PPD with the wider and more flexible cavity of the gamma-CD "triggers" a self-associative scheme of the initially formed supramolecular building blocks, namely, singly occupied complexes, leading to the formation of nanotubular superstructures. It is found that these linear arrays are constituted from more than 17 gamma-CD units which are held together with the aid of dimers of PPD. Interestingly, our results supported that two distinct dimeric forms of PPD play the role of the "shaft" between adjacent cyclodextrin units. The topology of the dimers in the interlinking space of gamma-CD units is such that PPD molecules are held in suitable proximity, resulting, upon excitation, in the observation of dual excimer emission.

Coordination-Driven Self Assembly of a Brilliantly Fluorescent Rhomboid Cavitand Composed of Bodipy-Dye Subunits

“…No aggregation phenomena were observed in a good solvent such as tetrachloroethane at ambient temperatures, as demonstrated by time-resolved fluorescence depolarization spectroscopy in conjunction with the Stokes−Einstein−Debye (SED) equation. By applying the spherical rotor anisotropy model to the experimental I VV and I VH data of 3 at 23 °C, the rotational reorientation time τ r of 3 (0.70 μM) was determined to be 655 ± 40 ps (Figure and SI). This experimental value corresponds to a rotating spheroid with a molecular volume of ∼1690 Å 3 which is in accordance, within experimental error, with the one calculated from the monomeric unit of the X-ray crystal structure of 3 (∼1780 Å 3 including van der Waals radii).…”

Cooperative Self-Assembly Enables Two-Dimensional H-type Aggregation of a Sterically Crowded Perylene-Bisimide Dimer

“…Once the nucleus M 2 is formed, the development of aggregates is modeled to proceed in a similar fashion as the K 2 – K model predicts, i.e., by the successive addition of the active unit M 1 to the growing self-assembled aggregate, ,− i.e., isodesmically, each step of which ( n ⩾ 3) preserves the same association constant ( K A ), as shown in Scheme and described by the equations given in the Supporting Information. In fact, as shown in Scheme , subsequent attachment of monomeric units (mauve-colored M 1 ) along the direction perpendicular to the mean plane of PBIs ( a axis) is favored by the concerted action of ionic (core of S ) and π–π or C–H ···· π interactions (terminal PBIs of S ), providing collectively the driving force for building up the PBI columnar direction.…”

Designing fluoroprobes through Förster resonance energy transfer: surface modification of nanoparticles through “click” chemistry

“…1. Previous efforts have indicated that various forms of CD are effective in forming inclusion complexes with molecules that incorporate the 1,3,4-oxadiazole ring, [37][38][39] while carbazole-containing molecules have been the focus of a number of studies on the alteration of its luminance characteristics with inclusion in CDs. [40][41][42][43][44][45] The far right image in Fig.…”

“…This relative affinity to water and the influence of b-CD in incorporating AD1 and AD2 was confirmed by a preliminary fluorescence analysis of the b-CD:AD1 and b-CD:AD2 inclusion complexes; b-CD is less effective in raising the concentration of AD2 in an aqueous solution relative to AD1. The creation of supramolecular architectures between the b-CD and the oxadiazole 38 and dyes, 48 coupled with both an enhanced availability and reactivity at the transformation site, 49-51 may account for their higher grafting densities on the PA particles.…”