Brian D. Wagner

Fluorescence Enhancement of Curcumin upon Inclusion into Cucurbituril

The effect of the macrocyclic host compounds cucurbit[n]urils (Qn), with on the fluorescence of the biologically active compound curcumin has been studied. Curcumin, the main constituent of the Indian spice turmeric, is of growing interest because of its wide-ranging pharmaceutical properties. This compound forms strong 2:1 host–guest inclusion complexes with Q6 (the original cucurbituril), with an overall equilibrium constant of (1.9±0.8)×104 M−2. It is postulated that a Q6 host partially encapsulates each of the two phenyl groups at the ends of the curcumin molecule. The difference in magnitude of the equilibrium constants K 1 (72±2 M−1) and K 2 (260±120 M−1) for stepwise encapsulation of the two ends of the curcumin molecule indicates that encapsulation by the first Q6 significantly alters its entire electronic structure, resulting in a more favorable second encapsulation. A very large enhancement of the fluorescence of curcumin results from this complex formation, on the order of 5.0; this is a significant fraction of the polarity sensitivity factor (PSF) of 39 measured for curcumin, that is the ratio of fluorescence intensity in ethanol vs. water. Surprisingly, no such enhancement could be observed in the case of Q7, indicating that the interactions between the guest and the host cavity are not favorable in this case, contrary to expectations. Similarly, no enhancement was observed in the case of Q5, which is not unexpected, because of the extremely small size of the host cavity and portal in this case.

Subpicosecond pump–probe measurements of the electronic relaxation rates of the S1 states of azulene and related compounds in polar and nonpolar solvents

The lifetimes of the S1 states of azulene, azulene‐d8, 4,6,8‐trimethylazulene, and guaiazulene (1,4‐dimethyl‐7‐isopropylazulene) have been measured in three nonviscous solvents of different polarity and structure using a two‐photon, two‐color, pump–probe method with subpicosecond time resolution. A significant solvent effect is measured. The rate constants for S1■S0 internal conversion in all four compounds in all three solvents exhibit one common S1–S0 energy gap law correlation, indicating that variations in the electronic relaxation rates are governed exclusively by changes in the Franck–Condon factors for the transition. No effect is observed when the exciting wavelength is changed, indicating that vibrational relaxation is occurring on a time scale which is faster than that of electronic relaxation in these systems. No significant deuterium isotope effect is measured in azulene, indicating that high frequency C–H(D) stretching vibrations do not act as significant accepting modes in the radiationless ...

Perturbative treatments of pump–probe laser‐molecule interactions with applications to azulene and trimethylazulene

Semianalytic perturbative approaches for investigating the spectroscopy, and the underlying dynamics, associated with fast time‐resolved, two‐photon, two‐color, pump–probe, low intensity laser‐molecule interactions are developed and discussed. In particular the perturbation theory is developed with emphasis on molecular models associated with pump–probe experiments on the S0→S1→S2 two‐photon transitions in azulene and trimethylazulene. The experiments are discussed and the theory is used to determine the lifetimes of the intermediate S1 states by analyzing the experimental two‐photon fluorescence signals from the S2 states as a function of the time‐delay between the pump and probe lasers. The advantages of using this approach relative to the traditional methods for determining the lifetime of the S1 states are discussed. The dependence of the two‐photon fluorescence signals on the angle between the polarization vectors of the pump and probe lasers, for fixed time delay between the lasers, is also consider...

The effect of solvent viscosity on the population relaxation times of the S1 state of azulene and related compounds

Abstract The S1 population decay times of azulene and 4,6,8-trimethylazulene have been measured in solvents with a wide range of viscosities using a two-photon, two-color, pump—probe method. The S1 lifetimes vary as expected on the basis of the energy gap law. No effect of solvent viscosity is observed, contrary to a previous suggestion. Data for 2-chloroazulene in non-viscous solvents are also reported.

Fluorescence-Based Comparative Binding Studies of the Supramolecular Host Properties of PAMAM Dendrimers Using Anilinonaphthalene Sulfonates: Unusual Host-Dependent Fluorescence Titration Behavior

This work describes the fluorescence enhancement of the anilinonaphthalene sulfonate probes 1,8-ANS, 2,6-ANS, and 2,6-TNS via complexation with PAMAM dendrimer hosts of Generation 4, 5 and 6. The use of this set of three very closely related probes allows for comparative binding studies, with specific pairs of probes differing only in shape (1,8-ANS and 2,6-ANS), or in the presence of a methyl substituent (2,6-TNS vs. 2,6-ANS). The fluorescence of all three probes was significantly enhanced upon binding with PAMAM dendrimers, however in all cases except one, a very unusual spike was consistently observed in the host fluorescence titration plots (fluorescence enhancement vs. host concentration) at low dendrimer concentration. This unprecedented fluorescence titration curve shape makes fitting the data to a simple model such as 1:1 or 2:1 host: guest complexation very difficult; thus only qualitative comparisons of the relative binding of the three guests could be made based on host titrations. In the case of G4 and G5 dendrimers, the order of binding strength was qualitatively determined to be 1,8-ANS < 2,6-ANS indicating that the more streamlined 2,6-substituted probes are a better match for the dendrimer cavity shape than the bulkier 1,8-substituted probe. This order of binding strength was also indicated by double fluorometric titration experiments, involving both host and guest titrations. Further double fluorometric titration experiments on 2,6-ANS in G4 dendrimer revealed a host concentration-dependent change in the nature of the host: guest complexation, with multiple guests complexed per host molecule at very low host concentrations, but less than one guest per host at higher concentrations.