Effect of the complexation with cucurbit[6]uril on the spectroscopic properties of 1-aminopyrene

VALERIA N. SUELDO OCCELLO; ALICIA V. VEGLIA

Mendoza

Conferencia; 21st I-APS Conference; 2011

Inter-American Phothochemical Society

Cucurbit[6]uril (CB6) is a macrocyclic host molecule comprised of six glycoluril units bridged by methylene groups defining a highly rigid and symmetric structure. The inner surface of the cavity is hydrophobic, whereas both portals are identical and surrounded by carbonyl groups permitting ion-dipole or hydrogen bond interactions. Many organic molecules have shown changes in their spectroscopic properties due the formation of supramolecular species with CB6. In this work, the effect of the addition of the macrocycle on the photophysical properties of 1-aminopyrene was studied in HCOOH 55 % for the CB6 solubilization.  In aqueous solution 1-aminopyrene exists in two prototropic forms: the neutral specie, 1-PYRNH2, and the protonated form, 1-PYRNH3+ (pKa = 3,6). In the presence of CB6, the UV-Visible absorption spectrum of 1-PYRNH3+ exhibits a bathochromic shift (5 nm) and the absorbance decreases suggesting an interaction between this organic compound and the macrocycle. The value of the association constant determined in HCOOH 55 % w/v at 25.0ºC was KA = (2.5 ± 0.2) x 102 M-1. The guest-host 1:1 stoichiometry was confirmed by the linear double reciprocal plot. The fluorescence emission spectrum of a solution of 1-PYRNH3+ in HCOOH 55 % w/v shows a dual fluorescent behaviour: a strong fluorescence band centered at 440 nm attributed to the basic form and a weak band at wavelength of emission < 410 nm related to the acidic specie. This effect is associated to the difference between the pKa values of the ground and excited states (pKa (1-PYRNH3+) > pKa* (1-PYRNH3+)). The pKa value of the excited state estimated from the Förster cycle was pKa* = - 2.0. The short wavelength band enhances gradually upon successive addition of CB6, whereas the long wavelength fluorescence band shows a considerable decrease with increasing CB6 concentration. A global constant, that involves association and acid dissociation constants, was determined from the fluorescent data, KA = (14 ± 1) x 102 M-1 (HCOOH 55 % w/v at 25.0ºC). Therefore, the complexation between 1-PYRNH3+ and the macrocycle reduces its acidity in the excited state (pKa* (1-PYRNH3+-CB6) > pKa* (1-PYRNH3+)), resulting in the significant decrease of the emission of the neutral excited state and the enhancement of the emission from the protonated excited state.