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 unitsbridged by methylene groups defining a highly rigid and symmetric structure. The innersurface of the cavity is hydrophobic, whereas both portals are identical and surrounded bycarbonyl groups permitting ion-dipole or hydrogen bond interactions. Many organicmolecules have shown changes in their spectroscopic properties due the formation ofsupramolecular 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 UVVisibleabsorption spectrum of 1-PYRNH3+ exhibits a bathochromic shift (5 nm) and theabsorbance decreases suggesting an interaction between this organic compound and themacrocycle. The value of the association constant determined in HCOOH 55 % w/v at 25.0ºCwas KA = (2.5 ± 0.2) x 102 M-1. The guest-host 1:1 stoichiometry was confirmed by the lineardouble reciprocal plot.The fluorescence emission spectrum of a solution of 1-PYRNH3+ in HCOOH 55 % w/vshows a dual fluorescent behaviour: a strong fluorescence band centered at 440 nm attributedto the basic form and a weak band at λem < 410 nm related to the acidic specie. This effect isassociated 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 theFörster cycle was pKa* = - 2.0. The short wavelength band enhances gradually uponsuccessive addition of CB6, whereas the long wavelength fluorescence band shows aconsiderable decrease with increasing CB6 concentration. A global constant, that involvesassociation 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 inthe excited state (pKa* (1-PYRNH3+-CB6) > pKa* (1-PYRNH3+)), resulting in the significantdecrease of the emission of the neutral excited state and the enhancement of the emissionfrom the protonated excited state.