Effect of encapsulation in cyclodextrin nanocavities on the photophysical properties of bithiophene-5-carboxamidine derivatives

The objective of this work is to investigate the impact of encapsulation on the photophysical characteristics of the hydrochloride salt of 5-(5-(4-methoxyphenyl)thiophen-2-yl)thiophene-2-carboxamidine (I) and 5-(5-(3,5-dimethoxyphenyl)thiophen-2-yl)thiophene-2-carboxamidine (II) compounds in neutral aqueous solution and when added to different cyclodextrin derivatives (CDs) such as α-cyclodextrin (α-CD), β-cyclodextrin (β-CD), methyl-β-cyclodextrin (Mβ-CD) and 2-(hydroxypropyl) β-cyclodextrin (HPβ-CD) investigated by steady-state absorption, fluorescence emission, and time-resolved fluorescence emission techniques. Significant changes in fluorescence quantum yield were detected, accompanied by a blue shift similar to what occurs when the solvent is switched from aqueous to non-aqueous media. The observed changes in the fluorescence emission spectra indicate that both (I) and (II) form 1:1 inclusion complex with the cyclodextrin derivatives under study. Fluorescence decay measurements show that the contribution of the ICT state (a₁) decreases while the contribution of the LE state (a₂) increases with added concentrations of α-CD, β-CD, Mβ-CD and HPβ-CD. ¹H NMR measurements show noticeable chemical shifts in the cyclodextrin interior protons only in the case of β-CD and small or distorted shifts in other types of CDs. In contrast, changes in the chemical shift of I or II protons were very pronounced despite their very low solubility in D₂O. This study highlights how cavity size and surface functionalization influence excited-state behaviour. The results indicate that larger and more functionalized cyclodextrins (Mβ-CD and HPβ-CD) exhibit different stabilization effects compared to smaller, unmodified α-CD and β-CD.