Tuning dual emission behavior of p-dialkylaminobenzonitriles by supramolecular interactions with cyclodextrin hosts

Abstract

Ground state absorption and steady-state and time-resolved fluorescence measurements have been carried out to understand the host-guest interactions of p-diethylaminobenzonitrile (DEABN) and p-dimethylaminobenzonitrile (DMABN) dyes with alpha-cyclodextrin (alpha-CD) and beta-cyclodextrin (beta-CD) hosts. DEABN and DMABN dyes show both locally excited (LE) state and intramolecular charge transfer (ICT) state emissions in solution. The LE and ICT emissions of the dyes are seen to get modulated in the presence of alpha-CD and beta-CD hosts. The results indicate that the dyes form 1 : 1 inclusion complexes with both the hosts. Comparing the binding constants and the fluorescence characteristics of different dye x CD systems it is inferred that DEABN adopts a completely different orientation on complexation with alpha-CD than in the other cases of dye.CD systems. It is indicated that while in all other cases of dye x CD systems the N,N-dialkyl group of the dyes enters the host cavity leaving the C[triple bond, length as m-dash]N group projected out into the water phase, the DEABN dye enters the alpha-CD cavity (smallest CD) with its C[triple bond, length as m-dash]N group entering the host cavity. The differences in the orientation of the dye in the host cavities is understood to be determined by the requirement of maximum van der Waals contact of the encapsulated dye with the host cavity for maximum stability of the complex and the relative sizes of the substituents of the dye compared to the host cavities. From the observation that the binding constants for the present dye x CD systems are not that significantly high, it is inferred that the hydrophobic interaction mainly govern the inclusion complex formation in the present systems.

Triple Emission from p‐Dimethylaminobenzonitrile–Cucurbit[8]uril Triggers the Elusive Excimer Emission

“…Photophysical parameters of uncomplexed DMABN, the CB7·DMABN complex, and the CB8·(DMABN) 2 dimer (all in water). [c] Same value as reported in Refs [21,22]…”

“…[19][20][21] Addition of CB5 or CB6 had no effect on the dual emission of DMABN: these cucurbiturils are too small to encapsulate the aryl groups of the dye and to form sufficiently stable inclusion complexes. The total (LE + ICT) fluorescence quantum yield (F f ) for the dye in aqueous solution is known to be small, but nonnegligible (0.0007), with a lifetime of 0.3 ns measured at the ICT fluorescence band (the LE fluorescence is much weaker but known to have the same lifetime as the ICT band).…”

“…[1,2] The intramolecular chargetransfer (ICT) state, which gives rise to a red-shifted emission, is favored in polar media, but coexists with the initially populated locally excited (LE) state in nonpolar environments. Experimental investigations involving differently substituted or structurally confined derivatives of DMABN, [13] as well as in media of different polarity and viscosity, [8,9,17] including the gas phase, [10] micelles, [18] solvent clusters, and supramolecular host cavities, [19][20][21] have invariably afforded evidence for the well-known dual emission, with the LE band centered around 350 nm and the ICT band around 450-500 nm. [2][3][4][5][12][13][14][15] Interestingly, excimer emission, which is a competing process for numerous other aromatic chromophores such as anthracene and pyrenes, [16] has not been characterized for DMABN in the solution phase.…”

pH‐Responsive Indicator Displacement Assay of Acetylcholine Based on Acridine–p‐Sulfonatocalix[4]arene Supramolecular System: Fluorescence Off/On Switching and Reversible pKa Shift

“…Supramolecular host‐guest chemistry is an extremely active research area due to their diverse applications in the fields of drug delivery, drug formulations, food industry, pharmaceuticals, nanomedicines, photodynamic therapy, photostabilization, catalysis, nanotechnology, functional materials, optical sensors, on–off switches, etc . Host‐guest assemblies are formed by combining discrete molecular components in ordered manner involving dynamic noncovalent interactions . Numerous weak noncovalent interactions, such as hydrophobic, van der Waals, hydrogen‐bonding, electrostatic, dipole–dipole, etc., can collectively contribute in the molecular recognition for the hosts towards the guest, dynamically leading to the development of various smart host−guest systems with excellent molecular recognition properties and strong responses towards various external stimuli, such as desired analyte molecules, pH, ionic strength, temperature, light, etc .…”

“…Host‐guest assemblies are formed by combining discrete molecular components in ordered manner involving dynamic noncovalent interactions . Numerous weak noncovalent interactions, such as hydrophobic, van der Waals, hydrogen‐bonding, electrostatic, dipole–dipole, etc., can collectively contribute in the molecular recognition for the hosts towards the guest, dynamically leading to the development of various smart host−guest systems with excellent molecular recognition properties and strong responses towards various external stimuli, such as desired analyte molecules, pH, ionic strength, temperature, light, etc . Studies on stimuli responsive “ON/OFF” optical sensors, involving macrocyclic hosts and fluorogenic guests in combination with suitable external stimuli as the triggers, are currently undergoing great advances, displaying binding and release mechanism through fluorescence indicator displacement methodology …”