Modulation in the acidity constant of acridine dye with cucurbiturils: stimuli-responsive pKa tuning and dye relocation into live cells.

The noncovalent host-guest interactions of the cationic (AcH+) and neutral (Ac) forms of an acridine dye with macrocyclic hosts such as cucurbit[7]uril (CB7) and cucurbit[8]uril (CB8) have been investigated to evaluate the effect of cavity size on the photophysical properties and the protolytic equilibrium of the acridine dye. The cationic form undergoes significant complexation with CB7 (Keq = 106 M-1), causing a sharp decrease in the fluorescence intensity, whereas the neutral Ac form of the dye undergoes weak complexation with CB7 (Keq = 103 M-1) and the binding constant is lowered by three orders of magnitude compared to that of the CB7-AcH+ system. The Job plot revealed that both forms form a 1 : 1 complex with CB7. On the other hand, the AcH+ form shows strong emission quenching on interaction with CB8 and the formation of the 1 : 2 CB8 : AcH+ complex has been confirmed from the Job plot. The strong affinity of CB7 and CB8 to the protonated form resulted in a large upward pKa shift (ΔpKa ∼ 3.4 units for CB7 and ∼1.3 units for CB8) in the dye. Taking advantage of the above modulations in the fluorescence and pKa values, adamantylamine-induced fluorescence regeneration, controlled pKa tuning and dye relocation from the CB7 cavity to cell lines have been established for the first time, which find potential applications in fluorescence off-on sensing and drug delivery.

Metal ion-induced supramolecular pKa tuning and fluorescence regeneration of a p-sulfonatocalixarene encapsulated neutral red dye.

The host-guest interactions and the consequent modulation in the prototropic equilibrium of a phenazine dye, neutral red, with p-sulfonatocalix[4]arene (SCX4) and p-sulfonatocalix[6]arene (SCX6) macrocyclic hosts have been investigated. Both the neutral (NR) and cationic (NRH+) forms of dyes formed inclusion complexes with SCX6, with a larger binding constant for the latter (K = 8.6 × 105 M-1versus 4.8 × 103 M-1) due to the cation receptor behavior of the calixarenes. The distinct differences in the binding constant of NR and NRH+ provided a finite tuning of pKa between 6.5 and 8.8, through a competitive binding with metal ions. Importantly, the fluorescence quenching observed in the SCX-neutral red interactions stands in contrast to the fluorescence enhancement observed with other macrocyclic hosts, such as ß-cyclodextrin and cucurbit[7]uril. This is due to the unique portal stacking interaction of NRH+ with the SCXs, compared to the axial inclusion geometry documented for the other macrocycles. The electron transfer from the SCX to the neutral red dye is adjudged to be the effective excited-state relaxation pathway leading to fluorescence quenching. In combination with the metal-ion induced fluorescence regeneration and tuning the pKa value, the SCX-neutral red system finds potential applications in drug delivery, photodynamic therapy, catalysis, and sensor applications.

Contrasting solvent polarity effect on the photophysical properties of two newly synthesized aminostyryl dyes in the lower and in the higher solvent polarity regions.

Solvent polarity effect on the photophysical properties of two newly synthesized aminostyryl-thiazoloquinoxaline dyes, one with a flexible diphenylamino group, namely, N,N-diphenyl-4-[2-(thiazolo[4,5-b]quinoxalin-2-yl)vinyl]aniline (TQ1), and the other with a rigid julolidinylamino group, namely, (9-[2-(thiazolo[4,5-b]quinoxalin-2-yl)vinyl]julolidine) (TQ2), have been investigated in different aprotic solvents and solvent mixtures. From the polarity dependent changes in the absorption and fluorescence spectral properties, it is indicated that the fluorescent states of the dyes are of intramolecular charge transfer (ICT) character. For both the dyes, the photophysical properties like fluorescence quantum yields (Phi(f)), fluorescence lifetimes (tau(f)), radiative rate constants (k(f) = Phi(f)/tau(f)), and nonradiative rate constants (k(nr) = 1/tau(f) - Phi(f)/tau(f)) show clearly contrasting solvent polarity effects in the lower and in the higher solvent polarity region, causing an interesting reversal in the properties below and above an intermediate solvent polarity. It is inferred that the domination of the cis-trans isomerization in the lower solvent polarity region and that of the twisted intramolecular charge transfer (TICT) state formation in the higher solvent polarity region are responsible for the observed contrasting solvent polarity effects on the photophysical properties of the two dyes. As both isomerization and TICT state formation causes an enhancement in the nonradiative decay rate of the excited dyes and both the processes become less significant at the intermediate solvent polarity region, the two dyes show their largest Phi(f) and tau(f) values at intermediate solvent polarities. Suitable mechanistic schemes have been proposed and qualitative potential energy diagrams have been presented to explain the observed results with the changes in the polarity of the solvents used.

Effect of donor orientation on ultrafast intermolecular electron transfer in coumarin-amine systems.

Effect of donor amine orientation on nondiffusive ultrafast intermolecular electron transfer (ET) reactions in coumarin-amine systems has been investigated using femtosecond fluorescence upconversion measurements. Intermolecular ET from different aromatic and aliphatic amines used as donor solvents to the excited coumarin-151 (C151) acceptor occurs with ultrafast rates such that the shortest fluorescence lifetime component (tau(1)) is the measure of the fastest ET rate (tau(1)=tau(ET) (fast)=(k(ET) (fast))(-1)), assigned to the C151-amine contact pairs in which amine donors are properly oriented with respect to C151 to maximize the acceptor-donor electronic coupling (V(el)). It is interestingly observed that as the amine solvents are diluted by suitable diluents (either keeping solvent dielectric constant similar or with increasing dielectric constant), the tau(1) remains almost in the similar range as long as the amine dilution does not cross a certain critical limit, which in terms of the amine mole fraction (x(A)) is found to be approximately 0.4 for aromatic amines and approximately 0.8 for aliphatic amines. Beyond these dilutions in the two respective cases of the amine systems, the tau(1) values are seen to increase very sharply. The large difference in the critical x(A) values involving aromatic and aliphatic amine donors has been rationalized in terms of the largely different orientational restrictions for the ET reactions as imposed by the aliphatic (n-type) and aromatic (pi-type) nature of the amine donors [A. K. Satpati et al., J. Mol. Struct. 878, 84 (2008)]. Since the highest occupied molecular orbital (HOMO) of the n-type aliphatic amines is mostly centralized at the amino nitrogen, only some specific orientations of these amines with respect to the close-contact acceptor dye [also of pi-character; A. K. Satpati et al., J. Mol. Struct. 878, 84 (2008) and E. W. Castner et al., J. Phys. Chem. A 104, 2869 (2000)] can give suitable V(el) and thus ultrafast ET reaction. In contrary, the HOMO of the pi-type aromatic amines is largely distributed throughout the whole molecule and thus most of the orientations of these amines can give significant V(el) for ultrafast ET reactions with close-contact C151 dyes.

Host-guest complexation of neutral red with macrocyclic host molecules: contrasting pK(a) shifts and binding affinities for cucurbit[7]uril and beta-cyclodextrin.

The photophysical properties of the phenazine-based dye neutral red were investigated in aqueous solution in the presence of the macrocyclic host molecule cucurbit[7]uril (CB7) using ground-state absorption as well as steady-state and time-resolved fluorescence measurements. The results are contrasted to those previously obtained for beta-cyclodextrin (beta-CD; Singh et al. J. Phys. Chem. A 2004, 108, 1465). Both the neutral (NR) and cationic (NRH+) forms of the dye formed inclusion complexes with CB7, with the larger binding constant for the latter (K = 6.5 x 10(3) M(-1) versus 6.0 x 10(5) M(-1)). This result differed from that for beta-CD, where only the neutral form of the dye was reported to undergo sizable inclusion complex formation. From the difference in binding constants and the pK(a) value of protonated neutral red in the absence of CB7 (6.8), an increased pK(a) value of the dye when complexed by CB7 was projected (approximately 8.8). This shift differed again from the behavior of the dye with beta-CD, where a decreased pK(a) value (ca. 6.1) was reported. The photophysical properties of both NR and NRH+ forms showed significant changes in the presence of CB7. Fluorescence anisotropy studies indicated that the inclusion complexes of both forms of the dye rotate as a whole, giving rotational relaxation times much larger than that expected for the free dye in aqueous solution. The thermodynamic parameters for the NRH+.CB7 complex were investigated in temperature-dependent binding studies, suggesting an entropic driving force for complexation related to desolvation of the cation and the removal of high-energy water molecules from the CB7 cavity.