Encapsulation of labetalol, pseudoephedrine in ß-cyclodextrin cavity: spectral and molecular modeling studies.

The absorption and fluorescence spectra of labetalol and pseudoephedrine have been studied in different polarities of solvents and ß-cyclodextrin (ß-CD). The inclusion complexation with ß-CD is investigated by UV-visible, steady state and time resolved fluorescence spectra and PM3 method. In protic solvents, the normal emission originates from a locally excited state and the longer wavelength emission is due to intramolecular charge transfer (TICT). Labetalol forms a 1:2 complex and pseudoephedrine forms 1:1 complex with ß-CD. Nanosecond time-resolved studies indicated that both molecules show triexponential decay. Thermodynamic parameters (ΔG, ΔH, ΔS) and HOMO, LUMO orbital investigations confirm the stability of the inclusion complex. The geometry of the most stable complex shows that the aromatic ring is deeply self included inside the ß-CD cavity and intermolecular hydrogen bonds were established between host and guest molecules. This suggests that hydrophobic effect and hydrogen bond play an important role in the inclusion process.

Dual fluorescence of fast blue RR and fast violet B: effects of solvents and cyclodextrin complexation.

Absorption and steady-state and time-resolved fluorescence spectra of fast blue RR (FBRR) and fast violet B (FVB) were studied in solvents with different polarities and in the presence of α- and ß-cyclodextrins (CDs). Dual emission observed in nonpolar solvents suggested that the energy of the intramolecular charge transfer (ICT) state is lower than that of the locally excited state. The normal Stokes-shifted band originated from the locally excited state, and the large Stokes-shifted band was due to the emission from a planar intramolecular charge transfer (PICT) state. The ratio of the PICT emission to the normal emission increased with ß-CD concentration, whereas it was constant upon addition of α-CD. This behavior is in accordance with CD-dependent decay times of PICT and normal emissions, indicating the formation of different 1:1 FBRR/CD inclusion complexes. The rise time for the PICT emission increased with ß-CD concentration, whereas no rise time was observed in the case of the α-CD complex. The size of the dimethoxyaniline ring suggested that the orientation of FBRR in the ß-CD complex was different from that in the α-CD complex. The benzamido moiety of FBRR is deeply encapsulated in the CD cavity, whereas the aniline ring is exposed to the hydrophilic part. Semiempirical quantum-mechanical (ΔE, ΔG, ΔH, ΔS, and HOMO-LUMO) calculations were also carried out to assign the encapsulation of FBRR and FVB.

Azonium-ammonium tautomerism and inclusion complexation of 1-(2,4-diamino phenylazo) naphthalene and 4-aminoazobenzene.

Spectral characteristics of 1-(2,4-diamino phenylazo) naphthalene (FBRR, fat brown RR) and 4-aminoazobenzene (AAB) have been studied in various solvents, varying hydrogen ion concentrations and in ß-cyclodextrin (ß-CD). The inclusion complex of FBRR and AAB with ß-CD were analysed by UV-visible, fluorometry and CAche-DFT methods. Solvent study reveals that only azo tautomer is present in both compounds and the large red shifted absorption and emission maxima of FBRR indicate naphthalene ring effectively increases the π-π* transition. Unusual red shift is observed in acid solutions suggests azonium-ammonium tautomer is present in both molecules. In ß-CD solutions, the large hypsochromic shift is observed in S(0) and S(1) states indicates ortho amino group of FBRR molecule is entrapped in the ß-CD cavity and the large bathochromic shift for AAB in the S(1) state indicates 1:1 inclusion complex is formed.

Unusual spectral shifts of imipramine and carbamazepine drugs.

The absorption and fluorescence spectra of imipramine and carbamazepine have been recorded in solvents of different polarity and ß-cyclodextrin (ß-CD). The inclusion complexes for both drugs are investigated by UV-visible, fluorimetry and DFT. Solvents study shows isotropic polarizability structure is present in imipramine while the amide group inhibits the above structure in carbamazepine. The band width half a maximum of carbamazepine decreased in polar solvents suggest that different species present in non-polar solvents and among that one of this species is affected in protic solvents. Both drugs form two different 1:2 inclusion complexes with ß-CD. The structured longer wavelength emission in ß-CD solution suggests viscosity plays major roles in the inclusion complex. This study also confirms van der Waals forces and hydrophobic interactions are the driving forces in imipramine and hydrogen bonding interactions play major roles in carbamazepine.

Intramolecular proton transfer effects on 2,6-diaminopyridine.

The photophysical behaviour of 2,6-diaminopyridine (DAP) has been studied in solvents of different polarity, pH, beta-cyclodextrin (beta-CD) and compared with 2-amino pyridine (2AP). The inclusion complex of both molecules with beta-CD are analysed by UV-visible, fluorimetry, FT-IR, (1)H NMR, SEM and AM1 methods. The solvent studies shows i) DAP gives more red shifted absorption and emission maxima than 2AP molecule and ii) addition of amino group in 2AP effectively increase the resonance interaction in the pyridine ring. A regular red shift observed in acidic pH solutions suggests intramolecular proton transfer (IPT) present in both molecules. beta-CD studies indicates i) in pH approximately 7, a regular red shifted absorption and emission maxima observed in AP molecules suggests pyridine ring encapsulated in to the beta-CD cavity (1:1 inclusion complex formed) and ii) in pH approximately 1, a blue shifted absorption maxima noticed in 2AP, is due to protonated amino group deeply encapsulated in to the hydrophobic part of the beta-CD cavity.