Spectroscopic Studies of the Interaction of Tert-Butylamine and n-Propylamine with the O-Cyclodextrin : Pyrene Complex A. YVETTE WILL,* ARSENIO MUNOZ DE LA PE~A,t THILIVHALI T. NDOU,:~ and ISIAH M. WARNER*§ Department of Chemistry, Emory University, Atlanta, Georgia 30322 The influence of amines on the aqueous beta-cyclodextrin (/~-CD):pyrene complex with the use of steady-state fluorescence measurements is ex- amined. A 2:1 B-CD:pyrene complex is reported in the presence of either tert-butylamine (TBA) or n-propylamine (PA). At low amine concentra- tions, the role of the amine is to shield pyrene from the bulk aqueous phase. However, at higher TBA concentrations, there exists a compet- itive equilibrium between the amine and pyrene for the ~-CD cavity. The formation constants for the various complexes in the presence of either amine were estimated to be of comparable order of magnitude. Index Headings: Pyrene; Cyclodextrins; Amines; Fluorescence; Nuclear magnetic resonance. INTRODUCTION Cyclodextrins (CDs) are cyclic oligosaccharides com- posed of glucopyranose units linked together by oxygen bridges at the 1 and 4 positions. This class of organized media possesses a hydrophilic exterior and a hydropho- bic cavity which enables the CD to extract a variety of organic guest molecules of appropriate size and hydro- phobicity from the bulk aqueous solution. 1-7This process has been documented to occur by hydrophobic interac- tions, hydrogen bonding, and van der Waals forces. 8 The phenomena of inclusion of various substrates within CD cavities have been investigated by the use of fluorescence spectroscopy, 9-12 absorption techniques, 13,~4 circular dichroism, t5,~6 and nuclear magnetic resonance (NMR) spectroscopy. 17 A number of these studies have involved the use of polycyclic aromatic hydrocarbons as guest molecules?s-22 Pyrene is a suitable fluorescence probe because of its vibronic fine structure and sensitiv- ity to microenvironmental changes. It has a long fluo- rescence lifetime and exhibits several vibronic bands over an emission wavelength region of 370 to 400 nm. The ratio of the intensity of the first band to that of the third band (I/III) has been used to assess the polarity of the microenvironment of pyrene. 23 It has been shown that a decrease in the I/III ratio usually suggests a decrease in polarity of the microenvironment around pyrene. In our laboratory, we have reported on the complex- ation processes for the CD:pyrene systems in aqueous media. 24 Stoichiometries of 1:1 and 2:1 for the ~-CD: Received 26 October 1992. * Present address: Department of Chemistry, Louisiana State Univer- sity, Baton Rouge, LA 70803. t Present address: Department of Analytical Chemistry, University of Extremadura, Badajoz, Spain. $ Present address: Gillette Research Institute, Gaithersburg, MD 20879. § Author to whom correspondence should be sent. pyrene and the ~-CD:pyrene complexes, respectively, were shown to be formed. This result is in agreement with the work of several researchersY ,26This work has also shown that no complex is formed between a-CD and pyrene, due primarily to size incompatibility. The addition of third components, such as alcohols, to the ~-CD:pyrene and/~-CD:pyrene binary complexes produces higher apparent formation constants. 27-3°Other studies by Kano et al. 31,32 proposed the formation of three- component complexes between ~-CD, pyrene, and small aliphatic amines, such as diethylamine (DEA). They also suggested that both ~-CD and fl-CD accelerated the flu- orescence quenching of pyrene by smaller amines, such as dimethylamine (DMA) and trimethylamine (TMA). In the latter study by Kano et al., 32 it was proposed that pyrene binds to the primary hydroxyl rim of the fl-CD cavity, forming a pyrene-capped CD complex with TMA or DMA occupying the residual void space inside the CD cavity. This is possible since the pyrene molecule is too large to fit entirely inside the fl-CD cavity. However, use of molecular modeling has indicated that pyrene is more likely to enter the larger side of/3-CD. 24 Kano et al. suggested that static quenching occurred in the systems described above, 31,32as shown by the Stern-Volmer plots for fluorescence intensities and lifetime data. These au- thors also reported that the effects of the CD on the fluorescence quenching of pyrene are dependent on the structure and geometry of the quencher molecule. For example, a TMA molecule is able to rotate more freely inside the ~-CD cavity than is a DEA molecule. Conse- quently, the quenching of pyrene by TMA is more effi- cient than that observed with the use of DEA. The work presented in this paper compares the effects of n-propylamine (PA) and tert-butylamine (TBA) on the fluorescence emission of the ~-CD:pyrene complex. Variations of the I/III ratio with varying ~-CD concen- tration in the presence of the amine were used to estimate the equilibrium constants. The stoichiometry between the amine and ~-CD for the ternary complex is also ex- amined. EXPERIMENTAL Apparatus. Steady-state fluorescence measurements were acquired with a SPEX-Fluorolog Model F2T21I spectrofluorometer equipped with a thermostated cell housing. The solutions were excited at 335 nm, and ex- citation and emission slit widths of 8.6 and 1.7 nm, re- spectively, were employed. All measurements were per- formed at 21.0 ± 0.1°C. Volume 47, Number 3, 1993 0003-7028/93/4703-027752.00/0 APPLIED SPECTROSCOPY 277 © 1993 Society for Applied Spectroscopy