Journal of Fluorescence, Vol. 10, No. 4, 2000 On the Intramolecular Excimer Formation of 1,10-Bis(1- Pyrene)Decane in Organized Media Marilena Vasilescu, 1,3 Mats Almgren, 2 and Daniel Angelescu 1 Received September 28, 1999; revised March 10, 2000; accepted March 28, 2000 The formation of an intramolecular excimer of 1,10-bis(1-pyrene)decane in sodium dodecyl sulfate (SDS)/bovine serum albumin (BSA) and SDS/poly(ethylene oxide) (PEO) solutions was studied by steady-state and time-resolved fluorescence methods to determine the effect of viscosity. The viscosity-dependent ratio between intramolecular excimer and monomer fluorescence intensities of 1,10-bis(1-pyrene)decane was obtained by steady-state fluorescence measurements. The results show that in micelle-like clusters formed in SDS/BSA and SDS/PEO systems, the microviscosity is higher than in free micelles. It was found that Birks’ kinetic model was not valid in this case, and the monomer and excimer fluorescence decays had to be fitted by a sum of three exponentials. The excimer formation kinetics in constrained systems is more complex than in homogeneous solutions, but it is possible to find an empirical parameter depending on viscosity. KEY WORDS: Intramolecular excimer; microviscosity; fluorescent probe method; micelles; 1,10-bis(1- pyrene)decane. INTRODUCTION other types of measurements is not consistent with the results. As expected, fluorescence polarization, ESR, and NMR [4] result in different values for the viscosity, It is well known that the derivatives of di[1-pyren- because of the micelle intrinsic anisotropy, distinct inter- yl]alkanes, Py(CH) 2 ) n Py, may form intramolecular exci- face properties, and different probe solubilization regions. mers whose fluorescence is sensitive to the viscosity of Therefore, the method of intramolecular excimer emis- the medium [1,2]. The I E /I M ratio (I M = the intensity of sion can be applied to micellar solutions only for qualita- monomer fluorescence, I E = the intensity of excimer tive information regarding the variation of microviscosity fluorescence) is used as a viscosity measure, the value due to modification of some environmental conditions. of which is calibrated using “calibration solutions” (mix- Steady-state measurements of monomer and excimer tures of glycerin–ethanol or hexadecane–liquid paraffin). fluorescence offer the advantage of simplicity; however, Application of this method for determination of the one has to take into account that some factors may have “microviscosity” of micelles showed that the results var- a direct bearing on the results. If there is a restricted ied as a function not only of the values of n (e.g., Ref. solubilization of the probe, a fraction of the probe may 6 or 9), but also of the different sets of calibration solu- tions used [3]. The use of other fluorescent probes or of remain in the solvent under the form of microcrystals, while in micelles of cationic surfactants with quaternary ammonium head groups, the aromatic probes are easily 1 Institute of Physical Chemistry, Splaiul Independentei 202, Bucharest soluble due to the affinity for the head group [5,6], and 77208, Romania. in anionic surfactants micelles they have a longer solubili- 2 Uppsala University, Department of Physical Chemistry, Uppsala, zation time [7]. The freezing (at liquid nitrogen tempera- Sweden. 3 To whom correspondence should be addressed. ture) and thawing (at 23°C) of a SDS micellar solution 339 1053-0509/00/1200-0339$18.00/0  2000 Plenum Publishing Corporation