J. Phys. Chem. B 2007, 111, 95-106
95
Properties of Confined and Sheared Rhodamine B Films Studied by SFA-FECO Spectroscopy Anna R. Godfrey Alig, Delphine Gourdon, and Jacob Israelachvili* Department of Chemical Engineering, UniVersity of California, Santa Barbara, California 93106 ReceiVed: February 14, 2005; In Final Form: October 24, 2006
We have used a surface forces apparatus and multiple beam interferometry to measure the absorbance of thin films of rhodamine B in water/ethylene glycol solutions while applying and measuring normal and lateral (shear) forces. Both normal and shear forces induced changes in the absorption spectra indicating a change in molecular alignment, and rhodamine-rhodamine and rhodamine-surface interactions. We also measured differences in the absorbance spectra in different regions of the contact indicating, as expected, that the stresses are not uniform throughout the contact area. We also observed crystallization (solidification) parallel to the shearing direction.
I. Introduction A. Optical Properties of Rhodamine B in Aqueous Solutions. Rhodamine B is commonly used in tunable dye lasers as well as a label in fluorescent measurements. It is well-known that it has different molecular forms which depend on the pH and concentration of the solution,1-12 and that it readily forms dimers in aqueous solution.1,3,5,13-23 Shown in Figure 1 are the most prevalent monomer and dimer forms. It is generally agreed that in a neutral solution both the cationic and zwitterionic forms of the dye are present and that both forms dimerize at high concentrations.5,7,8,10-12,16,24-26 The absorbance spectra of the cationic and zwitterionic monomers are very similar at neutral pH and can be distinguished by adding a small amount of acid or base to the solution. As shown in Figure 1a,b, the rhodamine B molecule has a planar xanthene base with a carboxyphenyl group that is perpendicular to the base. The transition moment of the dye is parallel to the long axis of the xanthene molecule as shown by the dashed lines in Figure 1a,b. The phenyl group is able to rotate around the phenyl-xanthene bond, and the carboxyl group can rotate around the C-C bond. The surface area of the dye as determined from surface coverage experiments is about 150160 Å,2,27-30 and the apparent molecular radius of the dye is 6-8 Å.31-34 The dimers that form are either H-dimers, which have a sandwich-type head-to-head structure as shown in Figure 1c, or J-dimers, which are known to have an oblique head-totail structure (Figure 1d), or tilted parallel structures with a tilt angle less than 54.7° (Figure 1e).10,35-39 It is not completely understood what interactions occur between the monomer groups to form the dimers, but it is believed that in the cationic H-dimers the carboxy group of one monomer interacts with the oxygen of the xanthene in the other monomer.3,40,41 According to simple exciton theory,17,18,42 the sandwich-type H-dimer structure causes a blue shift in the absorbance spectrum. A theoretically possible “linear” structure causes a red shift.43 The oblique and tilted J-dimers cause a bimodal splitting in the absorbance spectrum with the lower energy peak being more intense.5,17-19,35-39,42,44,45 If there is a twist angle between the * To whom correspondence should be addressed. E-mail: jacob@ engineering.ucsb.edu.
Figure 1. Different monomer and dimer forms of the rhodamine B dye molecule. The transition moment of the dye is indicated by the dashed line in parts a and b, and the rotational bonds are indicated with curved arrows. (a) Cationic monomer form. (b) Zwitterionic monomer form. (c) Schematic of the head-to-head H-dimer (sandwich) configuration constructed from two cationic monomer units. (d) Schematic of the head-to-tail oblique J-dimer configuration constructed from two cationic monomer units. (e) Schematic of the tilted J-dimer configuration constructed from two cationic monomer units with a tilt angle
