Abstract
The fluorescence characteristics of two esterified (RTME and R123) and two unesterified (R110 and RB) rhodamines were determined in water as a function of pH, and their partitioning between water (W) and an organic solvent phase (OS) was determined. Only esterified dyes are potential anticancer agents capable of interacting with biological receptors. Since they may do this as part of the ionization and hydrogen-bonding tautomeric properties of their molecular forms in solution, their fluorescence spectra were reloved, and then compared with those of the unesterified dyes. RTME and R123 exist in water as the esterified fluorescent form Re until Re is transformed via the zwitterionic R ± species into a non-fluorescent photoproduct RX in alkaline pH. From fluorometric titrations we respectively find p
K
a(S
1) = 11.7 (RTME) 11.5 (R123) for the
▪ equilibria. R110 and RB exist in water as cationic R + H, zwitterionic R± and lactonic RO molecular structures, both R + H and R ± being fluorescent; all these species are in tenuous equilibria R + H
▪ RO depending of pH. We find respectively p
K
b(S
1)=3.5 (R110), 3.0 (RB) and p
K
c(S
1) = 11.4 (R110), 11.8 (RB). Corresponding p
K
1(S
0) values calculated from the Forster cycle are only slightly smaller than their related p
K
i(S
1) values. Typical bleaching of R110 and RB occurs when aprotic and proton-attracting solvents (PAS) transform the R + H or R ± species into the RO inner lactone in a process mostly controlled by the basicity of the solvent. The solute-solvent interactions change considerably upon excitation so that RO and (R + H, R ±) are the predominat forms in the S
0 and S
1 states respectively. The dyes are more and more partitioned into the OS phase of an OS-W mixture as the size of their
N-alkyl substitutes is increased. Two biphasic (1:1) systems were studied with OS being
n-octanol, then cyclohexane. The largest dye RB is more trapped in the OS phases than the other dyes whatever the pH of the W phase is. When the W phase is at neutral pH, RB is almost totally trapped in octanol and its extraction percentage
R
RB=0.97 is twice that of the other dyes. The partition coefficients measured for R110, RB, RTME, R123 in octanol are respectively
P=0.8, 30, 1.2, 1.2; RB is again more partitioned in cyclohexane (
E
RB=0.85) than R110 (
E
R110=0.07) when esterified dyes are insoluble.
