MOLECULAR ASSOCIATION OF PENTANOLS IN N-HEPTANE-IV - A PHOTOCHROMIC REACTION PROBE

A photochromic reaction of a fulgide, Aberchrome 540 dye has been used to probe the molecular association of 1-,2-,and t-, pentanol in n-hep tane. The kinetics of the photobleaching of the red dye (C-form) to th e amber product (E-form) has been investigated spectrophotometrically over a low concentration range of < 0.3 mole fraction (F), at the temp erature span of 303 to 323 K. Analyses of the results show that the ph otobleaching of the red dye follows a pseudo-first-order reaction, all owing for the extraction of the apparent rate constants (k) and the ap parent activation energies (E(a)) Factors affecting the reaction are d iscussed. These factors are the solvent polarity effect, the solvent v iscosity effect and the hydrogen bonding effect, deriving from the int ermolecular hydrogen bond formation between the dye and the solvent mo lecules. The latter effect has been partially supported by the IR stud y of the dye in a number of solvents. Plots of E(a) against concentrat ion (F) show extreme value for each pentanol. This result has been use d to show the plausible presence of dominant multimers in the solution s. It appears that linear multimers are important for the 1-pentanol a nd 2-pentanol solutions. For the case of t-pentanol, E(a) Of the react ion reaches a maximum at F=0.05. This is the same concentration at whi ch the activation energy of viscous now, E(v) was found to be maximum in the low concentration range. An explanation of the observed maximum E(a) is offered and it has led to the suggestion that the cyclic mult imers may be the dominant species. Such a view is consistent with prev ious results from this laboratory i.e., the Kerr effect, viscometric a nd the NMR studies.