DYNAMICS WITHIN A SINGLE MOLECULAR LAYER - AGGREGATION, RELAXATION, AND THE ABSENCE OF MOTION

We report on the transient and steady-state optical responses of the c hromophore 2,2'-bithiophene-5,5'-diylbis(phosphonic acid) (BDP) incorp orated within a single zirconium-phosphonate layer as a function of ch romophore density. While the dilute solution optical response of BDP r eveals no anomalous behavior, its characteristics are substantially di fferent when confined within a monolayer. We vary the concentrations o f layer constituents to determine the extent of interaction between BD P moieties within a single monolayer. We observe limited initial aggre gation of BDP, the extent of which is determined largely by the condit ions under which the monolayer is formed. Over time, the fractional co ntribution of BDP aggregates to the total optical response decreases t o a limiting value, implicating surface adsorption site density as the dominant factor in determining the morphology of the organobis(phosph onate) layer. Motional relaxation measurements of BDP within the layer show that the chromophores are immobile on the hundreds-of-picosecond s time scale of our experiments.