SELF-ASSEMBLED CHROMOPHORIC NLO-ACTIVE STRUCTURES - 2ND-HARMONIC GENERATION AND X-RAY PHOTOELECTRON SPECTROSCOPIC STUDIES OF NUCLEOPHILIC-SUBSTITUTION AND ION-EXCHANGE PROCESSES ON BENZYL HALIDE-FUNCTIONALIZEDSURFACES

The progress and extent of nucleophilic substitution and ion exchange reactions of self-assembled chromophoric monolayers are studied by X-r ay photoelectron (XPS) and second harmonic generation (SHG) spectrosco py. Self-assembled monolayers prepared from 2-[4-(chloromethyl)phenyl] ethyl trichlorosilane (1) on glass substrates are susceptible to nucle ophilic substitution of similar to 90% of the surface-confined benzyli c chloride functionalities with the ''hypernucleophile'' 4-(dimethylam ino)pyridine; however, only similar to 60% of the densely packed benzy l chloride groups undergo reaction with the high-beta chromophore prec ursor -[4-[N,N-bis(3-hydroxypropyl)amino]styryl]pyridine (2a). Quatern ization of a benzylic monolayer with this molecule yields a monolayer having a bulk second-order NLO response (chi((2))) of 3 x 10(-7) esu a t lambda(0) = 1064 nm, corresponding to a near-maximum chromophore cov erage of similar to 2 x 10(14) molecules/cm(2). The kinetics of this s ubstitution reaction and associated structural modifications are studi ed in real time by in situ polarized SHG techniques, which reveal non- Langmuirian kinetics and a rapidly increasing chromophore tilt angle w ith increasing coverage. The quaternization kinetics can be fit to a p henomenological biexponential rate equation with k'(1) approximate to 2 x 10(-2) L mol(-1) s(-1) and k'(2) approximate to 2 x 10(-3) L mol(- 1) s(-1) and to a coverage-dependent activation energy needed (E(A) = E(0) + E(b) theta), yielding a perturbative energy E(b) of 6-8 kJ mol( -1). Both models are compatible with increasing repulsive interactions between chromophores at high coverages. The charge-compensating chlor ide counterions within monolayers having dense chromophore packing can be ion exchanged with iodide, up to a maximum of similar to 40% of av ailable chloride ions. The introduction of larger anions (sulfanilate, ethyl orange, eosin B) is observed in less densely packed films; howe ver, the ion exchange process is completely inhibited in monolayers ca pped with a siloxane overlayer. In all cases, exchange of the chloride leads to significant increases in the second-harmonic generation effi ciency, up to 45% on exchange with eosin B. In the case of iodide and sulfanilate substitution for chloride, the increase in the second-orde r response upon ion exchange is attributable to the incoming anion ass uming a position within the monolayer microstructure different from th at of the displaced anion.