Self-assembled chromophoric NLO-active structures. Second-harmonic generation and X-ray photoelectron spectroscopic studies of nucleophilic substitution and ion exchange processes on benzyl halide-functionalized surfaces

Stephen B. Roscoe, Shlomo Yitzchaik, Ashok K. Kakkar, Tobin J Marks, Zuyan Xu, Tongguang Zhang, Weiping Lin, George K. Wong

Research output: Contribution to journalArticle

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Abstract

The progress and extent of nucleophilic substitution and ion exchange reactions of self-assembled chromophoric monolayers are studied by X-ray photoelectron (XPS) and second harmonic generation (SHG) spectroscopy. Self-assembled monolayers prepared from 2-[4-(chloromethyl)phenyl]ethyl trichlorosilane (1) on glass substrates are susceptible to nucleophilic substitution of ∼90% of the surface-confined benzylic chloride functionalities with the "hypernucleophile" 4-(dimethylamino)pyridine; however, only ∼60% of the densely packed benzyl chloride groups undergo reaction with the high-β chromophore precursor 4′-[4-[N,N-bis(3-hydroxypropyl)amino]styryl]pyridine (2a). Quaternization of a benzylic monolayer with this molecule yields a monolayer having a bulk second-order NLO response (χ(2)) of 3 x 10-7 esu at λ0= 1064 nm, corresponding to a near-maximum chromophore coverage of ∼2 x 1014 molecules/cm2. The kinetics of this substitution reaction and associated structural modifications are studied in real time by in situ polarized SHG techniques, which reveal non-Langmuirian kinetics and a rapidly increasing chromophore tilt angle with increasing coverage. The quaternization kinetics can be fit to a phenomenological biexponential rate equation with k′1 ≈ 2 x 10-2 L mol-1 s-1 and k′2 ≈ 2 x 10-3 L mol-1 s-1 and to a coverage-dependent activation energy model (EA = E0 + Ebθ), yielding a perturbative energy Eb of 6-8 kJ mol-1. Both models are compatible with increasing repulsive interactions between chromophores at high coverages. The charge-compensating chloride counterions within monolayers having dense chromophore packing can be ion exchanged with iodide, up to a maximum of ∼40% of available chloride ions. The introduction of larger anions (sulfanilate, ethyl orange, eosin B) is observed in less densely packed films; however, the ion exchange process is completely inhibited in monolayers capped with a siloxane overlayer. In all cases, exchange of the chloride leads to significant increases in the second-harmonic generation efficiency, up to 45% on exchange with eosin B. In the case of iodide and sulfanilate substitution for chloride, the increase in the second-order response upon ion exchange is attributable to the incoming anion assuming a position within the monolayer microstructure different from that of the displaced anion.

Original languageEnglish
Pages (from-to)5338-5349
Number of pages12
JournalLangmuir
Volume12
Issue number22
Publication statusPublished - Oct 30 1996

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Chromophores
Harmonic generation
Photoelectrons
chromophores
halides
Monolayers
Ion exchange
harmonic generations
photoelectrons
Substitution reactions
chlorides
substitutes
Eosine I Bluish
Chlorides
X rays
Anions
Negative ions
Iodides
Self assembled monolayers
ions

ASJC Scopus subject areas

  • Colloid and Surface Chemistry
  • Physical and Theoretical Chemistry

Cite this

Self-assembled chromophoric NLO-active structures. Second-harmonic generation and X-ray photoelectron spectroscopic studies of nucleophilic substitution and ion exchange processes on benzyl halide-functionalized surfaces. / Roscoe, Stephen B.; Yitzchaik, Shlomo; Kakkar, Ashok K.; Marks, Tobin J; Xu, Zuyan; Zhang, Tongguang; Lin, Weiping; Wong, George K.

In: Langmuir, Vol. 12, No. 22, 30.10.1996, p. 5338-5349.

Research output: Contribution to journalArticle

Roscoe, Stephen B. ; Yitzchaik, Shlomo ; Kakkar, Ashok K. ; Marks, Tobin J ; Xu, Zuyan ; Zhang, Tongguang ; Lin, Weiping ; Wong, George K. / Self-assembled chromophoric NLO-active structures. Second-harmonic generation and X-ray photoelectron spectroscopic studies of nucleophilic substitution and ion exchange processes on benzyl halide-functionalized surfaces. In: Langmuir. 1996 ; Vol. 12, No. 22. pp. 5338-5349.
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abstract = "The progress and extent of nucleophilic substitution and ion exchange reactions of self-assembled chromophoric monolayers are studied by X-ray photoelectron (XPS) and second harmonic generation (SHG) spectroscopy. Self-assembled monolayers prepared from 2-[4-(chloromethyl)phenyl]ethyl trichlorosilane (1) on glass substrates are susceptible to nucleophilic substitution of ∼90{\%} of the surface-confined benzylic chloride functionalities with the {"}hypernucleophile{"} 4-(dimethylamino)pyridine; however, only ∼60{\%} of the densely packed benzyl chloride groups undergo reaction with the high-β chromophore precursor 4′-[4-[N,N-bis(3-hydroxypropyl)amino]styryl]pyridine (2a). Quaternization of a benzylic monolayer with this molecule yields a monolayer having a bulk second-order NLO response (χ(2)) of 3 x 10-7 esu at λ0= 1064 nm, corresponding to a near-maximum chromophore coverage of ∼2 x 1014 molecules/cm2. The kinetics of this substitution reaction and associated structural modifications are studied in real time by in situ polarized SHG techniques, which reveal non-Langmuirian kinetics and a rapidly increasing chromophore tilt angle with increasing coverage. The quaternization kinetics can be fit to a phenomenological biexponential rate equation with k′1 ≈ 2 x 10-2 L mol-1 s-1 and k′2 ≈ 2 x 10-3 L mol-1 s-1 and to a coverage-dependent activation energy model (EA = E0 + Ebθ), yielding a perturbative energy Eb of 6-8 kJ mol-1. Both models are compatible with increasing repulsive interactions between chromophores at high coverages. The charge-compensating chloride counterions within monolayers having dense chromophore packing can be ion exchanged with iodide, up to a maximum of ∼40{\%} of available chloride ions. The introduction of larger anions (sulfanilate, ethyl orange, eosin B) is observed in less densely packed films; however, the ion exchange process is completely inhibited in monolayers capped with a siloxane overlayer. In all cases, exchange of the chloride leads to significant increases in the second-harmonic generation efficiency, up to 45{\%} on exchange with eosin B. In the case of iodide and sulfanilate substitution for chloride, the increase in the second-order response upon ion exchange is attributable to the incoming anion assuming a position within the monolayer microstructure different from that of the displaced anion.",
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N2 - The progress and extent of nucleophilic substitution and ion exchange reactions of self-assembled chromophoric monolayers are studied by X-ray photoelectron (XPS) and second harmonic generation (SHG) spectroscopy. Self-assembled monolayers prepared from 2-[4-(chloromethyl)phenyl]ethyl trichlorosilane (1) on glass substrates are susceptible to nucleophilic substitution of ∼90% of the surface-confined benzylic chloride functionalities with the "hypernucleophile" 4-(dimethylamino)pyridine; however, only ∼60% of the densely packed benzyl chloride groups undergo reaction with the high-β chromophore precursor 4′-[4-[N,N-bis(3-hydroxypropyl)amino]styryl]pyridine (2a). Quaternization of a benzylic monolayer with this molecule yields a monolayer having a bulk second-order NLO response (χ(2)) of 3 x 10-7 esu at λ0= 1064 nm, corresponding to a near-maximum chromophore coverage of ∼2 x 1014 molecules/cm2. The kinetics of this substitution reaction and associated structural modifications are studied in real time by in situ polarized SHG techniques, which reveal non-Langmuirian kinetics and a rapidly increasing chromophore tilt angle with increasing coverage. The quaternization kinetics can be fit to a phenomenological biexponential rate equation with k′1 ≈ 2 x 10-2 L mol-1 s-1 and k′2 ≈ 2 x 10-3 L mol-1 s-1 and to a coverage-dependent activation energy model (EA = E0 + Ebθ), yielding a perturbative energy Eb of 6-8 kJ mol-1. Both models are compatible with increasing repulsive interactions between chromophores at high coverages. The charge-compensating chloride counterions within monolayers having dense chromophore packing can be ion exchanged with iodide, up to a maximum of ∼40% of available chloride ions. The introduction of larger anions (sulfanilate, ethyl orange, eosin B) is observed in less densely packed films; however, the ion exchange process is completely inhibited in monolayers capped with a siloxane overlayer. In all cases, exchange of the chloride leads to significant increases in the second-harmonic generation efficiency, up to 45% on exchange with eosin B. In the case of iodide and sulfanilate substitution for chloride, the increase in the second-order response upon ion exchange is attributable to the incoming anion assuming a position within the monolayer microstructure different from that of the displaced anion.

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