Photochemistry Channels of Merocyanine Encapsulated in Sol-Gel Glasses
- PDF / 401,252 Bytes
- 6 Pages / 414.72 x 648 pts Page_size
- 88 Downloads / 165 Views
A. M. MAHLOUDJI, L. B. MEURET, JR. AND C. T. LIN Department of Chemistry, Northern Illinois University, DeKalb, IL 60115-2862
ABSTRACT Photophysical properties of 1-Docosyl-4-(4-hydroxystyryl)pyridinium bromide (SB), a merocyanine dye in solution and encapsulated in sol-gel derived glass are investigated at 298 and 77 K. In solution, the absorption spectra of SB display an equilibrium between the quinolinium and benzoid forms. The equilibrium can be shifted to either quinolinium or benzoid form under an acidic or basic condition, respectively. The emission spectra of SB, on the other hand, give not only the quinolinium and benzoid forms but also the quinoid form which emits at 500 nm. The existence of excited state quinoid form of SB is also evident in the excitation spectrum while the emission at 500 nm is monitored. Both in solution and in xerogel, the quinoid form of SB is shown to be photochemically unstable as compared to the benzoid form. It is proposed that the photoexcited quinolinium form of SB is a proton dissociative species which transforms readily to become the quinoid form. The results indicate that photochemistry channels of SB are originated from the quinoid form. Moreover, the benzoid form of SB (photochemically stable) exhibits large hyperpolarizability due to its chargetransfer characteristic, and is a desired molecular form for nonlinear optical (NLO) applications. The material processing techniques for stabilizing the benzoid form of SB in optically transparent sol-gel glasses are illustrated for the first time.
INTRODUCTION
Rapid strides have been made1 in the design of new organic or organometallic materials (e.g., monomers, single crystals, thin films, polymers, counterion salts, guest-host composites and sol-gel glasses) having the unique nonlinear optical (NLO) properties. It is believed 2 that the structure-property relationships are essential for a better understanding of how to synthesize and process new compounds with enhanced second-order P3 and X(2), and third-order y and X(3) hyperpolarizabilities. The relationship of P to structural parameters depends on the presence of low-lying charge-transfer (CT) excited states possessing a large oscillator strength and a large change in dipole moment between ground and excited states. This has usually been accomplished 3. 4 by having strong electron-donating (D) substituents coupled to strong electronaccepting (A) groups via an extended conjugated x-electron system, i.e., D-+-(CH=CH)n+--A (referred to as D,A-DDP). It is imperative that the NLO molecules reside in a noncentrosymmetric environment if the molecular hyperpolarizability P3 is to lead to an observable bulk effect X(2). During the past few years, the temporal stability of poled polymer films with large X(2) response has been improved to the point of practical electro-optic device application. 5 The third-order y and X(1) materials do not require a noncentrosymmetric alignment. The second hyperpolarizability y is generally related to the extent of electron delocalization in molecul
Data Loading...
