The Raman spectrum of the lowest electronic excited state of R~( b p y ) ,~+ (bpy = 2,2'-bipyridine) shows features that have been attributed to the reduced character of 2,2'-bipyridine in the excited species.'S2 Significantly, near coincidence in the frequency of Raman bands for the excited complex and the ground state 2,2'-bipyridine radical anion is observed,'s2 suggesting that the excited state, which is associated with a metal-to-ligand chargetransfer transition in ab~orption,~ can be viewed as having the (1) (a) Dallinger, R. F.; Woodruff, W. H. J. Am. Chem. SOC. 1979, 101, 4391. (b) Bradley, P. G.; Kress, N.; Hornberger, B. A,; Dallinger, R. F.; Woodruff, W. H. Ibid. 1981, 103, 7441 and references cited therein. (2) Forster, M.; Hester, R. E. Chem. Phys. Lett. 1981, 81, 42. for grant support to M.S.W. and for support of the MIT Regional Laser Facility. Synthesis of fac-XRe(CO),(bpy) by David P. Summers is gratefully acknowledged. Registry No.fac-C1Re(CO)3(bpy), 55658-96-3;fac-BrRe(CO),(bpy), 40231-87-6; Ru(bpy),,+, 15158-62-0. (8) The near-UV absorption spectrum of excited Ru(bpy)32+ closely resembles that of the radical anion of 2,2'-bipyridine; see: Creutz, C.; Chou, M.; Netzel, T. L.; Okumura, M.; Sutin, N.

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Photosensitive films composed of dye, initiator, acrylic monomers, and polymeric film-forming binder, and their use in recording volume phase transmission and reflection holograms are described. Systematic variation of monomer-binder combinations reveals that the maximum attainable index modulation (hologram efficiency) increases with increasing difference between the refractive indices of monomer and binder. Addition of plasticizer is also useful for increasing index modulation. Thermal and wet chemical processing methods for altering hologram properties are described.

New families of transmission and reflection holographic photopolymer materials and their performance are described. The materials are composed of polymeric binders, monomers, initiation system, and sensitizing dyes. The physical and holographic properties may be controlled by choice of components. Photopolymerization and diffusion of monomers is the proposed mechanism for recording of refractive index modulation. The materials are sensitized from the Uv to the red with typical exposure energies form 10 to 100 mJ/cm2. Hologram recording consists of exposure, UV cure, and heat processing. Significant refractive index modulation occurs during exposure. Processed holograms are insensitive to humidity and temperature. Holographic properties and performance of various formulations are discussed, as well as applications of these materials.