Chemical. CAS 134339-08-5. Formula C35H45F6N2P. MW 638.72. Cyanine dye. Spectral Data lambdaex=747nm lambdaem=774nm. Cyanine dyes were first synthesized over a century ago, used to sensitize photographic plates and comprise a diverse family of fluorophores. With their narrow absorption and fluorescence spectral profiles, together with high oscillator strengths for the lowest-energy absorption transition, these dyes are ideal candidates for fluorescence standards for the far-red region.