ABSTRACT
Near-infrared organic compounds All truths are easy to understand once they are discovered.
Galileo Galilei (1564-1642)
Organic compounds that absorb or emit in the near-infrared (NIR) spectral region typically contain a conjugated π-system as a chromophore, in which the π-electron should be effectively delocalized within the entire chromophoric entity. In this chapter, organic compounds containing the rylene, polymethine, and meropolymethine, donor-acceptor chromophores, and metal complex chromophores are discussed. In addition, a new class of selective NIR-absorbing colorless chromophores is presented.
The simplest and most representative of π-conjugated polyene system is trans-polyacetylene. However, due to its bond fixation, its absorption wavelength is converged to a certain value; about 600 nm even with a longer chain length [1]. Cyclic π-conjugated polyene is another example. The absorption maxima of cyclic polyene and cyclic polyene-yne can extend into the NIR region only when its conformation is rigid and locked [2], although its long wavelength absorbance is quite weak (log ε ≈ 3). Structurally, benzene can be considered to be a cyclic polyene and then acenes are also chromophores that are structurally related to the polyene as they are derived by means of annulations of benzene. Owing to more effective π-electron delocalization, the low-energy absorption peaks are red shifted as the number of annulated benzene rings increases. However, higher homologous acenes show a limited bathochromic shift (e.g., pentacence, λabsmax = 700 nm) due to their insufficient stability. The annulated arenes derived formally by connecting all the peri-positions of naphthalene have been designated by Clar as rylenes or oligo(peri-naphthalene)s [3]. Rylenes are another form of the extended polyene system. Going from perylene to pentarylene, this series of soluble rylenes exhibits a gradual bathochromic shift toward the NIR region [4]. A penta(peri-naphthalene) or pentarylene derivative absorbs at 750 nm, implying that further
