Selective NIR chromophores: bis(pyrrolopyrrole) cyanines.

New applications have recently led to an increased interest in near infrared (NIR) dyes. Both, strong NIR absorption and fluorescence emission are important phenomena. While fluorescence is exploited especially for labelling purposes in microscopy, NIR absorption has a broad range of applications mainly in material science. Examples of growing fields of interest are NIR light emitting diodes and the use of NIR-photosensitizers in dye-sensitized solar cells. In the latter cases, the aim is to utilize the NIR part of the solar spectrum in order to generate photocurrents. In many applications, especially selective NIR absorbers which absorb strongly in the NIR while exhibiting negligible absorption in the visible spectral range are required. Dyes with these properties find use as NIR absorbers in paints and windows to block off heat, in laser protecting glasses, as dyes for laser polymer welding of transparent components, or as anti-forgery markers. In all these examples it is required that the dye does not influence the spectral properties of the components in the visible. Thus, photostable dyes with strong, narrowbandwidth NIR absorptions and negligible absorption in the visible spectral range are needed. Common strategies to achieve bathochromic shifts of the absorption maximum of a dye are the extension of the chromophoric system or the introduction of donor and acceptor groups into the chromophore. Cyanine dyes and the rylene dyes are the most popular classes showing significant bathochromic shifts upon extension of their π-system. In cyanine dyes, however, the chemical stability decreases with increasing extension. By contrast, rylene dyes are chemically very stable, but, due to comparably large bondlength changes accompanying the S0→S1 transition, they show rather intense 01 and 02 vibronic bands (Franck-Condon principle). Thus, their S0→S1 absorption is extended into the visible range. Other important classes of NIR dyes, showing strong and narrowband NIR absorption are squarine dyes, bodipys and some aza[18]annulene derivatives such as porphyrins phthalocyanines and especially naphtolocyanines. We recently described pyrrolopyrrole cyanine (PPCy) dyes as a new class of NIR dyes and fluorophores. PPCys are synthesized by a condensation reaction of diketopyrrolopyrrole (DPP) 1 and heteroaromatic acetonitriles (HAA) 3. Complexation with either BF2 or BPh2 yields fluorophores which exhibit narrowband absorption between 650 and 900 nm and fluorescence emission with high quantum yields. By isolation of the monoactivated DPP 2, selectivly one of the carbonyl groups of the DPP can be reacted with a HAA 3 to the half converted products 4. Further reaction with another acetonitrile derivative 3 leads to PPCys with an asymmetric substitution pattern. This stepwise reaction scheme allows for the introduction of just one functional group. Here, we describe the synthesis of a new class of NIR chromophores with unprecedented spectral properties. They have very strong and narrowband NIR absorption while featuring negligible visible absorption. The absorption coefficients of these dyes in the NIR belong to the strongest ever reported for organic fluorophores, being twice as high as those of recently reported rylene dyes. Our previous approaches to shift the absorption of the PPCys bathochromically concerned the modification of the heteroaromatic endgroup (A) and/or the complexation agent. The selective synthesis of the halfconverted DPPs 4 opens a strategy to enlarge the chromophore of the PPCys by reacting two equivalents of 4 with a bifunctional bridging heteroaromatic acetonitrile such as 5 (Scheme 1.) The resulting condensation products contain two pyrrolopyrrole moieties and can therefore be called bis-pyrrolopyrrole cyanine dyes.