Fused Pyrene–Diporphyrins: Shifting Near‐Infrared Absorption to 1.5 μm and Beyond

Porphyrins have been explored for a number of potential optoelectronic applications that require strong absorption in the near-infrared (NIR) spectral region; these applications include organic electronics, nonlinear optics, and telecommunication technologies. Porphyrins have also been investigated as active materials in photovoltaic cells because of their high efficiency of charge separation and transport, strong absorbance in the visible region, high chemical stability, and the ease with which their optoelectronic properties can be tuned with chemical modification. The absorption bands of porphyrins are not readily shifted into the deepred and NIR spectral regions, and also tend to be narrow, thus minimizing their overlap with the solar spectrum. Triply bridged, (b,meso,b), porphyrin tapes (Figure 1a, n= 0–22) show marked red-shifts in the porphyrin absorption bands, which extend deep into the NIR region. Triply fused porphyrins with n= 1,2 give absorbance in the mid-NIR region (i.e., conventional wavelengths for telecommunications, ca. 1.5 mm), however, these porphyrins are difficult to synthesize, have low solubility, and are isolated only in small quantities. Triply connected porphyrin dimers (Figure 1a, n= 0) have a strong absorbance at l= 1050 nm, are photoand chemically stable, have a high solubility, and can be easily prepared from monoporphyrins. Development of new organic dyes based on these accessible porphyrin dimers with absorption at the wavelengths for telecommunications (l= 1.5 mm) still remains a challenge. Extending the size of p conjugation in porphyrin systems results in most cases in a bathochromic (red) shift of the absorption. The conjugation of porphyrin dimers can be extended through several modes of substitution involving the meso, (b,b), (b,meso) and (b,meso,b) positions. For diporphyrins substituted with two alkyne groups at the terminal meso positions, the Q band is red-shifted by 130 nm (l= 1181 nm) relative to the parent dimer. In contrast, extending the conjugation in porphyrin dimers by benzannulating b,bpyrrolic positions red-shifts the Q band by only 18 nm, and the resulting compounds have poor solubility. Recently, it has been shown that anthracene rings can be fused to porphyrin dimers through the (b,meso,b) mode, which leads to a red-shift of the Q band to 1495 nm. However, the anthracene-fused diporphyrin exhibits the same undesirable difficulties found with higher porphyrin tapes, for example, synthetic difficulty, low yields and low solubility. Moreover, fusion of anthracene rings is limited only to alkoxy-substituted derivatives. The effects of aromatic ring fusion to porphyrin tapes in a (meso,b) mode have not been explored. We have analyzed the structures of the diporphyin core (Figure 1b), a (b,meso,b) triply fused aromatic system (Figure 1c), and a (b,meso) doubly fused molecule (Figure 1d) using standard DFT methods. Significant bathochromic shifts of the lowestenergy transition are expected in all cases. Unlike the case of anthracene-fused porphyrins and porphyrin tapes, in which the planarity causes aggregation and low solubility, the pyrene–(b,meso)-fused diporphyrin displays out-of-plane distortion that is known to improve solubility and processibility in conjugated aromatics. By taking into account the predicted bathochromic shift, distortion from planarity, and ease of synthesis, the (b,meso)-fused pyrene diporphyrin from Figure 1. a) General structure of triply fused porphyrins. b)–d) Structures of diporphyrin hybrids calculated at B3LYP/6-31G with calculated red-shifts of the lowest-energy transitions compared to the parent diporphyrin.