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Preparation and Study of Novel Near-Infrared Dyes Derived from Ferrocene

There is at present considerable a strong scientific and commercial interest in near-infrared (NIR) absorbing dyes (lambdamax > 700 nm) which arises from their large range of potential applications in optical imaging systems, infrared photography, optical lasers, biological probes, as well as in medicine for photodynamic cancer therapy.1, 2 Near-infrared dyes are of particular interest for biological diagnostic and therapeutic applications since there is little interference from other species in the NIR region. This has particular importance in in vivo diagnostic applications where the gains in sensitivity mean that the concentration of potentially toxic reagents can be minimised. A prerequisite for absorption in the NIR region is a small HOMO-LUMO gap. One way of realising this is by the end-capping of conjugated organic chromophores with strong donor and acceptor substituents which allows the possibility of intramolecular charge transfer.3 This is similar to the requirements of non-linear optical materials such as those based on the stilbene framework with which our group has previously worked.4 So, with this in mind we have prepared in good overall yealds a variety of organometallic dyes containing multiple ferrocene units as the strong donor group, conjugatively linked to carbocationic centres which act as strong electron acceptor groups. The carbocations can be generated from the corresponding carbinols by protonation with strong acid such as CF3CO2H or HPF6. The absorption of their chloroform solutions in the near infrared region between 800 and 1100 nm have been studied. However, despite absorbing in the NIR region, these dyes quickly revert to the carbinols from which they are derived. Now, by making suitable modifications to these carbinols we have succeeded in preparing extremely stable carbenium ions with the desired properties. Dendrimers in which chromophores are incorporated have been the subject of much investigation and the usefulness of dendrimers as light-harvesting antennas, fluorescent chemosensors, and organic light-emitting diodes, and in drug delivery has been demonstrated by many research groups.5 Little has been done in connection with NIR absorbing materials, howewer, and we are now investigating the extension of our work with ferrocenyl carbocationic NIR dyes to the synthesis of dendrimeric molecules in which the chromophores are attached to the periphery of the dendrimer and preliminary work will be reported.

Ferrocene; NIR-Dyes; Dendrimers

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