Push-pull dyes for medical imaging applications

Linear organic conjugated molecules linked to an electron donor (D) at one end and an electron acceptor (A) at the opposite end are recognized as one class of nonlinear optical materials for potential application in energy conversion devices, organic electronics, optical communication, and information storage [1]. Upon photoexcitation, in fact, a large change in the charge distribution is induced in the excited-state, resulting in a intramolecular charge-transfer (ICT) from the donor side to the acceptor side, and in a large transition dipole moment [2,3]. The luminescence of the ICT excited state should present a large Stokes shift and this particular photophysical behaviour can be of interest in projecting a wavelength shifter for spectral conversion from DUV to visible light. This is at the basis of the STRING (STRuctured scintillators for medical imagING) project [4] that aims at developing structured hybrid organic / inorganic ceramic scintillators with faster emission, higher conversion efficiency and reduced production costs for PET (Positron Emission Tomography) scanners, with respect to nowadays state-of-the-art gadolinium silicate or lutetium silicate single crystals doped with light emitting Ce(3+) ions, currently used for the detection of the high energy photons. Within the STRING project we synthesised and photophysically characterized a series of organic molecules constituted of a 1,2-diphenyl-acetylene core and a donor group and an acceptor group directly linked to the ?-conjugated backbone (Figure 1). The photophysical properties have been investigated in solvents of different polarity and in solid matrices at different temperature by steady-state and time-resolved techniques. In these systems, tuning of absorption and emission energies is achieved via the push-pull effect, which influences to different degrees the energy levels of ICT excited states. The resulting strong interaction of the large transition dipoles with the surrounding solvent molecules is correlated to the large Stokes shifts of the fluorescence spectra observed in solution. All the investigated compounds were found to be highly luminescent in condensed media. Acknowledgements. We thank the EC (STRP-032636, STRING) and the CNR (PM.P04.010, MACOL) for financial support.