Fused porphyrin oligomers have a number of attractive photophysical properties, however their rapid decay limits their potential application. Until now it was assumed that this rapid decay is due to the small energy gap, however new research by Harry Anderson at Oxford University, UK, and colleagues shows that this is not the case.   

1. Please explain, for a non-specialist, the significance of your article.   

When edge-fused porphyrin dimers absorb light they form singlet excited states which rapidly decay back to the ground state, without giving significant fluorescence.  It has been widely assumed that this rapid non-radiative deactivation is simply a consequence of the small energy of the excited state, according to the "energy gap law".  By studying a deuterated version of the chromophore we have eliminated this explanation.  The synthetic route to our deuterated porphyrin dimer may also interest people.  We discovered that it is easy to make deuterated dipyrromethane, and this compound could be used to make a wide range of other deuterated porphyrins.   

2. What has motivated you to conduct this work?   

The original motivation for the work was very practical:  We hoped that deuteration of the porphyrin dimer would slow down non-radiative deactivation of the singlet excited state, leading to more formation of triplet exited states, which have strong absorption in the near-infrared wavelength region.  This would have given a material with useful "reverse-saturable absorption", i.e. its absorption would increase with increasing light intensity, as the triplet state becomes populated, making it useful for protecting delicate sensors from high intensity light.   

There is also a more general motivation for this project: edge-fused porphyrin dimers have remarkable absorption spectra and nonlinear optical behaviour (strong two-photon absorption and excited-state absorption) yet it is difficult to imagine how these properties could lead to applications because the excited states are so short-lived.  Any way of preventing the rapid deactivation of these excited states should make it possible to apply these dyes in a wide range of areas.   

3. Where do you see this work developing in the future?   

This work shows that deuteration is not the way to avoid rapid non-radiative deactivation in these compounds.  It shows that there must be a specific non-radiative decay channel in this type of chromophore, and that the fast non-radiative decay is not simply a consequence of the small energy of the excited state.  This encourages us to explore the synthesis and photophysics of other highly conjugated porphyrin-based chromophores.   

4. Are there any particular challenges facing future research in this area?   

It is very difficult to predict how rapidly the excited state of a dye will undergo non-radiative decay, so the design of target structures in this area is challenging.  Their synthesis tends to be challenging too, and it generally involves a lot of serendipity.

Richard Kelly