The fluorescence of an organogel mixed with a photochromic compound can be switched off by thermal heating, UV irradiation, or both, thus forming an OR binary logic gate.
'Smart' gels that can reversibly change their structures or properties in response to an external signal are being investigated for a variety of applications — fluorescence switching, for example, could be used for optical memory devices. Such gels have been formed by incorporating stimuli-responsive moieties into low-molecular-mass organogelators (LMOGs), but they have only been able to react to single factors so far.
Now, Soo Young Park and co-workers at Seoul National University have prepared1 a mixture that switches its fluorescence in response to dual input — heating and UV irradiation. An LMOG known to be fluorescent only in the gel state was combined with a photochromic compound. This compound bears a ring that can be reversibly isomerized into a closed (non-fluorescent) or open (fluorescent) form, by UV or visible light irradiation respectively. In solution, the mixture of LMOG with either isomer is non-fluorescent. In the gel state, however, they behaved differently: an energy transfer between the closed ring and the LMOG quenched the fluorescence.
This system can therefore be seen as an OR molecular logic gate, where the output signal (fluorescence quenching) was triggered by heating and/or UV irradiation. The researchers used the mixture to record and erase fluorescent images, showing its promise for erasable optical data storage.
Chung, J. W., Yoon, S.-J., Lim, S.-J., An, B.-K. & Park, S. Y. Dual-mode switching in highly fluorescent organogels: binary logic gates with optical/thermal inputs. Angew. Chem. Int. Ed. 10.1002/anie.200902777 (2009).