© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1 www.advmat.de www.MaterialsViews.com wileyonlinelibrary.com RESEARCH NEWS Graeme Copley, Thomas A. Moore,* Ana L. Moore,* and Devens Gust* Analog Applications of Photochemical Switches Dr. G. Copley, Prof. T. A. Moore, Prof. A. L. Moore, Prof. D. Gust Department of Chemistry and Biochemistry Arizona State University Tempe, AZ 85287, USA E-mail: gust@asu.edu; tmoore@asu.edu; amoore@asu.edu DOI: 10.1002/adma.201201744 1. Introduction External stimuli such as the addition of chemicals or exposure to light can be used to convert molecules reversibly between dif- ferent forms. This general concept has allowed the construction of multistate molecular switches that employ chemicals and/or light as inputs and outputs. The idea of a molecular switch can be extended to design molecules that perform more complex roles such as Boolean logic gates, half-adders, multiplexers and demultiplexers, encoders-decoders, keypad locks, latches/flip- flops and multivalued logic devices. [1–9] All photonic molecular devices, which rely on only optical inputs and outputs, offer several special advantages; they do not rely on molecular diffu- sion for transport, they can operate in the solid state, physical access for the addition of chemicals or wires is not required and they do not generate undesired byproducts that have deleterious effects on recycling and reproducibility. Photochromic molecules (photochromes) are often used as components of photochemical molecular switches. Photo- chromes are reversibly isomerized between two metastable forms using light. A photochrome can be used to control the photochemistry or photophysics of attached chromophores by quenching excited states via energy transfer or photoinduced electron transfer, creating excited states via energy transfer, mediating energy transfer between other chromophores, and modifying electronic coupling between chromophores, donors and acceptors. The systems described above are binary devices. The photochromes or other molec- ular switching elements are employed as digital on-off switches, and therefore per- form in ways similar to transistors in elec- tronic devices when the transistors are used as binary switches. Much less attention has been paid to the applications of molecular switches as analog devices. Analog devices have continuously variable output levels controlled by continuously variable inputs. Examples are volume controls, light dimmer switches, audio amplifiers, etc. The function of such devices is sometimes mimicked by digital systems with many discrete outputs. Tran- sistors, which are often thought of as the ultimate binary device, can be configured to function as analog devices. For example, a field-effect transistor can function as an amplifier, in which cur- rent flowing from an input (source) to an output (drain) is con- trolled by the variable voltage at a second input (gate). Individual molecules, such as photochromes, are inherently digital, as they can exist stably in one discrete, quantized state or another. However, ensembles of molecular switches can perform as analog systems because measurements of their properties are ensemble averages, and essentially continuously variable. A typical photochrome, for example, exists in one state that is coloured in the visible or another that features absorp- tion only in the ultraviolet. A solution of such a photochrome can display continuously variable light absorption in the visible, as determined by the ratio of the two photoisomeric forms of the molecule. It is possible to design and prepare molecular “devices” that make use of this analog behaviour. Here, we will illustrate with three examples from our laboratories. One is a molecular pho- tonic analogue of a field-effect transistor, and the other two mimic aspects of photosynthetic photoprotection. 2. A Photochemical Molecular Signal Transducer Molecular hexad 1 ( Figure 1) successfully mimics, on the molec- ular level, a transistor amplifier or triode vacuum tube. [10] To understand this behaviour, it is essential to first understand the fundamental photophysical properties of the hexad. The central hexaphenylbenzene core of 1 serves as a template for organizing the five bisphenylethynyl-anthracene (BPEA) fluorophores and the single dithienylethene (DTE) photochrome. An important feature of the hexaphenylbenzene core is that, due to steric effects, rota- tion of the six peripheral aryl groups is hindered, and the periph- eral rings are at very steep angles to the central ring. This feature strictly limits interchromophore distances and orientations. Molecules that change their structure in response to a stimulus such as light or an added chemical can act as molecular switches. Such switches can be chemically linked to other active moieties to create molecular “devices” for various purposes. There has been much activity of late in the use of molecular switches such as photochromes in the construction of molecular logic gates that carry out binary or digital functions. However, ensembles of such molecules can also act as analog devices. Here, examples of a molecular photonic signal transducer and two mimics of photosynthetic photoregula- tory processes are discussed. Adv. Mater. 2012, DOI: 10.1002/adma.201201744