Sequential logic operations with surface-confined polypyridyl complexes displaying molecular random access memory features.

The processing of molecular information is essential for organisms to respond to external/internal stimuli. For example, in vision, a single molecule of 11-cis-retinal is photoisomerized to all-trans-retinal, which starts a cascade of signal transduction pathways that eventually enables us to see. The fact that molecules can be implemented for processing information akin to electronic systems was recognized and demonstrated by the construction of a photo-ionic AND gate by de Silva et al. This opened up an exciting research area that led to a variety of molecular logic systems such as logic gates, half-adders and subtractors, multiplexers, and encoders. Bio-inspired systems have also attracted much attention. The output of these combinatorial systems is exclusively a Boolean function of the current inputs. In contrast, the output of sequential systems is determined by the current state of the system, which is usually a function of the previous input and the present input. This situation thus requires that the molecular-based system must remember information about the previous input, and hence, functions as a basic memory element. Consequently, sequential logic systems are commonly used in the construction of memory devices, delay and storage elements, and finite-state machines. The demonstration of sequential logic operations with molecularbased systems is relatively rare, and includes circuits, molecular keypad locks, 13] and finite-state machines. Furthermore, previous studies on molecular-based logic are almost exclusively based on solution-based chemistry. Recently, we reported the proof-of-principle that 1-based monolayers (Scheme 1) can perform combinatorial logic operations. The system mimics the input and output characteristics of electronic circuitry when using chemical reagents as inputs and the formal oxidation state of the system as the output. Here, we demonstrate a fundamentally new concept towards reversible and reconfigurable sequential logic operations by addressing the memory function of the 1-based monolayers. Interestingly, not only were we able to generate sequential logic circuits with one, two, and even three chemical inputs, but we were also able to use this sequential logic approach to model the memory function of random access memory (RAM). Moreover, by keeping the starting state static or dynamic, delicate control is obtained regarding which kind of logic is performed—combinatorial or sequential logic. A dynamic starting state generates sequential circuits, whereas a static starting state produces combinatorial circuits. For sequential operations with the 1-based monolayer, the presence or absence of an arbitrary chemical input is defined as a logical 1 or 0, respectively. The output or state is dependent on the formal oxidation state of the system, which is monitored by UV/Vis spectroscopy in the transmission mode. The logical outputs 1 and 0 are defined as Os and Os, respectively (See the Supporting Information). For example, a one-input sequential system was designed with Cr ions in an aqueous solution at pH< 1 as the input. The four possible combinations were demonstrated with the same monolayer (Table 1). Only when Cr ions are present and the monolayer is in state 1 (Os) can the logic gate change to state 0 (Os; Table 1, see also Figure S1 in the Supporting Information). Since the current state is variable, the output Scheme 1. The osmium polypyridyl complex used in this study.