Multistate/Multifunctional Molecular-Level Systems: Light and pH Switching between the Various Forms of a Synthetic Flavylium Salt

Abstract

The photochromic properties of the 4'-hydroxyflavylium ion (AH ) have been investigated in aqueous solution. This system can be interconverted between as many as ten different forms by light excitation and/or pH changes. In neutral or moderately acidic solution the thermodynamically stable form of this compound is trans-2,4'-dihydroxychalcone (Ct). Light excitation of Ct in acidic or neutral solution gives rise to cis-chalcone Cc (F 0.04 at 365 nm), which undergoes slow equilibration with three other forms, namely hemiacetal B, flavylium cation AH , and quinoidal base A. The relative amounts of the photoproducts depend on pH. At pH3 the only product is the colored AH . In neutral and moderately acidic solutions, the four photoproducts revert back to Ct by a slow thermal reaction (k 4.0 Â 10 À5 s À1, half-life 27 hours at 25 8C, pH 3), whose rate can be accelerated by increasing the temperature (k 1.8 Â 10À2 s À1 at 75 8C, pH 3), and by exploiting the photochemical conversion of Cc to Ct (F b 0.16 at 313 nm). At pH`1, AH is the thermodynamically stable form of the system. A pH jump from 1 to 12 causes the complete conversion of AH to the Cc 2À dianion. This species is relatively stable (half-life 400 hours at pH 12 and 25 8C). Its thermal and photochemical reactions (F 0.17 at 313 nm) lead to Ct 2À , which is the thermodynamically stable form of the system in basic solution. Ct 2À does not undergo any photochemical reaction. All the observed processes are fully reversible and accompanied by large changes in the absorption and emission spectra. The flavylium salt investigated represents a multistate/multifunctional molecular-level system. It has properties required by optical memory devices with multiple storage in two different memory levels and nondestructive readout capacity through a write ± lock ± read ± unlock ± erase cycle. Its light-and/or pH-induced transformations can be taken as a basis for simple logic operations and create an intricate network of chemical processes.

Control of Noise in Chemical and Biochemical Information Processing

“…Digital logic functions of several-gate device components [128][129][130][131][132][133][134] have been realized, such as keypad lock and memory units. [85,[135][136][137][138][139][140][141][142][143][144][145] Chemical-computing systems can function at the single-molecule [146] nano-scale devices, [147] as well as perform parallel computations by numerous molecules. [148] Chemical computing shows great promise, [149][150][151] though, as most unconventional computing approaches, [152] it is not being developed as an alternative to the speed and versatility of Si-computers.…”

All-optical processing with molecular switches

“…Simple logic functions (16) have been implemented at the molecular level, operating individual molecular switches (17)(18)(19)(20)(21)(22)(23)(24)(25)(26)(27)(28)(29)(30)(31)(32) or ensembles of communicating molecules (33)(34)(35). These chemical systems elaborate chemical, electrical, and͞or optical signals with logic algorithms that are dictated by the design of the molecular components.…”

Photochromic Properties of 3-Methyl-Substituted Flavylium Salts

“…Systems of this kind, capable of existing in several forms that can be interconverted by different external stimuli, are referred to as multistate/ multifunctional systems. [15] An interesting aspect of such systems is that they exhibit properties suitable for playing the role of optical memories with multiple storage and nondestructive readout capacities (write-lock-read-unlock-erase cycles, Figure 2, a). In these systems, light is used to convert a species, X, into another species, Y (write); a second stimulus (such as a proton, as in Figure 2, a) is then employed to transform Y (which would otherwise be reconverted back into X either thermally or by a direct photon-reading process) into Z, another stable state of the system (lock), that can be optically detected without being destroyed (read).…”

“…We have already reported that the 4Ј-methoxyflavylium ion may be used as a basis for a write-lock-read-unlock-erase photochromic cycle [14] and that suitably designed 4Ј-substituted flavylium ions may i) behave as logic gates, [15,17] ii) exhibit intricate patterns of chemical processes, [15] and iii) even be used as components of a rudimentary neural network. [17] In order to extend our knowledge on the reversible transformation processes that take place in flavylium compounds and to improve the stability of the written information when such compounds are used for optical information storage, we have investigated three compounds of the 3-methylflavylium family: namely 3-methylflavylium, 4Ј-hydroxy-3-methylflavylium, and 4Ј,7-dihydroxy-3-methylflavylium ( Figure 3).…”