Photochemical "triode" molecular signal transducer

Rapid amplitude-modulation of a diarylethene photoswitch: en route to contrast-enhanced fluorescence imaging

A water soluble diarylethene (DAE) derivative that displays exceptionally intense fluorescence from the colorless open form has been synthesized and characterized using UV/vis spectroscopy and fluorescence microscopy. We show that the bright emission from the open form can be rapidly switched using amplitude modulated red light, that is, by light at wavelengths longer than those absorbed by the fluorescent species. This is highly appealing in any context where undesired background fluorescence disturbs the measurement, e.g., the autofluorescence commonly observed in fluorescence microscopy. We show that this scheme is conveniently applicable using lock-in detection, and that robust amplitude modulation of the probe fluorescence is indeed possible also in cell studies using fluorescence microscopy.

A water soluble diarylethene derivative displaying exceptionally bright fluorescence in the open isomeric form has been used for emission amplitude-modulation. We apply this scheme in fluorescence microscopy, aiming to suppress undesired background.

Toward Two-Photon Absorbing Dyes with Unusually Potentiated Nonlinear Fluorescence Response

The combination of two two-photon-induced processes in a Förster resonance energy transfer (FRET)-operated photochromic fluorene-dithienylethene dyad lays the foundation for the observation of a quartic dependence of the fluorescence signal on the excitation light intensity. While this photophysical behavior is predicted for a four-photon absorbing dye, the herein proposed approach opens the way to use two-photon absorbing dyes, reaching the same performance. Hence, the spatial resolution limit, being a critical parameter for applications in fluorescence imaging or data storage with common two-photon absorbing dyes, is dramatically improved.

Excitation-Wavelength-Dependent Photoinduced Electron Transfer in a π-Conjugated Diblock Oligomer

Control of photoinduced electron transfer through selective excitation of a π-conjugated diblock oligomeric system featuring tetrathiophene (T₄) and tetra(phenylene ethynylene) (PE₄) donor blocks capped with a naphthalene diimide (NDI) acceptor (T₄PE₄NDI) is demonstrated. Each π-conjugated oligomeric segment has its own discrete ionization potential, electron affinity, and optical band gap which provides an absorption profile that has specific wavelengths that offer selective excitation of the PE₄ and T₄ blocks. Therefore, T₄PE₄NDI can be selectively excited to form a charge-separated state via ultrafast photoinduced electron transfer from the PE₄ segment to NDI when excited at 370 nm, but it does not produce a charge-separated state when excited at 420 nm (T₄). Picosecond transient absorption techniques were performed to probe the excited-state dynamics, revealing ultrafast charge separation (∼4 ps) occurring from the PE₄ segment to NDI when excited at 370 nm, followed by delocalization of the hole over the T₄ segment. On the contrary, electron transfer is suppressed with excitation at longer wavelengths (≥420 nm), where the spectrum is dominated by the T₄ unit. The rate of electron transfer and charge recombination was investigated versus the length of the PE bridge unit in oligomers featuring zero and two PE units (T₄NDI and T₄PE₂NDI). The rate of charge recombination decreases from 1.2 × 10¹¹ to 1.0 × 10⁹ s^-1 with increasing bridge length between the T₄ and NDI components (T₄NDI to T₄PE₄NDI). Furthermore, wavelength-dependent photoinduced electron transfer was not observed in either T₄NDI or T₄PE₂NDI due to an insufficient PE_n bridge length. This work demonstrates the ability to use optical wavelength to control photoinduced electron transfer in a fully π-conjugated oligomer.

Ruthenium(II)-Catalyzed Microwave-Promoted Multiple C-H Activation in Synthesis of Hexa(heteroaryl)benzenes in Water

A series of hexa(heteroaryl)benzenes were synthesized by the Ru(II)-carboxylate-catalyzed multiple C-H activation of benzenes carrying pyridyl, pyrimidyl, or pyrazolyl directing groups using N-heteroaryl bromides as coupling partners. The reactions proceeded with high selectivity under microwave irradiation in water. Iterative penta-arylation could be implemented via activation of C-H bonds of generated intermediates by cascade chelation assistance of in situ installed pyridyl groups. This strategy provides multidentate ligands for selective complexation of transition metals and potential building of photoredox systems.

Coupling carbon nanomaterials with photochromic molecules for the generation of optically responsive materials

Multifunctional carbon-based nanomaterials offer routes towards the realization of smart and high-performing (opto)electronic (nano)devices, sensors and logic gates. Meanwhile photochromic molecules exhibit reversible transformation between two forms, induced by the absorption of electromagnetic radiation. By combining carbon-based nanomaterials with photochromic molecules, one can achieve reversible changes in geometrical structure, electronic properties and nanoscale mechanics triggering by light. This thus enables a reversible modulation of numerous physical and chemical properties of the carbon-based nanomaterials towards the fabrication of cognitive devices. This review examines the state of the art with respect to these responsive materials, and seeks to identify future directions for investigation.

A multi-stimuli responsive switch as a fluorescent molecular analogue of transistors

Although the quantum nature of molecules makes them specially suitable for mimicking the operation of digital electronic elements, molecular compounds can also be envisioned to emulate the behavior of analog devices. In this work we report a novel fluorescent three-state switch capable of reproducing the analog response of transistors, an ubiquitous device in modern electronics. Exploiting the redox and thermal sensitivity of this compound, the amplitude of its fluorescence emission can be continuously modulated, in a similar way as the output current in a transistor is amplified by the gate-to-source voltage.

An acido- and photochromic molecular device that mimics triode action

A molecular triode that is operated by pH and light can be implemented with spiropyrans.

Energy redistribution dynamics in triarylamine–triarylborane containing hexaarylbenzenes

Hexaarylbenzenes with triarylamine donors and triarylborane acceptors show rapid energy transfer (∼3 ps) as proved by pump–probe spectroscopy with fs-time resolution.

Water-soluble naphthalimide-based ‘Pourbaix sensors’: pH and redox-activated fluorescent AND logic gates based on photoinduced electron transfer

Naphthalimide-based ‘Pourbaix sensors’ for redox potential and pH fluoresce with a lifetime of 8.5 ns while photoinduced electron transfer occurs on a time scale of 20 ps.

Reversible energy-transfer switching on a DNA scaffold

We show that FRET between Pacific Blue (PB) and Alexa488 (A488) covalently attached to a DNA scaffold can be reversibly controlled by photochromic switching of a spiropyran derivative. With the spiropyran in the closed spiro isomeric form, FRET occurs freely between PB and A488. UV-induced isomerization to the open merocyanine form shuts down the FRET process by efficient quenching of the PB excited state. The process is reversed by exposure to visible light, triggering the isomerization to the spiro isomer.

Molecules with a sense of logic: a progress report

Ones and zeros can be handled by molecules through the input-control of their signaling features. The progress in this exciting field during the last five years is covered in this tutorial review.

Current developments in fluorescent PET (photoinduced electron transfer) sensors and switches

A fluorophore can be combined with a receptor according to a molecular engineering design in order to yield fluorescent sensing and switching devices.

Taking baby steps in molecular logic-based computation

Constructs of fluorophores, receptors, spacers, ¹O₂ sensitizers, enzymes and oligonucleotides play their part in advancing the field of molecular logic-based computation.

On the relation of energy and electron transfer in multidimensional chromophores based on polychlorinated triphenylmethyl radicals and triarylamines

Chromophores with many donors and acceptors show electron transfer which is identical to energy transfer.

Supramolecular photochemistry applied to artificial photosynthesis and molecular logic devices

Supramolecular photochemical systems consist of photochemically active components such as chromophores, electron donors or electron acceptors that are associated via non-covalent or covalent interactions and that interact in some functional way. Examples of interactions are singlet–singlet energy transfer, triplet–triplet energy transfer, photoinduced electron transfer, quantum coherence and spin–spin magnetic interactions. Supramolecular photochemical “devices” may have applications in areas such as solar energy conversion, molecular logic, computation and data storage, biomedicine, sensing, imaging, and displays. This short review illustrates supramolecular photochemistry with examples drawn from artificial photosynthesis, molecular logic, analog photochemical devices and models for avian magnetic orientation.

Modulating short wavelength fluorescence with long wavelength light

Two molecules in which the intensity of shorter-wavelength fluorescence from a strong fluorophore is modulated by longer-wavelength irradiation of an attached merocyanine-spirooxazine reverse photochromic moiety have been synthesized and studied. This unusual fluorescence behavior is the result of quenching of fluorophore fluorescence by the thermally stable, open, zwitterionic form of the spirooxazine, whereas the photogenerated closed, spirocyclic form has no effect on the fluorophore excited state. The population ratio of the closed and open forms of the spirooxazine is controlled by the intensity of the longer-wavelength modulated light. Both square wave and sine wave modulation were investigated. Because the merocyanine-spirooxazine is an unusual reverse photochrome with a thermally stable long-wavelength absorbing form and a short-wavelength absorbing photogenerated isomer with a very short lifetime, this phenomenon does not require irradiation of the molecules with potentially damaging ultraviolet light, and rapid modulation of fluorescence is possible. Molecules demonstrating these properties may be useful in fluorescent probes, as their use can discriminate between probe fluorescence and various types of adventitious "autofluorescence" from other molecules in the system being studied.

Modification of fluorescent photoinduced electron transfer (PET) sensors/switches to produce molecular photo-ionic triode action

The fluorophore-spacer₁-receptor₁-spacer₂-receptor₂ system (where receptor₂ alone is photoredox-inactive) shows ionically tunable proton-induced fluorescence off-on switching, which is reminiscent of thermionic triode behavior. This also represents a new extension to modular switch systems based on photoinduced electron transfer (PET) towards the emulation of analogue electronic devices.

Analog applications of photochemical switches

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 photoregulatory processes are discussed.

All-photonic multifunctional molecular logic device

Photochromes are photoswitchable, bistable chromophores which, like transistors, can implement binary logic operations. When several photochromes are combined in one molecule, interactions between them such as energy and electron transfer allow design of simple Boolean logic gates and more complex logic devices with all-photonic inputs and outputs. Selective isomerization of individual photochromes can be achieved using light of different wavelengths, and logic outputs can employ absorption and emission properties at different wavelengths, thus allowing a single molecular species to perform several different functions, even simultaneously. Here, we report a molecule consisting of three linked photochromes that can be configured as AND, XOR, INH, half-adder, half-subtractor, multiplexer, demultiplexer, encoder, decoder, keypad lock, and logically reversible transfer gate logic devices, all with a common initial state. The system demonstrates the advantages of light-responsive molecules as multifunctional, reconfigurable nanoscale logic devices that represent an approach to true molecular information processing units.

The Ultrafast Pathway of Photon-Induced Electrocyclic Ring-Opening Reactions: The Case of 1,3-Cyclohexadiene

The photochemically induced electrocyclic ring-opening reaction of 1,3-cyclohexadiene to 1,3,5-hexatriene serves as a prototype for many important reactions in chemistry and in biological systems. Based on experimental and computational studies, a detailed picture of the reaction has now emerged: Excitation to the Franck-Condon region places the molecule on a steeply repulsive part of the 1B potential energy surface, which propels the molecule in exactly the conrotatory direction that conforms to the Woodward-Hoffmann rules of orbital symmetry. Bypassing a cusp in a symmetry-breaking direction, the wave packet enters the 2A state within 55 fs. It continues to move directly toward the 2A/1A conical intersection, where it crosses in approximately 80 fs to the ground state. This article summarizes the published experimental and theoretical work to describe the current understanding of the reaction while pointing to important questions that remain to be addressed.