Bridging the visible: the modulation of the absorption by more than 450 nm

Naphthopyran molecular switches and their emergent mechanochemical reactivity

Naphthopyran molecular switches undergo a ring-opening reaction upon external stimulation to generate intensely colored merocyanine dyes. Their unique modularity and synthetic accessibility afford exceptional control over their properties and stimuli-responsive behavior. Commercial applications of naphthopyrans as photoswitches in photochromic ophthalmic lenses have spurred an extensive body of work exploring naphthopyran-merocyanine structure-property relationships. The recently discovered mechanochromic behavior of naphthopyrans has led to their emergent application in the field of polymer mechanochemistry, enabling advances in the design of force-responsive materials as well as fundamental insights into mechanochemical reactivity. The structure-property relationships established in the photochemical literature serve as a convenient blueprint for the design of naphthopyran molecular force probes with precisely tuned properties. On the other hand, the mechanochemical reactivity of naphthopyran diverges in many cases from the conventional photochemical pathways, resulting in unexpected properties and opportunities for deeper understanding and innovation in polymer mechanochemistry. Here, we highlight the features of the naphthopyran scaffold that render it a powerful platform for the design of mechanochromic materials and review recent advances in naphthopyran mechanochemistry.

Bridged-Imidazole Dimer Exhibiting Three-State Negative Photochromism with a Single Photochromic Unit

Photochromic molecules that can exhibit multiple states of photochromism in a single photochromic unit are considered more attractive than traditional bistable photochromic molecules because they can offer more versatility and control in photoresponsive systems. We have synthesized a negative photochromic 1-(1-naphthyl)pyrenyl-bridged imidazole dimer (NPy-ImD) that has three different isomers: a colorless isomer, 6MR, a blue-colored isomer, 5MR-B, and a red-colored isomer, 5MR-R. NPy-ImD can interconvert between these isomers via a short-lived transient biradical, BR, upon photoirradiation. 5MR-R is the most stable isomer, and the energy levels of 6MR, 5MR-B, and BR are relatively close to each other. The colored isomers 5MR-R and 5MR-B are photochemically isomerized to 6MR via the short-lived BR upon irradiation with blue light and red light, respectively. The absorption bands of 5MR-R and 5MR-B are well separated by more than 150 nm, with a small overlap, which means they can be selectively excited with different light sources, visible light for 5MR-R and NIR light for 5MR-B. The colorless isomer 6MR is formed from the short-lived BR through a kinetically controlled reaction. 6MR and 5MR-B can then be converted to the more stable isomer 5MR-R through a thermodynamically controlled reaction, which is facilitated by the thermally accessible intermediate, BR. Notably, 5MR-R photoisomerizes to 6MR when irradiated with CW-UV light, whereas it photoisomerizes to 5MR-B by a two-photon process when irradiated with nanosecond UV laser pulses.

cis alkenes stabilized by intramolecular sulphurπ interactions

A series of alkenes with bistable isomers were obtained containing a thiophene/azoheteroaryl backbone. Visible light and heat-induced reversible cis ⇌ trans isomerizations were evidenced by UV-Vis and 1H NMR spectra. The stabilization of cis alkenes was attributed to intramolecular sulphurπ (Sπ) interactions, which were further supported by theoretical calculations.

Novel Diarylethene-Based Fluorescent Switching for the Detection of Al3+ and Construction of Logic Circuit

A novel photochromic diarylethene was synthesized successfully containing a phthalazine unit. Its multistate fluorescence switching properties were investigated by stimulating with UV/vis lights and Al3+/EDTA. The synthesized diarylethene displayed excellent selectivity to Al3+ with a distinct fluorescence change, revealing that it could be used as a sensor for fluorescence identification of Al3+, and a logic circuit was constructed by utilizing this diarylethene molecular platform. Moreover, it also exhibited a high accuracy for the determination of Al3+ in practical water samples.

A highly sensitive fluorescent sensor for Zn2+ based on diarylethene with an imidazole unit

A new sensitive sensor for Zn²⁺ based on diarylethene with an imidazole unit has been synthesized. Its photochromic and fluorescent behaviors have been systematically investigated by the stimulation of UV/vis lights and Zn²⁺ ion in THF solution. It displayed a dual-mode with a "turn on" fluorescence and color response to Zn²⁺. With the addition of Zn²⁺, the emission intensity enhanced 26-fold, accompanied by the fluorescent color changed from dark red to bright yellow. The 1:1 stoichiometry between the sensor and Zn²⁺ was verified by Job's plot and MS. The LOD for Zn²⁺ was determined to be 6.12 × 10^-9 mol L^-1. Furthermore, a logic circuit was designed by using the fluorescence at 578 nm as output and the combinational stimuli of UV/vis and Zn²⁺/EDTA as inputs.

Kinetics of photochemical reactions of biphotochromic compounds based on spironaphthopyran and enamine--conjugation effect

A novel biphotochromic compound (BPC) with two photochromic fragments, namely spironaphthopyran and hydroxyazomethine, was synthesized and studied by nanosecond laser flash photolysis using the excitation wavelengths of 337, 430, 470 and 500 nm in methanol and toluene. The photoexcitation of BPC results in the formation of at least two colored transients. The first one is the merocyanine form B (the maximum in the absorption spectrum is near 600 nm and the lifetime is 0.1 and 0.05 s in methanol and toluene, respectively) due to the spiro-bond break followed by isomerization. The second one is the trans-keto form A(Kt) (the maximum in the absorption spectra is near 480 nm and the lifetime is 0.1 and 5 ms in methanol and toluene, respectively) as a result of cis-enol or cis-keto tautomer transformations. The relative yields of B and A(Kt) depends essentially on the wavelength of excitation. The form A(Kt) is the key transient formed under excitation with the visible light, whereas its yield under excitation with UV light is comparable with that of B. The specific solvation by methanol molecules favors the spirocycle opening even under visible light excitation. The results obtained for novel BPC were compared with those for other BPC where the same fragments are combined in such a way that the form B is the major one under excitation with UV light whereas it virtually is not observed under visible light excitation. The difference in both BPC is discussed in terms of conjugation (π-coupling) between photochromic fragments.

Theoretical studies to investigate the effect of different cores and two different topologies on the optical and charge transfer properties of donor materials for organic solar cells

Three dimensional conjugated three- and four-armed molecules with core-D–π–A and core-A–π–D topologies were designed and studied by DFT, revealing that molecules with the core-A–π–D topology and N or C cores would result low reorganization energies and broad absorption bands for organic solar cells.

A multi-addressable diarylethene for the selective detection of Al3+ and the construction of a logic circuit

A multi-addressable diarylethene was designed and synthesized, which not only acted as a fluorescent sensor for Al³⁺ with high selectivity, but was also used to construct a combinational logic circuit.

P-Type Photochromism of New Helical Naphthopyrans: Synthesis and Photochemical, Photophysical and Theoretical Study

Two novel helical naphthopyrans have been synthesised. The helical scaffold has the interesting effect of increasing the thermal stability of the transoid-trans (TT) open isomer formed upon UV irradiation of the closed form (CF), which transforms these naphthopyrans from thermal to photochemical photochromes. The photochromic performance is excellent in both polar and apolar solvents and the conversion percentage from the CF to the TT form can be as high as 92.8 %. We propose a new method to determine the quantum yields of the photochemical processes that lead to transoid-cis (TC) and TT isomers, and their molar absorption coefficients. The thermal stability of the TT and TC isomers has been studied in different solvents. The quantum yields of fluorescence before and after irradiation, along with the decay lifetimes, have also been measured. TD-DFT calculations have been performed to determine the relative thermodynamic stability of the species involved in the photochromic mechanism and to rationalise their spectral properties.

Multiphotochromic molecular systems

We review molecular compounds encompassing several photochromic units with a focus on their functionalities.

Single molecule multiphotochromism with diarylethenes

In single photochromes, the two isomers that are interconverted in photoinduced reactions can serve as on and off states in a molecular switching device. The addition of several photochromic moieties onto a single molecule can allow the processing of more complex logical patterns. For example, an asymmetric triad could, in principle, store a byte, rather than a bit, of data. Because of the potential impact of multiphotochromic molecules in many research areas, over the past decade several groups have synthesized these coupled structures. The targets are easily addressable molecules that display increased contrast between the on and off states and in which all isomers have significantly distinguishable optical signatures. In this Account, we provide an overview of the multiswitchable molecular systems that incorporate at least one diarylethene group, which is the most successful thermally stable (P-type) organic photochrome. Up to this point, most systems have presented significant limitations. First of all, the reversibility of the processes is hindered by several side reactions more frequently than for single photochromes. Second, switching one part of the compound impedes the photoreactivity of other fragments in approximately 50% of the cases, and maximizing the electronic communication increases the probability of partial activity. In addition, most of the few synthesized operative systems only demonstrate cumulative absorption spectra rather than new features. Finally, it is impossible to selectively induce a chosen conversion because one wavelength might trigger several processes. We also emphasize the promising successes of asymmetric diarylethene dimers and trimers and molecules that combine two families of photochromes, such as diarylethene added to fulgimide or phenoxy-naphthacenequinone. In that framework, theoretical simulations offer complementary tools to investigate these structures, both to obtain structure/property relationships and to propose paths for the design of more efficient molecules. However, due to the size of the systems, researchers can only apply semiquantitative models. The investigation of the absorption spectra of the photochromes with time-dependent density functional theory (TD-DFT), the analysis of the topology of the LUMO + n (typically n = 1) of the closed-open hybrid, and an estimate of the steric stress in the hypothetical (ground-state) closed-closed structure serve as a useful combination of parameters to obtain initial insights regarding the photocyclization of the different open diarylethene groups. Nevertheless, because a first-order qualitative approach does not explore the potential energy surface of the photoexcited states, it remains inadequate for the investigation of some molecules.

Diarylethene-dihydroazulene multimode photochrome: a theoretical spectroscopic investigation

Using Time-Dependent Density Functional Theory, we have assessed the structural, energetic and spectroscopic properties of a hybrid diarylethene-dihydroazulene multi-addressable switch synthesized by Mrozek et al. [Chem. Commun., 1999, 1487-1488]. All possible closed/open structures have been considered and the different activation barriers along each reaction path have been evaluated. The determination of the energetic profiles allowed us to pinpoint the thermally possible reactions. To simulate the electronic absorption spectra of this compact two-way four-state hybrid compound, we relied on a PCM-TD-DFT approach combined with a molecular orbital analysis. Key insights are reached and the experimental photochromic properties of this multi-component switch are rationalised. From a more methodological point of view, this work also shows that range-separated hybrid functionals (CAM-B3LYP and ωB97XD) allow to reproduce the measured spectroscopic features with a remarkable accuracy.