One molecule to light it all: The era of dual-state emission

From cell imaging with precise subcellular localization to the fabrication of low-cost, more efficient optoelectronic devices for room illumination, Dual-State Emitter (DSE) molecules (DSEgens) can be considered as fine-tuned, tailored molecular entities capable of intensely fluorescing both in solution and in the solid state. The synthesis of these new kind of fluorophores requires innovative molecular blueprints as their successful development will enable a whole spectrum of possibilities for future uses in the fields of energy, analytical chemistry, or molecular biology. From cell imaging with precise subcellular localization to the fabrication of low-cost, more efficient optoelectronic devices for room illumination, Dual-State Emitter (DSE) molecules (DSEgens) can be considered as fine-tuned, tailored molecular entities capable of intensely fluorescing both in solution and in the solid state. The synthesis of these new kind of fluorophores requires innovative molecular blueprints as their successful development will enable a whole spectrum of possibilities for future uses in the fields of energy, analytical chemistry, or molecular biology. The design of organic fluorescent molecules has always been enticing due to their numerous superior features compared to purely inorganic compounds: higher stability in humid conditions, better solubility and processability, lighter weights, and emission tunability derived from structural changes in their core framework. These advantages have encouraged the search across a wide palette of applications, either in solution (for example, in biological bioimaging, like organelle probing) or in the solid state (for example, in optoelectronic devices like OLEDs or OPVs). Unsurprisingly, fluorescent organic molecules also have some drawbacks that need to be improved upon before reaching the market for the abovementioned purposes. Up until recently, traditional fluorophores could only be categorized into two large groups or classes. The first class includes those compounds that display an intense emission in dilute solutions but lack emission in the solid state due to detrimental π-π stacking interactions that emerge upon crystallization. This long-known phenomenon of fluorescence quenching is termed aggregation-caused quenching (ACQ) and occurs in highly used planar molecules like fluorescein, pyrene, and perylene derivatives, restricting their use to solution-only purposes. The second group comprises conjugated but twisted fluorophores that show strong emission only when they form aggregates or in their crystalline or amorphous forms, making them suitable only for solid-state applications. The latter group show a phenomenon termed aggregation-induced emission (AIE), which was first reported by Tang and coworkers in 2001. From that date, the scientific literature has witnessed a tremendous increase in reports with these compounds. Bearing in mind these two apparently mutually exclusive phenomena, we consider that the development of organic fluorophores that could emit in both states will be of huge interest from the fundamental science and applicative points of view. These compounds have been termed Dual-State Emitters or DSEgens and were envisioned as multipurpose molecules that could possibly be used in several applications as well as implemented in several technological devices, due to their rather small possibility of fluorescence quenching. Equally important is that achieving the dual-state emission in one molecule may also reduce several inconveniences derived from long, multiple-step synthetic methodologies in the synthesis of fluorophores, thus considerably reducing the time and effort employed in optimization of these methods. This preview will provide an overview of very recent remarkable examples of DSEgens to stimulate the discussion in this field. Although several research groups have reported DSEs, we consider that the synthetic strategies toward a more general design are still in their beginnings. We note that, in addition to the use of highly conjugated molecules, at least three additional features could increase the likelihood to obtain DSEgens for several applications: (1) the use of highly conjugated and twisted structures (Figure 1A), (2) the inclusion of aliphatic chains or voluminous substituents in the periphery of the molecules (Figure 1C), and/or (3) the inclusion of heteroatoms to achieve donor-acceptor (D-A) architectures (Figure 1E). The influence of each characteristic will be briefly detailed below. The first aspect is of great relevance because it allows the molecules to adopt slightly twisted conformations when they crystallize or during aggregation. These subtle changes in the shape of the molecules reduces the possibility for the existence of the deleterious π-π intermolecular interactions. It is important to note that although some degree of freedom is needed, an equilibrium should be established because obtaining molecules that experience extremely fast molecular rotations could enable undesired relaxation pathways and render a non-emissive compound. A good example of this delicate balance can be observed in the work by Zhou et al., which reported a novel one-pot synthetic route for the development of remarkable luminogens based on the V-shape furo[2,3-b]furans scaffolds and carbazole moieties with quantum yields of 92% and 42% in solution and solid-state emission, respectively (Figure 1A).1Zhou J. Huang M. Zhu X. Wan Y. One-pot synthesis of dual-state emission (DSE) luminogens containing the V-shape furo[2,3-b]furan scaffold.Chin. Chem. Lett. 2021; 32: 445-448Crossref Scopus (7) Google Scholar The second characteristic is the introduction of aliphatic chains or bulky groups in the periphery of the molecules; these substituents will enable “self-isolated” solid-state environments and improve the emission with better quantum yields. In addition, the inclusion of aliphatic chains will enhance the solubility of compounds, which would make them more solution-processable for device implementation. Ramamurthy and coworkers reported novel water-soluble acetylpyrene-tagged imidazolium salts that could be applied to forensic science. The pyrene portions in these molecules show emission in both states due to the presence of an imidazolium ring that prevents π-π interactions upon crystallization, thus enhancing the solid-state emission.7Nirmala M. Vadivel R. Chellappan S. Malecki J.G. Ramamurthy P. Water-Soluble Pyrene-Adorned Imidazolium Salts with Multicolor Solid-State Fluorescence: Synthesis, Structure, Photophysical Properties, and Application on the Detection of Latent Fingerprints.ACS Omega. 2021; 6: 10318-10332Crossref PubMed Scopus (1) Google Scholar Finally, the third feature, which is extremely useful for the technological application of these compounds, is based on the combination of strong push-pull intramolecular electronic effect. Joining electron-donating and electron-withdrawing components into a π-conjugated structure allows control over the Highest-Occupied Molecular Orbital (HOMO) and Lowest-Unoccupied Molecular Orbital (LUMO) energy levels. Among the most commonly used molecular blocks in D-A systems used in DSEgen designs, donor units such as tetraphenylethylene (TPE)8Ni Y. Zhang S. He X. Huang J. Kong L. Yang J. Yang J. Dual-state emission difluoroboron derivatives for selective detection of picric acid and reversible acid/base fluorescence switching.Anal. Methods. 2021; (in press. Published online May 5, 2021)https://doi.org/10.1039/d1ay00477hCrossref Scopus (7) Google Scholar or triphenylamine (TPA)9Gayathri P. Pannipara M. Al-Sehemi A.G. Anthony S.P. Recent advances in excited state intramolecular proton transfer mechanism-based solid state fluorescent materials and stimuli responsive fluorescence switching.CrystEngComm. 2021; 23: 3771-3789Crossref Google Scholar stand out because they also comply with the first aspect of presenting distorted conformations. Complementarily, using acceptors such as benzothiadiazole (BTD),3Cui L. Gong Y. Cheng C. Guo Y. Xiong W. Ji H. Jiang L. Zhao J. Che Y. Highly Photostable and Luminescent Donor-Acceptor Molecules for Ultrasensitive Detection of Sulfur Mustard.Adv. Sci. (Weinh.). 2021; 8: 2002615PubMed Google Scholar BODIPYs,10Nakano T. Sumida A. Naka K. Synthesis and Characterization of Boron Difluoride Complexes Bearing π-Expanded Pyridine Ligands as Organic Fluorochromes.J. Org. Chem. 2021; 86: 5690-5701Crossref PubMed Scopus (8) Google Scholar or more recently, squaraine dyes,5Xia G. Shao Q. Liang K. Wang Y. Jiang L. Wang H. A phenyl-removal strategy for accessing an efficient dual-state emitter in the red/NIR region guided by TDDFT calculations.J. Mater. Chem. C. 2020; 8: 13621-13626Crossref Google Scholar has been reported to produce excellent results both in solution and solid state when combined with the previously mentioned features. A significant example of the amalgamation of these characteristics is in the work by Cui et al., who designed promising DSEs with high photostability and photoluminescence using a slightly twisted D-A architecture with bulky alkyl chains. This compound could be applied as a fluorescence sensor for hazardous chemicals such as sulfur mustard (Figure 1B).3Cui L. Gong Y. Cheng C. Guo Y. Xiong W. Ji H. Jiang L. Zhao J. Che Y. Highly Photostable and Luminescent Donor-Acceptor Molecules for Ultrasensitive Detection of Sulfur Mustard.Adv. Sci. (Weinh.). 2021; 8: 2002615PubMed Google Scholar A subtle equilibrium between planarity, rigidity, and solubility can also be established through the presence of a radiative relaxation known as Excited-State Intermolecular Proton Transfer (ESIPT). In compounds that have this property, the equilibrium between tautomeric keto and enol forms and the subsequent proton transfer during the excited state causes the fluorescence emission with large red-shifting or even dual emission. A beautiful example is described by Pariat et al., using a series of 2-(2’-Hydroxyphenyl)benzazoles (Figure 1D). In these compounds, the crystal lattice reveals an intramolecular H-bond that induces a co-planar arrangement in the ground state. The nature of the heteroatoms in these dyes has a huge impact in the emission wavelength by stabilizing their first excited state and, consequently, the quantum yield values in both solution and the solid state.4Pariat T. Munch M. Durko-Maciag M. Mysliwiec J. Retailleau P. Vérité P.M. Jacquemin D. Massue J. Ulrich G. Impact of Heteroatom Substitution on Dual-State Emissive Rigidified 2-(2¢-hydroxyphenyl)benzazole Dyes: Towards Ultra-Bright ESIPT Fluorophores.Chemistry. 2021; 27: 3483-3495Crossref PubMed Scopus (15) Google Scholar The combination of the aforementioned strategies could lead to molecules capable of showing complex concomitant features, such as solvatochromism and mechanofluorochromism, desirable attributes for applications such as chemical sensors (Figure 1B) or anticounterfeiting/optical encoding (Figures 1A and 1F).6Zheng X. Wang J. Xiao D. Chen H. Lin Z. Ling Q. Highly emissive fused diarylmaleimides synthesized by a cascade reaction of selective bromination and visible-light-driven cyclization.Dyes Pigm. 2021; 187: 109113-109120Crossref Scopus (4) Google Scholar,2Zhang Y. Zhang T. Wang X. Kong L. Yang J. Indolo[3,2-b]carbazole derivatives with high fluorescent emission both in solution and aggregated states and mechanical-induced emission enhancement characteristic.Dyes Pigm. 2021; 188: 109230Crossref Scopus (6) Google Scholar Even though dual-state emission is an advantageous and attractive feature to achieve unique optical properties in one molecule, synthesizing these compounds is still a formidable challenge. We believe that this preview will help incentivize the design of new DSE compounds for unseen applications in the fields of medicine, energy, and organic electronics. L.A.R.-C. (849196) and A.N.-H. (957838) thank CONACYT for their Ph.D. scholarships.