Polymers and Materials
Turn on Light Emission by "Waving Magic Wand"
by Ben Zhong Tang
March 28, 2016
Luminescence is an increasingly important property of molecules and polymers (see “What Is Luminescence?”). Luminescent materials are used in devices ranging from organic light-emitting diodes (OLEDs) to biological imaging systems.
New luminescent molecules are usually produced in chemical reactions that assemble π-conjugated building blocks by forming covalent bonds. This strategy, although common, can be troublesome, expensive, and synthetically demanding. It can also involve difficulties in product isolation and purification.
Can a light emitter be readily generated by simpler means? Two research groups recently developed ways to turn on light emission by mixing inexpensive commodity reagents with nonluminescent compounds. They created amazing effects that metaphorically turn a “dark stone” into “shining gold” by waving a “magic wand.”
Perchlorate makes a difference
The ionic dinuclear iridium(III) complex 1 shown in Figure 1 is nonluminescent when it is dissolved in “good” solvents. The quantum yield of luminescence (ΦL) of 1 in a 1:4 v/v mixture of acetonitrile and water, for example, is practically nil (ΦL ≈ 0.03%). The excited states of the isolated molecular species of 1 in the dilute aqueous solution may be nonradiatively annihilated by structural distortion in the triplet state.
What Is Luminescence?
Luminescence is the emission of light by a substance when its molecules are excited by an external energy source and then relax back to the ground state via a radiative decay process.
Photoluminescence is an example of luminescence in which the energy source is photonic, such as a beam of ultraviolet light. Photoluminescent emission can be fluorescent or phosphorescent.
Fluorescence is the photoluminescence process in which singlet excitons (molecules in an excited state) decay radiatively back to the ground state. The typical fluorescence lifetime is in the nanosecond range.
Phosphorescence is the photoluminescence process in which triplet excitons decay radiatively back to the ground state. Phosphorescence lifetimes are much longer, ranging from milliseconds to seconds or even hours.
Dongxia Zhu, Zhongmin Su, Martin R. Bryce, and colleagues at Northeast Normal University (Changchun, China) and Durham University (UK) transformed 1 into an efficient luminogen simply by mixing it with perchlorate (ClO4–), an anion that is commercially available as its sodium salt. Adding ClO4– to the aqueous mixture of 1 quickly replaces PF6– with ClO4– to give complex 2. The newly formed complex is immiscible in water, and its molecules aggregate into nanoparticles.
In the aggregates, structural distortion is restricted, and light emission is turned on. The aggregates of 2 efficiently phosphoresce with a ΦL value of 13%, ≈430-fold greater than that of soluble form 1. (Chem. Commun. DOI: 10.1039/c5cc07187a)
Turn on the light with B(C6F5)3
Carbonyl-containing molecules such as aldehydes and ketones are normally nonfluorescent. Their n–π* singlet states readily funnel photoexcited electrons into π–π* triplet states, effectively shutting the channel for fluorescent decay. A. Stephen K. Hashmi, Carlos Romero-Nieto, and coauthors at Heidelberg University (Germany) and King Abdulaziz University (Jeddah, Saudi Arabia) developed a straightforward protocol for efficiently turning on light emission from benzaldehydes (3a–k in Figure 2) by simple coordination with the Lewis acid tris(pentafluorophenyl)borane [B(C6F5)3].
Neither B(C6F5)3 nor the benzaldehydes fluoresce the solid state. Mixing B(C6F5)3 with 3a–k under ambient conditions forms substituted benzaldehyde–borane adducts (4 a–k) instantly and quantitatively.
Solid aggregates of the aldehyde adducts emit efficiently, with ΦL values as high as 64%. The variety of substituents in 4b–k produces emission colors from blue to red that cover almost the whole visible spectral region (Figure 3).
In their structural studies, the authors found very short F–F distances (2.83–2.94 Å) between the B(C6F5)3 rings in the adducts. This noncovalent intermolecular interaction blocks nonradiative decay processes and enables adducts in the 4 series to emit efficiently. (Angew. Chem., Int. Ed. DOI:10.1002/anie.201508461)
Life becomes easier
The common practice for developing new luminescent materials has been to join various conjugated units at the molecular level via chemical reactions. The studies discussed here clearly demonstrate that new light emitters can be easily generated by mixing nonemitting components, particularly commercially available ions or compounds. The simplicity of these methods makes life much easier in terms of preparing functional materials with efficient light emission, especially for practical solid-state applications.