Photochemistry

Multifunctional Luminogens

by Ben Zhong Tang

June 27, 2016

Luminescent materials are critically important in a variety of high-tech innovations. In particular, a large research effort is devoted to creating luminogenic molecules with multiple functionalities.

Two groups of scientists recently developed series of organic and organometallic luminogens that simultaneously show the photophysical effects of aggregation-induced emission (AIE) and thermally activated delayed fluorescence (TADF). Another group has produced carbon dots (CDs) with triple-mode light emission: fluorescence, phosphorescence, and upconversion photoluminescence. Their findings are reported here.

Pure organics

Figure 1

Organic luminogens often exhibit aggregation-caused quenching (ACQ) or the AIE effect. In an ACQ system, the luminogens emit in the solution state but become nonluminescent as solids. The behavior of AIE luminogens is the reverse: They emit in the solid state and are nonemissive in solution.

Xinhai Zhu, Yiqian Wan, and co-workers at Sun Yat-sen University (Guangzhou, China) synthesized a series of organic luminogens such as 13 in Figure 1. (Mesyl is methanesulfonyl; p-tosyl is p-toluenesulfonyl.) Their photophysical behavior changes with variations in their molecular structures.

The luminogens are all molecular rotors with multiple rotating units. Luminogen 1 is an ACQ dye: The quantum yields of fluorescence (ΦF) of its solution in benzene (a nonpolar solvent) and its solid powder are 13 and 2%, respectively. If the R group is an electron acceptor, the luminogen is AIE-active. Thus 2 is totally nonemissive (ΦF = 0%) in benzene, but its solid powder is emissive (ΦF = 39%).

Some luminogens exhibit strong solvatochromic effects; they emit efficiently when they are dissolved in polar solvents but not in nonpolar ones. Luminogen 3, for example, displays a ΦF value of 100% in water, whereas its benzene solution is much less emissive at 7%. The luminogen powder is also emissive. Thus 3 exhibits AIE-like emission when it is dissolved in a nonpolar solvent and in the solid state. (Chem. Sci. DOI: 10.1039/c6sc01254j)

Organic–inorganic hybrids

Figure 2

Boron clusters such as carboranes are used as building blocks to form molecular hybrids or organometallic complexes with unique functional properties. Using o-carborane as the central core, a team led by Takuma Yasuda at Kyushu University (Fukuoka, Japan) prepared a group of “Janus” hybrid triads that contain a single boron cluster functionalized with an electron donor and an electron acceptor (4 and 5 in Figure 2) or two electron donors (6). (Janus is the Greek god with two opposing faces; a Janus molecule has two or more contrasting functionalities.)

The electron donor groups are 9-phenylcarbazole (4 and 6) and triphenylamine (5). The acceptors are 2,4,6-triphenyl-1,3,5-triazine (4 and 5) and o-carborane itself.

None of the dilute solutions of the triads are emissive (ΦF = 1–3%). Their solid films, however, are highly luminescent (ΦF as high as 97%); therefore, the triads show marked AIE activity. The triads can harvest singlet (S1) and triplet (T1) excitons and display the TADF effect via the upconversion of nonradiative T1 excitons to emissive S1 excitons in the aggregated state.

Because of the combined effects of AIE and TADF (or aggregation-induced delayed fluorescence), high external electroluminescence quantum efficiencies of >10% can be attained in organic light-emitting diodes that use 46 as nondoped emitting layers. These efficiencies are much greater than those that are obtained from the conventional fluorophores. (Angew. Chem., Int. Ed. DOI: 10.1002/anie.201603232)

Carbon dots

Figure 3

CDs are small nanoparticles composed of carbonaceous materials; they exhibit attractive properties such as high stability, good conductivity, low toxicity, environmental benignancy, and luminescence behaviors comparable to quantum dots. CDs have been extensively investigated because of their strong, tunable fluorescence. Hengwei Lin and coauthors at Ningbo Institute of Materials Technology & Engineering, Chongqing University, and Southeast University (Nanjing, all in China) recently added to the CD family by producing CDs with triple-mode emissions.

It is very difficult for a single luminogen to display multimodal light emission. The authors achieved simultaneous fluorescence, phosphorescence, and upconversion photoluminescence at ambient temperature in CDs prepared by the solvothermal reaction of m-phenylenediamine (7 in Figure 3).

The CDs were dispersed in a transparent poly(vinyl alcohol) (PVA) matrix to form CD–PVA composite films. When excited by an ultraviolet lamp (365 nm) and a near-infrared pulse laser (800 nm), the films emitted blue fluorescence and cyan upconversion photoluminescence, respectively.

When the CD–PVA films were illuminated for a few seconds by the UV lamp, they emitted a green afterglow that was visible to the naked eye. This phenomenon is known as room-temperature phosphorescence. (Angew. Chem., Int. Ed. DOI: 10.1002/anie.201602445)

How do they work?

The light-emitting performance of multifunctional luminogens is impressive, but their working mechanisms are not fully understood. For example, why is luminogen 1 ACQ-active and 2 AIE-active? Intramolecular hydrogen bonding (H-bonding) in solution and intermolecular π–π interactions in the solid state may be the causes of the ACQ behavior of 1.

The electron-withdrawing unit introduced into the molecular structure of 2 may weaken the H-bonds and activate intramolecular rotation, thus making its solution nonemissive. In its aggregated state, though, the twisted conformation (again the result of H-bond weakening) hampers intermolecular π–π interactions, making the aggregates emissive. The validity of this hypothesis, however, must be proved experimentally.

For CDs, a key mechanistic question is: What is the component responsible for their light emission? CDs are nanoparticles composed of uncertain structures. They are insoluble in almost all solvents; this intractability has prevented clear structural characterization.

In the CDs developed by Lin et al., the authors hypothesized that the π–π* transition of the C=C bonds and the n–π* transitions of the C–N and C=N bonds, as well as structural rigidification by the H-bonds between the CD particles and the PVA chains, account for their light emission. It was recently found that CDs prepared from nonaromatic compounds that contain heteroatoms (e.g., O and N) at relatively low temperatures are also luminescent, suggesting that heteroatom clusters formed in the CD systems may play an important role in their light emission processes.