Conventional optical imaging, particularly fluorescence imaging, often encounters significant background noise due to tissue autofluorescence under real-time light excitation. To address this issue, a novel optical imaging strategy called delayed photon emission that captures optical signals after light excitation has been developed.
Delayed photon emission staggers the time window of energy excitation and signal acquisition, and only collects the delayed optical signal after removal of excitation source, thus minimizing the background noise during signal acquisition. This imaging strategy relies on molecular probes exhibiting slow photon release kinetics after absorbing excitation energy to achieve delayed emission. Elucidation on the energy conversion mechanism under excitation by various energy sources and the photophysical or photochemical mechanism with the potential for delayed photon emission is significant for designing and developing such molecular probes.
Recently, we reviewed recent advancements in organic molecular probes designed for delayed photon emission using various energy sources. We discuss distinct mechanisms, and molecular design strategies, and offer insights into the future development of organic molecular probes for enhanced delayed photon emission.
1. Three photophysical/photochemical mechanisms for delay photon emission
The delayed photon emission behavior of molecular probes can be observed at various stages from excitation to emission and can be categorized into three mechanisms in the organic system: (1) Charge separation and recombination, which uses the charge diffusion process to gradually produce exciplex emission; (2) Generation, stabilization and conversion of the triplet excitons, which utilizes crossing between energy states and additional stabilizing energies in traps to extend excitons lifetime; (3) Generation and decomposition of chemical traps, where photoenergy is transformed into chemical energy to create metastable emissive intermediates.
Figure 1. Schematic mechanism of charge separation and recombination process for delayed photon release.
Figure 2. Schematic mechanism for delayed photon emission via generation, stabilization, and conversion of triplet excitons.
Figure 3. Schematic mechanism of delayed photon emission via generation and decomposition of chemical traps.
2. X-ray/ultrasound energy activation facilitates deep-penetrating in vivo imaging of delayed photon emission
Another challenge in optical imaging is the limited tissue penetration depth of light, which severely restricts the efficiency of energy delivery, leading to a reduced penetration depth for delayed photon emission. In contrast, X-ray and ultrasound serve as deep-tissue energy sources that facilitate the conversion of high-energy photons or mechanical waves into the potential energy of excitons or the chemical energy of intermediates, which has been proven of great feasibility in bioimaging.
(1) X-ray consists of high-energy photons with extremely short wavelengths that can penetrate through atomic intervals. Consequently, X-ray is inherently capable of delivering energy efficiently to targeted areas deep within tissues, enhancing energy storage for delayed emission. It is important to note that X-ray energy is generally too high for valence shell electrons to absorb. While a fraction of high-energy electrons can be generated via Compton scattering, only the inner electrons, which are seated in a deep potential well, can be activated through the photoelectric effect and subsequently ionized by X-ray. This activation process generates electron-hole pairs and initiates a cascade of secondary electron excitations through Auger–Meitner decay or intermolecular Coulombic decay. These secondary excitations subsequently relax into a non-equilibrium distribution before photon emission occurs.
Figure 4. Mechanism and designing strategies for X-ray-activated delayed photon emission.
(2) Different from the direct generation of excitons or separated charges under light or X-ray activation, the energy transformation via ultrasound relies significantly on the transmission medium. This transformation can manifest as sonoluminescence in water, a phenomenon where spontaneous luminescence is triggered by plasma generation during bubble cavitation in a sound field. Under varying acoustic pressure amplitudes, gas-filled bubbles induced by the sound field undergo periodic expansions and energy extraction, followed by violent collapses. Once the collapsing bubble well attains supersonic velocities, microshock waves converge at the core of the bubble, leading to rapid compression. This compression heats the gas to high temperatures, causing ionization to form plasma, which in turn generates light. Although such sonoluminescence is highly restrained in direct imaging applications due to its low brightness and short duration, the cavitation process is capable of exciting nearby sonosensitizers to generate reactive species, storing the mechanical energy of ultrasound in the form of chemical energy. Another energy transformation strategy for ultrasound is interaction with mechanical-force-responsive materials to induce reactive species production and light emission. One common approach is acoustically mediatedpiezoelectric stimulation. High-frequency ultrasound can induce polarization in piezoelectric materials, creating an endogenous electric field that separates electrons and holes, thus transforming ultrasound’s mechanical energy into potential energy. The separated charges accumulate on opposite surfaces and engage in oxidation and reduction reactions with substrates in an aqueous solution, leading to the generation of ROS. This conversion process effectively translates mechanical energy into chemical energy. Both organic and inorganic piezoelectric materials have been proven as effective piezo catalysts for ROS generation, underscoring their potential in the fabrication of chemical traps and confirming the broad applicability of piezoelectric materials in ultrasound-activated delayed photon emission.
Figure 5. Mechanism and designing strategies for ultrasound-activated delayed photon emission.
Based on the energy conversion mechanism of X-ray and ultrasound activated molecular probes, we further summarized the design strategy of X-ray and ultrasound-activated delayed photon emission molecular probes and introduced the in vivo imaging application of delayed photon emission activated by these two deep energy sources.
The related review entitled “Light/X-ray/ultrasound activated delayed photon emission of organic molecular probes for optical imaging: mechanisms, design strategies, and biomedical applications”was published on Chemical Society Review on October 9, 2024 (Paper link: https://www.pubs.rsc.org/en/content/articlehtml/2024/cs/d4cs00599f, DOI: 10.1039/D4CS00599F). Prof. Xu Zhen and Prof. Xi-Qun Jiang from our department are the co-corresponding authors. PhD. Student Rui Qu is the first author. This research was supported by the National Natural Science Foundation of China.