One-axis Two-wing Guest-Host Strategy for Constructing Ultralong-lifetime Near-infrared Organic Phosphorescence Materials for Bioimaging

Organic near-infrared room temperature phosphorescence (RTP) materials have unparalleled advantages in bioimaging due to their excellent penetrability. However, limited by the energy gap law, organic matters with long wavelengths (> 700 nm) and long lifetimes (> 100 ms) have not been reported so far. In this work, we have obtained organic RTP materials with long wavelengths (657–732 nm) and long lifetimes (102–324 ms) for the first time through the one-axis two-wing guest-host strategy. The one axis refers to that the guest molecule has sufficient conjugation to reduce the lowest triplet energy level and the two wings refer to that the host assists the guest in exciton transfer and inhibits the non-radiative transition of guest excitons. These materials exhibit good tissue penetration in bioimaging. Thanks to the characteristic of long lifetime and long wavelength emissive RTP materials, the tumor imaging in living mice with a signal to background ratio value as high as 43 is successfully realized. This work provides a practical solution for the construction of organic RTP materials with both long wavelengths and long lifetimes.

Overcome the Energy Gap Law in Near-Infrared Emissive Aggregates via Nonadiabatic Coupling Suppression

<p>The pursuing of purely organic materials with high-efficiency near-infrared (NIR) emissions is fundamentally limited by the large non-radiative decay rates (<i>k</i>_nr) governed by the energy gap law. Here, we demonstrated a feasible and innovative strategy by employing intermolecular charge-transfer (CT) aggregates (CTA) to realize high-efficiency NIR emissions via nonadiabatic coupling suppression. The formation of CTA engenders intermolecular CT in the excited states; thereby, not only reducing the electronic nonadiabatic coupling and contributing to small <i>k</i>_nr for high-efficiency NIR photoluminescence, but also stabilizing excited-state energies and achieving thermally activated delayed fluorescence for high-efficiency NIR electroluminescence. This work provides new insights into aggregates and opens a new avenue for organic materials to overcome the energy gap law and achieve high-efficiency NIR emissions.<br></p>

Overcome the Energy Gap Law in Near-Infrared Emissive Aggregates via Nonadiabatic Coupling Suppression

<p>The pursuing of purely organic materials with high-efficiency near-infrared (NIR) emissions is fundamentally limited by the large non-radiative decay rates (<i>k</i>_nr) governed by the energy gap law. Here, we demonstrated a feasible and innovative strategy by employing intermolecular charge-transfer (CT) aggregates (CTA) to realize high-efficiency NIR emissions via nonadiabatic coupling suppression. The formation of CTA engenders intermolecular CT in the excited states; thereby, not only reducing the electronic nonadiabatic coupling and contributing to small <i>k</i>_nr for high-efficiency NIR photoluminescence, but also stabilizing excited-state energies and achieving thermally activated delayed fluorescence for high-efficiency NIR electroluminescence. This work provides new insights into aggregates and opens a new avenue for organic materials to overcome the energy gap law and achieve high-efficiency NIR emissions.<br></p>

Local Electric Field Controls Fluorescence Quantum Yield of Red and Far-Red Fluorescent Proteins

Genetically encoded probes with red-shifted absorption and fluorescence are highly desirable for imaging applications because they can report from deeper tissue layers with lower background and because they provide additional colors for multicolor imaging. Unfortunately, red and especially far-red fluorescent proteins have very low quantum yields, which undermines their other advantages. Elucidating the mechanism of nonradiative relaxation in red fluorescent proteins (RFPs) could help developing ones with higher quantum yields. Here we consider two possible mechanisms of fast nonradiative relaxation of electronic excitation in RFPs. The first, known as the energy gap law, predicts a steep exponential drop of fluorescence quantum yield with a systematic red shift of fluorescence frequency. In this case the relaxation of excitation occurs in the chromophore without any significant changes of its geometry. The second mechanism is related to a twisted intramolecular charge transfer in the excited state, followed by an ultrafast internal conversion. The chromophore twisting can strongly depend on the local electric field because the field can affect the activation energy. We present a spectroscopic method of evaluating local electric fields experienced by the chromophore in the protein environment. The method is based on linear and two-photon absorption spectroscopy, as well as on quantum-mechanically calculated parameters of the isolated chromophore. Using this method, which is substantiated by our molecular dynamics simulations, we obtain the components of electric field in the chromophore plane for seven different RFPs with the same chromophore structure. We find that in five of these RFPs, the nonradiative relaxation rate increases with the strength of the field along the chromophore axis directed from the center of imidazolinone ring to the center of phenolate ring. Furthermore, this rate depends on the corresponding electrostatic energy change (calculated from the known fields and charge displacements), in quantitative agreement with the Marcus theory of charge transfer. This result supports the dominant role of the twisted intramolecular charge transfer mechanism over the energy gap law for most of the studied RFPs. It provides important guidelines of how to shift the absorption wavelength of an RFP to the red, while keeping its brightness reasonably high.

Multiple Resonance Deep-red Emitters with Hybridized π-bonding/ non-bonding Orbitals to Surpass the Energy Gap Law

Efficient organic emitters in the deep-red to near infrared region are rare due to the ‘energy gap law’. Here, multiple boron (B)- and nitrogen (N)-atoms embedded polycyclic heteroaromatics featuring hybridized π-bonding/ non-bonding molecular orbitals are constructed, providing a way to overcome the above luminescent boundary. The introduction of B-phenyl-B and N-phenyl-N structures enhances the electronic coupling of those para-positioned atoms, forming restricted π-bonds on the phenyl-core for delocalized excited states and thus a narrow energy gap. The mutually ortho-positioned B- and N-atoms also induce a multiple resonance effect on the peripheral skeleton for the non-bonding orbitals, creating shallow potential energy surfaces to eliminate the high-frequency vibrational quenching. The corresponding deep-red emitters with peaks at 662 nm and 692 nm exhibit narrow full-width at half-maximums of 38 nm, high radiative decay rates of ~108 s-1, ~100% photo-luminance quantum yields and record-high maximum external quantum efficiencies of >28% in a normal planar organic light-emitting diode structure, simultaneously.

Equivalence between inverted regions of the energy gap law and inverted regions of donor–acceptor distances in photoinduced electron transfer processes in flavoproteins

Relationship between EXDL and SEGL.

Beating the Limitation of Energy Gap Law Utilizing Deep Red MR-TADF Emitter with Narrow Energy-Bandwidth

<p><b>The development of high-performance deep red/near-infrared organic light-emitting diodes is hindered by strong non-radiative processes as governed by the energy gap law. </b><b>Herein, a novel BN-containing skeleton featuring linear N-π-N and B-π-B structure is developed, establishing partial </b><b>bonding/antibonding character on phenyl core for enhanced electronics coupling of para-positioned B atoms as well as N atoms to narrow energy gaps. Also, the remained MR effect on the peripheral skeleton to maintain the MR effect to minimize the bonding/ antibonding character and suppress vibrational coupling between S₀ and S₁, thereby </b><b>fundamentally</b><b> overcoming the luminescent boundary set by the energy gap law. The target</b><b> molecules </b><b>R-BN and R-TBN exhibited extremely high</b><b> PLQYs of 100% with </b><b>emission wavelengths at 666 and 686 nm,</b><b> respectively. The narrow FWMHs of 38 nm observed also testify the effectiveness of vibronic suppression. The corresponding OLEDs afford</b><b> </b><b>record-high</b><b> EQEs over 28% with emission wavelength over 664 nm</b><b>. </b><b></b></p>