The way in which photons travel through biological tissues and subsequently become scattered or absorbed is a key limitation for traditional optical medical imaging techniques using visible light. In contrast, near-infrared wavelengths, in particular those above 1000 nm, penetrate deeper in tissues and undergo less scattering and cause less photo-damage, which describes the so-called “second biological transparency window”. Unfortunately, current dyes and imaging probes have severely limited absorption profiles at such long wavelengths, and molecular engineering of novel NIR-II dyes can be a tedious and unpredictable process, which limits access to this optical window and impedes further developments. Two-photon (2P) absorption not only provides convenient access to this window by doubling the absorption wavelength of dyes, but also increases the possible resolution. This review aims to provide an update on the available 2P instrumentation and 2P luminescent materials available for optical imaging in the NIR-II window.

A high efficiency of 7.47% is obtained in non-fullerene OSCs using PPDI-Se as the acceptor and PDBT-T1 as the donor.

High efficiencies of 6.85% and 7.63% are obtained in non-fullerene OSCs using TPE-PDI4-S/TPE-PDI4-Se as the acceptors and PBDB-T1 as the donor.

Figure 4. Cyclic voltammetrycurves of the three compoundsinC H 2 Cl 2 containing 0.1 m Bu 4 NPF 6 electrolyte. Scanning rate:50mVs À1 . Figure 5. Wave functions for the HOMO and LUMO of the three compounds.

Three novel N-heteroacene molecules (SDNU-1, SDNU-2 and SDNU-3)b ased on tetraazachrysene units as cores have been designed, synthesized and fully characterized. Their photophysical, electrochemical and fluorescence properties were investigated, and they exhibited blue to green emission in the solid state. Interestingly, SDNU-2 exhibited high solid photoluminescence quantum efficiencies (75.3 %), which is the highest value of N-heteroacenes derivatives to date. Two-photon absorption studies have been conducted by using the open and close apertureZ -san tech-nique. SDNU-3 showed as ignificant enhancement in the two-photon absorption cross-section with magnitudes as high as about 700 GM (1 GM = 1 10 À50 cm 4 s/photon)w hen excited with 800 nm light, which is the largest value based on ah eteroacene system measured by using aZ -scan experiment so far.W ea ttribute the outcomet os ufficient electronic coupling between the strong charge transfer of quadrupolars ubstituents and the tetraazachrysenec ore. Our result would provide an ew guideline to design novel efficient two-photon materials based on N-heteroacene cores.