Intersystem crossing (ISC) of triplet photosensitizers is a vital process for fundamental photochemistry and photodynamic therapy (PDT). Herein, we report the co‐existence of efficient ISC and long triplet excited lifetime in a heavy atom‐free bodipy helicene molecule. Via theoretical computation and time‐resolved EPR spectroscopy, we confirmed that the ISC of the bodipy results from its twisted molecular structure and reduced symmetry. The twisted bodipy shows intense long wavelength absorption (ϵ=1.76×10⁵ m⁻¹ cm⁻¹ at 630 nm), satisfactory triplet quantum yield (Φ_T=52 %), and long‐lived triplet state (τ_T=492 μs), leading to unprecedented performance as a triplet photosensitizer for PDT. Moreover, nanoparticles constructed with such helical bodipy show efficient PDT‐mediated antitumor immunity amplification with an ultra‐low dose (0.25 μg kg⁻¹), which is several hundred times lower than that of the existing PDT reagents.

A new method for the reconstruction of velocity maps from ion images is presented, which is based on the maximum entropy concept. In contrast to other methods used for Abel inversion the new method never applies an inversion or smoothing to the data. Instead, it iteratively finds the map which is the most likely cause for the observed data, using the correct likelihood criterion for data sampled from a Poissonian distribution. The entropy criterion minimizes the information content in this map, which hence contains no information for which there is no evidence in the data. Two implementations are proposed, and their performance is demonstrated with simulated and experimental data: Maximum Entropy Velocity Image Reconstruction (MEVIR) obtains a two-dimensional slice through the velocity distribution and can be compared directly to Abel inversion. Maximum Entropy Velocity Legendre Reconstruction (MEVELER) finds one-dimensional distribution functions Q_l(v) in an expansion of the velocity distribution in Legendre polynomials _l(cos θ) for the angular dependence. Both MEVIR and MEVELER can be used for the analysis of ion images with intensities as low as 0.01 counts per pixel, with MEVELER performing significantly better than MEVIR for images with low intensity. Both methods perform better than pBASEX, in particular for images with less than one average count per pixel.

Triplet photosensitizers (PSs) have attracted much attention in recent years, due to the significance of these kinds of compounds in fundamental photochemistry studies and applications in photocatalysis, photodynamic therapy (PDT) and triplet–triplet-annihilation upconversion (TTA-UC). Heavy-atom-free triplet PSs are of particular interest, because of the advantages of low cost, low toxicity and long-lived triplet states compared with triplet PSs that rely on the heavy-atom effect to enhance intersystem crossing (ISC). This review article summarizes recent developments in the design and application of heavy-atom-free triplet PSs, including those based on compact orthogonal electron donor–acceptor dyads, or compounds with twisted π-conjugation frameworks, or those that contain thiocarbonyl groups. The ISC mechanisms of these different types of triplet PSs are discussed, and all are based on enhanced spin–orbit coupling. Moreover, the electron spin selectivity of ISC, studied via time-resolved electron paramagnetic resonance (TREPR) spectra, is also introduced. Some new discoveries and the challenges in this area are discussed, for instance, the role of the triplet charge-transfer (³CT) state in the spin–orbit charge-transfer ISC (SOCT-ISC) mechanism, the relationship between the twisted π-conjugation frameworks and the ISC efficiency, and determination of the intrinsic triplet-state lifetimes.