SUMMARY We present a modular approach for analyzing calcium imaging recordings of large neuronal ensembles. Our goal is to simultaneously identify the locations of the neurons, demix spatially overlapping components, and denoise and deconvolve the spiking activity from the slow dynamics of the calcium indicator. Our approach relies on a constrained nonnegative matrix factorization that expresses the spatiotemporal fluorescence activity as the product of a spatial matrix that encodes the spatial footprint of each neuron in the optical field and a temporal matrix that characterizes the calcium concentration of each neuron over time. This framework is combined with a novel constrained deconvolution approach that extracts estimates of neural activity from fluorescence traces, to create a spatiotemporal processing algorithm that requires minimal parameter tuning. We demonstrate the general applicability of our method by applying it to in vitro and in vivo multineuronal imaging data, whole-brain light-sheet imaging data, and dendritic imaging data.

The photophysical properties of tetraphenylethene (TPE) compounds may differ widely depending on the substitution pattern, for example, with regard to the fluorescence quantum yield ϕf and the propensity to exhibit aggregation-induced emission (AIE). We report combined electronic structure calculations and nonadiabatic dynamics simulations to study the excited-state decay mechanisms of two TPE derivatives with four methyl substituents, either in the meta position (TPE-4mM, ϕf = 0.1%) or in the ortho position (TPE-4oM, ϕf = 64.3%). In both cases, two excited-state decay pathways may be relevant, namely, photoisomerization around the central ethylenic double bond and photocyclization involving two adjacent phenyl rings. In TPE-4mM, the barrierless S1 cyclization is favored; it is responsible for the ultralow fluorescence quantum yield observed experimentally. In TPE-4oM, both the S1 photocyclization and photoisomerization paths are blocked by non-negligible barriers, and fluorescence is thus feasible. Nonadiabatic dynamics simulations with more than 1000 surface hopping trajectories show ultrafast cyclization upon photoexcitation of TPE-4mM, whereas TPE-4oM remains unreactive during the 1 ps simulations. We discuss the chances for spectroscopic detection of the postulated cyclic photoproduct of TPE-4mM and the relevance of our findings for the AIE process.

Thermally activated delayed fluorescence (TADF) phenomena have been found in many organometallic complexes, but Au­(III) complexes with TADF are rarely reported, possibly due to the existence of efficient nonradiative channels for luminescence states. Recent experiments identified two cyclometalated Au­(III) aryl molecules with TADF; however, the underlying photophysical and luminescence mechanisms are elusive. Here, we have employed M06 and TD-M06 methods combined with polarizable continuum model and quantum mechanics/molecular mechanics approaches to comprehensively study the excited-state structures and properties of these two Au­(III) complexes in toluene solution and crystal phases, respectively. We have found that both S1 and T1 states are of ligand-to-ligand charge transfer character. The significant twisting between C∧N∧C and aryl groups leads to good separation and negligible overlap of the highest occupied molecular orbital and lowest unoccupied molecular orbital. This results in a pretty small S1–T1 energy gap, which, in conjunction with strong spin–orbit coupling, facilitates the reverse intersystem crossing (rISC) process. In terms of the results of electronic structure calculations, we have calculated the related radiative and nonradiative rates. The forward intersystem crossing (ISC) and rISC processes are estimated to occur on the timescale of 1010 s–1, which is significantly faster than the fluorescence and phosphorescence emission rates (106 and 103 s–1). The faster rISC process relative to the phosphorescence one enables the TADF process. The low-frequency vibrational modes are found to have important contribution to the Huang–Rhys factors and to enhance the ISC and rISC rates. Moreover, environmental effects are found to be important and cannot be completely ignored in realistic simulations. Finally, the substituted −F and −OEt groups have a small influence on geometric structures but visible effects on electronic structures and related radiative and nonradiative rates, which implies that the TADF performance of the Au­(III) complexes could be further enhanced through chemical tailoring or tuning these substituting groups.

Herein, we have employed B3LYP and TD-B3LYP methods with the QM/MM approach to study the thermally activated delayed fluorescence (TADF) phenomenon of two Cu(i) complexes bearing 5-(2-pyridyl)-tetrazolate (PyrTet) and phosphine (POP) ligands in the gas phase, solution, and crystal form. On the basis of spectroscopic properties, ground- and excited-state geometric and electronic structures, and related radiative and nonradiative rates, we have found that (1) the S1 and T1 excited states have clear metal-to-ligand charge transfer character from the Cu(i) atom to the PyrTet group; (2) the S1 and T1 states have a very small energy gap ΔES1-T1, less than 0.18 eV, which makes the forward and reverse intersystem crossing ISC and rISC processes between the S1 and T1 states very efficient; and (3) the low-frequency vibrational modes related to the torsional motion of the POP and PyrTet groups are found to have significant Huang-Rhys factors and are responsible for the efficient ISC and rISC rates. However, the corresponding Huang-Rhys factors are remarkably suppressed in the crystal compared with those in the gas phase and in solution due to the rigidity of the crystal surroundings; as a result, the ISC and rISC rates are accordingly reduced slightly in the crystal. This comparison also demonstrates that the surrounding effects are very important for modulating the photophysical properties of the Cu(i) complexes. Finally, our work gives helpful insights into the TADF mechanism of the Cu(i) compounds, which could assist in rationally designing TADF materials with excellent performance.

The dark- and light-adapted states of YtvA LOV domains exhibit distinct excited-state behavior. We have employed high-level QM(MS-CASPT2)/MM calculations to study the photochemical reactions of the dark- and light-adapted states. The photoreaction from the dark-adapted state starts with an S →T intersystem crossing followed by a triplet-state hydrogen transfer from the thiol to the flavin moiety that produces a diradical intermediate, and a subsequent internal conversion that triggers a barrierless C-S bond formation in the S state. The energy profiles for these transformations are different for the four conformers of the dark-adapted state considered. The photochemistry of the light-adapted state does not involve the triplet state: photoexcitation to the S state triggers C-S bond cleavage followed by recombination in the S state; both these processes are essentially barrierless and thus ultrafast. The present work offers new mechanistic insights into the photoresponse of flavin-containing blue-light photoreceptors.

The Pd complex PdN3N exhibits an unusual dual emission of roomtemperature phosphorescence (RTP) and thermally activated delayed fluorescence (TADF), but the mechanism is elusive. Herein, we employed both density functional theory (DFT) and time-dependent DFT (TD-DFT) methods to explore excited-state properties of this Pd complex, which shows that the S 0 , S 1 , T 1 , and T 2 states are involved in the luminescence. Both the S 1 → T 1 and S 1 → T 2 intersystem crossing (ISC) processes are more efficient than the S 1 fluorescence and insensitive to temperature. However, the direct T 1 → S 1 and T 2 -mediated T 1 → T 2 → S 1 reverse ISC (rISC) processes change remarkably with temperature. At 300 K, these two processes are more efficient than the T 1 phosphorescence and therefore enable TADF. Importantly, the T 1 → S 1 rISC and T 1 phosphorescence rates are comparable at 300 K, which leads to dual emissions of TADF and RTP, whereas these two channels become blocked at 100 K so that only the T 1 phosphorescence is recorded experimentally.

Pigment Yellow 101 (PY101) is widely used as a typical pigment due to its excellent excited-state properties. However, the origin of its photostability is still elusive. In this work, we have systematically investigated the photodynamics of PY101 by performing combined electronic structure calculations and trajectory-based nonadiabatic dynamics simulations. On the basis of the results, we have found that upon photoexcitation to the S state, PY101 undergoes an essentially barrierless excited-state intramolecular single proton transfer generating an S keto species. In the keto region, there is an energetically accessible S/S conical intersection that funnels the system to the S state quickly. In the S state, the keto species either goes back to its trans-enol species through a ground-state reverse hydrogen transfer or arrives at the cis-keto region. In addition, we have found an additional excited-state decay channel for the S enol species, which is directly linked to an S/S conical intersection located in the enol region. This mechanism has also been confirmed by our dynamics simulations, in which about 54% of the trajectories decay to the S state via the enol S/S conical intersection; while the remaining ones employ the keto S/S conical intersection. The gained mechanistic information helps us understand the photostability of the PY101 chromophore and its variants with the same molecular scaffold.

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