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Film morphology of emissive layers is crucial to the performance and stability of solution-processable organic light-emitting diodes (OLEDs). Compared to the interpenetration of conjugated polymer chain, small molecular emitter with a flexible side chain always presents easily aggregation upon external treatment, and caused π-electronic coupling, which is undesirable for the efficiency and stability of deep-blue OLEDs. Herein, we proposed a side-chain coupling strategy to enhance the film morphological an emission stability of solution-processable small molecular deep-blue emitter. In contrary to “parent” MC8TPA, the crosslinkable styryl and vinyl units were introduced as ended unit at the side-chain of CmTPA and OEYTPA. Interestingly, CmTPA and OEYTPA films present a relatively stable morphology and uniform deep-blue emission after thermal annealing (160 °C) in the atmosphere, different to the discontinuous MC8TPA annealed film. Besides, compared to the CmTPA and OEYTPA ones, serious polaron formation in the MC8TPA annealed film also negative to the deep-blue emission, according to transient absorption analysis. Therefore, both CmTPA and OEYTPA annealed film obtained at 140 °C present an excellent deep-blue ASE behavior with a 445 nm, but absence for MC8TPA ones, associated with the disruption of annealed films. Finally, enhancement of device performance based on CmTPA and OEYTPA film (∼40%) after thermal annealing with a similar performance curves also confirmed the assumption above. Therefore, these results also supported the effectiveness of our side-chain coupling strategy for optoelectronic applications.

Until recently, the many classes of non-fullerene acceptor (NFA) molecules and polymers exhibited lower power conversion efficiencies than polymer:fullerene counterparts in organic photovoltaic (OPV) cells. We sought to understand the shortcomings of such NFAs, and provide insights for material design in the new era of efficient NFA acceptors. Our study applies broadband transient absorption spectroscopy to a series of donor:acceptor blends, comprising a fixed donor component (PPDT2FBT), and a varying acceptor component of a NFA polymer (N2200), an earlier generation NFA small molecule (NIDCS-HO), or [6,6]-phenyl-C71-butyric acid methyl ester (PCBM) for reference. Our results show that the poor performance of the polymer:polymer blend is explained by incomplete exciton dissociation and rapid geminate recombination. In the case of the small-molecule NFA (NIDCS-HO), we reveal the presence of CT-excitons in the NFA phase, as well as a significant photocurrent generation channel via hole transfer from the NFA. We find that a simple kinetic model of measured charge separation and recombination rates can adequately account for observed photoccurent quantum efficiencies, but the advantage of hole-transfer mediated charge photogeneration (which is less available in fullerene-based devices) may be offset by higher disorder and sub-optimal thin-film morphology for these NFA materials. These results provide valuable insight into how new intrinsic excited states and interfacial excited states need to be accounted for in charge generation dynamics of NFA based OPVs.

Selenium nucleic acids possess unique properties and have been demonstrated to have a wide range of applications such as DNA X-ray crystallography and novel medical therapies. Yet, as a heavy atom, selenium substitution may easily alter the photophysical properties of nucleic acid by red-shifting absorption spectra and introducing effective intersystem crossing to triplet excited states. In this paper, excited state dynamics of a natural occurring selenium substituted uracil (2-selenuracil, 2SeU) is studied by using femtosecond transient absorption spectroscopy as well as quantum chemistry calculation. Ultrafast intersystem crossing to the lowest triplet state (T1) and effective non-radiative decay of this state to the ground state (S0) are demonstrated in neutral form 2SeU. However, the triplet lifetime of the deprotonated form 2SeU is found to be almost one order of magnitude longer than that in the neutral one. Quantum chemistry calculation indicates that the short triplet lifetime in 2SeU is due to excited state population decay through a crossing point between T1 and S0. In the deprotonated form, shorten of N1-C2 bond length makes the structural distortion more difficult and brings larger energy barrier on the pathway to the T1/S0 crossing point, resulting one order of magnitude increase of triplet state lifetime. Our study reveals one key factor to regulate the triplet lifetime of 2SeU and set the stage to further investigate the photophysical and photochemical properties of 2SeU substituted DNA/RNA duplexes.

Recent studies have shown that efficient free carrier (FC) generation with a small voltage loss can be achieved in organic solar cells (OSCs); however, the photophysical insights underpinning this remain unclear. Herein, we examined the mechanisms underlying the FC generation in a state-of-the-art OSC consisting of PM6 and Y6 as electron donor and acceptor, respectively, wherein the energy offset between the lowest excited singlet state and the charge transfer state is as small as ∼0.12 eV. We used transient absorption spectroscopy to track the time evolution of electroabsorption caused by electron–hole pairs generated at donor/acceptor interfaces. After hole transfer from Y6 to PM6, we observed slow yet efficient spatial charge dissociation on a time scale of picoseconds. Based on temperature-dependence measurements, we found that this slow yet efficient FC generation is driven by downhill energy relaxation of charges through the energy cascade generated near the interfaces. We provide here direct experimental evidence for the FC generation mechanism in the very topical PM6/Y6 blend system., 【研究成果】プラスチック太陽電池の発電機構を解明 --オフセットが小さくても光電変換効率が高い理由を分析--. 京都大学プレスリリース. 2022-02-24.

Aggregation-induced emission enhancement (AIEE) is a process recently exploited in solid-state materials and organic luminophores, and it is explained by tight-molecular packaging. However, solution-phase AIEE and its formation mechanism have not been widely explored. This work investigated AIEE phenomena in two donor-acceptor-donor-type benzodiazole-based molecules (the organic building block in metal-organic frameworks) with an acetylene and phenyl π-conjugated backbone tapered with a carboxylic acid group at either end. This was done using time-resolved electronic and vibrational spectroscopy in conjunction with time-dependent density functional theory (TD-DFT) calculations. Fluorescence up-conversion spectroscopy and time-correlated single-photon counting conclusively showed an intramolecular charge transfer-driven aggregate emission enhancement. This is shown by a red spectral shift of the emission spectra as well as an increase in the fluorescence lifetime from 746 ps at 1.0 × 10-11 to 2.48 ns at 2.0 × 10-3 M. The TD-DFT calculations showed that a restricted intramolecular rotation mechanism is responsible for the enhanced emission. The femtosecond infrared (IR) transient absorption results directly revealed the structural dynamics of aggregate formation, as evident from the evolution of the C≡C vibrational marker mode of the acetylene unit upon photoexcitation. Moreover, the IR data clearly indicated that the aggregation process occurred over a time scale of 10 ps, which is consistent with the fluorescence up-conversion results. Interestingly, time-resolved results and DFT calculations clearly demonstrated that both acetylene bonds and the sulfur atom are the key requirements to achieve such a controllable aggregation-induced fluorescence enhancement. The finding of the work not only shows how slight changes in the chemical structure of fluorescent chromophores could make a tremendous change in their optical behavior but also prompts a surge of research into a profound understanding of the mechanistic origins of this phenomenon. This may lead to the discovery of new chemical strategies that aim to synthesize novel chromophores with excellent optical properties for light-harvesting applications.

Molecular photoswitches capable of generating precise pH changes will allow pH-dependent processes to be controlled remotely and noninvasively with light. We introduce a series of new merocyanine photoswitches, which deliver reversible bulk pH changes up to 3.2 pH units (pH 6.5 to pH 3.3) upon irradiation with 450 nm light, displaying tunable and predictable timescales for thermal recovery. We present models to show that the key parameters for optimizing the bulk pH changes are measurable: the solubility of the photoswitch, the acidity of the merocyanine form, the thermal equilibrium position between the spiropyran and the merocyanine isomers, and the increased acidity under visible light irradiation. Using ultrafast transient absorption spectroscopy, we determined the quantum yields for the ring-closing reaction and found that the lifetimes of the transient cis-merocyanine isomers ranged from 30 to 550 ns. Quantum yields did not appear to be a limitation for bulk pH switching. The models we present use experimentally determined parameters and are, in principle, able to predict the change in pH obtained for any related merocyanine photoacid.

Semiconductor nanocrystals (NCs) are emerging luminescent materials with superior optical properties. However, the light-conversion application of NCs is restricted by reabsorption-induced fluorescent quenching. Here, a NC-NC Forster resonance energy transfer (FRET) system is developed by employing large CsPbBr3 NCs as donors and CdSe/CdS NCs as acceptors. The FRET systems using toluene and octadecene as solvents show decreases of 10% and 14%, respectively, in the integrated photoluminescence (PL) intensity, far below the reabsorption loss observed in concentrated CdSe/CdS NCs (>30%) at the same color purity. Notably, we demonstrate by transient absorption measurements that the styrene-mediated FRET system involves a Dexter energy transfer process, which enables the harvesting of triplet excitons and leads to an additional PL enhancement at system level by a maximum of 40% instead of fluorescence quenching. The remarkably improved light-conversion efficiency and antiquenching property make the proposed NC-NC system superior in light down-conversion applications.

Three A-π-A or D-π-D perylene diimide (PDI) derivatives with varied groups on π-conjugate were synthesized and characterized. The photophysical properties of these compounds were systematically studied by spectral experiments and density functional theory calculations. All compounds displayed intense absorption bands at 300-800 nm wavelengths. However, diverse groups on the π-conjugate influenced the UV-vis absorption. Electron-withdrawing groups on PDI-2 caused a slight red shift at the 350-400 nm wavelength and a blue shift after 400 nm wavelength. At the same time, the electron-donating substituents on PDI-3 caused an obvious red shift of this band. These PDI derivatives exhibited emission in solution at room temperature (λem = 500-850 nm). The quantum yield of PDI-3 decreased, while the electron-donating substituents were introduced to the π-conjugated motifs. However, the quantum yield of PDI-2 increased when electron-withdrawing substituents were introduced to the π-conjugated motifs. In addition, PDI-1 and PDI-2 exhibited broad triplet transient absorption in the visible region. These photophysical properties could help us to understand the relationship between structure and photophysical properties of perylene diimide derivatives and exploit more original perylene diimide-based optical functional materials.

Plasmonic Au nanoparticle photoelectrodes were fabricated and characterized with ultrafast transient absorption (TA) and photomodulated cyclic voltammetry (CV) to determine whether the plasmonic ph...

Femtosecond carrier cooling in the organohalide perovskite semiconductor CH3NH3PbI3 is measured using extreme ultraviolet (XUV) and optical transient absorption spectroscopy. XUV absorption between 44 and 58 eV measures transitions from the I 4d core to the valence and conduction bands and gives distinct signals for hole and electron dynamics. The core-to-valence-band signal directly maps the photoexcited hole distribution and provides a quantitative measurement of the hole temperature. The combination of XUV and optical probes reveals that upon excitation at 400 nm, the initial hole distribution is 3.5 times hotter than the electron distribution. At an initial carrier density of 1.4 × 1020 cm-3 both carriers are subject to a hot phonon bottleneck, but at 4.2 × 1019 cm-3 the holes cool to less than 1000 K within 400 fs. This result places significant constraints on the use of organohalide perovskites in hot-carrier photovoltaics.

Interlayer charge transfer (CT) between PtSe2 and WS2 is studied experimentally. Layer-selective pump-probe and photoluminescence quenching measurements reveal ultrafast interlayer CT in the heterostructure formed by bilayer PtSe2 and monolayer WS2, confirming its type-II band alignment. The CT facilitates the formation of the interlayer excitons with a lifetime of several hundred ps to 1 ns, a diffusion coefficient of 0.9 cm2 s-1, and a diffusion length reaching 200 nm. These results demonstrate the integration of PtSe2 with other materials in van der Waals heterostructures with novel charge-transfer properties and help develop fundamental understanding on the performance of various optoelectronic devices based on heterostructures involving PtSe2.

Time-resolved photoluminescence and transient absorption spectroscopy are used to examine the influence that long excited state lifetimes of Mn-doped nanocrystals have on the mechanisms of photoind...

With phenoxazine (PXZ) as the electron donor and perylene-3,4-dicarboximide (PMI) as the electron acceptor, we prepared a compact, orthogonal electron donor-acceptor dyad (PMI-PXZ) to study the spin-orbit charge transfer-induced intersystem crossing (SOCT-ISC). A weak charge transfer (CT) absorption band, due to S0 → 1CT transition, was observed (e = 2840 M-1 cm-1 at 554 nm, FWHM: 2850 cm-1), which is different from that of the previously reported analogue dyad with phenothiazine as the electron donor (PMI-PTZ), for which no CT absorption band was observed. A long-lived triplet state was observed (lifetime τT = 182 μs) with nanosecond transient absorption spectroscopy, and the singlet oxygen quantum yield (ΦΔ = 76%) is higher than that of the previously reported analogue dyad PMI-PTZ (ΦΔ = 57%). Ultrafast charge separation (ca. 0.14 ps) and slow charge recombination (1.4 ns) were observed with femtosecond transient absorption spectroscopy. With time-resolved electron paramagnetic resonance spectroscopy (TREPR), we confirmed the SOCT-ISC mechanism, and the electron spin polarization phase pattern of the triplet-state TREPR spectrum is (e, e, a, e, a, a), which is dramatically different from that of PMI-PTZ (a, e, a, e, a, e), indicating that the triplet-state TREPR spectrum of a specific chromophore in the electron donor-acceptor dyads is not only dependent on the geometry of the dyads but also dependent on the structure of the electron donor (or acceptor). Even one-atom variation in the donor structure may cause significant influence on the electron spin selectivity of the ISC of the electron donor-acceptor dyads.

Integrating multiple photosensitive properties into an “all-in-one” photosensitizer (PS) shows great promise for the treatment of cancers owing to synergistic effect among them. However, the development of such PSs, especially those that need a single laser source, remains a challenge. Herein, we report an orchestration of electron donors and acceptors in a propeller-like pentad, PBI-4Cz, where four carbazole (Cz) units are covalently linked to the ortho-positions of the perylene bisimide (PBI) core. Strong intramolecular donor-acceptor interaction significantly quenches the luminescence and largely extends the absorption spectra to near-infrared region. Excited-state dynamics investigated via femto- and nano-second transient absorption spectroscopy revealed exclusive charge separation of the PBI-4Cz within initial 0.5 ps when photoexcited regardless of which intermediate is involved. Energy dissipation of the resulting charge-separated state (PBI•−-4Cz•−) is subjected to the toggle between intersystem-crossing toward excited triplet states and charge recombination toward ground states. Relative importance of the two pathways can be tuned by micro-environmental polarity, which endows PBI-4Cz remarkable performances of singlet-oxygen generation (>90.0%) in toluene and photothermal conversion (∼28.6%) in DMSO. Harnessing intrinsic photostability and excited-state processes of heavy-atom-free PBI derivatives not only holds a promise for multifunctional phototheranostics, but also provides a prototype for designing high-performance PSs with tunable photoconversion pathways.

Strategic incorporation of a meta dimethylamino (-NMe2 ) group on the conformationally locked green fluorescent protein (GFP) model chromophore (m-NMe2 -LpHBDI) has drastically altered molecular electronic properties, counterintuitively enhancing fluorescence of only the neutral and cationic chromophores in aqueous solution. A ~200-fold decrease in fluorescence quantum yield of m-NMe2 -LpHBDI in alcohols (e.g., MeOH, EtOH, and 2-PrOH) supports this GFP-derived compound as a fluorescence turn-on water sensor, with large fluorescence intensity differences between H2 O and ROH emissions in various H2 O/ROH binary mixtures. A combination of steady-state electronic spectroscopy, femtosecond transient absorption, ground-state femtosecond stimulated Raman spectroscopy (FSRS), and quantum calculations elucidates an intermolecular hydrogen-bonding chain between a solvent -OH group and the chromophore phenolic ring -NMe2 and -OH functional groups, wherein fluorescence differences arise from an extended hydrogen-bonding network beyond the first solvation shell, as opposed to fluorescence quenching via a dark twisted intramolecular charge transfer state. The absence of a meta-NMe2 group twisting coordinate upon electronic excitation was corroborated by experiments on control samples without the meta-NMe2 group or with both meta-NMe2 and para-OH groups locked in a six-membered ring. These deep mechanistic insights stemming from GFP chromophore scaffold will enable rational design of organic, compact, and environmentally friendly water sensors.

Tin chalcogenides (SnX, X = S, Se)-based heterostructures (HSs) are promising materials for the construction of low-cost optoelectronic devices. Here, we report the synthesis of a SnSe/CdSe HS using the controlled cation exchange reaction. The (400) plane of SnSe and the (111) plane of CdSe confirm the formation of an interface between SnSe and CdSe. The Type I band alignment is estimated for the SnSe/CdSe HS with a small conduction band offset (CBO) of 0.72 eV through cyclic voltammetry measurements. Transient absorption (TA) studies demonstrate a drastic enhancement of the CdSe biexciton signal that points toward the hot carrier transfer from SnSe to CdSe in a short time scale. The fast growth and recovery of CdSe bleach in the presence of SnSe indicate charge transfer back to SnSe. The observed delocalization of carriers in these two systems is crucial for an optoelectronic device. Our findings provide new insights into the fabrication of cost-effective photovoltaic devices based on SnSe-based heterostructures.

We present the photoinduced size/structure transformation of [Au37-xAgx(PPh3)13Cl10]3+ (M37) into [Au25-yAgy(PPh3)10Cl8]+ (M25) cluster. Single-crystal X-ray diffraction revealed that M37 has a tri-icosahedron M36 metal core assembled via the fusion of three Au7Ag6 icosahedrons in a cyclic fashion and that the M36 core is further protected by phosphine and chloride ligands. The M37 cluster is found to be highly sensitive toward ambient light, and the M37 → M25 transition is observed with 530 nm irradiation, monitored by time-dependent UV-vis spectroscopy, electrospray ionization mass spectrometry (ESI-MS), and femtosecond transient absorption spectroscopy. Linear-response time-dependent DFT calculations indicated that the strong absorption of the M37 cluster close to 500 nm induces an antibonding-type configuration in the induced electron density within the plane of the three 8-electron systems, possibly promoting dissociation of one of the 8-electron superatoms. This theoretical result supports the experimental observation of the sensitivity of the M37 → M25 transition to 530 nm irradiation.

Development of metal dichalcogenides based air and water stable opto-electronic devices is a bottleneck in their commercial implementation. Here we address this issue by the direct catalyst-free deposition of fluorographene (FG) protective layer over monolayer MoS2 (MS), where such an atomic interface is found to be providing enormous photoresponse and chemical stability to the device. Electric field modulated (ionic liquid (IL) electrolyte based top-gated) photodetectors are developed with MS only and FG-MS heterostructure, where the MS photodetector has the responsivity of ~1.3 A/W (with VGS = 0 V while unstable with IL gating) while that of FG-MS is ~2000 A/W (with VGS = 0 V and ~8000 A/W with VGS = 1.5 V, and the detectivity ~1013). This giant photoresponse of the FG-MS is validated with the help of ultrafast transient absorption spectroscopy assisted charge carrier dynamics studies at different temperatures. The broad optical response (350–850 nm) of the FG-MS is found to be intact not only in IL based gating but also after exposing in water for a month or heat treated in air at 200 °C. Interfacing and capping with FG, developed via the direct growth method, are found to be ideal for realizing high shelf-life and good responsivity photodetectors and solar cells of several other monolayer TMDs too, while available for wafer-scale manufacture.

Femtosecond laser flash photolysis of 3-(1a,9b-dihydro-1H-cyclopropa[l]phenanthren-1-ylidene)tetrahydrofuran produces singlet 3-oxacyclopentylidenecarbene which reacts with acetonitrile solvent to form an ylide. This is the first direct detection of ylide formation by an alkylidenecarbene. This new type of ylide was observed to have a broad absorption band in the visible region with λmax ∼450 nm and a lifetime of ∼13.5 ps. As with other "conventional" carbenes (the divalent carbon atom is separately bound to two substituents), this ylide formation method could be also useful for detecting alkylidenecarbenes, especially those that do not absorb at wavelengths suitable for direct observation. Furthermore, the mechanisms by which 3-oxacyclopentylidenecarbene forms the ylide and the overall favorability of ylide formation, vis-a-vis ring expansion of the carbene to strained 3-oxacyclohexyne, were supported by results from density functional theory calculations.

The exciton dynamics of violanthrone-79 are investigated in solution and in the solidstate. In solution, the photo-prepared singlet is found to exhibit a strong ground-state bleachand stimulated emission feature, but when sensitized in its triplet state, exhibits only a narrowand weak ground-state bleach. As supported by density functional theory calculations,this is explained by the triplet state having absorptions in the same region, with a similaroscillator strength, as the ground state molecule. In solid films, the excited singlet isfound to survive only 100 ps, giving way to a long-lived transient absorption spectrum withcharacteristics reminiscent of the triplet in solution. This is interpreted in terms of singletfission in the solid film.

Chemical and spectroscopic characterization of the mononuclear photosensitizers [(DPEPhos)Cu(I)(MPyrT)]0/+ (CuL, CuLH) and their dinuclear analogues (Cu2L’, Cu2L'H2 ), backed by (TD)DFT and high‐level GW‐Bethe‐Salpeter equation calculations, exemplifies the complex influence of charge, nuclearity and structural flexibility on UV‐induced photophysical pathways. Ultrafast transient absorption and step‐scan FTIR spectroscopy reveal flattening distortion in the triplet state of CuLH as controlled by charge, which also appears to have a large impact on the symmetry of the long‐lived triplet states in Cu2L’ and Cu2L'H2 . Time‐resolved luminescence spectroscopy (solid state), supported by transient photodissociation spectroscopy (gas phase), confirm a lifetime of some tens of μs for the respective triplet states, as well as the energetics of thermally activated delayed luminescence, both being essential parameters for application of these materials based on earth‐abundant copper in photocatalysis and luminescent devices., The comprehensive results on two mononuclear pyridyl‐triazole/DPEPhos based Cu(I) complexes and its extension to their two dinuclear analogues paves the road to addressing the complex coupling of the respective monomeric units, involving various physical and chemical properties as charge, (electronic) structure and (long‐lived) excited state properties, relevant for applications.

The intermolecular interaction determines the photophysical properties of the organic aggregates, which are critical to the performance of organic photovoltaics. Here, excitonic coupling, an important intermolecular interaction in organic aggregates, between the π-stacking graphene quantum dots is studied by using transient absorption spectroscopy. We find that the spectral evolution of the ground state bleach arises from the dynamic variation of the excitonic coupling in the excited π-stacks. According to the spectral simulations, we demonstrate that the kinetics of the vibronic peak can be exploited as a probe to measure the dynamics of excitonic coupling in the excited π-stacks.

After binding to human serum albumin, bilirubin could undergo photo-isomerization and photo-induced cyclization process. The latter process would result the formation of a product, named as lumirubin. These photo induced behaviors are the fundamental of clinical therapy for neonatal jaundice. Previous studies have reported that the addition of long chain fatty acids is beneficial to the generation of lumirubin, yet no kinetic study has revealed the mechanism behind. In this study, how palmitic acid affects the photochemical reaction process of bilirubin in Human serum albumin (HSA) is studied by using femtosecond transient absorption and fluorescence up-conversion techniques. With the addition of palmitic acid, the excited population of bilirubin prefers to return to its hot ground state (S0) through a 4 ps decay channel rather than the intrinsic ultrafast decay pathways (< 1 ps). This effect prompts the Z-Z to E-Z isomerization at the S$_0$ state and then further increases the production yield of lumirubin. This is the first time to characterize the promoting effect of long chain fatty acid in the process of phototherapy with femtosecond time resolution spectroscopy and the results can provide useful information to benefit the relevant clinical study.

Achieving high-efficiency deep blue emitter with CIEy 10% has been a long-standing challenge for traditional fluorescent materials in organic light-emitting diodes (OLEDs). Here, we report the rational design and synthesis of two new deep blue luminogens: 4-(10-(4′-(9H-carbazol-9-yl)-2,5-dimethyl-[1,1′-biphenyl]-4-yl)anthracen-9-yl)benzonitrile (2M-ph-pCzAnBzt) and 4-(10-(4-(9H-carbazol-9-yl)-2,5-dimethylphenyl)anthracen-9-yl)benzonitrile (2M-pCzAnBzt). In particular, 2M-ph-pCzAnBzt produces saturated deep blue emissions in a non-doped electroluminescent device with an exceptionally high EQE of 10.44% and CIEx,y (0.151, 0.057). The unprecedented electroluminescent efficiency is attributed to the combined effects of higher-order reversed intersystem crossing and triplet-triplet up-conversion, which are supported by analysis of theoretical calculation, triplet sensitization experiments, as well as nanosecond transient absorption spectroscopy. This research offers a new approach to resolve the shortage of high efficiency deep blue fluorescent emitters.

MXenes, a new family of two-dimensional (2D) materials, have received extensive interest due to their fascinating physicochemical properties, such as outstanding light-to-heat conversion efficiency. However, the photothermal conversion mechanism of MXenes is still poorly understood. Here, by using femtosecond visible and mid-infrared transient absorption spectroscopy, the electronic energy dissipation dynamics of MXene (Ti3C2Tx) nanosheets dispersed in various solvents are carefully studied. Our results indicate that the lifetime of photoexcited MXene is strongly dependent on the surrounding environment. Especially, the interfacial electron-vibration coupling between the MXene nanosheets and the adjacent solvent molecules is directly observed following the ultrafast photoexcitation of MXene. It suggests that the interfacial interactions at the MXene-solvent interface play a critical role in the ultrafast energy transport dynamics of MXene, which offers a potentially feasible route for tailoring the light conversion properties of 2D systems.

A set of Pd(II) biladiene complexes bearing different combinations of methyl- and phenyl-substituents on the sp3-hybridized meso-carbon (the 10-position of the biladiene framework) was prepared and studied. In addition to a previously described Pd(II) biladiene complex bearing geminal dimethyl substituents a the 10-position (Pd[DMBil]), homologous Pd(II) biladienes bearing geminal methyl and phenyl substituents (Pd[MPBil1]) and geminal diphenyl groups(Pd[DPBil1]) were prepared and structurally characterized. Detailed electrochemical as well as steady-state and time-resolved spectroscopic experiments were undertaken to evaluate the influence of the substituents on the biladiene's tetrahedral meso-carbon. Although all three biladiene homologues are isostructural, Pd[MPBil1] and Pd[DPBil1] display more intense absorption profiles that shift slightly toward lower energies as geminal methyl groups are replaced by phenyl rings. All three biladiene homologues support a triplet photochemistry, and replacement of the geminal dimethyl substituents of Pd[DMBil1] (ΦΔ = 54%) with phenyl groups improves the ability of Pd[MPBil1] (ΦΔ = 76%) and Pd[DPBil1] (ΦΔ = 66%) to sensitize 1O2. Analysis of the excited-state dynamics of the Pd(II) biladienes by transient absorption spectroscopy shows that each complex supports a long-lived triplet excited-state (i.e., τ > 15 μs for each homologue) but that the ISC quantum yields (ΦT) varied as a function of biladiene substitution. The observed trend in ISC efficiency matches that for singlet oxygen sensitization quantum yields (ΦΔ) across the biladiene series considered in this work. The results of this study provide new insights to guide future development of biladiene based agents for PDT and other photochemical applications.

The time-resolved polarization-sensitive transient-absorption (TA) pump-probe (PP) spectra are simulated using on-the-fly surface-hopping nonadiabatic dynamics and the doorway-window representation of nonlinear spectroscopy. A dendrimer model system composed of two linear phenylene ethynylene units (2-ring and 3-ring) is taken as an example. The ground-state bleach (GSB), stimulated emission (SE), and excited-state absorption (ESA) contributions as well as the total TA PP signals are obtained and carefully analyzed. It is shown that intramolecular excited-state energy transfer from the 2-ring unit to the 3-ring unit can be conveniently identified by employing pump and probe pulses with different polarizations. Our results demonstrate that time-resolved polarization-sensitive TA PP signals provide a powerful tool for the elucidation of excited-state energy-transfer pathways, notably in molecular systems possessing several optically bright nonadiabatically coupled electronic states with different orientations of transition dipole moments.