Resonance Raman spectroscopy provides important insight into the initial nuclear motions of a molecule following optical excitation. The vibrational frequencies in the spectrum represent the ground-state structure, but the intensities of the Raman bands reflect the dynamics after excitation to an electronically excited state. Calculations are often necessary to assign vibrations in the experimental spectrum in order to correctly identify the excited-state dynamics, but there is a risk of misassignment when using calculations that do not include resonance-enhancement effects. Off-resonance calculations typically give frequencies that are accurate within 10-20 cm-1 but may not correctly represent the relative Raman scattering intensities of an experimental resonance Raman spectrum. Two approaches for including resonance-enhancement effects in the Raman spectrum are the Franck-Condon method and the gradient approximation method. This contribution examines both methods for obtaining resonance Raman intensities for a diarylethene-based molecular switch with 129 normal modes. Comparing experimental spectra measured both on- and off-resonance with simulated spectra using off-resonance, Franck-Condon, and gradient approximation methods highlights the need to include resonance-enhancement effects in the calculations in order to make accurate mode assignments. The gradient approximation gives good agreement with the experimental resonance Raman spectrum while avoiding potential complications and the computational cost of finding the optimized geometry and normal modes of the excited state. The vibrational assignments reveal key stretching motions involved in the initial excited-state dynamics of the molecular switch.

Most reported photocatalytic systems that primarily rely on the absorption of ultraviolet (UV) and short visible light irradiation face significant limitations, including shallow penetration in reaction media, competing absorption with substrates and catalysts, and incompatibility with light-sensitive molecules. These drawbacks can be largely avoided if the same reaction can be operated under near-infrared (NIR) light irradiation. Herein, we report a novel family of branched pyridinium-based photocatalysts designed with elegant donor-π-acceptor (D-π-A) architectures and exceptional two-photon absorption (TPA) capabilities in the deep-red and NIR regions. Among the three designed complexes, MPP-3arm demonstrates a remarkable TPA cross section and hence efficient NIR-driven photocatalytic condensation of aromatic diamines and aldehydes. Notably, it also exhibits a unique viscosity-dependent photocatalytic performance, attributed to restricted rotational mobility in a highly viscous environment, a feature rarely explored in organic photocatalysis. Overall, this study presents the design of multibranched organic TPA photocatalysts and their potential of overcoming the limitations encountered in conventional photocatalysis, thus unlocking new opportunities in NIR light-driven chemical synthesis.

Applications involving two-photon activation, including two-photon fluorescence imaging, photodynamic therapy, and 3D data storage, require precise knowledge of the two-photon absorption (2PA) spectra of target chromophores. Broadband pump-probe spectroscopy using femtosecond laser pulses provides wavelength-dependent 2PA spectra with absolute cross sections, but the measurements are sometimes complicated by cross-phase modulation effects and dispersion of the broadband probe. Here, we introduce a single-shot approach that eliminates artifacts from cross-phase modulation and enables more rapid measurements by avoiding the need to scan the time delay between the pump and the probe pulses. The approach uses counterpropagating beams to automatically integrate over the full interaction between the two pulses as they cross. We demonstrate this single-shot approach for a common 2PA reference, coumarin 153 (C153), in three different solvents using the output from a Yb:KGW laser. This approach provides accurate 2PA cross sections that are more reliable and easier to obtain compared with scanning pump-probe methods using copropagating laser beams. The single-shot method for broadband two-photon absorption (BB-2PA) spectroscopy also has significant advantages compared with single-wavelength measurements, such as z-scan and two-photon fluorescence.

Conventional organic photocatalysis typically relies on ultraviolet and short‐wavelength visible photons as the energy source. However, this approach often suffers from competing light absorption by reactants, products, intermediates, and co‐catalysts, leading to reduced quantum efficiency and side reactions. To address this issue, we developed novel organic two‐photon‐absorbing (TPA) photosensitizers capable of functioning under deep red and near‐infrared light irradiation. Three model reactions including cyclization, Sonogashira Csp2‐Csp cross‐coupling, and Csp2‐N cross‐coupling reactions were selected to compare the performance of the new photosensitizers under both blue (427 nm) and deep red (660 nm) light irradiation. The obtained results unambiguously prove that for reactions involving blue light‐absorbing reactants, products, and/or co‐catalysts, deep red light source resulted in better performance than blue light when utilizing our TPA photosensitizers. This work highlights the potential of our metal‐free TPA photosensitizers as a sustainable and effective solution to mitigate the competing light absorption issue in photocatalysis, not only expanding the scope of organic photocatalysts but also reducing reliance on expensive Ru/Ir/Os‐based photosensitizers.

Generating wavelength-tunable picosecond laser pulses from an ultrafast laser source is essential for femtosecond stimulated Raman scattering (FSRS) measurements. Etalon filters produce narrowband (picosecond) pulses with an asymmetric temporal profile that is ideal for stimulated resonance Raman excitation. However, etalon filters with high finesse are typically only useful at the laser fundamental and harmonic frequencies due to very low throughput and limited tunability. Here, we show that a single etalon filter with low finesse (15 cm–1 bandwidth, 172 cm–1 free spectral range) provides an efficient and tunable option for generating Raman pump pulses over a wide range of wavelengths when used in combination with an optical parametric amplifier and a second harmonic generation (SHG) crystal that has an appropriate phase-matching bandwidth. Tuning the SHG wavelength to match individual transmission lines of the etalon filter gives asymmetric picosecond pump pulses over a range of 460-650 nm. The SHG crystal length determines the temporal rise time of the pulse, which is an important property for reducing background and increasing Raman signals compared with symmetric pulses having the same total energy. This approach provides a relatively simple and efficient method to generate optimally shaped picosecond pump pulses for resonance-enhanced FSRS measurements across the visible region of the spectrum.

Two-photon absorption (2PA) spectroscopy provides valuable information about the nonlinear properties of molecules. In contrast with single-wavelength methods, broadband 2PA spectroscopy using a pump-probe approach gives a continuous 2PA spectrum across a wide range of transition energies without tuning the excitation laser. This contribution shows how stimulated Raman scattering from the solvent can be used as a convenient and robust internal standard for obtaining accurate absolute 2PA cross sections using the broadband approach. Stimulated Raman scattering has the same pump-probe overlap dependence as 2PA, thus eliminating the need to measure the intensity-dependent overlap of the pump and probe directly. Eliminating the overlap represents an important improvement because intensity profiles are typically the largest source of uncertainty in the measurement of absolute 2PA cross sections using any method. Raman scattering cross sections are a fundamental property of the solvent and therefore provide a universal standard that can be applied any time the 2PA and Raman signals are present within the same probe wavelength range. We demonstrate this approach using sample solutions of coumarin 153 in methanol, DMSO, and toluene, as well as fluorescein in water.

The incorporation of trifluoromethyl groups into organicmoleculessuch as agrochemicals and pharmaceuticals has attracted a significantamount of interest because they will impact the binding ability, lipophilicity,metabolic stability, and chemical stability of the resulting molecules.Over the past few years, photocatalytic trifluoromethylation of arylalkenes has been reported, which typically requires precious Ru/Ir-containingphotocatalytic systems. Herein, we report a metal-free organic photocatalystcomposed of a bibenzothiazole core and two imine-bridged methoxyphenylsubstituents (dBIP-OMe), which is able to drivetrifluoromethylative bifunctionalization of alkenes photocatalyticallywithout the use of any sacrificial reagents. Mechanistic studies revealtwo consecutive single-electron-transfer steps between the excited dBIP-OMe ( * ), the CF3 precursor (Umemoto's reagent), and the alkene substrate.Substrate scope studies demonstrated that our trifluoromethylativebifunctionalization strategy using dBIP-OMe isapplicable for both aryl and aliphatic alkenes. Furthermore, a varietyof nucleophiles, such as H2O, acetate, cyanide, azide,etc., can be readily incorporated into the carbocation intermediateonce the foremost trifluoromethylation step is accomplished, substantiallybroadening the application scope of this photocatalytic method. Finally,taking advantage of the two-photon absorption capability of dBIP-OMe in the near-infrared region, we demonstratedthat the hydroxytrifluoromethylation of styrene could be achievedusing an inexpensive 740 nm LED as the sole light source.

Two-photon absorbing fluorescent probes have emerged as powerful imaging tools for subcellular-level monitoring of biological substances and processes, offering advantages such as deep light penetration, minimal photodamage, low autofluorescence, and high spatial resolution. However, existing two-photon absorbing probes still face several limitations, such as small two-photon absorption cross-section, poor water solubility, low membrane permeability, and potentially high toxicity. Herein, we report three small-molecule probes, namely MSP-1arm, Lyso-2arm, and Mito-3arm, composed of a pyridinium center (electron-acceptor) and various methoxystyrene "arms" (electron-donor). These probes exhibit excellent fluorescence quantum yield and decent aqueous solubility. Leveraging the inherent intramolecular charge transfer and excitonic coupling effect, these complexes demonstrate excellent two-photon absorption in the near-infrared region. Notably, Lyso-2arm and Mito-3arm exhibit distinct targeting abilities for lysosomes and mitochondria, respectively. In two-photon microscopy experiments, Mito-3arm outperforms a commercial two-photon absorbing dye in 2D monolayer HeLa cells, delivering enhanced resolution, broader NIR light excitation window, and higher signal-to-noise ratio. Moreover, the two-photon bioimaging of 3D human forebrain organoids confirms the successful deep tissue imaging capabilities of both Lyso-2arm and Mito-3arm. Overall, this work presents a rational design strategy in developing competent two-photon-absorbing probes by varying the number of conjugated "arms" for bioimaging applications.

Photoexcitation of cyclic ketones leads to the expulsion of carbon monoxide and a mixture of products derived from diradical intermediates. Here we show that synthetic utility of this process is improved if strained heterocyclic ketones are used. Photochemistry of 3-oxetanone and N-Boc-3-azetidinone has not been previously described. Decarbonylation of these 4-membered rings proceeds through a step-wise Norrish type I cleavage of the C-C bond from the singlet excited state. Ylides derived from both compounds are high-energy species that are kinetically stable long enough to undergo [3+2] cycloaddition with a variety of alkenes and produce substituted tetrahydrofurans and pyrrolidines. The reaction has a sufficiently wide scope to produce scaffolds that were either previously inaccessible or difficult to synthesize, thereby providing experimental access to new chemical space.

We prepared a collection of complex cycloheptatriene-containing azetidine lactones by applying two key photochemical reactions: “aza-Yang” cyclization and Buchner carbene insertion into aromatic rings. While photolysis of phenacyl amines leads to a rapid charge transfer and elimination, we found that a simple protonation of the amine enables the formation of azetidinols as single diastereomers. We provide evidence, through ultrafast spectroscopy, for the electron transfer from free amines in the excited state. Further, we characterize the aza-Yang reaction by establishing the dependence of the initial reaction rates on the rates of photon absorption. An unanticipated change in reactivity in morpholine analogues is explained through interactions with the tosylate anion. The Buchner reaction proceeds with a slight preference for one diastereomer over the other, and successful reaction requires electron-donating carbene-stabilizing substituents. Overall, 16 compounds were prepared over seven steps. Guided by an increase in structural complexity, efforts such as this one extend the reach of chemists into unexplored chemical space and provide useful quantities of new compounds for studies focused on their properties.

Resonance Raman spectroscopy probes the ultrafast dynamics of a diarylethene (DAE) molecular switch following excitation into the first two optical absorption bands. Mode-specific resonance enhancements for Raman excitation at visible (750-560 nm) and near-UV (420-390 nm) wavelengths compared with the calculated and experimental off-resonance Raman spectrum at 785 nm reveal different Franck-Condon active vibrations for the two electronically excited states. The resonance enhancements at visible wavelengths are consistent with initial motion on the first excited-state that promotes the cycloreversion reaction, whereas the enhancements for excitation at near-UV wavelengths highlight motions involving conjugated backbone and phenyl ring stretching modes that are orthogonal to the reaction coordinate. The results support a mechanism involving rapid internal conversion from the higher-lying state followed by cycloreversion on the first excited state. These observations provide new information about the reactivity of DAE derivatives following excitation in the visible and near-UV.

Some diarylethene molecular switches have a low quantum yield for cycloreversion when excited by a single photon, but react more efficiently following sequential two-photon excitation. The increase in reaction efficiency depends on both the relative time delay and the wavelength of the second photon. This paper examines the wavelength-dependent mechanism for sequential excitation using excited-state resonance Raman spectroscopy to probe the ultrafast (sub-30 fs) dynamics on the upper electronic state following secondary excitation. The approach uses femtosecond stimulated Raman scattering (FSRS) to measure the time-gated, excited-state resonance Raman spectrum in resonance with two different excited-state absorption bands. The relative intensities of the Raman bands reveal the initial dynamics in the higher-lying states, Sn, by providing information on the relative gradients of the potential energy surfaces that are accessed via secondary excitation. The excited-state resonance Raman spectra reveal specific modes that become enhanced depending on the Raman excitation wavelength, 750 or 400 nm. Many of the modes that become enhanced in the 750 nm FSRS spectrum are assigned as vibrational motions localized on the central cyclohexadiene ring. Many of the modes that become enhanced in the 400 nm FSRS spectrum are assigned as motions along the conjugated backbone and peripheral phenyl rings. These observations are consistent with earlier measurements that showed higher efficiency following secondary excitation into the lower excited-state absorption band and illustrate a powerful new way to probe the ultrafast dynamics of higher-lying excited states immediately following sequential two-photon excitation.

This Feature Article highlights the role of spatial confinement in controlling the fundamental behavior of molecules. Select examples illustrate the value of using space as a tool to control and understand excited-state dynamics through a combination of ultrafast spectroscopy and conventional steady-state methods. Molecules of interest were confined within a closed molecular capsule, derived from a cavitand known as octa acid (OA), whose internal void space is sufficient to accommodate molecules as long as tetracene and as wide as pyrene. The free space, i.e., the space that is left following the occupation of the guest within the host, is shown to play a significant role in altering the behavior of guest molecules in the excited state. The results reported here suggest that in addition to weak interactions that are commonly emphasized in supramolecular chemistry, the extent of empty space (i.e., the remaining void space within the capsule) is important in controlling the excited-state behavior of confined molecules on ultrafast time scales. For example, the role of free space in controlling the excited-state dynamics of guest molecules is highlighted by probing the cis-trans isomerization of stilbenes and azobenzenes within the OA capsule. Isomerization of both types of molecule are slowed when they are confined within a small space, with encapsulated azobenzenes taking a different reaction pathway compared to that in solution upon excitation to S2. In addition to steric constraints, confinement of reactive molecules in a small space helps to override the need for diffusion to bring the reactants together, thus enabling the measurement of processes that occur faster than the time scale for diffusion. The advantages of reducing free space and confining reactive molecules are illustrated by recording unprecedented excimer emission from anthracene and by measuring ultrafast electron transfer rates across the organic molecular wall. By monitoring the translational motion of anthracene pairs in a restricted space, it has been possible to document the pathway undertaken by excited anthracene from inception to the formation of the excimer on the excited-state surface. Similarly, ultrafast electron transfer experiments pursued here have established that the process is not hindered by a molecular wall. Apparently, the electron can cross the OA capsule wall provided the donor and acceptor are in close proximity. Measurements on the ultrafast time scale provide crucial insights for each of the examples presented here, emphasizing the value of both "space" and "time" in controlling and understanding the dynamics of excited molecules.

We prepared a collection of complex cycloheptatriene-containing azetidine lactones by ap- plying two key photochemical reactions: “aza-Yang” cyclization and Buchner carbene insertion into aromatic rings. While photolysis of phenacyl amines leads to a rapid charge transfer and elimination, we found that a simple protonation of the amine enables the formation of azetidinols as single diastereomers. We provide evidence, through ultrafast spectroscopy, for the electron transfer from free amines in the excited state. Further, we characterize aza-Yang re- action by establishing the dependence of initial reaction rates on rates of photon absorption. Unanticipated change in reactivity in morpholine analogs is explained through interactions with the tosylate anion. Buchner reaction proceeds with slight preference for one diastereomer over the other, and successful reaction requires electron-donating carbene-stabilizing substituents. Overall, sixteen compounds were prepared over seven steps. Guided by an increase in structural complexity, efforts such as this one extend reach of chemists into unexplored chemical space and provide useful quantities of new compounds for studies focused on their properties.

9 We prepared a collection of complex cycloheptatriene-containing azetidine lactones by ap10 plying two key photochemical reactions: “aza-Yang” cyclization and Buchner carbene insertion 11 into aromatic rings. While photolysis of phenacyl amines leads to a rapid charge transfer and 12 elimination, we found that a simple protonation of the amine enables the formation of aze13 tidinols as single diastereomers. We provide evidence, through ultrafast spectroscopy, for the 14 electron transfer from free amines in the excited state. Further, we characterize aza-Yang re15 action by establishing the dependence of initial reaction rates on rates of photon absorption. 16 Unanticipated change in reactivity in morpholine analogs is explained through interactions with 17 the tosylate anion. Buchner reaction proceeds with slight preference for one diastereomer over 18 the other, and successful reaction requires electron-donating carbene-stabilizing substituents. 19 Overall, sixteen compounds were prepared over seven steps. Guided by an increase in struc20 tural complexity, efforts such as this one extend reach of chemists into unexplored chemical 21 space and provide useful quantities of new compounds for studies focused on their properties. 22 Keywords— photochemistry, amines, spirocycles, azetidines, cycloheptatrienes, lactones, 23 complexity, kinetics, ultrafast spectroscopy 24 *Correspondence: zarko@ku.edu

Compared with experimental spectra, calculations for conjugated phenyl and thiophene oligomers tend to overestimate the ground state Raman intensities of higher-frequency vibrations (1200-1800 cm-1) relative to the intensities at lower frequencies (<1200 cm-1). The discrepancy was observed in previous benchmarking work that examined the method dependence of the calculated Raman spectra for a series of aromatic molecules. This paper further investigates the nature of the discrepancy by examining the role of anharmonic corrections and the dependence of the calculated Raman spectra on the inter-ring torsion angle for the representative molecules biphenyl (BP), 2-phenylthiophene (PT), and 2,2'-bithiophene (BT). Perturbative anharmonic corrections to the spectra calculated using density functional theory (DFT) provide only slightly better agreement with experiment. On the other hand, calculations at larger torsion angles give up to 30% improvement in the relative Raman intensities compared with the spectra calculated at the optimized geometries. The torsion-angle dependence of the Raman intensities is most pronounced for delocalized C-C and C-S stretching modes, and less pronounced for bending and ring distortion modes that do not involve inter-ring stretching. Higher-level calculations using the coupled cluster with single, double, and perturbative triple excitations [CCSD(T)] method indicate that DFT underestimates the energy barrier for torsion isomerization at small angles, and it overestimates the barriers at large angles, thus predicting minimum geometries at torsion angles that are too small. Therefore, the results suggest that the discrepancy in relative Raman intensities may be related to an overestimation of inter-ring conjugation by DFT, which also tends to favor geometries that are too planar.

Manganese tricarbonyl complexes are promising catalysts for CO2 reduction, but complexes in this family are often photosensitive and decompose rapidly upon exposure to visible light. In this report, synthetic and photochemical studies probe the initial steps of light-driven speciation for Mn(CO)3(Rbpy)Br complexes bearing a range of 4,4'-disubstituted 2,2'-bipyridyl ligands (Rbpy, where R = tBu, H, CF3, NO2). Transient absorption spectroscopy measurements for Mn(CO)3(Rbpy)Br coordination compounds with R = tBu, H, and CF3 in acetonitrile reveal ultrafast loss of a CO ligand on the femtosecond time scale, followed by solvent coordination on the picosecond time scale. The Mn(CO)3(NO2bpy)Br complex is unique among the four compounds in having a longer-lived excited state that does not undergo CO release or subsequent solvent coordination. The kinetics of photolysis and solvent coordination for light-sensitive complexes depend on the electronic properties of the disubstituted bipyridyl ligand. The results indicate that both metal-to-ligand charge-transfer (MLCT) and dissociative ligand-field (d-d) excited states play a role in the ultrafast photochemistry. Taken together, the findings suggest that more robust catalysts could be prepared with appropriately designed complexes that avoid crossing between the excited states that drive photochemical CO loss.

This paper examines the role of protonation on the photochromic reactions of multi-responsive phenyl amine diarylethene derivatives (PA-DAEs). Reversible protonation and deprotonation provide a secondary stimulus for controlling the properties of light-responsive PA-DAEs, including solubility and thermal stability. For this reason, the phenylamine substituted DAEs were synthesized using a novel and efficient microwave-assisted synthetic route. Steady-state spectroscopy results indicated that the photocyclization and photocycloreversion reactions were reversible at different pH values. Even though, the acidic condition caused red-shift of the visible light absorption bands and generated a new absorption band at near-IR to IR region in the closed-ring structure but made blue-shift in the absorption spectra of the open-ring isomers. Also, it was showed that the excess amount of proton locked back the photochromic reaction. Results also indicated that the protonated closed-ring isomers of PA-DAEs are hydrophilic, whereas the deprotonated forms and protonated open-ring  forms are hydrophobic. In addition to photoswitching and solubility of PA-DAEs under irradiation at different light wavelengths, the thermal stability of P-type PA-DAEs was also monitored in the presence of trifluoroacetic acid at different temperatures as external stimuli.