Multiexcitonic and optically bright states in subunits of pentacene crystals: A hybrid DFT/MRCI and molecular mechanics study.

A hybrid quantum mechanics/molecular mechanics setup was used to model electronically excited pentacene in the crystal phase. Particularly interesting in the context of singlet fission (SF) is the energetic location of the antiferromagnetically coupled multiexcitonic singlet state, 1(TT), and the ferromagnetically coupled analog in relation to the optically bright singlet state. To provide photophysical properties of the accessible spin manifold, combined density functional theory and multi-reference configuration interaction calculations were performed on pentacene dimers and a trimer, electrostatically embedded in the crystal. The likelihood of a quintet intermediate in the SF process was estimated by computing singlet-quintet electron spin-spin couplings employing the Breit-Pauli Hamiltonian. The performance of the applied methods was assessed on the pentacene monomer. The character of the optically bright state and the energetic location of the 1(TT) state depend strongly on the relative orientation of the pentacene units. In the V-shaped dimers and in the trimer, the optically bright state is dominated by local and charge transfer (CT) excitations, with admixtures of doubly excited configurations. The CT excitations gain weight upon geometry relaxation, thus supporting a CT-mediated SF mechanism as the primary step of the SF process. For the slip-stacked dimer, the energetic order of the bright and the 1(TT) states swaps upon geometry relaxation, indicating strong nonadiabatic coupling close to the Franck-Condon region-a prerequisite for a coherent SF process. The multiexcitonic singlet, triplet, and quintet states are energetically too far apart and their spin-spin couplings are too small to bring about a noteworthy multiplicity mixing.

A Thermally Activated Delayed Fluorescence Emitter Investigated by Time-Resolved Near-Infrared Spectroscopy.

Emitters for organic light-emitting diodes (OLEDs) based on thermally activated delayed fluorescence (TADF) require small singlet (S1 )-triplet (T1 ) energy gaps as well as fast intersystem crossing (ISC) transitions. These transitions can be mediated by vibronic mixing with higher excited states Sn and Tn (n=2, 3, 4, …). For a prototypical TADF emitter consisting of a triarylamine and a dicyanobenzene moiety (TAA-DCN) it is shown that these higher states can be located energetically by time-resolved near-infrared (NIR) spectroscopy.

Enhanced Solid-State Fluorescence of Flavin Derivatives by Incorporation in the Metal-Organic Frameworks MIL-53(Al) and MOF-5.

The flavin derivatives 10-methyl-isoalloxazine (MIA) and 6-fluoro-10-methyl-isoalloxazine (6F-MIA) were incorporated in two alternative metal-organic frameworks, (MOFs) MIL-53(Al) and MOF-5. We used a post-synthetic, diffusion-based incorporation into microcrystalline MIL-53 powders with one-dimensional (1D) pores and an in-situ approach during the synthesis of MOF-5 with its 3D channel network. The maximum amount of flavin dye incorporation is 3.9 wt% for MIA@MIL-53(Al) and 1.5 wt% for 6F-MIA@MIL-53(Al), 0.85 wt% for MIA@MOF-5 and 5.2 wt% for 6F-MIA@MOF-5. For the high incorporation yields the probability to have more than one dye molecule in a pore volume is significant. As compared to the flavins in solution, the fluorescence spectrum of these flavin@MOF composites is broadened at the bathocromic side especially for MIA. Time-resolved spectroscopy showed that multi-exponential fluorescence lifetimes were needed to describe the decays. The fluorescence-weighted lifetime of flavin@MOF of 4 ± 1 ns also corresponds to those in solution but is significantly prolonged compared to the solid flavin dyes with less than 1 ns, thereby confirming the concept of "solid solutions" for dye@MOF composites. The fluorescence quantum yield (ΦF) of the flavin@MOF composites is about half of the solution but is significantly higher compared to the solid flavin dyes. Both the fluorescence lifetime and quantum yield of flavin@MOF decrease with the flavin loading in MIL-53 due to the formation of various J-aggregates. Theoretical calculations using plane-wave and QM/MM methods are in good correspondence with the experimental results and explain the electronic structures as well as the photophysical properties of crystalline MIA and the flavin@MOF composites. In the solid flavins, π-stacking interactions of the molecules lead to a charge transfer state with low oscillator strength resulting in aggregation-caused quenching (ACQ) with low lifetimes and quantum yields. In the MOF pores, single flavin molecules represent a major population and the computed MIA@MOF structures do not find π-stacking interactions with the pore walls but only weak van-der-Waals contacts which reasons the enhanced fluorescence lifetime and quantum yield of the flavins in the composites compared to their neat solid state. To analyze the orientation of flavins in MOFs, we measured fluorescence anisotropy images of single flavin@MOF-5 crystals and a static ensemble flavin@MIL53 microcrystals, respectively. Based on image information, anisotropy distributions and overall curve of the time-resolved anisotropy curves combined with theoretical calculations, we can prove that all fluorescent flavins species have a defined and rather homogeneous orientation in the MOF framework. In MIL-53, the transition dipole moments of flavins are orientated along the 1D channel axis, whereas in MOF-5 we resolved an average orientation that is tilted with respect to the cubic crystal lattice. Notably, the more hydrophobic 6F-MIA exhibits a higher degree order than MIA. The flexible MOF MIL-53(Al) was optimized essentially to the experimental large-pore form in the guest-free state with QuantumEspresso (QE) and with MIA molecules in the pores the structure contracted to close to the experimental narrow-pore form which was also confirmed by PXRD. In summary, the incorporation of flavins in MOFs yields solid-state materials with enhanced rigidity, stabilized conformation, defined orientation and reduced aggregations of the flavins, leading to increased fluorescence lifetime and quantum yield as controllable photo-luminescent and photo-physical properties.

Coordinatively unsaturated metal sites (open metal sites) in metal-organic frameworks: design and applications.

Metal-organic frameworks (MOFs) can contain open metal sites (OMS) or coordinatively unsaturated sites (CUS) or open coordination sites (OCS) when vacant Lewis acid sites on the metal ions or cluster nodes have been generated. This review combines for the first time all aspects of OMS in MOFs, starting from different preparation strategies over theoretical studies on the effects of OMS with host-guest interactions up to distinct OMS-MOF applications. In the experimental part the focus of this review is on MOFs with proven OMS formation which are not only invoked but are clearly verified by analytical methods.

Capture and Separation of SO2 Traces in Metal-Organic Frameworks via Pre-Synthetic Pore Environment Tailoring by Methyl Groups.

Herein, we report a pre-synthetic pore environment design strategy to achieve stable methyl-functionalized metal-organic frameworks (MOFs) for preferential SO2 binding and thus enhanced low (partial) pressure SO2 adsorption and SO2 /CO2 separation. The enhanced sorption performance is for the first time attributed to an optimal pore size by increasing methyl group densities at the benzenedicarboxylate linker in [Ni2 (BDC-X)2 DABCO] (BDC-X=mono-, di-, and tetramethyl-1,4-benzenedicarboxylate/terephthalate; DABCO=1,4-diazabicyclo[2,2,2]octane). Monte Carlo simulations and first-principles density functional theory (DFT) calculations demonstrate the key role of methyl groups within the pore surface on the preferential SO2 affinity over the parent MOF. The SO2 separation potential by methyl-functionalized MOFs has been validated by gas sorption isotherms, ideal adsorbed solution theory calculations, simulated and experimental breakthrough curves, and DFT calculations.

Encapsulation of Phosphorescent Pt(II) Complexes in Zn-Based Metal-Organic Frameworks toward Oxygen-Sensing Porous Materials.

In this work, we synthesized two tailored phosphorescent Pt(II) complexes bearing a cyclometalating tridentate thiazole-based C^N*N pincer luminophore (L) and exchangeable chlorido ([PtCl(L)]) or cyanido ([PtCN(L)]) coligands. While both complexes showed photoluminescence from metal-perturbed ligand-centered triplet states (3MP-LC), [PtCN(L)] reached the highest phosphorescence quantum yields and displayed a significant sensitivity toward quenching by 3O2. We encapsulated them into two Zn-based metal-organic frameworks, namely, MOF-5 and ZIF-8. The incorporation of the organometallic compounds in the resulting composites [PtCl(L)]@ZIF-8, [PtCN(L)]@ZIF-8, [PtCl(L)]@MOF-5, and [PtCN(L)]@MOF-5 was verified by powder X-ray diffractometry, scanning electron microscopy, time-resolved photoluminescence spectroscopy and microscopy, as well as N2- and Ar-gas sorption studies. The amount of encapsulated complex was determined by graphite furnace atomic absorption spectroscopy, showing a maximum loading of 3.7 wt %. If compared with their solid state forms, the solid-solution composites showed prolonged 3O2-sensitive excited state lifetimes for the complexes at room temperature, reaching up to 18.4 µs under an Ar atmosphere, which is comparable with the behavior of the complex in liquid solutions or even frozen glassy matrices at 77 K.

In vitro quantification of botulinum neurotoxin type A1 using immobilized nerve cell-mimicking nanoreactors in a microfluidic platform.

The bacterial toxin botulinum neurotoxin A (BoNT/A) is not only an extremely toxic substance but also a potent pharmaceutical compound that is used in a wide spectrum of neurological disorders and cosmetic applications. The quantification of the toxin is extremely challenging due to its extraordinary high physiological potency and is further complicated by the toxin's three key functionalities that are necessary for its activity: receptor binding, internalization-translocation, and catalytic activity. So far, the industrial standard to measure the active toxin has been the mouse bioassay (MBA) that is considered today as outdated due to ethical issues. Therefore, recent introductions of cell-based assays were highly anticipated; their impact however remains limited due to their labor-intensive implementation. This report describes a new in vitro approach that combines a nanosensor based on the use of nerve cell-mimicking nanoreactors (NMN) with microfluidic technology. The nanosensor was able to measure all three key functionalities, and therefore suitable to quantify the amount of physiologically active BoNT/A. The integration of such a sensor in a microfluidic device allowed the detection and quantification of BoNT/A amounts in a much shorter time than the MBA (<10 h vs. 2-4 days). Lastly, the system was also able to reliably quantify physiologically active BoNT/A within a simple final pharmaceutical formulation. This complete in vitro testing system and its unique combination of a highly sensitive nanosensor and microfluidic technology represent a significant ethical advancement over in vivo measures and a possible alternative to cell-based in vitro detection methods.

Impact of fluorination on the photophysics of the flavin chromophore: a quantum chemical perspective.

10-Methylisoalloxazine (MIA) and its mono-fluorinated derivatives (6-9F-MIA) were investigated by means of quantum chemistry, looking into the influence of fluorination on fluorescence, absorption and inter-system crossing (ISC). A maximized fluorescence quantum yield (ΦFl) of this chromophore is desirable for application as a potential fluorescence marker in biodiagnostics/photobiological studies. An enhanced triplet quantum yield ΦT on the other hand may open a perspective for photodynamic therapies (PDT) in cancer treatment. Determination of equilibrium geometries was carried out employing (time-dependent) Kohn-Sham density functional theory and electronic properties were obtained using a combined density functional theory and multi-reference configuration interaction (DFT/MRCI) method. In the gas phase, El-Sayed-favored 1(ππ*) [radiolysis arrow - arrow with voltage kink] 3(nπ*)-ISC enables population transfer to the triplet domain on a timescale of 109 s-1, i.e. significantly faster than fluorescence (kFl ≈ 107 s-1). Two different models were applied to investigate the influence of aqueous medium on absorption and relaxation: the implicit solvation model A is the well-established conductor-like screening model (COSMO) and hybrid model B combines quantum mechanical micro-hydration and conductor-like screening. A polar, protic environment leads to a significant blue-shift of the nπ* potentials, slowing down the ISC process to 107-108 s-1, now enabled by vibronic spin-orbit coupling. Simple principles are derived that demonstrate the effect of fluorination at different positions on the spectroscopic properties. These principles can be utilized with respect to multiply fluorinated derivatives and even further substitution to enlarge effects on the population decay and quantum yields.

On the large apparent Stokes shift of phthalimides.

The photophysics of N-methylphthalimide (MP) in solution (cyclohexane, ethanol, acetonitrile, and water) was characterized by steady state as well as time resolved fluorescence and absorption spectroscopy. In all solvents the compound exhibits an unusually large Stokes shift of ∼10 000 cm-1. It is attributed to an ultrafast (<100 fs) depletion of the initially excited state, which results in the population of a weakly emitting state. Quantum chemical computations (DFT-MRCI) support this. They identify two energetically low-lying singlet ππ* excitations of different oscillator strength. Whereas the Stokes shift and thereby the ultrafast depletion of the initial excitation are hardly affected by the solvent later processes respond strongly. The fluorescence lifetime varies from ∼10 ps (cyclohexane) to ∼3 ns (water). This is attributed to a varying energetic accessibility of nπ* excitations.

Fluorinated azobenzenes as supramolecular halogen-bonding building blocks.

ortho-Fluoroazobenzenes are a remarkable example of bistable photoswitches, addressable by visible light. Symmetrical, highly fluorinated azobenzenes bearing an iodine substituent in para-position were shown to be suitable supramolecular building blocks both in solution and in the solid state in combination with neutral halogen bonding acceptors, such as lutidines. Therefore, we investigate the photochemistry of a series of azobenzene photoswitches. Upon introduction of iodoethynyl groups, the halogen bonding donor properties are significantly strengthened in solution. However, the bathochromic shift of the π→π* band leads to a partial overlap with the n→π* band, making it slightly more difficult to address. The introduction of iodine substituents is furthermore accompanied with a diminishing thermal half-life. A series of three azobenzenes with different halogen bonding donor properties are discussed in relation to their changing photophysical properties, rationalized by DFT calculations.

The effect of solvent relaxation in the ultrafast time-resolved spectroscopy of solvated benzophenone.

Benzophenone (BP) despite its relatively simple molecular structure is a paradigmatic sensitizer, featuring both photocatalytic and photobiological effects due to its rather complex photophysical properties. In this contribution we report an original theoretical approach to model realistic, ultra-fast spectroscopy data, which requires describing intra- and intermolecular energy and structural relaxation. In particular we explicitly simulate time-resolved pump-probe spectra using a combination of state-of-the art hybrid quantum mechanics/molecular mechanics dynamics to treat relaxation and vibrational effects. The comparison with experimental transient absorption data demonstrates the efficiency and accuracy of our approach. Furthermore the explicit inclusion of the solvent, water for simulation and methanol for experiment, allows us, despite the inherent different behavior of the two, to underline the role played by the H-bonding relaxation in the first hundreds of femtoseconds after optical excitation. Finally we predict for the first time the two-dimensional electronic spectrum (2DES) of BP taking into account the vibrational effects and hence modelling partially symmetric and asymmetric ultrafast broadening.

Molcas 8: New capabilities for multiconfigurational quantum chemical calculations across the periodic table.

In this report, we summarize and describe the recent unique updates and additions to the Molcas quantum chemistry program suite as contained in release version 8. These updates include natural and spin orbitals for studies of magnetic properties, local and linear scaling methods for the Douglas-Kroll-Hess transformation, the generalized active space concept in MCSCF methods, a combination of multiconfigurational wave functions with density functional theory in the MC-PDFT method, additional methods for computation of magnetic properties, methods for diabatization, analytical gradients of state average complete active space SCF in association with density fitting, methods for constrained fragment optimization, large-scale parallel multireference configuration interaction including analytic gradients via the interface to the Columbus package, and approximations of the CASPT2 method to be used for computations of large systems. In addition, the report includes the description of a computational machinery for nonlinear optical spectroscopy through an interface to the QM/MM package Cobramm. Further, a module to run molecular dynamics simulations is added, two surface hopping algorithms are included to enable nonadiabatic calculations, and the DQ method for diabatization is added. Finally, we report on the subject of improvements with respects to alternative file options and parallelization.

Nerve cell-mimicking liposomes as biosensor for botulinum neurotoxin complete physiological activity.

Botulinum neurotoxins (BoNT) are the most toxic substances known, and their neurotoxic properties and paralysing effects are exploited for medical treatment of a wide spectrum of disorders. To accurately quantify the potency of a pharmaceutical BoNT preparation, its physiological key activities (binding to membrane receptor, translocation, and proteolytic degradation of SNARE proteins) need to be determined. To date, this was only possible using animal models, or, to a limited extent, cell-based assays. We here report a novel in vitro system for BoNT/B analysis, based on nerve-cell mimicking liposomes presenting motoneuronal membrane receptors required for BoNT binding. Following triggered membrane translocation of the toxin's Light Chain, the endopeptidase activity can be quantitatively monitored employing a FRET-based reporter assay within the functionalized liposomes. We were able to detect BoNT/B physiological activity at picomolar concentrations in short time, opening the possibility for future replacement of animal experimentation in pharmaceutical BoNT testing.

Conical intersection dynamics of the primary photoisomerization event in vision.

Ever since the conversion of the 11-cis retinal chromophore to its all-trans form in rhodopsin was identified as the primary photochemical event in vision, experimentalists and theoreticians have tried to unravel the molecular details of this process. The high quantum yield of 0.65 (ref. 2), the production of the primary ground-state rhodopsin photoproduct within a mere 200 fs (refs 3-7), and the storage of considerable energy in the first stable bathorhodopsin intermediate all suggest an unusually fast and efficient photoactivated one-way reaction. Rhodopsin's unique reactivity is generally attributed to a conical intersection between the potential energy surfaces of the ground and excited electronic states enabling the efficient and ultrafast conversion of photon energy into chemical energy. But obtaining direct experimental evidence for the involvement of a conical intersection is challenging: the energy gap between the electronic states of the reacting molecule changes significantly over an ultrashort timescale, which calls for observational methods that combine high temporal resolution with a broad spectral observation window. Here we show that ultrafast optical spectroscopy with sub-20-fs time resolution and spectral coverage from the visible to the near-infrared allows us to follow the dynamics leading to the conical intersection in rhodopsin isomerization. We track coherent wave-packet motion from the photoexcited Franck-Condon region to the photoproduct by monitoring the loss of reactant emission and the subsequent appearance of photoproduct absorption, and find excellent agreement between the experimental observations and molecular dynamics calculations that involve a true electronic state crossing. Taken together, these findings constitute the most compelling evidence to date for the existence and importance of conical intersections in visual photochemistry.

The photoformation of a phthalide: a ketene intermediate traced by FSRS.

The photo-isomerization of o-acetylbenzaldehyde (oABA) in acetonitrile was studied by femto- and nanosecond transient absorption spectroscopy. Spectroscopic signatures are assigned with the aid of TD-DFT, TD-CAM-DFT and DFT-MRCI computations. The isomerization yields a lactone, 3-methylphthalide (3MP), with a quantum yield of 0.3 (30%). As evidenced by femtosecond stimulated Raman spectroscopy (FSRS), the isomerization proceeds via a ketene intermediate. It is formed within ∼2-3 ps after photo-excitation. Intersystem crossing (ISC) populating the triplet state of oABA seems to compete with the ketene formation. Experiments on the non-reactive meta- and para-derivatives, which undergo efficient ISC with time constants of 5 ps, support this statement. The triplet state of oABA also contributes to the ketene formation, presumably involving a biradical intermediate. The ketene exhibits a lifetime of 1.4 µs and generates an additional intermediate in the cascade towards the lactone.

Wavepacket splitting and two-pathway deactivation in the photoexcited visual pigment isorhodopsin.

Isorhodopsin is the visual pigment analogue of rhodopsin. It shares the same opsin environment but it embeds 9-cis retinal instead of 11-cis. Its photoisomerization is three times slower and less effective. The mechanistic rationale behind this observation is revealed by combining high-level quantum-mechanical/molecular-mechanical simulations with ultrafast optical spectroscopy with sub-20 fs time resolution and spectral coverage extended to the near-infrared. Whereas in rhodopsin the photoexcited wavepacket has ballistic motion through a single conical intersection seam region between the ground and excited states, in isorhodopsin it branches into two competitive deactivation pathways involving distinct conical intersection funnels. One is rapidly accessed but unreactive. The other is slower, as it features extended steric interactions with the environment, but it is productive as it follows forward bicycle pedal motion.

Modeling pH-Dependent Biomolecular Photochemistry.

The tuning mechanism of pH can be extremely challenging to model computationally in complex biological systems, especially with respect to the photochemical properties. This article reports a protocol aimed at modeling pH-dependent photodynamics using a combination of constant-pH molecular dynamics and semiclassical nonadiabatic molecular dynamics simulations. With retinal photoisomerization in Anabaena sensory rhodopsin (ASR) as a testbed, we show that our protocol produces pH-dependent photochemical properties, such as the isomerization quantum yield or decay rates. We decompose our results into single-titrated residue contributions, identifying some key tuning amino acids. Additionally, we assess the validity of the single protonation state picture to represent the system at a given pH and propose the most populated protein charge state as a compromise between cost and accuracy.

Periodic decay in the photoisomerisation of p-aminoazobenzene.

Semiempirical OM2/MRCI surface-hopping simulations have been performed to study the E→Z and Z→E isomerisations of p-aminoazobenzene upon photoexcitation to the S1 state (nπ*). The overall mechanism is similar to the one found previously for the unsubstituted parent system, although there is a moderate speedup of the decay to the ground state because of the steeper excited-state potential between the Franck-Condon region and the conical intersection seam. The decay dynamics to the ground state shows an oscillatory pattern that can be attributed to an out-of-plane rotation of the N2 moiety. The reaction is thus initially driven by N2 rotation, which triggers phenyl rotations around the C-N bonds. The Z isomer is produced most effectively when the phenyl rings rotate in phase.

Femtosecond Spectroscopy on a Dibenzophenazine-Cored Macrocycle Exhibiting Thermally Activated Delayed Fluorescence.

The photophysics of a thermally activated delayed fluorescence (TADF) emitting macrocycle consisting of two dibenzo[a,j]phenazine acceptor moieties bridged by two N,N,N',N'-tetraphenylene-1,4-diamine donor units was scrutinized in solution by steady-state and time-resolved spectroscopy. The fluorescence lifetime of the compound proved to be strongly solvent-dependent. It ranges from 6.3 ns in cyclohexane to 34 ps in dimethyl sulfoxide. In polar solvents the fluorescence decay is predominantly due to internal conversion. In non-polar ones radiative decay and intersystem crossing contribute. Contrary to the behaviour in polymer matrices (S. Izumi et al., J. Am. Chem. Soc. 2020, 142, 1482) the excited state decay is not predominantly due to prompt and delayed fluorescence. The solvent-dependent behaviour is analyzed with the aid of quantum chemical computations.

Sampling excited state dynamics: influence of HOOP mode excitations in a retinal model.

Zero point energy and classical thermal sampling techniques are compared in semi-classical photodynamics of the pentadienyliminium cation, a minimal retinal model. Using both methods, the effects of vibrational hydrogen-out-of-plane (HOOP) excitations on the photo-reactivity are probed at the ab initio CASSCF level. With 2376 individual trajectories the calculations reveal a clear picture of the relation between the excited state reaction coordinate, surface crossing and product generation. The productivity is strongly coupled with hydrogen torsion and the number of hopping attempts before the molecule finally decays.