A Facile Access to Green Fluorescent Albumin Derivatives.

A Morita-Baylis-Hillman Adduct (MBHA) derivative bearing a triphenylamine moiety was found to react with human serum albumin (HSA) shifting its emission from the blue to the green-yellow thus leading to green fluorescent albumin (GFA) derivatives and enlarging the platform of probes for aggregation-induced fluorescent-based detection techniques. A possible interaction of MBHA derivative 7 with a lipophilic pocket within the HSA structure was suggested by docking studies. DLS experiments showed that the reaction with HSA induce a conformational change of the protein contributing to the aggregation process of GFA derivatives. The results of investigations on the biological properties suggested that GFA retained the ability of binding drug molecules such as warfarin and diazepam. Finally, cytotoxicity evaluation studies suggested that, although the MBHA derivative 7 at 0.1 µg/mL affected the percentage of cell viability in comparison to the negative control, it cannot be considered cytotoxic, whereas at all the other concentrations≥0.5 µg/mL resulted cytotoxic at different extent.

Alkylated green fluorescent protein chromophores: dynamics in the gas phase and in aqueous solution.

Fluorescent labelling of macromolecular samples, including using the green fluorescent protein (GFP), has revolutionised the field of bioimaging. The ongoing development of fluorescent proteins require a detailed understanding of the photophysics of the biochromophore, and how chemical derivatisation influences the excited state dynamics. Here, we investigate the photophysical properties associated with the S1 state of three alkylated derivatives of the chromophore in GFP, in the gas phase using time-resolved photoelectron imaging, and in water using femtosecond fluorescence upconversion. The gas-phase lifetimes (1.6-10 ps), which are associated with the intrinsic (environment independent) dynamics, are substantially longer than the lifetimes in water (0.06-3 ps), attributed to stabilisation of both twisted intermediate structures and conical intersection seams in the condensed phase. In the gas phase, alkylation on the 3 and 5 positions of the phenyl ring slows the dynamics due to inertial effects, while a 'pre-twist' of the methine bridge through alkylation on the 2 and 6 positions significantly shortens the excited state lifetimes. Formation of a minor, long-lived (≫ 40 ps) excited state population in the gas phase is attributed to intersystem crossing to a triplet state, accessed because of a T1/S1 degeneracy in the so-called P-trap potential energy minimum associated with torsion of the single-bond in the bridging unit connecting to the phenoxide ring. A small amount of intersystem crossing is supported through TD-DFT molecular dynamics trajectories and MS-CASPT2 calculations. No such intersystem crossing occurs in water at T = 300 K or in ethanol at T ≈ 77 K, due to a significantly altered potential energy surface and P-trap geometry.

Through-Space Interaction of Tetraphenylethylene: What, Where, and How.

Electronic conjugation through covalent bonds is generally considered as the basis for the electronic transition of organic luminescent materials. Tetraphenylethylene (TPE), an efficient fluorophore with aggregation-induced emission character, fluoresces blue emission in the aggregate state, and such photoluminescence is always ascribed to the through-bond conjugation (TBC) among the four phenyl rings and the central C═C bond. However, in this work, systematic spectroscopic studies and DFT theoretical simulation reveal that the intramolecular through-space interaction (TSI) between two vicinal phenyl rings generates the bright blue emission in TPE but not the TBC effect. Furthermore, the evaluation of excited-state decay dynamics suggests the significance of photoinduced isomerization in the nonradiative decay of TPE in the solution state. More importantly, different from the traditional qualitative description for TSI, the quantitative elucidation of the TSI is realized through the atoms-in-molecules analysis; meanwhile, a theoretical solid-state model for TPE and other multirotor systems for studying the electronic configuration is preliminarily established. The mechanistic model of TSI delineated in this work provides a new strategy to design luminescent materials beyond the traditional theory of TBC and expands the quantum understanding of molecular behavior to the aggregate level.

Action spectroscopy of the isolated red Kaede fluorescent protein chromophore.

Incorporation of fluorescent proteins into biochemical systems has revolutionized the field of bioimaging. In a bottom-up approach, understanding the photophysics of fluorescent proteins requires detailed investigations of the light-absorbing chromophore, which can be achieved by studying the chromophore in isolation. This paper reports a photodissociation action spectroscopy study on the deprotonated anion of the red Kaede fluorescent protein chromophore, demonstrating that at least three isomers-assigned to deprotomers-are generated in the gas phase. Deprotomer-selected action spectra are recorded over the S1 ← S0 band using an instrument with differential mobility spectrometry coupled with photodissociation spectroscopy. The spectrum for the principal phenoxide deprotomer spans the 480-660 nm range with a maximum response at ≈610 nm. The imidazolate deprotomer has a blue-shifted action spectrum with a maximum response at ≈545 nm. The action spectra are consistent with excited state coupled-cluster calculations of excitation wavelengths for the deprotomers. A third gas-phase species with a distinct action spectrum is tentatively assigned to an imidazole tautomer of the principal phenoxide deprotomer. This study highlights the need for isomer-selective methods when studying the photophysics of biochromophores possessing several deprotonation sites.

Stability and Optical Absorption of a Comprehensive Virtual Library of Minimal Eumelanin Oligomer Models*.

Eumelanin is responsible for photoprotection in living organisms. It is made of 5,6-dihydroxyindole (DHI) oligomers. However, lack of detailed structural knowledge limits understanding its function and exploiting its potential in material science. To uncover the relationship between structural stability and optical properties, we have studied a virtual library of 830 DHI dimers. We find a preference for oxidized, polycyclic structures which speaks in favor of graphite-like structures for the larger oligomers, and propose an electrocyclic formation mechanism. Besides widely considered quinone oxidation patterns, also structures with interfragment double bonds and zwitterionic resonance structures are stable. Future theoretical melanine models will have to cover this diversity, and we introduce a new representative set of 49 stable dimers. Some stable oxidized dimers have absorption energies as low as 1.3 eV. They may be present as substructures in the naturally found oligomers and contribute to the absorption spectrum of the biopolymer.

An nπ* gated decay mediates excited-state lifetimes of isolated azaindoles.

Aiming to serve as a guide to understand the relaxation mechanisms of more complex aza-aromatic compounds, such as purine bases, we have studied the non-radiative channels of a set of azaindole structural isomers: 4-, 5-, 6- and 7-azaindole (AI). The relaxation of the isolated molecules, after excitation at the low energy portion of their spectra, has been tracked by femtosecond time-resolved ionization, and the decay paths have been obtained with MS-CASPT2//TD-DFT calculations. Although the ultrashort measured lifetimes for 5- and 6-AI are in contrast to the long-living excited state found in 7-AI, the calculations describe a common relaxation pathway. Along it, the initially excited ππ* states decay to the ground state through a conical intersection accessed through an nπ* state that functions as a gate state. The work reveals that the position of the nitrogen atoms in the purine ring determines the barrier to access the gate state and therefore, the rate of the non-radiative relaxation.

Unusual Through-Space Interactions between Oxygen Atoms that Mediate Inverse Morphochromism of an AIE Luminogen.

We have studied the photophysics of tetrafurylethene, an aggregation-induced emission luminogen with exceptionally short intramolecular O-O distances of 2.80 Šand a significant red-shifted morphochromism (27 nm) when going from the aggregate to the crystal. The short O-O distances, which are substantially smaller than the sum of the van der Waals radii (3.04 Å), are due to the fact that the oxygen atoms act as an electronic bridge connecting the furan rings on opposite ends of the central double bond, giving rise to a circular delocalization of the π-electron density across the rings. In the excited state the O-O distance is further reduced to 2.70 Å; the increased O-O interaction causes a narrowing of the HOMO-LUMO gap, resulting in the red morphochromism of the emission. Our results show the structural origin of the red-shifted emission lies in close O-O contacts, paving the way for understanding the clusteroluminescence of oxygen-rich non-conjugated systems that emit visible light.

Picosecond Switchable Azo Dyes.

Azo dyes that combine electron-withdrawing thiazole/benzothiazole heterocycles and electron-donating amino groups within the very same covalent skeleton exhibit relaxation times for their thermal isomerization kinetics within milli- and microsecond timescales at room temperature. Notably, the thermal back reaction of the corresponding benzothiazolium and thiazolium salts occurred much faster, within the picosecond temporal domain. In fact, these new light-sensitive platforms are the first molecular azo derivatives capable of reversible switching between their trans and cis isomers in a subnanosecond timescale under ambient conditions. In addition, theoretical calculations revealed very low activation energies for the isomerization process, in accordance with the fast subnanosecond kinetics that were observed experimentally.

Effects of Intra- and Intermolecular Hydrogen Bonding on O-H Bond Photodissociation Pathways of a Catechol Derivative.

The catechol functional motif is thought to play both a structural and photochemical role in the ubiquitous natural pigment, eumelanin. Intramolecular and intermolecular hydrogen bonding interactions lead to a variety of geometries involving the two O-H groups in catechol, but its photophysical behavior in these situations has not been comprehensively characterized. Toward this end, we monitor the UV-induced O-H bond photodissociation reaction in an exemplar catechol derivative, 4- tert-butylcatechol, possessing different intramolecular and intermolecular hydrogen bonding geometries using femtosecond transient absorption spectroscopy measurements in the UV-visible and mid-infrared regions following 265 nm photoexcitation. Three different hydrogen bonding arrangements are obtained by tuning solution complexation equilibria of the catechol with the hydrogen bond acceptor, diethyl ether (Et2O), and are verified computationally. We find that intermolecular hydrogen bonding to the free O-H group in catechol increases its first excited singlet state (S1) lifetime by 2 orders of magnitude (i.e., ∼ 16 to 1410 ps), and that O-H bond dissociation is prevented because Et2O is a poor hydrogen atom acceptor. Complexation of both O-H groups with multiple Et2O molecules further elongates the S1 lifetime to 1670 ps due to shifting of the solution equilibria that describe complex formation. Weakening of the characteristic, intramolecular hydrogen bond of the catechol derivative by intermolecular hydrogen bonding to one or more Et2O molecules does not enhance the rate of O-H bond dissociation.

Self-Assembled Cofacial Zinc-Porphyrin Supramolecular Nanocapsules as Tuneable 1 O2 Photosensitizers.

We demonstrate the benefits of using cofacial Zn-porphyrins as structural synthons in coordination-driven self-assembled prisms to produce cage-like singlet oxygen (1 O2 ) photosensitizers with tunable properties. In particular, we describe the photosensitizing and emission properties of palladium- and copper-based supramolecular capsules, and demonstrate that the nature of the bridging metal nodes in these discrete self-assembled prisms strongly influences 1 O2 generation at the Zn-porphyrin centers. The PdII -based prism is a particularly robust photosensitizer, whereas the CuII self-assembled prism is a dormant photosensitizer that could be switched to a ON state upon disassembly of the suprastructure. Furthermore, the well-defined cavity within the prisms allowed encapsulation of pyridine-based ligands and fullerene derivatives, which led to a remarkable guest tuning of the 1 O2 production.

Quantum Mechanical Studies on the Photophysics and the Photochemistry of Nucleic Acids and Nucleobases.

The photophysics and photochemistry of DNA is of great importance due to the potential damage of the genetic code by UV light. Quantum mechanical studies have played a key role in interpretating the results of modern time-resolved pump-probe spectroscopy, and in elucidating the main photoactivated reactive paths. This review provides a concise, complete picture of the computational studies carried out, approximately, in the past decade. We start with an overview of the photophysics of the nucleobases in the gas phase and in solution. We discuss the proposed mechanisms for ultrafast decay to the ground state, that involve conical intersections, consider the role of triplet states, and analyze how the solvent modulates the photophysics. Then we move to larger systems, from dinucleotides to single- and double-stranded oligonucleotides. We focus on the possible role of charge transfer and delocalized or excitonic states in the photophysics of these systems and discuss the main photochemical paths. We finish with an outlook on the current challenges in the field and future directions of research.

Thermally induced hopping model for long-range triplet excitation energy transfer in DNA.

We present a theoretical model for long-range triplet excitation energy transfer in DNA sequences of alternating adenine-thymine steps. It consists of thermally induced hops through thymine bases. Intrastrand hops between thymines separated by an AT step are preferred to interstrand hops between thymines in consecutive steps. Our multi-step mechanism is consistent with recent results (L. Antusch, N. Gass and H.-A. Wagenknecht, Angew. Chem., Int. Ed., 2017, 56, 1385-1389) describing a shallow dependence of triplet sensitized DNA damage relative to the distance between the sensitizer and the reacting thymine-thymine pair.

Theoretical study of non-Hammett vs. Hammett behaviour in the thermolysis and photolysis of arylchlorodiazirines.

Arylchlorodiazirines (ACDA) are thermal and photochemical precursors of carbenes that form these molecules via nitrogen elimination. We have studied this reaction with multireference quantum chemical methods (CASSCF and CASPT2) for a series of ACDA derivatives with different substitution at the aromatic ring. The calculations explain the different reactivity trends found in the ground and excited state, with good correlation between the calculated barriers and the experimental reaction rates. The ground state mechanism can be described as a reverse cycloaddition with small charge transfer from the aromatic ring to the diazirine moiety. This is consistent with the lack of correlation between the Hammett σ descriptors and the experimental rates. In contrast, the excited state reaction is the cleavage of a single C-N bond mediated by small barriers of 4-6 kcal mol-1. The reaction path goes through a conical intersection with the ground state, which facilitates radiationless decay and explains the disappearance of the transient absorption signal measured experimentally. This leads to a diazomethane intermediate that ultimately yields the carbene. Electronically, excitation to S1 is characterized initially by significant charge transfer from the phenyl ring to the diazirine. The charge transfer is reversed during the C-N cleavage reaction, and this explains the preferential stabilization of the excited-state minimum by polar solvents and electron-donating substituents. Therefore, our calculations reproduce and explain the relationship found experimentally between the Hammett σ+ parameters and the life time of S1 (Y. L. Zhang, et al. J. Am. Chem. Soc., 2009, 131, 16652-16653).

What Controls Photocatalytic Water Oxidation on Rutile TiO2(110) under Ultra-High-Vacuum Conditions?

The photocatalytic O-H dissociation of water absorbed on a rutile TiO2(110) surface in ultrahigh vacuum (UHV) is studied with spin-polarized density functional theory and a hybrid exchange-correlation functional (HSE06), treating the excited-state species as excitons with triplet multiplicity. This system is a model for the photocatalytic oxidation of water by TiO2 in an aqueous medium, which is relevant for the oxygen evolution reaction and photodegradation of organic pollutants. We provide a comprehensive mechanistic picture where the most representative paths correspond to excitonic configurations with the hole located on three- and two-coordinate surface oxygen atoms (O3s and O2s). Our picture explains the formation of the species observed experimentally. At near band gap excitation, the O3s path leads to the generation of hydroxyl anions which diffuse on the surface, without net oxidation. In contrast, free hydroxyl radicals are formed at supra band gap excitation (e.g., 266 nm) from an interfacial exciton that undergoes O-H dissociation. The oxidation efficiency is low because the path associated with the O2s exciton, which is the most favored one thermodynamically, is unreactive because of a high propensity for charge recombination. Our results are also relevant to understand the reactivity in the liquid phase. We assign the photoluminescence measured for atomically flat TiO2(110) surfaces in an aqueous medium to the O3s exciton, in line with the proposal based on experiments, and we have identified a species derived from the O2s exciton with an activated O2s-Ti bond that may be relevant in photocatalytic applications in an aqueous medium.

Direct estimation of the transfer integral for photoinduced electron transfer from TD DFT calculations.

The rate of photoinduced ET in molecular systems is controlled by electronic coupling of the locally excited and charge transfer states. We generalize the Bixon-Jortner-Verhoeven expression for electronic coupling to systems with a small energy gap and derive the transfer integral for charge separation in two model heterojunctions using the excitation energies and oscillator strengths computed with TD DFT. The estimated couplings are in good agreement with the reference values.

Planarizing cytosine: The S1 state structure, vibrations, and nonradiative dynamics of jet-cooled 5,6-trimethylenecytosine.

We measure the S0 → S1 spectrum and time-resolved S1 state nonradiative dynamics of the "clamped" cytosine derivative 5,6-trimethylenecytosine (TMCyt) in a supersonic jet, using two-color resonant two-photon ionization (R2PI), UV/UV holeburning, and ns time-resolved pump/delayed ionization. The experiments are complemented with spin-component scaled second-order approximate coupled cluster (SCS-CC2), time-dependent density functional theory, and multi-state second-order perturbation-theory (MS-CASPT2) ab initio calculations. While the R2PI spectrum of cytosine breaks off ∼500 cm-1 above its 000 band, that of TMCyt extends up to +4400 cm-1 higher, with over a hundred resolved vibronic bands. Thus, clamping the cytosine C5-C6 bond allows us to explore the S1 state vibrations and S0 → S1 geometry changes in detail. The TMCyt S1 state out-of-plane vibrations ν1', ν3', and ν5' lie below 420 cm-1, and the in-plane ν11', ν12', and ν23' vibrational fundamentals appear at 450, 470, and 944 cm-1. S0 → S1 vibronic simulations based on SCS-CC2 calculations agree well with experiment if the calculated ν1', ν3', and ν5' frequencies are reduced by a factor of 2-3. MS-CASPT2 calculations predict that the ethylene-type S1 ⇝ S0 conical intersection (CI) increases from +366 cm-1 in cytosine to >6000 cm-1 in TMCyt, explaining the long lifetime and extended S0 → S1 spectrum. The lowest-energy S1 ⇝ S0 CI of TMCyt is the "amino out-of-plane" (OPX) intersection, calculated at +4190 cm-1. The experimental S1 ⇝ S0 internal conversion rate constant at the S1(v'=0) level is kIC=0.98-2.2⋅108 s-1, which is ∼10 times smaller than in 1-methylcytosine and cytosine. The S1(v'=0) level relaxes into the T1(3ππ*) state by intersystem crossing with kISC=0.41-1.6⋅108 s-1. The T1 state energy is measured to lie 24 580±560 cm-1 above the S0 state. The S1(v'=0) lifetime is τ=2.9 ns, resulting in an estimated fluorescence quantum yield of Φfl=24%. Intense two-color R2PI spectra of the TMCyt amino-enol tautomers appear above 36 000 cm-1. A sharp S1 ionization threshold is observed for amino-keto TMCyt, yielding an adiabatic ionization energy of 8.114±0.002 eV.

Excitonic Interfacial Proton-Coupled Electron Transfer Mechanism in the Photocatalytic Oxidation of Methanol to Formaldehyde on TiO2(110).

CH3OH on a single-crystal rutile TiO2(110) surface is a widely studied model system for heterogeneous photocatalysis. Using spin-polarized density functional theory with a hybrid functional (HSE06), we study the photocatalytic oxidation of CH3OH adsorbed at a coordinately unsaturated Ti site as an excited-state process with triplet spin multiplicity. The oxidation to CH2O is stepwise and involves a CH3O intermediate. The first O-H dissociation step follows an excitonic interfacial proton-coupled electron transfer mechanism where the hole-electron (h-e) pair generated during the excitation is bound, and the h is transferred to the adsorbate. The O-H dissociation paths associated with other h-e pairs are unreactive, and the moderate experimental efficiency is due to the different reactivity of the h-e pairs. The excited-state CH3O intermediate further deactivates through a seam of intersection between the ground and excited states. It can follow three different paths, regeneration of adsorbed CH3OH or formation of the ground-state CH3O anion or an adsorbed CH2O radical anion. The third channel corresponds to photochemical CH2O formation from CH3OH, where a single photon induces one electron oxidation and transfer of two protons. These results expand the current view on the photocatalysis of CH3OH on TiO2(110) by highlighting the role of excitons and showing that adsorbed CH3OH may also be an active species in the photocatalytic oxidation to CH2O.

Multiple Decay Mechanisms and 2D-UV Spectroscopic Fingerprints of Singlet Excited Solvated Adenine-Uracil Monophosphate.

The decay channels of singlet excited adenine uracil monophosphate (ApU) in water are studied with CASPT2//CASSCF:MM potential energy calculations and simulation of the 2D-UV spectroscopic fingerprints with the aim of elucidating the role of the different electronic states of the stacked conformer in the excited state dynamics. The adenine (1) La state can decay without a barrier to a conical intersection with the ground state. In contrast, the adenine (1) Lb and uracil S(U) states have minima that are separated from the intersections by sizeable barriers. Depending on the backbone conformation, the CT state can undergo inter-base hydrogen transfer and decay to the ground state through a conical intersection, or it can yield a long-lived minimum stabilized by a hydrogen bond between the two ribose rings. This suggests that the (1) Lb , S(U) and CT states of the stacked conformer may all contribute to the experimental lifetimes of 18 and 240 ps. We have also simulated the time evolution of the 2D-UV spectra and provide the specific fingerprint of each species in a recommended probe window between 25 000 and 38 000 cm(-1) in which decongested, clearly distinguishable spectra can be obtained. This is expected to allow the mechanistic scenarios to be discerned in the near future with the help of the corresponding experiments. Our results reveal the complexity of the photophysics of the relatively small ApU system, and the potential of 2D-UV spectroscopy to disentangle the photophysics of multichromophoric systems.

Synthesis and Isomeric Analysis of RuII Complexes Bearing Pentadentate Scaffolds.

A RuII-pentadentate polypyridyl complex [RuII(κ-N5-bpy2PYMe)Cl]+ (1+, bpy2PYMe = 1-(2-pyridyl)-1,1-bis(6-2,2'-bipyridyl)ethane) and its aqua derivative [RuII(κ-N5-bpy2PYMe)(H2O)]2+ (22+) were synthesized and characterized by experimental and computational methods. In MeOH, 1+ exists as two isomers in different proportions, cis (70%) and trans (30%), which are interconverted under thermal and photochemical conditions by a sequence of processes: chlorido decoordination, decoordination/recoordination of a pyridyl group, and chlorido recoordination. Under oxidative conditions in dichloromethane, trans-12+ generates a [RuIII(κ-N4-bpy2PYMe)Cl2]+ intermediate after the exchange of a pyridyl ligand by a Cl- counterion, which explains the trans/cis isomerization observed when the system is taken back to Ru(II). On the contrary, cis-12+ is in direct equilibrium with trans-12+, with absence of the κ-N4-bis-chlorido RuIII-intermediate. All these equilibria were modeled by density functional theory calculations. Interestingly, the aqua derivative is obtained as a pure trans-[RuII(κ-N5-bpy2PYMe)(H2O)]2+ isomer (trans-22+), while the addition of a methyl substituent to a single bpy of the pentadentate ligand leads to the formation of a single cis isomer for both chlorido and aqua derivatives [RuII(κ-N5-bpy(bpyMe)PYMe)Cl]+ (3+) and [RuII(κ-N5-bpy(bpyMe)PYMe)(H2O)]2+ (42+) due to the steric constraints imposed by the modified ligand. This system was also structurally and electrochemically compared to the previously reported [RuII(PY5Me2)X]n+ system (X = Cl, n = 1 (5+); X = H2O, n = 2 (62+)), which also contains a κ-N5-RuII coordination environment, and to the newly synthesized [RuII(PY4Im)X]n+ complexes (X = Cl, n = 1 (7+); X = H2O, n = 2 (82+)), which possess an electron-rich κ-N4C-RuII site due to the replacement of a pyridyl group by an imidazolic carbene.

Early events in the photochemistry of 5-diazo Meldrum's acid: formation of a product manifold in C-N bound and pre-dissociated intersection seam regions.

5-Diazo Meldrum's acid (DMA) undergoes a photo-induced Wolff rearrangement (WR). Recent gas-phase experiments have identified three photochemical products formed in a sub-ps scale after irradiation, a carbene formed after nitrogen loss, a ketene formed after WR and a second carbene formed after nitrogen and CO elimination (A. Steinbacher, et al. Phys. Chem. Chem. Phys., 2014, 16, 7290-7298). In this work, ground- and excited-state potential energy surfaces (PESs) have been investigated at the MS-CASPT2//CASSCF level. The key element of the PESs is an extended S0/S1 conical intersection seam along the C-N dissociation coordinate. The C-N predissociated region of the seam is accessed after excitation to the bright S2 state, and decay paths from the seam to the three primary products have been characterized. For the ketene and carbene II products, we show two possible formation pathways, a direct and a stepwise one, which suggests that these products may be formed in a bi-modal fashion. We have also characterized two possible mechanisms for triplet formation, one occurring before C-N dissociation involving a (S1/T2/T1) crossing region, and another one through the carbene. In contrast, excitation to S1 leads to a C-N bound region of the seam from where DMA regeneration or diazirine formation is possible, with a preference for the first case. The results are in good agreement with experimental data. Together with our previous work on diazonaphthoquinone, they show the importance of an extended seam in the photochemistry of α-diazoketones.