Marco Garavelli

Triplet Pathways in Diarylethene Photochromism: Photophysical and Computational Study of Dyads Containing Ruthenium(II) Polypyridine and 1,2-Bis(2-methylbenzothiophene-3-yl)maleimide Units

A 1,2-bis(2-methylbenzothiophene-3-yl)maleimide model ( DAE) and two dyads in which this photochromic unit is coupled, via a direct nitrogen-carbon bond ( Ru-DAE) or through an intervening methylene group ( Ru-CH 2-DAE ), to a ruthenium polypyridine chromophore have been synthesized. The photochemistry and photophysics of these systems have been thoroughly characterized in acetonitrile by a combination of stationary and time-resolved (nano- and femtosecond) spectroscopic methods. The diarylethene model DAE undergoes photocyclization by excitation at 448 nm, with 35% photoconversion at stationary state. The quantum yield increases from 0.22 to 0.33 upon deaeration. Photochemical cycloreversion (quantum yield, 0.51) can be carried out to completion upon excitation at lambda > 500 nm. Photocyclization takes place both from the excited singlet state (S 1), as an ultrafast (ca. 0.5 ps) process, and from the triplet state (T 1) in the microsecond time scale. In Ru-DAE and Ru-CH 2-DAE dyads, efficient photocyclization following light absorption by the ruthenium chromophore occurs with oxygen-sensitive quantum yield (0.44 and 0.22, in deaerated and aerated solution, respectively). The photoconversion efficiency is almost unitary (90%), much higher than for the photochromic DAE alone. Efficient quenching of both Ru-based MLCT phosphorescence and DAE fluorescence is observed. A complete kinetic characterization has been obtained by ps-ns time-resolved spectroscopy. Besides prompt photocyclization (0.5 ps), fast singlet energy transfer takes place from the excited diarylethene to the Ru(II) chromophore (30 ps in Ru-DAE, 150 ps in Ru-CH 2-DAE ). In the Ru(II) chromophore, prompt intersystem crossing to the MLCT triplet state is followed by triplet energy transfer to the diarylethene (1.5 ns in Ru-DAE, 40 ns in Ru-CH 2-DAE ). The triplet state of the diarylethene moiety undergoes cyclization in a microsecond time scale. The experimental results are complemented with a combined ab initio and DFT computational study whereby the potential energy surfaces (PES) for ground state (S 0) and lowest triplet state (T 1) of the diarylethene are investigated along the reaction coordinate for photocyclization/cycloreversion. At the DFT level of theory, the transition-state structures on S 0 and T 1 are similar and lean, along the reaction coordinate, toward the closed-ring form. At the transition-state geometry, the S 0 and T 1 PES are almost degenerate. Whereas on S 0 a large barrier (ca. 45 kcal mol (-1)) separates the open- and closed-ring minima, on T 1 the barriers to isomerization are modest, cyclization barrier (ca. 8 kcal mol (-1)) being smaller than cycloreversion barrier (ca. 14 kcal mol (-1)). These features account for the efficient sensitized photocyclization and inefficient sensitized cycloreversion observed with Ru-DAE. Triplet cyclization is viewed as a nonadiabatic process originating on T 1 at open-ring geometry, proceeding via intersystem crossing at transition-state geometry, and completing on S 0 at closed-ring geometry. A computational study of the prototypical model 1,2-bis(3-thienyl)ethene is used to benchmark DFT results against ab initio CASSCF//CASPT2 results and to demonstrate the generality of the main topological features of the S 0 and T 1 PES obtained for DAE. Altogether, the results provide strong experimental evidence and theoretical rationale for the triplet pathway in the photocyclization of photochromic diarylethenes.

The Different Photoisomerization Efficiency of Azobenzene in the Lowest nπ* and ππ* Singlets: The Role of a Phantom State

Azobenzene E<==>Z photoisomerization, following excitation to the bright S(pi pi*) state, is investigated by means of ab initio CASSCF optimizations and perturbative CASPT2 corrections. Specifically, by elucidating the S(pi pi*) deactivation paths, we explain the mechanism responsible for azobenzene photoisomerization, the lower isomerization quantum yields observed for the S(pi pi*) excitation than for the S1(n pi*) excitation in the isolated molecule, and the recovery of the Kasha rule observed in sterically hindered azobenzenes. We find that a doubly excited state is a photoreaction intermediate that plays a very important role in the decay of the bright S(pi pi*). We show that this doubly excited state, which is immediately populated by molecules excited to S(pi pi*), drives the photoisomerization along the torsion path and also induces a fast internal conversion to the S1(n pi*) at a variety of geometries, thus shaping (all the most important features of) the S(pi pi*) decay pathway and photoreactivity. We reach this conclusion by determining the critical structures, the minimum energy paths originating on the bright S(pi pi*) state and on other relevant excited states including S1(n pi*), and by characterizing the conical intersection seams that are important in deciding the photochemical outcome. The model is consistent with the most recent time-resolved spectroscopic and photochemical data.

Electrostatic Control of the Photoisomerization Efficiency and Optical Properties in Visual Pigments: On the Role of Counterion Quenching

Hybrid QM(CASPT2//CASSCF/6-31G*)/MM(Amber) computations have been used to map the photoisomerization path of the retinal chromophore in Rhodopsin and explore the reasons behind the photoactivity efficiency and spectral control in the visual pigments. It is shown that while the electrostatic environment plays a central role in properly tuning the optical properties of the chromophore, it is also critical in biasing the ultrafast photochemical event: it controls the slope of the photoisomerization channel as well as the accessibility of the S(1)/S(0) crossing space triggering the ultrafast decay. The roles of the E113 counterion, the E181 residue, and the other amino acids of the protein pocket are explicitly analyzed: it appears that counterion quenching by the protein environment plays a key role in setting up the chromophores optical properties and its photochemical efficiency. A unified scenario is presented that discloses the relationship between spectroscopic and mechanistic properties in rhodopsins and allows us to draw a solid mechanism for spectral tuning in color vision pigments: a tunable counterion shielding appears as the elective mechanism for L<-->M spectral modulation, while a retinal conformational control must dictate S absorption. Finally, it is suggested that this model may contribute to shed new light into mutations-related vision deficiencies that opens innovative perspectives for experimental biomolecular investigations in this field.

Deciphering low energy deactivation channels in adenine.

The radiationless decay paths of 9H-adenine in its lowest excited states (1)npi*, (1)L(b)((1)pipi*), and (1)L(a)((1)pipi*) and in dissociative (1)pisigma* states have been mapped in vacuo at the CASPT2//CASSCF resolution. The minimum energy path (MEP) of the (1)L(a) state, which shows the strongest absorption below 5 eV, is found to decrease monotonically along the puckering coordinate from the vertical excitation to a S(0)/(1)L(a) conical intersection (CI). The vertically excited (1)npi* and (1)L(b) states are found to relax to the respective minima and to require some energy to reach CIs with S(0). This picture suggests that (1)L(a) alone is responsible of both components of the ultrafast biexponential decay (with tau(1) < 0.1 ps and tau(2) < 1 ps) recently observed in time-resolved pump-probe resonant ionization and fluorescence spectroscopy, and that the (1)npi* and (1)L(b) states do not act as important intermediates in the (1)L(a) decay process. We find that the (1)L(a)-->(1)pisigma(N9H)* internal conversion can be followed by N(9)-H photocleavage, albeit with tiny quantum yield. The amino N(10)-H bond photocleavage is hindered by the high barrier encountered along the N(10)-H bond-breaking path in the (1)pisigma(N10H)* state.

Multistate Photo-Induced Relaxation and Photoisomerization Ability of Fumaramide Threads: A Computational and Experimental Study

Fumaric and maleic amides are the photoactive units of an important and widely investigated class of photocontrollable rotaxanes as they trigger ring shuttling via a cis-trans photoisomerization. Here, ultrafast decay and photoinduced isomerization in isolated fumaramide and solvated nitrogen-substituted fumaramides (that are employed as threads in those rotaxanes) have been investigated by means of CASPT2//CASSCF computational and time-resolved spectroscopic techniques, respectively. A complex multistate network of competitive deactivation channels, involving both internal conversion and intersystem crossing (ISC) processes, has been detected and characterized that accounts for the picosecond decay and photochemical/photophysical properties observed in the singlet as well as triplet (photosensitized) photochemistry of fumaramides threads. Interestingly, singlet photochemistry appears to follow a non-Kasha rule model, where nonequilibrium dynamical factors control the outcome of the photochemical process: accessible high energy portions of extended crossing seams turn out to drive the deactivation process and ground-state recovery. Concurrently, extended singlet/triplet degenerate regions of twisted molecular structures with significant spin-orbit-coupling values account for ultrafast (picosecond time scale) ISC processes that lead to higher photoisomerization efficiencies. This model discloses the principles behind the intrinsic photochemical reactivity of fumaramide and its control.

Computational evidence for the catalytic mechanism of glutaminyl cyclase. A DFT investigation

The results of a DFT theoretical investigation on the catalytic mechanism of the QC enzyme are presented. A rather large model‐system is used. It includes the most important residues that are believed to play a key‐role in the catalysis. The computational results show that the rate‐determining step of the catalytic process is not the nucleophilic attack leading to the cycle formation (a very easy and fast process with a negligible barrier of 0.8 kcal mol−1), but a proton transfer, which is assisted by the Glu201 residue acting as a proton shuttle (general base and general acid). A complex network of hydrogen bonds (involving Asp248 and other residues) contribute to lower the activation barrier for the proton shift which affords the formation of an ammonia molecule bonded to the substrate. The ammonia molecule is a good leaving group which is easily expelled from the substrate in the last step of the catalytic cycle, but remains anchored to the enzyme as a ligand of the zinc cation. The metal plays a key‐role in assisting the nucleophilic attack (electrostatic catalysis) since it polarizes the substrate γ‐amide carbonyl group (its electrophilic character increases). Also, the strength of the nucleophilic nitrogen (substrate α‐amino group) is enhanced by hydrogen bonds involving the Glu201 residue. The computations outline the important role of Trp329 in helping the substrate binding process and stabilizing the cyclization transition state. Proteins, 2008.

Modeling the photophysics and photochromic potential of 1,2-dihydronaphthalene (DHN): a combined CASPT2//CASSCF-topological and MMVB-dynamical investigation.

The photochemical ring opening of 1,2-dihydronaphthalene (DHN) was investigated using two complementary computational approaches. CASPT2//CASSCF minimum energy paths were characterized for reaction channels on the three lowest-energy singlet excited states, describing initial evolution of the spectroscopic bright (ionic) state and its subsequent decay to dark (covalent) states of benzene-like and hexatriene-like character. Although the benzene-like state is unreactive and can radiate, the hexatriene-like state has indirect access to a low-energy conical intersection seam, at which radiationless decay to the ground state and subsequent product formation can take place. An MMVB molecular dynamics simulation was carried out on the reactive hexatriene-like excited state, suggesting that intramolecular vibrational energy redistribution (IVR) controls the radiationless decay and the photoproduct distribution (which is qualitatively reproduced).

Substituent controlled spectroscopy and excited state topography of retinal chromophore models: fluorinated and methoxy-substituted protonated Schiff bases

Ab initio multireference second-order perturbation theory computations are used to explore the spectroscopic behaviour (i.e. absorption and emission) and the structure of two isolated (i.e. in vacuo) short chain retinal chromophore models (i.e. the 2-cis-penta-2,4-dieniminium and all-trans-epta-2,4,6-trieniminium cations), which have been modified using fluorine or methoxyl substituents as representative examples of electron withdrawing and electron releasing groups, respectively. A systematic analysis has been performed for the systems substituted in all positions along the chain. Significant effects on the excited state structure of the chromophore and its absorption and emission features are unveiled by comparison with previously reported results for the corresponding unsubstituted cations in vacuo. Indeed, it is demonstrated that (i) substituents may affect the chromophore absorption/emission energy depending on the substituted position along the chain and the specific electronic effect of the substituent (with fluorine and methoxyl inducing opposite effects), (ii) substituted odd- and even-numbered positions along the chain behave in a different way in agreement with the experiments, and (iii) the initially relaxed geometry in the excited state, addressing the excited state dynamics of the chromophore out of the Franck–Condon region, may change. A rationale for these effects is shown, which provides a crude basis for understanding experimental spectroscopic observations and allows one to speculate and draw conjectures about the intrinsic photochemical behaviour (namely the photoisomerization efficiency/selectivity) of substituted retinal protonated Schiff bases.

Computational DFT Investigation of Vicinal Amide Group Anchimeric Assistance in Ether Cleavage

Density functional theory (DFT) computations in solvent have been used to investigate the mechanism of anchimeric assistance (by a vicinal amide group) in the acid-induced ether cleavage. The calculations were carried out at the B3LYP/6-31G* level of theory via full geometry optimizations within the IEF-PCM continuum solvent model. Two different mechanisms have been investigated here that were previously hypothesized for the rate-determining step of this process: the first (mechanism A1) involves a protonated amide and an ethereal oxygen as the nucleophile, while the second (mechanism A2) involves protonation of the ethereal oxygen followed by a nucleophilic attack of the amide. Computations clearly show that the second (involving protonation of the less basic site) is the most favorite route and leads to the formation of an oxazolidinic intermediate that triggers ether hydrolysis. Results are produced that are in excellent agreement with the experiments, and a rationale for them is provided, which represents a general interpretative basis for similar anchimerically assisted processes, such as the ones characterizing the glycosidic activity of two very important classes of enzymes: beta-hexosaminidases and O-GlcNAcases.

COBRAMM: A Tunable QM/MM Approach to Complex Molecular Architectures. Modelling the Excited and Ground State Properties of Sized Molecular Systems

This contribution describes a new implementation of a general hybrid approach with a modular structure (called COBRAMM: Computations in Bologna Relating Ab‐initio and Molecular Mechanics Methods) that is able to integrate some specialized softwares and acts as a flexible computational environment, thus increasing the flexibility/efficiency of both QM, and MM, and QM/MM calculations. Specifically, QM/MM ground and excited states geometry optimizations, frequency calculations, conical intersection searches and adiabatic/non‐adiabatic molecular dynamics can be performed on a large molecular system, that can be split up to three different layers corresponding to different levels of accuracy. Here we report, together with a description of the method and its implementation, some test examples on very different chemical problems, which span the wide and diversified area of chemistry (from ground to excited states topics) and show the flexibility, general applicability and accuracy of the presented hybrid approac...