Demystifying the Origin of Vibrational Coherence Transfer Between the S1 and T1 States of the Pt-pop Complex

We demonstrate that spin-vibronic coupling is the most significant mechanism in vibrational coherence transfer (VCT) from the singlet (S₁) to the triplet (T₁) state of the [Pt₂(P₂O₅H₂)₄]^4- complex. Our time-dependent correlation function-based study shows that the rate of intersystem crossing (k_ISC) through direct spin-orbit coupling is negligibly small, making VCT vanishingly small due to the ultrashort decoherence time (2.5 ps). However, the inclusion of the spin-vibronic contribution to the net k_ISC in selective normal modes along the Pt-Pt axis increases the k_ISC to such an extent that VCT becomes feasible. Our results suggest that k_ISC for the S₁ →T₂ (τ_ISC = 1.084 ps) is much faster than the S₁ → T₁ (τ_ISC = 763.4 ps) and S₁ → T₃ (τ_ISC = 13.38 ps) in CH₃CN solvent, indicating that VCT is possible from the low-lying excited singlet (S₁) to the triplet (T₁) state through the intermediate T₂ state. This is the first example where VCT occurs solely due to spin-vibronic interactions. This finding can pave the way for new types of photocatalysis.

Isolating reactive metal-based species in Metal-Organic Frameworks - viable strategies and opportunities

Structural insight into reactive species can be achieved via strategies such as matrix isolation in frozen glasses, whereby species are kinetically trapped, or by confinement within the cavities of host molecules. More recently, Metal-Organic Frameworks (MOFs) have been used as molecular scaffolds to isolate reactive metal-based species within their ordered pore networks. These studies have uncovered new reactivity, allowed observation of novel metal-based complexes and clusters, and elucidated the nature of metal-centred reactions responsible for catalysis. This perspective considers strategies by which metal species can be introduced into MOFs and highlights some of the advantages and limitations of each approach. Furthermore, the growing body of work whereby reactive species can be isolated and structurally characterised within a MOF matrix will be reviewed, including discussion of salient examples and the provision of useful guidelines for the design of new systems. Novel approaches that facilitate detailed structural analysis of reactive chemical moieties are of considerable interest as the knowledge garnered underpins our understanding of reactivity and thus guides the synthesis of materials with unprecedented functionality.

Anisotropy enhanced X-ray scattering from solvated transition metal complexes

Time-resolved X-ray scattering patterns from photoexcited molecules in solution are in many cases anisotropic at the ultrafast time scales accessible at X-ray free-electron lasers (XFELs). This anisotropy arises from the interaction of a linearly polarized UV-Vis pump laser pulse with the sample, which induces anisotropic structural changes that can be captured by femtosecond X-ray pulses. In this work, a method for quantitative analysis of the anisotropic scattering signal arising from an ensemble of molecules is described, and it is demonstrated how its use can enhance the structural sensitivity of the time-resolved X-ray scattering experiment. This method is applied on time-resolved X-ray scattering patterns measured upon photoexcitation of a solvated di-platinum complex at an XFEL, and the key parameters involved are explored. It is shown that a combined analysis of the anisotropic and isotropic difference scattering signals in this experiment allows a more precise determination of the main photoinduced structural change in the solute, i.e. the change in Pt-Pt bond length, and yields more information on the excitation channels than the analysis of the isotropic scattering only. Finally, it is discussed how the anisotropic transient response of the solvent can enable the determination of key experimental parameters such as the instrument response function.

Photochemical reactions of metal complexes in the solid state

This perspective focusses on the solid-state reactivity and structural transformation driven by photochemical methods in discrete metal complexes, organometallic compounds, metallo-macrocycles and cages. Changes in the metal-metal bond distances, racemization of chiral centres, fusion of cages, formation of coordination polymers, expected [2 + 2] and [4 + 4] cycloaddition products, unusual phenyl-olefin dimerization, and linkage isomerization of -SO₂, -NO & -NO₂ ligands cause the structural transformations. Of these, [2 + 2] photo-cycloaddition reactions have been widely studied and the photoreactions are made possible by various supramolecular interactions including hydrogen bonds, metallophilic, ππ and C-Hπ interactions, ligand design and metallic clips to bring the reactive functional groups closely into correct orientation close to the transition state. These photoreactions are often accompanied by crystal bending, mechanical motion, and changes in the magnetic and photoluminescence properties. In several cases, the single crystals have been preserved at the end of the reactions, which are known as single-crystal-to-single-crystal (SCSC) reactions.

The dramatic development of X-ray photocrystallography over the past six decades

A short description of some of the paradigm-changing developments of the study of light-induced structural changes in molecular crystals is presented. The review is by no means comprehensive. The extensive literature on the subject should be consulted for further information.

Synchrotron ultrafast techniques for photoactive transition metal complexes

In the last decade, the use of time-resolved X-ray techniques has revealed the structure of light-generated transient species for a wide range of samples, from small organic molecules to proteins. Time resolutions of the order of 100 ps are typically reached, allowing one to monitor thermally equilibrated excited states and capture their structure as a function of time. This review aims at providing a general overview of the application of time-resolved X-ray solution scattering (TR-XSS) and time-resolved X-ray absorption spectroscopy (TR-XAS), the two techniques prevalently employed in the investigation of light-triggered structural changes of transition metal complexes. In particular, we herein describe the fundamental physical principles for static XSS and XAS and illustrate the theory of time-resolved XSS and XAS together with data acquisition and analysis strategies. Selected pioneering examples of photoactive transition metal complexes studied by TR-XSS and TR-XAS are discussed in depth.

Photocrystallographic identification of metastable nitrito linkage isomers in a series of nickel(II) complexes

Single crystal photocrystallographic experiments and solid state Raman spectroscopy have been used to determine the low temperature, metastable structures of the nickel(ii) nitrito complexes [Ni(aep)(2)(η(1)-ONO)(2)] 1#O (aep = 1-(2-aminoethyl)piperidine), [Ni(aem)(2)(η(1)-ONO)(2)] 2#O (aem = 1-(2-aminoethyl)morpholine), and [Ni(aepy)(2)(η(1)-ONO)(2)] 3#O (aepy = 1-(2-aminoethyl)pyrrolidine and where the #O denotes the oxygen-bound nitrito metastable molecule). These linkage isomers of the equivalent nitro complexes [Ni(aep)(2)(η(1)-NO(2))(2)] 1, [Ni(aem)(2)(η(1)-NO(2))(2)] 2 and [Ni(aepy)(2)(η(1)-NO(2))(2)] 3 are formed by LED irradiation at temperatures below 120 K. The behavior of the three complexes upon irradiation is generally similar, but some subtle differences have been observed. From the crystallographic studies all three complexes 1-3 exhibit the endo-nitrito linkage isomer upon irradiation, however, for 3# (a crystal structure that contains components of both 3 and 3#O) an exo-nitrito isomer is also observed. Under conditions of 90-100 K, with blue light, the conversion percentages to the nitrito isomers, 1#O, 2#O and 3#O were 16%, 22% and 30%, respectively. At temperatures below 110 K all three nitrito isomers were stable for over four hours but while 2#O and 3#O could be detected at temperatures down to 30 K, at temperatures below 60 K the metastable structure 1#O appeared to be quenched and only the nitro isomer 1 was identified in the crystal. The solid state Raman spectra for 1#, 2# and 3# confirmed the photocrystallographic results with the nitrito isomers being identified from the O-N-O deformation vibrations.

Polymorphic crystal approach to changing the emission of [AuCl(PPh3)2], analyzed by direct observation of the photoexcited structures by X-ray photocrystallography

The photoexcited charge-transferred state of [AuCl(PPh(3))(2)] in a novel polymorphic crystal form was directly observed by X-ray photocrystallographic analysis. Its photoexcited state was completely different from the one generated in the known crystal of [AuCl(PPh(3))(2)]; the photoexcited bond-shrunk state was generated in the known crystal. This difference in the generated photoexcited state was clearly reflected by the difference in emission color. While the known crystal form showed green phosphorescence, the novel form showed blue phosphorescence under UV irradiation. The difference in the generated photoexcited state was due to the differences in steric hindrance in the crystal; bond shortening by photoexcitation was sterically allowed in the known form, while on the other hand, it was restricted in the novel form. Therefore, instead of the bond-shrunk state, the charge-transferred excited state became the lowest triplet state, and the emission color changed from green to blue (i.e., a blue shift of the emission wavelength was observed). These results mean that the photoexcited structure and the emission color of [AuCl(PPh(3))(2)] can be controlled by designing the molecular environment in the crystal.

Time-resolved synchrotron diffraction and theoretical studies of very short-lived photo-induced molecular species

Definitive experimental results on the geometry of fleeting species are at the time of writing still limited to monochromatic data collection, but methods for modifications of the polychromatic Laue data to increase their accuracy and their suitability for pump-probe experiments have been implemented and are reviewed. In the monochromatic experiments summarized, excited-state conversion percentages are small when neat crystals are used, but are higher when photoactive species are embedded in an inert framework in supramolecular crystals. With polychromatic techniques and increasing source brightness, smaller samples down to tenths of a micrometre or less can be used, increasing homogeneity of exposure and the fractional population of the excited species. Experiments described include a series of transition metal complexes and a fully organic example involving excimer formation. In the final section, experimental findings are compared with those from theoretical calculations on the isolated species. Qualitative agreement is generally obtained, but the theoretical results are strongly dependent on the details of the calculation, indicating the need for further systematic analysis.

Photocrystallography

This review describes the development and application of a new crystallographic technique that is starting to enable the three-dimensional structural determination of molecules in their photo-activated states. So called `photocrystallography' has wide applicability, particularly in the currently exciting area of photonics, and a discussion of this applied potential is put into context in this article. Studies are classified into four groups: photo-structural changes that are (i) irreversible; (ii) long-lived but reversible under certain conditions; (iii) transient with photo-active lifetimes of the order of microseconds; (iv) very short lived, existing at the nanosecond or even picosecond level. As photo-structural changes relative to the `ground state' can be subtle, this article necessarily concentrates on small-molecule single-crystal X-ray diffraction given that high atomic resolution is possible. That said, where it is pertinent, references are also made to related major advances in photo-induced macromolecular crystallography. The review concludes with an outlook on this new research area, including the future possibility of studying even more ephemeral, femtosecond-lived, photo-active species.

Structure Determination of Triplet Diphenylcarbenes by in Situ X-ray Crystallographic Analysis

Crystalline-state photoreactions of the following diphenyldiazomethanes were investigated by in situ X-ray crystallography, spectroscopy, and theoretical calculations: bis(2,4,6-trichlorophenyl)diazomethane (1-N2), bis(2,4,6-tribromophenyl)diazomethane (2-N2), bis(2,6-dibromo-4-methylphenyl)diazomethane (3-N2), bis(2,6-dibromo-4-tert-butylphenyl)diazomethane (4-N2), (2,4,6-tribromophenyl)-(2,6-dimethyl-4-tert-butylphenyl)diazomethane (5-N2), bis(4-bromophenyl)diazomethane(6-N2), and diazofluorene (7-N2). Crystal structures of photoinduced triplet diphenylcarbenes (DPCs) of 1, 2, and 4 were determined. We found remarkable differences between their structural information obtained in the crystalline state and that previously obtained spectroscopically in a glass matrix. Although the triplet DPCs of 1, 2, and 4 have significantly different stabilities in solution, only subtle differences in their structural parameters, except for their C(:)-Ar bond lengths, are observed. It is noteworthy that the average bond length of C(:)-Ar for 4 (1.374 A) is considerably shorter than those for (3)1 and (3)2 (1.430 and 1.428 A, respectively), provided that the two C(:)-Ar bonds being compared were chemically equivalent. The most likely explanations for the small and large differences in bond lengths in 1, 2, and 4 may be derived from the packing effect. The packing patterns of 1 and 2 are identical, but that of 4 is totally different from those of 1 and 2. Moreover, these results are interpreted as indicating that triplet DPCs undergo relaxation upon softening of the environments. Theoretical calculations indicate that the potential energy surface of triplet DPCs in terms of the carbene angle is extremely flat and changes in the angles have little effect on the energies. Triplet DPCs with a sterically congested carbene center are trapped in a structure dictated by the precursor structure in a rigid matrix, even if this is not the thermodynamically most stable geometry, but undergo geometrical relaxation upon softening the matrix to relieve steric compression. ESR studies indicate that the interplanar angles are more flexible than the bond angles.

Photocrystallographic structure determination of a new geometric isomer of [Ru(NH3)4(H2O)(η1-OSO)][MeC6H4SO3]2

The structure of a new metastable geometric isomer of [Ru(NH3)4(H2O)(SO2)][MeC6H4SO3]2 in which the SO2 group is coordinated through a single oxygen in an eta1-OSO bonding mode has been determined at 13 K; the new isomer was obtained as a 36% component of the structure within a single crystal upon irradiation using a tungsten lamp.

Shedding light on the structure of a photoinduced transient excimer by time-resolved diffraction

Time-resolved single-crystal diffraction performed with synchrotron radiation shows that the 53(1) micros phosphorescent state, generated in the crystalline phase of trimeric {[3,5-(CF3)(2)Pyrazolate]Cu}(3) molecules by exposure to 355 nm of light at 17 K, is due to the formation of an excimer rather than the shortening of the intramolecular Cu...Cu distances within the trimeric units, or the formation of a continuous chain of interacting molecules. One of the intermolecular Cu...Cu distances contracts by 0.56 Angstroms from 4.018(1) to 3.46(1) Angstroms;, whereas the interplanar spacing of the trimers is reduced by 0.65 Angstroms; from 3.952(1) to 3.33(1) Angstroms. Density-functional theory calculations support the formation of a Cu...Cu bond through the intermetallic transfer of a Cu 3d electron to a molecular orbital with a large 4p contribution on the reacting Cu atoms.

Synthetic Control of Pt···Pt Separation and Photophysics of Binuclear Platinum Complexes

A new series of luminescent mu-pyrazolate-bridged cyclometalated platinum binuclear complexes having the formula CwedgeNPt(mu-pz')2PtCwedgeN (CwedgeN = 2-(2,4-difluorophenyl)pyridyl, pz' = pyrazolate for 1, 3,5-dimethylpyrazolate for 2, 3-methyl-5-tert-butylpyrazolate for 3, and 3,5-bis(tert-butyl)pyrazolate for 4) have been synthesized and characterized. The two Pt(CwedgeN) moieties are bridged by two mu-pyrazolate ligands in an exo-bidentate fashion. A mononuclear complex with an isolated Pt center, CwedgeNPt(pz)2BEt2, 5, is also described. The X-ray crystal structures of 1-4 show the following Pt-Pt spacings: 1 = 3.3763(7) A, 2 = 3.1914(9) A, 3 = 3.0457(7) A, and 4 = 2.8343(6) A. At 77 K, the emission energy of the complexes varies from blue (for 1, 2, and 5) to green (for 3) to red (for 4). The changes in the photophysical properties of the binuclear complexes can be correlated with the decreasing Pt-Pt distance; the emissive state changes from a mixed ligand center triplet/metal-to-ligand charge transfer excited state (for 1 and 2) to a lower-energy, Pt-Pt metal-metal-to-ligand charge transfer state (for 3 and 4).

A very large Rh–Rh bond shortening on excitation of the [Rh2(1,8-diisocyano-p-menthane)4]2+ion by time-resolved synchrotron X-ray diffraction

A very large Rh-Rh contraction of approximately 0.85 A occurs on excitation of the [Rh(2)(1,8-diisocyano-p-menthane)(4)](2+) ion to its triplet state.

Effect of chelating vs. bridging coordination of chiral short-bite P–X–P (X = C, N, O) ligands in enantioselective palladium-catalysed allylic substitution reactions

The chiral short-bite ligands (Ra,Ra)-bis(dinaphthylphosphonito)methane, (Ra,Ra)-1, (Ra,Ra)-bis-dinaphthylpyrophosphite, (Ra,Ra)-2, (Sc)-bis(diphenylphosphino)-sec-butylamine, (Sc)-3, (Ra,Ra)-bis(dinaphthylphosphonito)phenylamine, (Ra,Ra)-4a, (Ra,Ra,Sc)-bis(dinaphthylphosphonito)-sec-butylamine, (Ra,Ra,Sc)-4b, and (Ra,Sc)-(dinaphthylphosphonito)(diphenylphosphino)-sec-butylamine, (Ra,Sc)-5, have been synthesised. The cationic palladium-allyl mononuclear chelate, [Pd(eta3-PhCHCHCHPh)(mu-L-Lshort-bite)]PF6 [L-Lshort-bite=(Sc)-3, (Ra,Ra)-4a, (Ra,Ra,Sc)-4b and (Ra,Sc)-5 for complexes, and, respectively] and binuclear bridged [Pd(eta3-PhCHCHCHPh)(mu-Ra,Ra-2)]2(PF6)2, 12, have been isolated. The short-bite chiral ligands synthesised have been tested in the palladium-allyl catalysed substitution reaction of 1,3-diphenylallyl acetate with dimethyl malonate. The catalytic system was studied, in solution, by a multinuclear NMR technique. In the catalytically active species formed with (Ra,Ra)-2 ligand, [Pd(eta3-PhCHCHCHPh)(Ra,Ra-2)]2(PF6)2, 12, the palladium(II) centres are bridged by two ligands which are forced to adopt a nearly cis-coordination to allow coordination of the allyl-moiety. Semiempirical calculations on a biphenyl-model molecule, similar to the species 12, indicate that this situation induces a strain and rigid conformation in the chiral ligands, which produce differences in the terminal allyl carbon atoms. As consequence, the catalytic product was obtained with an enantiomeric excess of 57.1% in the S form. A low e.e. value was obtained when the (Ra,Ra)-1, (Sc)-3, (Ra,Ra)-4a, (Ra,Ra,Sc)-4b and (Ra,Sc)-5 ligands have been tested in the same palladium-catalysed reaction.

What can time-resolved diffraction tell us about transient species?: excited-state structure determination at atomic resolution

The author describes his work for which he coined the word 'photocrystallography', a technique which consists of using a laser to pump, or excite, a molecular crystal while the X-ray diffractometer probes its structure at the atomic level. The technique is being used to study highly reactive excited molecules that exist for just millionths or even billionths of a second using very intense light sources at the National Synchrotron Light Source at Brookhaven National Laboratory and the Advanced Photon Source at Argonne National Laboratory.