Multiscale Mechanical Performance of Wood: From Nano- to Macro-Scale across Structure Hierarchy and Size Effects.

This review describes methods and results of studying the mechanical properties of wood at all scales: from nano- to macro-scale. The connection between the mechanical properties of material and its structure at all these levels is explored. It is shown that the existing size effects in the mechanical properties of wood, in a range of the characteristic sizes of the structure of about six orders of magnitude, correspond to the empirical Hall-Petch relation. This "law" was revealed more than 60 years ago in metals and alloys and later in other materials. The nature, as well as the particular type of the size dependences in different classes of materials can vary, but the general trend, "the smaller the stronger", remains true both for wood and for other cellulose-containing materials. The possible mechanisms of the size effects in wood are being discussed. The correlations between the mechanical and thermophysical properties of wood are described. Several examples are used to demonstrate the possibility to forecast the macromechanical properties of wood by means of contactless thermographic express methods based on measuring temperature diffusivity. The research technique for dendrochronological and dendroclimatological studies by means of the analysis of microhardness and Young's modulus radial dependences in annual growth rings is described.

Relationship between Thermal Diffusivity and Mechanical Properties of Wood.

This paper describes an experimental study of the relationships between thermal diffusivity and mechanical characteristics including Brinell hardness, microhardness, and Young's modulus of common pine (Pinus sylvestris L.), pedunculate oak (Quercus robur L.), and small-leaf lime (Tilia cordata Mill.) wood. A dependence of Brinell hardness and thermal diffusivity tensor components upon humidity for common pine wood is found. The results of the measurement of Brinell hardness, microhardness, Young's modulus, and main components of thermal diffusivity tensor for three perpendicular cuts are found to be correlated. It is shown that the mechanical properties correlate better with the ratio of longitude to transversal thermal diffusivity coefficients than with the respective individual absolute values. The mechanical characteristics with the highest correlation with the abovementioned ratio are found to be the ratio of Young's moduli in longitude and transversal directions. Our technique allows a comparative express assessment of wood mechanical properties by means of a contactless non-destructive measurement of its thermal properties using dynamic thermal imaging instead of laborious and material-consuming destructive mechanical tests.

Ultrafast dynamics in LMCT and intraconfigurational excited states in hexahaloiridates(iv), models for heavy transition metal complexes and building blocks of quantum correlated materials.

The population and structural dynamics of IrCl62- is studied in acetonitrile and aqueous solutions in comparison to isoelectronic IrBr62- using ultrafast broadband, dispersed transient absorption, with both octahedra excited with 85 fs pulses at four different wavelengths, encompassing the first seven t2g-based electronic states. Ligand-to-metal charge transfer (LMCT) 420 or 490 nm excitation of IrCl62- into Uu'(2T2u) + Eu''(2T2u) states, superimposed due to Ham effect, or Uu'(2T1u), respectively, leads to symmetry lowering due to Jahn-Teller effect in these excited states with the subsequent 100 fs decay into Ug'(2T1g). This first LMCT state is formed vibrationally coherent in the 104 cm-1 t2g (scissor) or 243 cm-1 eg (out-of-phase-stretch) Jahn-Teller modes for the respective excitation wavelength. Direct excitation into Ug'(2T1g) at 600 nm and the intraconfigurational lowest excited Ug'(2T2g) state at 1900 nm helped to establish that Ug'(2T1g) decays via back electron transfer into Ug'(2T2g) (time constants, 3.55 ps in acetonitrile and 0.9 ps in water), and the decay of Ug'(2T2g) into the ground state is the rate-limiting relaxation step. The relaxation cascade of IrBr62- is similar with short-lived (≤100 fs) higher LMCT states, but the vibrational coherence is only observed in the Jahn-Teller t2g mode. Faster back electron transfer for IrBr62- is explained by the energy gap law. The intraconfigurational Ug'(2T2g) states, which are ∼5100 cm-1 above the ground state for both complexes, have a sub-nanosecond lifetime largely independent of the ligand nature (∼350 ps, acetonitrile).

Femtosecond dynamics of metal-centered and ligand-to-metal charge-transfer (t2g-based) electronic excited states in various solvents: A comprehensive study of IrBr6 2.

The photophysical properties of intraconfigurational metal-centered (MC) and ligand-to-metal charge transfer (LMCT) states were studied in a prototype low spin heavy d5 transition metal complex, IrBr6 2-. The femtosecond-to-picosecond dynamics of this complex was investigated in solutions of drastically different polarity (acetonitrile, chloroform, and water) by means of ultrafast broadband transient absorption spectroscopy. We observed that the system, when excited into the third excited [second LMCT, 2Uu'(T1u)] state, undergoes distortion from the Franck-Condon geometry along the t2g vibrational mode as a result of the Jahn-Teller effect, followed by rapid internal conversion to populate (90 fs) the second excited [first LMCT, 2Ug'(T1g)] state. Vibrational decoherence and vibrational relaxation (∼400 fs) in 2Ug'(T1g) precede the decay of this state via internal conversion (time constants, 2.8 and 3 ps in CH3CN and CHCl3 and 0.76 ps in water), which can also be viewed as back electron transfer and which leads into the intraconfigurational MC 2Ug'(T2g) state. This is the lowest-excited state, from which the system returns to the ground state. This MC state is metastable in both CH3CN and CHCl3 (lifetime, ∼360 ps), but is quenched via OH-mediated energy transfer in aqueous environments, with the lifetime shortening up to 21 ps in aqueous solutions. The cascade relaxation mechanism is the same upon excitation into the second excited state. Excitation of IrBr6 2- in chloroform into higher 2Uu'(T2u), 2Eu″(T2u), and 2Eg'(T1g) states is observed to populate the third excited 2Uu'(T1u) state within 100 fs. These experiments allow us to resolve the ultrafast relaxation coordinate and emphasize that the excited-state Jahn-Teller effect is a driving force in the ultrafast dynamics, even for heavy transition metal complexes with very significant spin-orbit interactions.

Ultrafast Excited-State Dynamics of Ligand-Field and Ligand-to-Metal Charge-Transfer States of CuCl42- in Solution: A Detailed Transient Absorption Study.

Ultrafast excited-state dynamics of CuCl42- in acetonitrile is studied by femtosecond broadband transient absorption spectroscopy following excitation of the complex into all ligand-field (LF or d-d) states and into the two ligand-to-metal charge transfer (LMCT) states corresponding to the most intense steady-state absorption bands. The LF excited states are found to be nonreactive. The lowest-lying 2E LF excited state has a lifetime less than 150 fs, and the lifetimes of the second (2B1) and the third (2A1) LF excited states are 1 and 5 ps, respectively. All three LF states decay directly into the ground 2B2 state. Such significant differences in excited-state decay time constants were rationalized computationally through time-dependent density functional theory (TD-DFT) computations. TD-DFT mapping of the relaxation pathway along the symmetric Cl-Cu-Cl umbrella bending vibration gives evidence for a conical intersection between the 2E excited state and the ground 2B2 state. The LMCT states decay within 200 fs with the primary deactivation mode consistent to be Cu-Cl stretch. A fraction of the CuCl42- complexes excited into the LMCT states undergoes ionic dissociation to form products that survive longer than 1 ns. The remaining fraction undergoes internal conversion, which can be viewed as back electron transfer, populating the lower vibrationally hot LF states. The LF states populated from the LMCT states exhibit the same lifetimes as the Franck-Condon LF states and likewise decay directly into the ground state.

Direct photoisomerization of CH2I2vs. CHBr3 in the gas phase: a joint 50 fs experimental and multireference resonance-theoretical study.

Femtosecond transient absorption measurements powered by 40 fs laser pulses reveal that ultrafast isomerization takes place upon S1 excitation of both CH2I2 and CHBr3 in the gas phase. The photochemical conversion process is direct and intramolecular, i.e., it proceeds without caging media that have long been implicated in the photo-induced isomerization of polyhalogenated alkanes in condensed phases. Using multistate complete active space second order perturbation theory (MS-CASPT2) calculations, we investigate the structure of the photochemical reaction paths connecting the photoexcited species to their corresponding isomeric forms. Unconstrained minimum energy paths computed starting from the S1 Franck-Condon points lead to S1/S0 conical intersections, which directly connect the parent CHBr3 and CH2I2 molecules to their isomeric forms. Changes in the chemical bonding picture along the S1/S0 isomerization reaction path are described using multireference average coupled pair functional (MRACPF) calculations in conjunction with natural resonance theory (NRT) analysis. These calculations reveal a complex interplay between covalent, radical, ylidic, and ion-pair dominant resonance structures throughout the nonadiabatic photochemical isomerization processes described in this work.

Solution-state photophysics of N-carbazolyl benzoate esters: dual emission and order of states in twisted push-pull chromophores.

The stepwise photoinduced charge transfer in a series of N-carbazolyl benzoate ester push-pull chromophores has been studied in solution. Dual emission from the locally excited (LE, the lowest-energy singlet excited state of 1Lb nature localized on the carbazole donor) and the highly polarized, intramolecular charge-transfer states of (pre)-twisted type (TICT states) is observed in non-polar and polar solvents. Ultrafast transient spectroscopy reveals that the excitation into the 1Lb LE state is followed by rapid (∼ps) charge separation into an emissive TICT state. Excitation into the second singlet excited state localized on the carbazole (S2) with 1La nature results in sub-100 fs population of both 1Lb and TICT states.

Probing the fate of lowest-energy near-infrared metal-centered electronic excited states: CuCl(4)(2-) and IrBr(6)(2-).

Ultrafast transient absorption spectroscopy is used to investigate the radiationless relaxation dynamics of CuCl4(2-) and IrBr6(2-) complexes directly promoted into their lowest-energy excited metal-centered states upon near-infrared femtosecond excitation at 2000 nm. Both the excited CuCl4(2-) (2)E and IrBr6(2-) (2)Ug'(T2g) states undergo internal conversion to the ground electronic states, yet with significantly different lifetimes (55 fs and 360 ps, respectively) despite the fact that the (2)E and (2)Ug'(T2g) states are separated by the same energy gap (∼5000 cm(-1)) from the respective ground state. This difference likely arises from the predominance of the Jahn-Teller effect in a Cu(2+) ion and the spin-orbit coupling effect in an Ir(4+) ion. The approach documented in this work may be used for elucidating the role of low-energy metal-centered states in relaxation cascades of a number of coordination compounds, allowing for design of efficient light-triggered metal complexes.