1,721,122 research outputs found
Capturing photo-induced structural changes in a donor–bridge–acceptor dyad crystal, that also exhibits elastic bending under mechanical pressure, using pump–probe photocrystallography
No full textControlling different external stimuli to tune the photophysical properties of purely organic molecular crystals toward designing multifunctional materials remains a challenge. Over the years, pyrene-based fluorophores owing to their high charge carrier mobility, complex photoelectronic characteristics in the excited state, and high fluorescence quantum yield, have come up as an attractive choice for researchers while designing luminescent molecular crystals. Recently, we have employed a pair of polymorphic D-B-A dyad crystals where electron-rich N,N ¢-dimethylaniline (DMA) acts as an electron donor (D) is attached to pyrene (Py) that acts as an acceptor (A), through -CH2-CH2- group, to study photo-induced charge transfer (PCT) mechanism by time-resolved pump-probe photocrystallography [1]. These D-B-A dyads sometimes behave like semiconductors and if designed properly can be useful as an efficient means of charge transfer and the creation of long-lived charge-separated states necessary for designing smart optoelectronic materials. The light-driven processes are mostly investigated in the D-B-A dyads due to the obvious reason of the matching of UV-Vis radiation wavelength with the electronic excitations and the need to investigate the excited state electron/hole transfer akin to the one observed in the naturalphotosynthesis process. For the present study, one such D-B-A molecule with an aromatic ring as the bridging (B) group has been employed (Fig. 1a). Since, the aromatic ring in the bridging group has the potential to play the role of a hopping station during electron transfer from DMA moiety to pyrene, measurements at different pump-probe delays were employed. The TCSPCmeasurements show that the photoinduced transient excited state is decaying bi-exponentially with a ~22ns long intramolecular charge transfer (ICT) state and a long-lived excimer state (~85ns) (Fig. 1b). The TDDFT calculations show a spatially separated HOMO and LUMO with a HOMO-LUMO gap of 3.46 eV, suggesting the CT nature of the HOMO-LUMO transition (Fig. 1c). The needle-shaped single crystals were also exhibiting elastic bending up to ~49° upon pressing with a needle (Fig. 1d). These single crystals were employed for time-resolved photocrystallography measurements at the BioCARS beamline at the APS using different pump-probe delays, where a short 35ps laser pump pulse was followed by a ~100ps X-ray probe pulse on the single crystals (Fig. 1e-f). The pump-probe photocrystallography datasets collected at different pump-probe delays show very interesting structural changes directly correlating the photo-chemical processes in solid-state. The photo-difference map from an 18ns pump-probe photocrytallography dataset shows how atomic shifts take place in a photo-induced, intramolecular charge-separated state (Fig. 1g). The results will be also useful in accessing the changes in the intermolecular interactions at different pump-probe delays which in turn will help design solid-state, smart materials
Ultrafast Processes in Isomeric Pyrene-N-methylacetamides: Formation of Hydrogen Bond Induced Static Excimers with Varied Coupling Strength
No full textPyrene based molecules are inclined to form excimers through self-association upon photoexcitation. In this work, the pyrene core is functionalized with N-methylacetamide group at the position 1 or 2, to develop pyren-1-methylacetamide (PyMA1) and pyren-2-methylacetamide (PyMA2), respectively. Upon photoexcitation, PyMA1 and PyMA2, at 1.0mM,in toluene, have formed predominantly static excimers. The steady state spectroscopic studies have showed that the excitonic coupling of PyMA1 dimers are much stronger in solution than its isomeric counterpart, PyMA2. The transient absorption (TA) measurements over fs-ps regime (fs-TA) showed that the formation of static excimers with the strongly-coupled pre-associated dimers, in PyMA1, happens in approx. 560fs, whereas, the excimers for the weakly-coupled pre-associated dimers in PyMA2 have formed in much slower time scale (approx. 65ps). The introduction of methylacetamide group at the position 1 or 2 on pyrene ring, was believed to have allowed forming hydrogen bonded excimers with different degrees of excitonic coupling
Investigation of encapsulated water wire within self-assembled hydrophilic nanochannels, in a modified -amino acid crystals: Tracking thermally induced changes of intermolecular interactions within a crystalline hydrate
No full textNanostructures formed by the self-assembly of modified/unmodified amino acids have the potential to be useful in several biological/nonbiological applications. In that regard, the greater conformational space provided by -amino acids, owing to their additional backbone torsional degrees of freedom and enhanced proteolytic stability, compared to their -counterparts, should be explored. Though, modified single amino acid-based nanomaterials such as nanobelts or hydrogels are developed by utilizing the monosubstituted -amino acids derived from the backbone homologation of phenylalanine (Phe). Examples of a single -amino acid-based porous nanostructure capable of accommodating solvent molecules are not really known. The crystal structures of a modified (R)Phe residue, Boc-(R)Phe-OH, at different temperatures, showed that hydrogen-bonded water molecules are forming a wire inside hydrophilic nanochannels. The dynamics of intermolecular interactions between the water wire and the inner wall of the channel with relation to the temperature change was investigated by analyzing the natural bonding orbital (NBO) calculation results performed with the single crystal structures obtained at different temperature points. The NBO results showed that from 325 K onward, the strength of water–water interactions in the water wire are getting weaker, whereas, for the water–inner wall interactions, it getting stronger, suggesting a favorable change in the orientation of water molecules with temperatures, for the latter
Short- vs . long-range elastic distortion: structural dynamics of a [2 × 2] tetrairon( ii ) spin crossover grid complex observed by time-resolved X-Ray crystallography
No full textUpon irradiation with a ps laser pulse, a [2 × 2] tetrairon(
ii
) spin crossover grid shows two types of elastic distortions: a short-range distortion in ps regime and a long-range distortion in the ns time scale.Spin crossover complexes (SCO) are among the most studied molecular switches due to their potential use in displays, sensors, actuators and memory components. A prerequisite to using these materials is the understanding of the structural changes following the spin transition at out-of-equilibrium conditions. So far, out-of-equilibrium studies in SCO solids have been focused on mononuclear complexes, though a growing number of oligonuclear SCO complexes showing cooperative effects are being reported. Here, we use time-resolved pink Laue crystallography to study the out-of-equilibrium dynamics of a [2 × 2] tetranuclear metallogrid of the form [FeII4L
Me
4
](BF
4
)
4
·2MeCN ([L
Me
]
−
= 4-methyl-3,5-bis{6-(2,2′-bipyridyl)}pyrazolate). The out-of-equilibrium spin state switching induced by a ps laser pulse demonstrates that the metallogrid exhibits a multi-step response similar to that reported for mononuclear complexes. Contrary to the mononuclear complexes, the metallogrid shows two types of elastic distortions at different time scales. The first is a short-range distortion that propagates over the entire Fe
4
grid complex during the ps time scale, and it is caused by the rearrangement of the coordination sphere of the photo-switching ion and the constant feedback between strongly linked metal ions. The second is a long-range distortion caused by the anisotropic expansion of the lattice during the ns time scale, observed in mononuclear materials. The structural analysis demonstrates that the long-range prevails over the short-range distortion, inducing the largest deformation of both the entire grid and the coordination sphere of each metal ion. The present study sheds light on the out-of -equilibrium dynamics of a non-cooperative oligonuclear complex.Deutsche Forschungsgemeinschaft https://doi.org/10.13039/50110000165
Crystallographic studies of the spin state transition of three Fe II metallogrids: thermal versus ultra-fast photoswitching
No full textMetal-to-metal communication during the spin state transition of a [2 × 2] Fe( ii ) metallogrid at equilibrium and out-of-equilibrium conditions
No full textSpin crossover (SCO) complexes are prototypes of materials with bi- or multi-stability in the solid state. The structural evolution during their spin transition is a key feature to establish the foundations of how to utilize this type of material. So far, ultrafast time-resolved structural investigations of SCO solids have been focused on monometallic complexes, though an increasing number of oligometallic SCO complexes showing cooperativity effects are being reported. Here, we used single crystal X-ray crystallography and time-resolved pink Laue photocrystallography to study the molecular reorganisation during the thermal and photoinduced SCO of a [2 × 2] tetranuclear metallogrid of the form [FeL](BF)·2MeCN ([L]− = 4-methyl-3,5-bis{6-(2,2′-bipyridyl)}pyrazolate). A multitemperature crystallographic investigation on single crystals reveals an effective communication between the metal centres during thermal SCO, observed by the simultaneous transformation of the coordination polyhedra of both crystallographic-symmetry independent metal atoms accompanying the SCO in only one of them. Time-resolved photocrystallography results reveal the different molecular responses between mononuclear and oligonuclear complexes, after light irradiation with a picosecond laser pulse. While mononuclear SCO complexes reorganise once during the first nanosecond after excitation, the tetranuclear metallogrid exhibits a multiple structural rearrangement in the same span of time. Such behaviour is attributed to the elastic communication between metal atoms, which allows the propagation of a short-range elastic distortion over the entire Fe grid complex. The present study sheds light on the importance of strong elastic coupling of metal atoms during the correlated spin transition of oligometallic complexes
Photo-induced electron transfer in pyrene-(CH 2 ) 2 - N , N ′-dimethylaniline: time-resolved pink Laue X-ray diffraction studies on crystalline polymorphs
No full text) spin crossover defect grid complex
No full textA triiron(II) metallogrid was studied upon temperature increase and light irradiation. While lacking thermal spin transition, time-resolved photocrystallography revealed the photoinduced and elastic steps of the out-of-equilibrium dynamics.The structural evolution of spin crossover (SCO) complexes during their spin transition at equilibrium and out-of-equilibrium conditions needs to be understood to enable their successful utilisation in displays, actuators and memory components. In this study, diffraction techniques were employed to study the structural changes accompanying the temperature increase and the light irradiation of a defect [2 × 2] triiron( ii ) metallogrid of the form [FeII3L H 2 (HL H ) 2 ](BF 4 ) 4 ·4MeCN (FE3), L H = 3,5-bis{6-(2,2′-bipyridyl)}pyrazole. Although a multi-temperature crystallographic investigation on single crystals evidenced that the compound does not exhibit a thermal spin transition, the structural analysis of the defect grid suggests that the flexibility of the grid, provided by a metal-devoid vertex, leads to interesting characteristics that can be used for intermolecular cooperativity in related thermally responsive systems. Time-resolved photocrystallography results reveal that upon excitation with a ps laser pulse, the defect grid shows the first two steps of the out-of-equilibrium process, namely the photoinduced and elastic steps, occurring at the ps and ns time scales, respectively. Similar to a previously reported [2 × 2] tetrairon( ii ) metallogrid, FE3 exhibits a local distortion of the entire grid during the photoinduced step and a long-range distortion of the lattice during the elastic step. Although the lifetime of the pure photoinduced high spin (HS) state is longer in the tetranuclear grid than in the defect grid, suggesting that the global nuclearity plays a crucial role for the lifetime of the photoinduced species, the influence of the co-crystalising solvent on the lifetime of the photoinduced HS state remains unknown. This study sheds light on the out-of-equilibrium dynamics of a thermally silent defect triiron SCO metallogrid.National Institutes of Health https://doi.org/10.13039/100000002Deutsche Forschungsgemeinschaft https://doi.org/10.13039/50110000165
Ultrafast Processes in Isomeric Pyrene-N-methylacetamides: Formation of Hydrogen Bond Induced Static Excimers with Varied Coupling Strength
Full text linkPyrene based molecules are inclined to form excimers through self-association
upon photoexcitation. In this work, the pyrene core is functionalized with
N-methylacetamide group at the position 1 or 2, to develop
pyren-1-methylacetamide (PyMA1) and pyren-2-methylacetamide (PyMA2),
respectively. Upon photoexcitation, PyMA1 and PyMA2, at 1.0mM, in toluene, have
formed predominantly static excimers. The steady state spectroscopic studies
have showed that the excitonic coupling of PyMA1 dimers are much stronger in
solution than its isomeric counterpart, PyMA2. The transient absorption (TA)
measurements over fs-ps regime (fs-TA) showed that the formation of static
excimers with the strongly-coupled pre-associated dimers, in PyMA1, happens in
approx. 560fs, whereas, the excimers for the weakly-coupled pre-associated
dimers in PyMA2 have formed in much slower time scale (approx. 65ps). The
introduction of methylacetamide group at the position 1 or 2 on pyrene ring,
was believed to have allowed forming hydrogen bonded excimers with different
degrees of excitonic coupling
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