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Two new charge-transfer compounds, [{Ru2(O2CPh-x-F)4}2(BTDA-TCNQ)]·n(solv) (x-F-PhCO2−= x-fluorobenzoate; x= o-, 4; p-, 5; BTDA-TCNQ = bis[1,2,5]thiadizolotetracyanoquinodimethane), have been prepared from reactions of paddlewheel diruthenium(II, II) complexes, [Ru2II,II(O2CPh-x-F)4] (x= o-, p-) and BTDA-TCNQ. Compounds 4and 5crystallize as two-dimensional (2-D) layered structures composed of [Ru2] units and BTDA-TCNQ in a 2 : 1 ratio. The isomer with the m-fluorobenzoate-bridged [Ru2] unit (3) exhibits a three-dimensional (3-D) structure. The oxidation states of the constituent units were evaluated from their crystal structures, and assigned to be close to the charge-polarized state represented formally as [Ru25+]–[BTDA-TCNQ−]–[Ru24+] including two types of [Ru2] units that are electronically and structurally different and a one-electron-transferred BTDA-TCNQ radical anion. The data indicate that a 1-e−transfer from one site of [Ru2] units to BTDA-TCNQ has likely occurred. The units are paramagnetic with heterospin states for [Ru2II,II] (S= 1), [Ru2II,III] (S= 3/2), and BTDA-TCNQ−(S= 1/2), which lead to magnetic ordering in the layer. In the case of 4, antiferromagnetic ordering occurs at TN= 93 K due to inter-layer antiferromagnetic interactions which involve two steps, namely canting of the ordered spins at Tc1= 87 K followed by rearrangement of the canted spin states at Tc2= 13 K. Compound 5exhibits 3-D ferromagnetic ordering at Tc= 83 K. Consequently, 4and 5behave as magnets although the origin of their spontaneous magnetization is different. The bulk magnetic properties of 4and 5are in contrast to what was observed in a similar layered compound [{Ru2(O2CCF3)4}2TCNQF4] (2; TCNQF4= tetrafluoro-7,7,8,8-tetracyanoquino-dimethane). This compound is an antiferromagnet (at H= 0) with TN= 95 K with the behavior being strongly dependent on the environment between the layers: in particular the stacking motif can either be “in-registry” with an overlap between analogous groups (for 2) or “out-of-registry” which involves a misalignment of similar groups between layers (for 4and 5). [ABSTRACT FROM AUTHOR]

Benzoate-bridged paddlewheel diruthenium(II,II) complexes ([RuII,II2(R n ArCO 2 ) 4 (L ax ) 2 ] (L ax = axial ligand); [RuII,II2]) exhibit reversible redox activity involving the oxidized species [RuII,III2] + . The redox activity can be finely tuned over a broad potential range by altering the substituent R on the benzoate-bridging ligand R n ArCO 2 - . The electronic contributions of the substituents R depend on their type and position, as was empirically demonstrated by Hammett for substituents at the meta - and para -positions. However, the substituent effect at the ortho -position is not solely determined by the electronic contribution of R but also by steric hindrance between the o -substituents and adjacent carboxylate groups. Nevertheless, an OH group at the o -position did not provide any steric hindrance, leading to a strong electron-withdrawing effect owing to intramolecular hydrogen bonding between the o -OH group and the adjacent carboxylate group, despite the electron-donating ability of the m - and p -OH groups. The OH group at the o -position induced a significant shift in the redox potential and HOMO energy levels of the [RuII,II2] complexes, thereby stabilizing the [RuII,II2] state. The redox potential and HOMO can be adjusted by introducing additional substituents, such as F, Cl, Me, OMe, and CF 3 groups, to cover a wide range, in accordance with an extended Hammett law that considers the contribution of the o -position.

Molecule-based hybrid layered magnets provide an ideal platform for investigating the long-range spin-ordering process in low-dimensional magnetic systems. Within this context, a promising area of research is spin-sandwiched hybrid layered magnets. These materials offer the potential to explore how the spin, which is sandwiched between magnetic layers, is influenced by the internal magnetic fields generated by the magnetic layers. Herein, we report a layered ferrimagnet with intercalated biferrocenium ([bifc] + ) radicals, [bifc][{Ru 2 (2,3,5,6-F 4 ArCO 2 ) 4 } 2 (TCNQF 2 )] (1, TCNQF 2 = 2,5-difluorotetracyano- p -quinodimethane). The [{Ru 2 (2,3,5,6-F 4 ArCO 2 ) 4 } 2 (TCNQF 2 )] - moiety acts as a ferrimagnetic layer with S T = 3/2, composed of a paddlewheel [Ru 2 II,II (2,3,5,6-F 4 ArCO 2 ) 4 ] (2,3,5,6-F 4 ArCO 2 - = 2,3,5,6-tetrafluorobenzoate) with S = 1 and 2,5-difluoro-7,7,8,8-tetracyanoquinodimethanate (TCNQF 2 ˙ - ) units with S = 1/2 in a 2 : 1 ratio. The isostructural paramagnetic compound [bifc][{Rh 2 (2,3,5,6-F 4 ArCO 2 ) 4 } 2 (TCNQF 2 )] (2) consisting of diamagnetic [Rh 2 II,II (2,3,5,6-F 4 ArCO 2 ) 4 ] components was also synthesized. An investigation of the properties of 2 revealed minimal magnetic interaction between the [bifc] + and TCNQF 2 ˙ - components. Compound 1 displayed long-range ferrimagnetic ordering at the Curie temperature of 105 K without any frequency dependence on alternating current (AC) susceptibility, due to the combination of predominant ferrimagnetic ordering within the layer and interlayer ferromagnetic dipole interactions. However, subsequent stepwise magnetic ordering involving a strong AC frequency dependence was observed upon further cooling. These dynamic behaviors are associated with the ordering of two types of anisotropic [bifc] + spins between the ferrimagnetic layers, indicating that [bifc] + spin ordering is sensitive to anisotropic internal magnetic fields generated by the ferrimagnetic layers., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

The increased anthropogenic emission level of CO 2 urges the development of CO 2 -responsive materials, but is it possible to regulate the inherent electronic properties through weak physisorption of a ubiquitous gas such as CO 2 ? Herein, we intended to answer this imperative question by the first case of CO 2 -actuated variable spin-state stabilisation in an interdigitated Hofmann-type coordination polymer [Fe II Pd(CN) 4 L 2 ] (1, L = methyl isonicotinate), showing a wide shift in transition temperature ( T eq ) from 178 K at P CO 2 = 0 kPa to 229 K at P CO 2 = 100 kPa. Interestingly, the emergence of a stepped behaviour in the heating process below P CO 2 = 10 kPa and overlapping magnetic susceptibility values above P CO 2 = 10 kPa elucidate the selective LS state stabilisation solely correlated with the extent of CO 2 accommodation. Based on the magnetic response and phase transition diagrams obtained under respective P CO 2 , a plausible scenario of the spin-state switching can be interpreted as (1 ls + ) → (1 hs + ) → 1 hs at P CO 2 ≤ 10 kPa, → 1 hs at 100 kPa < P CO 2 ≥ 32 kPa and → → 1 hs at 100 kPa, where 1 and 1' represent CO 2 -free and CO 2 -encapsulated states, respectively. The cooperative CO 2 sorption with SCO based on the varied CO 2 pressure corroborates a novel case for developing CO 2 -responsive magnetic materials henceforth., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Understanding physicochemical property changes based on reaction kinetics is required to design materials exhibiting desired functions at arbitrary timings. In this work, we investigated the photodimerization of anthracene derivatives in single crystals. Single crystals of 9-cyanoanthracene (9CA) and 9-anthraldehyde (9AA) exhibited reaction front propagation on the optical length scale, while 9-methylanthracene and 9-acetylanthracene crystals underwent spatially homogeneous conversion. Moreover, the sigmoidal behavior in the absorbance change associated with the reaction was much pronounced in the case of 9CA and 9AA and correlated with the observation of heterogeneous reaction progress. A kinetic analysis based on the Finke-Watzky model showed that the effective quantum yield of the photochemical reaction changes by more than an order of magnitude during the course of the reaction in 9CA and 9AA. Both the reaction front propagation and nonlinear kinetic behavior could be rationalized in terms of the difference in the cooperativity of the reactions. We propose a plausible mechanism for the heterogeneous reaction progress in single crystals that depends on the magnitude of the conformational change required for reaction. Our results provide useful information to understand the connection between photochemical reaction progress in the crystalline phase and the dynamic changes in the physicochemical properties., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Polycyclic aromatic hydrocarbons (PAHs) are fragments of graphene that have attracted considerable attention as a new class of carbon-based materials. The functionalization of edge positions in PAHs is important to enable the modulation of physical and chemical properties essential for various applications. However, straightforward methods that combine functional group tolerance and regioselectivity remain sought after. Here we report a photochemical approach for the direct alkylation of carbon-hydrogen bonds in PAHs that takes place in a regiospecific manner, an outcome that has never been achieved in related thermal reactions. A reaction mechanism involving a single electron transfer process from photo-excited PAHs to sulfones, and a rationale for the origin of regioselectivity are proposed on the basis of spectroscopic analyses and theoretical calculations., Competing Interests: The authors declare no conflicts of interest., (This journal is © The Royal Society of Chemistry.)

In [Mn(5-MeOsalen)(Cl)] 2 (dibenzo[24]crown-8), dibenzo[24]crown-8 formed a supramolecule via multi-point interactions with the [Mn(5-MeOsalen)(Cl)] dimer. The dimer was magnetically isolated with S T = 4 and weak interdimer magnetic interactions. The crystal exhibited single-molecule magnet behaviour with an anisotropic barrier of 26(1) K, which is the highest among the Mn-salen series reported to date.

A new synthetic route for constructing functional paddlewheel diruthenium(II,II) complexes ([RuII,II2]) was developed by utilizing Schiff base condensation reactions of formyl-substituted benzoate-bridged [RuII,II2] complexes with various aromatic monoamines under mild conditions. Cyclic voltammetry and DFT calculations revealed that the attached Schiff base groups significantly affected the electronic states of the resulting [RuII,II2] complexes.

The resonance between an electronic transition of a micro/nanoscale object and an incident photon flux can modify the radiation force exerted on that object, especially at an interface. It has been theoretically proposed that a non-linear stimulated emission process can also induce an optical force, however its direction will be opposite to conventional photon scattering/absorption processes. In this work, we experimentally and theoretically demonstrate that a stimulated emission process can induce a repulsive pulling optical force on a single trapped dye-doped particle. Moreover, we successfully integrate both attractive pushing (excited state absorption) and repulsive pulling (stimulated emission) resonance forces to control the overall exerted optical force on an object, validating the proposed non-linear optical resonance theory. Indeed, the results presented here will enable the optical manipulation of the exerted optical force with exquisite control and ultimately enable single particle manipulation., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Iron(ii) spin cross-over (SCO) compounds combine a thermally driven transition from the diamagnetic low-spin (LS) state to the paramagnetic high-spin (HS) state with a distinct change in the crystal lattice volume. Inversely, if the crystal lattice volume was modulated post-synthetically, the spin state of the compound could be tunable, resulting in the inverse effect for SCO. Herein, we demonstrate such a spin-state tuning in a breathing cyanido-bridged porous coordination polymer (PCP), where the volume change resulting from guest-induced gate-opening and -closing directly affects its spin state. We report the synthesis of a three-dimensional coordination framework {[Fe II (4-CNpy) 4 ] 2 [W IV (CN) 8 ]·4H 2 O} n (1·4H 2 O; 4-CNpy = 4-cyanopyridine), which demonstrates a SCO phenomenon characterized by strong elastic frustration. This leads to a 48 K wide hysteresis loop above 140 K, but below this temperature results in a very gradual and incomplete SCO transition. 1·4H 2 O was activated under mild conditions, producing the nonporous {[Fe II (4-CNpy) 4 ] 2 [W IV (CN) 8 ]} n (1) via a single-crystal-to-single-crystal process involving a 7.3% volume decrease, which shows complete and nonhysteretic SCO at T 1/2 = 93 K. The low-temperature photoswitching behavior in 1 and 1·4H 2 O manifested the characteristic elasticity of the frameworks; 1 can be quantitatively converted into a metastable HS state after 638 nm light irradiation, while the photoactivation of 1·4H 2 O is only partial. Furthermore, nonporous 1 adsorbed CO 2 molecules in a gated process, leading to {[Fe II (4-CNpy) 4 ] 2 [W IV (CN) 8 ]·4CO 2 } n (1·4CO 2 ), which resulted in a 15% volume increase and stabilization of the HS state in the whole temperature range down to 2 K. The demonstrated post-synthetic guest-exchange employing common gases is an efficient approach for tuning the spin state in breathing SCO-PCPs., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

The phenomenon of crystal melting by light irradiation, known as photo-induced crystal-to-liquid transition (PCLT), can dramatically change material properties with high spatiotemporal resolution. However, the diversity of compounds exhibiting PCLT is severely limited, which hampers further functionalisation of PCLT-active materials and the fundamental understandings of PCLT. Here, we report on heteroaromatic 1,2-diketones as the new class of PCLT-active compounds, whose PCLT is based on conformational isomerisation. In particular, one of the diketones demonstrates luminescence evolution prior to crystal melting. Thus, the diketone crystal exhibits dynamic multistep changes in the luminescence colour and intensity during continuous ultraviolet irradiation. This luminescence evolution can be ascribed to the sequential PCLT processes of crystal loosening and conformational isomerisation before macroscopic melting. Single-crystal X-ray structural analysis, thermal analysis, and theoretical calculations of two PCLT-active and one inactive diketones revealed weaker intermolecular interactions for the PCLT-active crystals. In particular, we observed a characteristic packing motif for the PCLT-active crystals, consisting of an ordered layer of diketone core and a disordered layer of triisopropylsilyl moieties. Our results demonstrate the integration of photofunction with PCLT, provide fundamental insights into the melting process of molecular crystals, and will diversify the molecular design of PCLT-active materials beyond classical photochromic scaffolds such as azobenzenes., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

A luminescent three-dimensional coordination polymer (CP) of [Cd II (pmd){Ag I (CN) 2 } 2 ] (1; pmd = pyrimidine) comprising two different coordination modes of Ag + ions was synthesised herein. 1 exhibited thermochromic luminescence, accompanied by positive thermal elongation of the Ag⋯Ag distance. Moreover, 1 showed a bright phosphorescence with the highest photoluminescence quantum yield ( Φ em ), approximately 60% at room temperature, among previously reported phosphorescent Ag-based CPs or metal-organic frameworks.

Magnetism of layered magnets depends on the inter-layer through-space magnetic interactions ( J NNNI ). Using guest sorption to address inter-layer pores in bulk-layered magnets is an efficient approach to magnetism control because the guest-delicate inter-layer distance ( l trans ) is a variable parameter for modulating J NNNI . Herein, we demonstrated magnetic changes induced by the adsorption of CO 2 , N 2 , and O 2 gases in various isostructural layered magnets with a π-stacked pillared-layer framework, , (M = Co, 1, Fe, 2, Cr, 3; Cp* = η 5 -C 5 Me 5 ; 2,3,5,6-F 4 PhCO 2 - = 2,3,5,6-tetrafluorobenzoate; TCNQ = 7,7,8,8-tetracyano- p -quinodimethane). Each compound had almost identical adsorption capability for the three types of gases; only CO 2 adsorption was found to have a gated profile. A breathing-like structural modulation involving the extension of l trans occurred after the insertion of gases into the isolated pores between the [Ru 2 ] 2 -TCNQ ferrimagnetic layers, which is more significant for CO 2 than for O 2 and N 2 , due to the CO 2 -gated transition. While adsorbent 1 with M = Co ( S = 0) was an antiferromagnet with T N = 75 K, 1⊃CO 2 was a ferrimagnet with T C = 76 K, whereas 1⊃N 2 and 1⊃O 2 were antiferromagnets with T N = 68 K. The guest-insertion effect was similarly confirmed in 2 and 3, and was characteristically dependent on the type of sandwiched spin in as M = Fe ( S = 1/2) and Cr ( S = 3/2), respectively. This study reveals that common gases such as CO 2 , O 2 , and N 2 can serve as crucial triggers for the change in magnetism as a function of variable parameter l trans ., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Chiral weak ferromagnets were developed in organic-inorganic hybrid systems of one-dimensional manganese(II) chloride hydrate with chiral organic cations of ( S )- and ( R )-β-methylphenethylammonium (= ( S )- and ( R )-MPA + ), [( S )/( R )-MPA] 2 [MnCl 4 (H 2 O)]. The weak ferromagnetic phase found below 3.2 K could be attributed to a spin canting antiferromagnetic state induced by Dzyaloshinskii-Moriya interactions owing to spatial inversion symmetry breaking in chirality introduced systems. This is the first chiral weak ferromagnet found in a spin chain of deformed perovskite derivatives with a corner-sharing structure of transition metal halide hydrates.

A light-harvesting strategy is crucial for the utilisation of solar energy. In this study, we addressed the expanding light-harvesting (LH) wavelength of photosynthetic LH complex 2 (LH2, from Rhodoblastus acidophilus strain 10050) through covalent conjugation with extrinsic chromophores. To further understand the conjugation architecture and mechanism of excitation energy transfer (EET), we examined the effects of the linker length and spectral overlap integral between the emission and absorption spectra of the energy donor and acceptor pigments. In the former case, contrary to the intuition based on the Förster resonance energy transfer (FRET) theory, the observed energy transfer rate was similar regardless of the linker length, and the energy transfer efficiency increased with longer linkers. In the latter case, despite the energy transfer rate increases at higher spectral overlaps, it was quantitatively inconsistent with the FRET theory. The mechanism of EET beyond the FRET theory was discussed in terms of the higher-lying exciton state of B850, which mediates efficient EET despite the small spectral overlap. This systematic investigation provides insights for the development of efficient artificial photosynthetic systems.

Pump-repump-probe spectroscopy with a burst mode of photoexcitation was applied to the direct observation of the photoionization dynamics of perylene in the solution phase. Irradiation of a pump pulse train generated with birefringent crystals effectively accumulated an intermediate S 1 state and a repump pulse triggered photoionization in the higher excited state, ensuring sufficiently large signal intensity to probe. Two-photon excitation to the energy level, which is 0.7 eV lower than the ionization potential in the gas phase, results in instantaneous formation of the radical cation of perylene in acetonitrile, unlike aromatic amines in previous reports. In addition, subsequent recombination dynamics between the radical cation and ejected electron was monitored in polar and nonpolar solvents. The ultrafast recombination in nonpolar solvents suggests that the distribution of the distance in the cation-ejected electron pair largely evolves even in acetonitrile in the femtosecond timescale, in which the solvation is not completed and the dielectric constant is still low. The recombination process in acetonitrile was well reproduced with simulations based on the Smoluchowski equation taking account of the transient change in the dielectric constant by solvation.

The synthesis of a series of trans -heteroleptic paddlewheel diruthenium(II,II) complexes with various hydroxy-substituted benzoate ligands, [Ru 2 ((OH) x PhCO 2 ) 2 (2,6-(CF 3 ) 2 PhCO 2 ) 2 (THF) 2 ] ([RuII,II2]) as tetrahydrofuran (THF) adducts is reported, where (OH) x PhCO 2 - stands for o -hydroxybenzoate (o-OH), m -hydroxybenzoate (m-OH), p -hydroxybenzoate (p-OH), 2,3-dihydroxybenzoate (2,3-(OH)2), 2,4-dihydroxybenzoate (2,4-(OH)2), 2,5-dihydroxybenzoate (2,5-(OH)2), 2,6-dihydroxybenzoate (2,6-(OH)2), or 3,4-dihydroxybenzoate (3,4-(OH)2), and 2,6-(CF 3 ) 2 PhCO 2 - represents 2,6-bis(trifluoromethyl)benzoate. In this heteroleptic series, the redox potential ( E 1/2 ) of the [RuII,II2]/[RuII,III2] + couple in THF varies over a wide range, from -18 mV ( vs. Ag/Ag + ) for p-OH to 432 mV for 2,6-(OH)2. The redox properties are linearly dependent on the acidity (p K a ) of the OH-substituted benzoic acids, but do not depend on the number of ortho -substituted hydroxy ( o -OH) groups. This indicates that the steric effect of o -substituents is irrelevant in the case of hydroxyl groups, owing to the formation of intramolecular hydrogen bonds between the o -OH group and carboxylate oxygens. The value of the Hammett constant σ o for the o -OH substituent was determined to be 0.667, indicating a strongly electron-withdrawing character, contrary to the expectation of electron-donating character for an OH group. The redox properties of the compounds were well explained in a framework of Hammett analyses and were also consistent with their HOMO energy levels evaluated by DFT calculations based on the atomic coordinates. The unexpected electron-withdrawing character of the o -OH groups could be attributed to the direct effect of intramolecular hydrogen bonding on the charge density on the carboxylate oxygen.

Because supramolecular polymerization of emissive π-conjugated molecules depends strongly on π-π stacking interaction, the formation of well-defined one-dimensional nanostructures often results in a decrease or only a small increase of emission efficiency. This is also true for our barbiturate-based supramolecular polymers wherein hydrogen-bonded rosettes of barbiturates stack quasi-one-dimensionally through π-π stacking interaction. Herein we report supramolecular polymerization-induced emission of two regioisomeric 2,3-diphenylthiophene derivatives functionalized with barbituric acid and tri(dodecyloxy)benzyl wedge units. In CHCl 3 , both compounds are molecularly dissolved and accordingly poorly emissive due to a torsion-induced non-radiative decay. In methylcyclohexane-rich conditions, these barbiturates self-assemble to form crystalline nanofibers and exhibit strongly enhanced emission through supramolecular polymerization driven by hydrogen-bonding. Our structural analysis suggests that the barbiturates form a tape-like hydrogen-bonding motif, which is rationalized by considering that the twisted geometries of 2,3-diphenylthiophene cores prevend the competing rosettes from stacking into columnar supramolecular polymers. We also found that a small difference in the molecular polarity originating from the substitutional position of the thiophene core influences interchain association of the supramolecular polymers, affording different luminescent soft materials, gel and nanosheet., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Here, we present single crystalline capsules of a photoresponsive molecule produced by simple recrystallization from organic solutions without direct human processing. During the crystal growth process, a movie was taken of the capsule taking in the organic solution. The capsules responded rapidly (<1 s) to the UV light stimuli and released the captured solution or solute. In principle, they can take in any substance dissolved in organic solvents, and their size can be controlled. Moreover, the capsule can be broken by multi-photon excitation using a near-infrared laser within the biological window. Furthermore, because the molecular packing in the crystal is unidirectional, the response can be controlled by the polarization of the light. This study shows the new potential of photoresponsive molecules., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

A novel porous magnet consisting of cationic two-dimensional (2-D) layers extended by Fe III -CN-Ni II linkages and pseudo-pillar dianions was synthesized. The size-selective guest adsorption behaviour of water and methanol molecules originates from the narrow bottle-neck-type pores in the flexible pseudo-pillared-layer structure, which results in the switching of the magnetic phases from antiferromagnetic to ferromagnetic, involving significant changes in the interlayer distance.

Canted antiferromagnetism (AFM) is considered an effective tool for designing single-chain magnets (SCMs) in homometallic chain systems. The family of manganese(iii) (MnIII) salen-type Schiff-base complexes is an outstanding building-unit candidate for designing SCMs because such complexes possess relatively large uniaxial magnetic anisotropy in the out-of-plane direction. However, SCM behaviour in simple alternating chains based on monomeric MnIII salen-type complexes has not been studied extensively. Herein, we report the SCM behaviour of canted AFM in an alternating chain of an acetate-bridged MnIII salen-type complex.

The precise control of intra-lattice multiple electron transfers was demonstrated in the solvated and desolvated species of the tetraoxolene-bridged Fe honeycomb layer system, (NPr4)2[Fe2(Cl2An)3]·(solv) (Cl2Ann- = 2,5-dichloro-3,6-dihydroxy-1,4-benzoquinonate; NPr4+ = tetrapropylammonium cation), by the site-doping of the Cl2Ann- bridging unit using X2Ann- units with X = Br or F.

Electron transfer reactions driven by two-photon ionization in the higher excited state were investigated via transient absorption spectroscopy, with the aim to develop a method for creating the charge-separated (CS) state with a large formation rate, high energy level, and long lifetime. In the proof-in-principle experiments using pyrene and biphenyl as a model system, femtosecond transient absorption spectroscopy revealed that intense irradiation of an ultraviolet laser pulse at 355 nm efficiently pumps up pyrene into a higher excited state via a stepwise two-photon absorption, and then an ionization process takes place. An electron ejected from pyrene is directly captured by biphenyl with a time constant of 200 fs without the diffusion process of the electron in solution. The energy level of the CS state (Py + -Bp - ) thus formed was estimated to be higher than that of the S 1 state of pyrene by 0.53 eV. In addition, the subsequent ionic dissociation without a remarkable geminate recombination in the sub-nanosecond to nanosecond time region effectively avoids the quantity loss of the CS state. By applying the two-photon excitation method, we experimentally achieved ultrafast formation of the long-lived CS state at a high energy beyond the traditional framework of electron transfer reactions.

Herein, we synthesized four isostructural flexible interdigitated hydrophobic metal-organic frameworks that differ only in their central metal ion. The water-adsorption properties are dependent on the metal species, which was revealed to be caused by coordination distortion around the metal ions.

The electrocyclic reaction dynamics of a photochromic dithiazolylarylene derivative, 2,3-dithiazolylbenzothiophene (DTA) was investigated by using time-resolved transient absorption and fluorescence spectroscopies. The closed-ring isomer of DTA undergoes cycloreversion through the conical intersection mediating the potential energy surfaces of the excited and ground states, which is in agreement with the Woodward-Hoffmann rules for the electrocyclic reactions of 6π electron systems. On the other hand, a large portion of the open-ring isomer undergoes cyclization along the distinct reaction scheme, in which the cyclization takes place in the excited state manifold leading to the formation of the excited state of the closed-ring isomer. The suppression of the geometrical motion of DTA due to the intramolecular interaction could open a new efficient reaction pathway resulting in the formation of the electronically excited state of the product., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Molecular materials whose electronic states are multiply varied depending on external stimuli are among the most promising targets for the development of multiply accessible molecular switches. Here, we report a honeycomb layer composed of tetraoxolene-bridged iron (Fe) subunits whose charge-ordered states are multiply variable via thermal treatments and solvation/desolvation with the crystallinity intact. The compound is (NPr 4 ) 2 [Fe 2 (Cl 2 An) 3 ] ( 1-d ; NPr 4 + = tetra- n -propylammonium; Cl 2 An 2- = 2,5-dichloro-3,6-dihydroxo-1,4-benzoquinonate), which possesses three charge-ordered states: a low-temperature (LT) phase [(Fe 3+ ) 2 (Cl 2 An 2- )(Cl 2 An˙ 3- ) 2 ] 2- ; an intermediate (IM) phase [(Fe 2.5+ ) 2 (Cl 2 An 2- )(Cl 2 An 2.5- ) 2 ] 2- ; and a high-temperature (HT) phase [(Fe 2+ ) 2 (Cl 2 An 2- ) 3 ] 2- that varies according to temperature. In addition, the LT phase of 1-d is reversibly changeable to another IM phase in its solvated compound 1 via a solvation/desolvation process at room temperature. This example demonstrates a new multiple-switching system based on electron transfer and host-guest chemistry in a charge-flexible metal-organic framework., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Photocyclization reaction dynamics of an inverse type diarylethene derivative was investigated in alkane solutions by means of ultrafast laser spectroscopies. Femtosecond transient absorption spectroscopy showed that the Franck-Condon state formed by photoexcitation is geometrically relaxed to a transient species within 100 fs and subsequently the cyclization process takes place with a time constant of 36 ps. This time constant is much longer than those in normal type derivatives. Steady-state and time-resolved fluorescence measurements with the aid of quantum chemical calculations revealed that there exist three kinds of conformers, one parallel and two anti-parallel forms, in the ground state. One of the anti-parallel conformers undergoes the cyclization reaction, while the other two conformers are nonreactive species and their major relaxation processes are radiative decay and intersystem crossing into the triplet states. The triplet states thus formed no longer undergo the cyclization reaction in the late time region.

Femtosecond transient absorption spectroscopy with off-resonant simultaneous and resonant stepwise two-photon excitation methods were applied to the direct observation of photoionization dynamics of a phenylenediamine derivative in n-hexane, ethanol and acetonitrile solutions. Upon the selective excitation of the solute via the off-resonant two-photon excitation to the energy level almost equivalent with the ionization potential in the gas phase, rapid appearance of the radical cation (within ca. 100-200 fs) was observed in polar and nonpolar solutions. On the other hand, in the case where the excited energy level from the ground state is 0.8 eV lower than the ionization potential in the gas phase, the radical cation appears only in polar solutions in sub-ps to ps time scales, indicating that the photoionization does not occur directly from the highly electronically excited state even in the polar solution. Comparison of the dynamics between ethanol and acetonitrile solutions strongly suggested that the solvation process of the precursor species leading to the ionization took a crucial role in the electron ejection process with lower energy in polar solutions.

A series of substituted benzoate-bridged dichromium(ii,ii) complexes [Cr2(RCO2)4(THF)2] ([Cr2]), where RCO2- is substituted benzoate, was synthesized and its structural and magnetic properties were investigated. The orbital energies, as well as magnetic coupling energies, were also investigated using computational approaches. The HOMO/LUMO energy levels of the complexes are strongly dependent on the acidity, i.e., pKa, of the corresponding benzoic acids (RCO2H), revealing a linear trend in the respective groups of non-ortho, mono-ortho, and bi-ortho substituted groups. The CrCr magnetic coupling constant (ES-T) is little affected by pKa; instead, the ES-T is associated with the HOMO/LUMO gap and strongly correlated with the CrCr distance.

A donor (D)/acceptor (A) assembly reaction using the paddlewheel-type diruthenium(ii,ii) complex [Ru2(3,5-F2PhCO2)4(THF)2] (3,5-F2PhCO2- = 3,5-difluorobenzoate; abbreviated hereafter as [Ru2]) as D and 7,7,8,8-tetracyano-p-quinodimethane derivatives (TCNQRx; 2,5-R-substituted TCNQ) as A in a DCM/TCE or DCE solvent system (DCM = dichloromethane, TCE = 1,1,2,2-tetrachloroethane, DCE = 1,2-dichloroethane) led to the formation of D2A-type two-dimensional layered compounds [{Ru2(3,5-F2PhCO2)4}2{TCNQRx}]·nsolv (Rx = H2 (1), Rx = Me2, (2), and Rx = (OEt)2 (3)). All the compounds had similar two-dimensional fishnet-type structures, where the two [Ru2] units were fully coordinated with the four cyano groups of TCNQRx. The compounds 1-3 were categorized as a one-electron transferred ionic state (1e-I) with D+-A--D formulation (D+[triple bond, length as m-dash][RuII,III2]+; A-[triple bond, length as m-dash]TCNQRx˙-). This subunit state formally derived a ferrimagnetically ordered state composed of up-spins with S = 1 for [RuII,II2] and S = 3/2 for [RuII,III2]+ and a down-spin of S = 1/2 for TCNQRx˙-, with antiferromagnetic superexchange interactions in each layer. Eventually, 1-3 became three-dimensional ferrimagnets with Curie temperatures TC = 93, 93, and 92 K, respectively, owing to the presence of interlayer ferromagnetic interactions. The interlayer translational distances (l2) for the three compounds were 10.56, 10.54, and 10.84 Å, respectively, which agreed with the empirical prediction based on the threshold value of l2 > 10.3 Å for a valid ferromagnetic interaction.

The one- and two-photon cycloreversion reactions of a fluorescent diarylethene derivative with oxidized benzothiophene moieties were investigated by means of ultrafast laser spectroscopy. Femtosecond transient absorption spectroscopy under the one-photon excitation condition revealed that the excited closed-ring isomer is simply deactivated into the initial ground state with a time constant of 2.6 ns without remarkable cycloreversion, the results of which are consistent with the very low cycloreversion reaction yield (<10-5) under steady-state light irradiation. On the other hand, an efficient cycloreversion reaction was observed under irradiation with a picosecond laser pulse at 532 nm. The excitation intensity dependence of the cycloreversion reaction indicates that a highly excited state attained by the stepwise two-photon absorption is responsible for the marked increase of the cycloreversion reaction, and the quantum yield at the highly excited state was estimated to be 0.018 from quantitative analysis, indicating that the reaction is enhanced by a factor of >1800.

Femtosecond excited-state dynamics of fullerene-C 60 nanoparticles (nC 60 ) having a mean size of 50 nm dispersed in pure water was studied by means of femtosecond transient absorption spectroscopy. The intermolecular charge-transfer (CT) excited state in solid C 60 was directly and firstly observed by femtosecond 350 nm and 420 nm excitations, and its intrinsic lifetime of 0.35 ps was found. The CT excited state relaxed to the locally excited S 1 state and excimers or directly to the ground state through geminate charge recombination. We also examined the laser fluence dependence of the CT excited-state dynamics. At a high laser fluence, the mutual interactions between neighboring CT excited states were observed immediately after the excitation. The interaction disappeared through the charge recombination in the geminate CT pair or between the neighboring CT excited states with a lifetime of 0.45 ps. After that, the locally excited S 1 state decayed with a few ps lifetime independent of the fluence. In this paper, the mechanism and dynamics of the intermolecular CT excited state generated by UV light excitation is discussed in detail.

Two ferromagnetic multinuclear Ni(ii) clusters, [Ni 4 L 1 4 (μ 3 -OMe) 4 (CH 3 OH) 4 ] (1) and [Ni 7 (HL 2 ) 6 (μ 3 -OMe) 6 ]Cl 2 (2), and a mononuclear complex [NiL 3 ]·2H 2 = benzyl 2-hydroxy-3-methoxybenzoate; H 1 = benzyl 2-hydroxy-3-methoxybenzoate; H 2 = 1-(2-hydroxy-3-methoxybenzamido)-2-(benzylideneamino)ethane; H 2 = 1-(2-hydroxy-3-methoxybenzamido)-2-(benzylideneamino)ethane; H 2 L 3 = 1-(2-hydroxy-3-methoxybenzamido)-2-(pyridylmethyleneamino)ethane) were synthesised, and their structures and magnetic behaviours were investigated in detail. 1 has a [Ni 4 O 4 ] cubane structure while 2 has a wheel-like [Ni@Ni 6 ] arrangement. The temperature-dependent magnetic susceptibilities for 1 and 2 are in accord with the presence of dominating intra-cluster ferromagnetic interactions between Ni(ii) ions. The best fits for their behaviours gave the following parameters: J 1 = 11.06 cm -1 , J 2 = 1.43 cm -1 , g = 2.29 for 1, and J 1 = 6.87 cm -1 , J 2 = -3.41 cm -1 , g = 2.24 for 2.

We synthesized cyano-bridged three dimensional metal-organic frameworks [M II (H 2 O) 2 Pt IV (CN) 4 Br 2 ] ∞ ·n(H 2 O) (1: M = Co, 2: M = Ni) with an NbO type topology. Although 1 and 2 have no accessible pores, these MOFs were found to undergo reductive elimination of the axial bromides on the Pt IV ions as well as removal of water molecules upon heating to form porous [M II Pt II (CN) 4 ] ∞ (1': M = Co, 2': M = Ni) with retention of their crystallinity.

We demonstrated one-colour control of activation, excitation and deactivation of a fluorescent diarylethene derivative by using a 532 nm CW laser at a weak output power. This one-colour control method was applied to single-molecule tracking in polymer films over a total duration of a few hours at room temperature and PALM for the nanostructures of polymer systems.

Two-dimensional layered compounds with different counteranions, [{Mn(salen)} 4 C6](BF 4 ) 2 ·2(CH 3 OH) (1) and [{Mn(salen)} 4 C6](PF 6 ) 2 ·2(CH 3 OH) (2) (salen 2- = N,N'-bis(salicylideneiminato), C6 2- = C 6 H 12 (COO) 2 ), were synthesized by assembling [Mn(salen)(H 2- ), were synthesized by assembling [Mn(salen)(H 2 O)]X (X - = BF 4 - H 6 - ) and C 6 H 12 (CO 2 - ) 2 (C6 2- H 4 -type C6 2- ions, forming a brick-wall-type network of [-{Mn 2 }-OCO-] chains alternately connected via C 6 H 12 linkers of C6 2- moieties. The counteranions for 1 and 2, i.e., BF 4 - and PF 6 - , respectively, are located between layers. Since the size of BF 4 - is smaller than that of PF 6 - , intra-layer inter-chain and inter-plane nearest-neighbor MnMn distances are shorter in 1 than in 2. The zigzag chain moiety of [-{Mn 2 }-OCO-] leads to a canted S = 2 spin arrangement with ferromagnetic coupling in the Mn III out-of-plane dimer moiety and antiferromagnetic coupling through -OCO- bridges. Due to strong uniaxial anisotropy of the Mn III ion, the [-{Mn 2 }-OCO-] chains could behave as a single-chain magnet (SCM), which exhibits slow relaxation of magnetization at low temperatures. Nevertheless, these compounds fall into an antiferromagnetic ground state at higher temperatures of T N = 4.6 and 3.8 K for 1 and 2, respectively, than active temperatures for SCM behavior. The spin flip field at 1.8 K is 2.7 and 1.8 kOe for 1 and 2, respectively, which is attributed to the inter-chain interactions tuned by the size of the counteranions. The relaxation times of magnetization become longer at the boundary between the antiferromagnetic phase and the paramagnetic phase.

Novel thin films composed of a donor (D)/acceptor (A) charge-transfer chain compound were fabricated by a layer-by-layer technique using complexation of a paddlewheel-type diruthenium(ii, ii) complex with an N,N'-dicyanoquinonediimine derivative on an ITO substrate with a pyridine-substituted phosphonate anchor. The stepwise growth of an electron-transfer D + A - -chain thin film was confirmed.

Carboxylate-ligand substitution reactions of paddlewheel-type diruthenium(ii, iii) complexes ([Ru(RCO2)4](+)) with 2,6-bis(trifluoromethyl)benzoate (2,6-(CF3)2PhCO2(-)) involving a selective reduction to [Ru] provide a series of trans-substituted paddlewheel-type diruthenium(ii, ii) complexes, [Ru(2,6-(CF3)2PhCO2)2(RCO2)2(THF)2] (R = CH3, ; C2H5, ; C3H7, ; C4H9, ; C(CH3)3, ; 2,3,5,6-F4Ph, ). Crystal structures of were determined, and their electronic states were investigated by cyclic voltammetry, density functional theory (DFT) and magnetic measurements. This is the first example of trans-heteroleptic carboxylate-bridged [Ru] complexes.

Pump-probe transient extinction spectroscopy was used to analyze 355 nm picosecond laser heating-induced phenomena in 60 nm-diameter aqueous gold nanoparticles (AuNPs) under a high pressure of 60 MPa. Kinetic spectroscopy revealed that a supercritical layer surrounding the AuNP nucleated with a lifetime of approximately 1 ns during its dynamic expansion and decay for a fluence of 19.6 mJ cm(-2). Moreover, in the post-mortem transmission electron micrographs we observed a number of fragments, a small percentage of size-reduced cores, and erupted particles among the intact particles after 60 shots, suggesting that evaporation occurred under laser illumination. The particle temperature calculation indicated that evaporation begins with a liquid droplet AuNP surrounded by a supercritical layer at temperatures below the boiling point of gold. By applying high pressure, we obtained a clear picture of the evaporation event, which was not possible at ambient pressure because bubble formation caused particle temperatures to rise uncontrollably. In this study, we shed light on the critical role of the supercritical layer formed around the AuNP under high pressure during laser-induced evaporation.

3D single-molecule tracking revealed that the translational diffusion of guest dyes in poly(2-hydroxyethyl acrylate) thin films on glass substrates was confined in a horizontal layer at a distance longer than 300-700 nm from the surface of the substrate. This peculiar long-range effect suggests that the interaction between the host polymer and the interface could affect the properties of polymers at a much longer distance than conventionally estimated.

A novel charge-disproportionation state with δ = 0.75 was observed in an electron-donor (D)/-acceptor (A) D(δ+)2A(2δ-) layered framework by chemically tuning the electron-donating affinity of D at the boundary between D(0.5+)2A(-) and D(+)2A(2-) phases, which was pressure-sensitive due to the formation of the D(+)2A(2-) oxidation state.

A series of paddlewheel diruthenium(ii, ii) complexes with various chlorine-substituted benzoate ligands (Cl-series) was synthesized as tetrahydrofuran (THF) adducts [Ru2(ClxPhCO2)4(THF)2]; where ClxPhCO2(-) = o-chlorobenzoate, ; m-chlorobenzoate, ; p-chlorobenzoate, ; 2,3-dichlorobenzoate, ; 2,4-dichlorobenzoate, ; 2,5-dichlorobenzoate, ; 2,6-dichlorobenzoate, ; 3,4-dichlorobenzoate, ; 3,5-dichlorobenzoate, ; 2,3,4-trichlorobenzoate, ; 2,3,5-trichlorobenzoate, ; 2,4,5-trichlorobenzoate, ; 3,4,5-trichlorobenzoate, ; 2,3,4,5-tetrachlorobenzoate, . This Cl-series and the previously synthesized F-series together with four new fluorine-substituted derivatives, [Ru2(FxPhCO2)4(THF)2] (where FxPhCO2(-) = 2,3-difluorobenzoate, ; 2,4-difluorobenzoate, ; 2,5-difluorobenzoate, ; 2,3,5-trifluorobenzoate, ), were experimentally characterized with respect to solid-state structure, magnetic properties and electrochemistry. By tuning the substituents of the benzoate ligands using chlorine or fluorine atoms, the redox potential (E1/2) for [Ru2(II,II)]/[Ru2(II,III)](+) varied over a wide range of potentials from -40 mV to 360 mV (vs. Ag/Ag(+) in THF). This was dependent on (i) the number of ortho-substituents, i.e. non-, mono- and di-o-substituted groups, with quasi-Hammett parameters for ortho-Cl and -F substitutions (σo = -0.272 and -0.217, respectively) and (ii) the general Hammett constants, σm and σp, for each group. The HOMO energy level calculated on the basis of the atomic coordinates of the solid-state structure was strongly affected by Cl- and F-substitutions as well as the redox potential in solution, which emphasizes the steric contribution of ortho-substituents in the energy level giving a deviation of EHOMO < 0.3 eV and <0.55 eV for the Cl- and F-series, respectively.

Room temperature ionic liquids (ILs) are a new type of solvent with peculiar properties. ILs are usually composed of an anion and a bulky cation with one or more alkyl chains to decrease the melting point. These structural peculiarities lead to the high viscosity and the heterogeneity of ILs, which could affect chemical reactions. In the present perspective, we will first introduce the experimentally observed nature of the heterogeneous liquid structure and then introduce recent developments in the study on electron transfer (ET) and charge transfer (CT) reactions in relation with the solvation and the heterogeneity of ILs. Because of the high viscosity of ILs, diffusive solvation is expected to be slow which could be the rate-limiting factor for ET and CT processes. However, ILs could provide a unique reaction field depending on the location of the solute within the heterogeneous liquid structure and the reaction could be faster than that expected from the bulk viscosity due to the fast fluctuation of the local environment.

The photoinduced electron transfer properties of two photo-hydrogen-evolving molecular devices (PHEMDs) [(bpy)2Ru(II)(phen-NHCO-bpy-R)Pt(II)Cl2](2+) (i.e., condensation products of [Ru(bpy)2(5-amino-phen)](2+) and (4-carboxy-4′-R-bpy)PtCl2; bpy = 2,2′-bipyridine, phen = 1,10-phenanthroline; RuPt-COOH for R = COOH and RuPt-CN for R = CN) were investigated. RuPt-CN demonstrates higher photocatalytic performance relative to RuPt-COOH arising from a larger driving force for the intramolecular photoinduced electron transfer (PET) associated with a stronger electron-withdrawing effect of R (ΔGPET = −0.43 eV for RuPt-CN and −0.16 eV for RuPt-COOH). This is the first study on PET events using ultrafast spectroscopy. Dramatic enhancement is achieved in the rate of PET in RuPt-CN (1.78 × 1010 s(−1)) relative to RuPt-COOH (3.1 × 109 s(−1)). For each system, the presence of three different conformers giving rise to three different PET rates is evidenced, which are also discussed with the DFT results. Formation of a charge-separated (CS) state [(bpy)2Ru(III)(phen-NHCO-bpy(−˙)-R)Pt(II)Cl2](2+) in the sub-picosecond time regime and recombination in the picosecond time regime are characterized spectrophotometrically. The CS-state formation was found to compete with reductive quenching of the triplet excited state by EDTA whose dianionic form ion-pairs with dicationic RuPt-COOH. Thus, a key intermediate [(bpy)2Ru(II)(phen-NHCO-bpy(−˙)-R)Pt(II)Cl2](+) (i.e., the one-electron-reduced species) prior to the H2 formation was found to be formed either via reduction of the CS state by EDTA or via formation of [(bpy)2Ru(II)(phen(−˙)-NHCO-bpy-R)Pt(II)Cl2](+) by reductive quenching of the triplet excited state. More importantly, it is also shown that some of the conformers in solution possess a CS lifetime sufficiently long to drive hydrogen evolution from water.

Pyrene-intercalated layered compounds, [{Ru2(O2CCF3)4}2(TCNQR(x))]·2(pyrene) (TCNQR(x) = 7,7,8,8-tetracyano-p-quinodimethane derivatives; R(x) = H4 and F4), were synthesized. Pyrene prohibits intralayer electron transfer of [Ru2(II,II)] → TCNQR(x), even in the compound with R(x) = F4, from occurring.

A porous one-dimensional coordination chain compound, [Ru(2)(p-F-PhCO(2))(4)(phz)] (; p-F-PhCO(2)(-) = para-fluorobenzoate; phz = phenazine) derived by drying its nitrobenzene-solvated compound, specifically adsorbs CO(2) at 195 K in a stepwise sorption manner.

A Mn(III)(2)Fe(II) chain with a [-Mn(III)-(O(Ph))(2)-Mn(III)-ON-Fe(II)-NO-] repeat unit was synthesized via the assembling reaction of Mn(III) salen-type dimers and Fe(II) pyridyloximate complexes, where the -(O(Ph))- and -ON- bridges represent a biphenolate bridge and an oximate bridge, respectively. The bulky counter anions, BPh(4)(-), which surround the chain to form a zeolite-like brick wall, successfully isolate the chains from a magnetic point of view. This compound is isostructural with Mn(2)Ni-BPh(4), which is an SCM with S(T) = 3 (H. Miyasaka, A. Saitoh, M. Yamashita and R. Clérac, Dalton Trans., 2008, 2422). Because the Fe(II) unit is diamagnetic, the Mn(III) dimer, which has the potential to be a single-molecule magnet (SMM), is nearly magnetically isolated, although a weak antiferromagnetic interaction with J/k(B)≈-0.1 K is perturbed between the Mn(III) dimers with S(T) = 4 via the Fe(II) unit. Ac susceptibility data shows that two types of relaxation of the magnetization are present, which are attributed to SMM and chain relaxations.

A diagram of energies between the HOMO of donor (D) and LUMO of acceptor (A) vs.ΔE(1/2)(DA) (= E(1/2)(D) - E(1/2)(A): E(1/2) = first-redox potential) clearly demonstrates the ionicity in the series of D/A assemblies, [{Ru(2)(CF(3)CO(2))(4)}(2)(TCNQR(x))]·n(solv) (TCNQR(x) = 2,5- or 2,3,5,6-R-substituted 7,7,8,8-tetracyano-p-quinodimethane; R(x) = H(4), F(2), Cl(2), Br(2), F(4), Me(2), (OMe)(2)).

We have investigated emissive behaviours of individual perylenebisimide derivatives, N,N'-dipropyl-1,6,7,12-tetrakis(4-tert-butylphenoxy)-3,4,9,10-perylenetetra-carboxydiimide (BP-PDI), in single ultrasmall droplets of n-octane at room temperature by using confocal and wide-field microscopic techniques. Single BP-PDIs in the small droplets show no distinguishable blinking in the time courses of fluorescence intensity. This is attributed to small probabilities of the formation of the long-lived ionized state leading to the off-state of the fluorescence. Temporal change in the degree of polarization of fluorescence and wide-field fluorescence images indicated short-time adsorption of the fluorescent molecules at the interfaces between n-octane and watery environments. Fluorescence correlation spectroscopy revealed that the adsorption/desorption processes took place at least in two different time scales, probably due to the difference in the adsorption geometry and/or in the interaction, such as van der Waals interaction and hydrogen bonding, between the dye and the interface.