Modulation of the lowest metal-to-ligand charge-transfer state in [Ru(bpy)2(N-N)]2+ systems by changing the N-N from hydrazone to azine: photophysical consequences

Two Ru(II) complexes, [Ru(bpy)2L](ClO4)2 (1) and [Ru(bpy)2L'](BF4)2 (2), where bpy is 2,2'-bipyridine, L is diacetyl dihydrazone, and L' 1:2 is the condensate of L and acetone, are synthesized. From X-ray crystal structures, both are found to contain distorted octahedral RuN(6)(2+) cores. NMR spectra show that the cations in 1 and 2 possess a C2 axis in solution. They display the expected metal-to-ligand charge transfer (1MLCT) band in the 400-500 nm region. Complex 1 is nonemissive at room temperature in solution as well as at 80 K. In contrast, complex 2 gives rise to an appreciable emission upon excitation at 440 nm. The room-temperature emission is centered at 730 nm (lambda(em)(max)) with a quantum yield (Phi(em)) of 0.002 and a lifetime (tau(em)) of 42 ns in an air-equilibrated methanol-ethanol solution. At 80 K, Phi(em) = 0.007 and tau(em) = 178 ns, with a lambda(em)(max) of 690 nm, which is close to the 0-0 transition, indicating an 3MLCT excited-state energy of 1.80 eV. The radiative rate constant (5 x 10(4) s(-1)) at room temperature and 80 K is almost temperature independent. From spectroelectrochemistry, it is found that bpy is easiest to reduce in 2 and that L is easiest in 1. The implications of this are that in 2 the lowest (3)MLCT state is localized on a bpy ligand and in 1 it is localized on L. Transient absorption results also support these assignments. As a consequence, even though 2 shows a fairly strong and long-lived emission from a Ru(II) --> bpy CT state, the Ru(II) --> L CT state in 1 shows no detectable emission even at 80 K.     

Effects of localized triplet exciton on reactivity of photoinduced omega-bond dissociation in naphthyl phenyl ketones having pi,pi* lowest triplet (T1) states studied by laser flash photolysis

Photochemical properties of photoinduced omega-bond dissociation in naphthyl phenyl ketones having a phenylthiyl moiety as a leaving group, p-(alpha-naphthoyl)benzyl phenyl sulfide (NBPS) and 4-benzoyl-1-naphthylmethyl phenyl sulfide (BNMPS), in solution were investigated by laser flash photolysis techniques. Both ketones were shown to undergo photoinduced omega-bond cleavage of the C-S bond to release the phenyl thiyl radical (PTR) at room temperature. Irrespective of excitation wavelengths of NBPS, a quantum yield (Phi(rad)) of the PTR formation was obtained to be 0.1, whereas that for BNMPS was found to depend on the excitation wavelength, i.e., absorption bands from the ground state (S0) to the excited singlet states, S3, S2, and S1 of BNMPS; Phi(rad)(S3) = 0.77 and Phi(rad)(S2) = Phi(rad)(S1) = 1.0. By using triplet sensitization of p-phenylbenzophenone (PBP), efficiencies (alpha(rad)) of the radical formation in the lowest triplet state (T1(pi,pi*)) of NBPS and BNMPS were determined to be 0 and 1.0, respectively. The agreement between Phi(rad)(S1) and alpha(rad) values for BNMPS indicates that the C-S bond dissociation occurs in the T1 state via the S1 state via a fast intersystem crossing from the S1 to the T1 state. The wavelength dependence of the radical yields upon direct excitation of BNMPS was interpreted in terms of the C-S bond cleavage in the S3 state competing with internal conversion from the S3 to the S2 state. The smaller value of Phi(rad)(S3) than those of Phi(rad)(S1) and Phi(rad)(S2) was proposed to originate from the geminate recombination of singlet radical pairs produced by the bond dissociation via the S3 state. Photoinduced omega-cleavage of NBPS was concluded to take place only in the S1(n,pi*) state. Difference in reactivity of omega-cleavage between the triplet states of NBPS and BNMPS was interpreted in terms of localized triplet exciton in the naphthoyl moieties.     

Ligand controlled dioxygen oxidation of rhenium nitrosyl complexes

The addition of an excess of phenyldiazomethane to chlorobenzene solutions of the cationic dinitrosyl bisphosphine rhenium(-I) complexes [Re(NO)2(PR3)2][BAr(F)4] (R = Cy 1a, R = (i)Pr 1b) gave the corresponding benzylidene complexes [Re{=CH(C6H5)}(NO)2(PR3)2][BAr(F)4] (2a and 2b) in good yields. The treatment of 2b with dioxygen resulted in the oxidation of one of the nitrosyl ligands into the corresponding eta2-nitrito (3b) and nitrato complexes (4b) both in the solid state and in solution. In the case of the tricyclohexylphosphine derivative 2a the analogous conversion was not observed. A mechanism for the reaction of 2b with O2 is proposed which is based on an initial SET to the O2 molecule and subsequent formation of a peroxynitrite complex followed by the formation of a dinuclear mU-N2O4 intermediate. This in turn would undergo fission of the peroxo bond to afford 3b. A related sequence of steps is anticipated for the transformation of 3b to 4b. Furthermore, a similar mechanism seems reasonable for the seemingly topochemical reaction of 2b to 3b and 4b in the solid state. The initial SET to dioxygen and subsequent formation of the peroxynitrite complex is supported by DFT calculations on the trimethylphosphine model complexes [Re=CH{C6H5})(NO)2(PMe3)2]n+ (n = 1 and 2).     

Femtosecond ground state recovery: measuring the intersystem crossing yield of polyspirobifluorene

Measurement of the quantum yield of triplet formation has been made for the prototypical conjugated polymer polyspirobifluorene in solution and solid state. An updated method has been described based on femtosecond time resolved ground state recovery following photoexcitation of the polymer. The two components to the recovery of the ground state due to the decay of the singlet and triplet excited states are clearly visible and from these it is possible to calculate Phi(T)=0.05+/-0.01 in solution, this gives k(isc)=5.4 x 10(7) s(-1) which compares favorably with other conjugated polymers. In polymer films an increased triplet yield of Phi(T)=0.12+/-0.02 is found to be independent of temperature, the increased yield is attributed to triplet recombination from charged states.     

Crystal‐to‐Crystal Synthesis of Helically Ordered Polymers of Trehalose by Topochemical Polymerization

The synthesis of crystalline helical polymers of trehalose via topochemical azide–alkyne cycloaddition (TAAC) of a trehalose‐based monomer is presented. An unsymmetrical trehalose derivative having azide and alkyne crystallizes in two different forms having almost similar packing. Upon heating, both the crystals undergo TAAC reaction to form crystalline polymers. Powder X‐ray diffraction (PXRD) studies revealed that the monomers in both the crystals polymerize in a crystal‐to‐crystal fashion; circular dichroism (CD) studies of the product crystals revealed that the formed polymer is helically ordered. This solvent‐free, catalyst‐free polymerization method that eliminates the tedious purification of the polymeric product exemplifies the advantage of topochemical polymerization reaction over traditional solution‐phase polymerization.     

Ground and lowest-lying electronic states of CoN. A multiconfigurational study

The lowest-lying X1Sigma+, a3Phi, b3II, c5Delta, A1Phi, and B1II electronic states of CoN have been investigated at the ab initio MRCI and MS-CASPT2 levels, with extended atomic basis sets and inclusion of scalar relativistic effects. Among the singlet states, the A1Phi and B1II states have been described for the first time. Potential energy curves, excitation energies, spectroscopic constants, and bonding character for all states are reported. Comparison with other early transition-metal nitrides (ScN, TiN, VN, and CrN), isoelectronic (NiC) and isovalent (RhN and IrN) species has been made, besides analyzing the B1II <=> X1+ electronic transition in terms of Franck-Condon factors, Einstein coefficients, and radiative lifetimes. At both levels of theory, the following energetic order has been obtained: X1Sigma+, a3Phi, b3II, c5Delta, A1Phi, and B1II, with good agreement with experimental results. In contrast, previous DFT and MRCI calculations predicted the ground state to be the 5Delta state.     

Solid-emissive fluorophores constructed by a non-planar heteropolycyclic structure with bulky substituents: synthesis and X-ray crystal structures

Novel solid-emissive indeno[1,2-b]benzo[4,5-e]pyran-11-one-type fluorophores having non-planar structures with sterically hindered substituents (R = butyl, phenyl, and thienyl) have been designed and conveniently synthesized. The fluorescence quantum yields of in 1,4-dioxane were (Phi = 0.053) > (Phi = 0.013) > (Phi = 0.003). On the other hand, the solid-state fluorescence quantum yields of the fluorophores were (Phi = 0.39) > (Phi = 0.15) > (Phi = 0.06). To elucidate the large differences in the quantum yields in solution and in the solid state and among the fluorophores , we performed time-resolved fluorescence spectroscopic measurements, semi-empirical molecular orbital calculations (AM1 and INDO/S), and X-ray crystallographic analyses of . The comparison of the values of the radiative and non-radiative rate constants determined by the time-resolved spectroscopic measurements in solution and in the crystalline state supported that non-radiative decay is reduced by restriction of the rotation of the phenyl and thienyl rings in the solid state. In addition, the X-ray crystal structures demonstrated that, in and , the non-planar structure with sterically hindered substituents prevents the fluorophores from forming short pi-pi contacts and produces strong solid-state fluorescence. On the other hand, in the crystal of , the formation of continuous intermolecular CH[dot dot dot]S bonding between neighboring fluorophores was found to increase short pi-pi contacts and reduce the fluorescence intensity.     

Synthesis and Reversible Hydration of a Pseudoprotein,a Fully Organic Polymeric Desiccant by Multiple Single‐Crystal‐to‐Single‐Crystal Transformations

A diphenylalanine derivative, N3‐Phe‐Phe‐NHCH2CCH, was designed for topochemical azide–alkyne cycloaddition (TAAC) polymerization. This dipeptide adopted β‐sheet arrangement as designed, in its crystals, but the azide and alkyne were not fitly aligned for their topochemical reaction. However, the voids present around these groups allowed them to attain a reactive geometry upon heating and their consequent TAAC polymerization to a pseudoprotein in a single‐crystal‐to‐single‐crystal (SCSC) fashion. This motion led to the creation of channels in the product crystal and it absorbed water from the surroundings to fill these channels as H‐bonded water wire. The pseudoprotein undergo reversible hydration/dehydration in SCSC fashion many times under mild conditions: hydration at low relative humidity and dehydration at low temperature. Vapor sorption analyses suggest that this fully organic polymer might be useful as an energy‐efficient desiccant material for controlling indoor humidity.     

Photophysical properties of a rhodium tetraphenylporphyrin-tin corrole dyad. The first example of a through metal-metal bond energy transfer

The luminescence spectroscopy study and the determination of the photophysical parameters for the M-M'-bonded rhodium meso-tetraphenylporphyrin-tin(2,3,7,13,17,18-hexamethyl-8,12-diethylcorrole) complex, (TPP)Rh-Sn(Me6Et2Cor) 1, was investigated. The emission bands as well as the lifetimes (tau(e)) and the quantum yields (Phi(e); at 77 K using 2MeTHF as solvent) were compared with those of (TPP)RhI 2 (TPP = tetraphenylporphyrin) and (Me6Et2Cor)SnCl 3 (Me6Et2Cor = 2,3,7,13,17,18-hexamethyl-8,12-diethylcorrole) which are the two chemical precursors of 1. The energy diagram has been established from the absorption, fluorescence and phosphorescence spectra. The Rh(TPP) and Sn(Me6Et2Cor) chromophores are the energy donor (D) and acceptor (A), respectively. The total absence of fluorescence in 1 (while fluorescence is observed in the tin derivative 3) indicates efficient excited state deactivation, presumably due to heavy atom effect and intramolecular energy transfer (ET). The large decreases in tau(P) and Phi(P) of the Rh(TPP) chromophore going from 2 to 1 indicate a significant intramolecular ET in the triplet states of 1 with an estimated rate ranging between 10(6) and 10(8) s(-1). Based on the comparison of transfer rates with other related dyads that exhibit similar D-A separations and no M-M' bond, and for which slower through space ET processes (10(2)-10(3) s(-1)) operate, a through M-M' bond ET has been unambiguously assigned to 1.     

Synthesis and Reversible Hydration of a Pseudoprotein,a Fully Organic Polymeric Desiccant by Multiple Single‐Crystal‐to‐Single‐Crystal Transformations

A diphenylalanine derivative, N3‐Phe‐Phe‐NHCH2CCH, was designed for topochemical azide–alkyne cycloaddition (TAAC) polymerization. This dipeptide adopted β‐sheet arrangement as designed, in its crystals, but the azide and alkyne were not fitly aligned for their topochemical reaction. However, the voids present around these groups allowed them to attain a reactive geometry upon heating and their consequent TAAC polymerization to a pseudoprotein in a single‐crystal‐to‐single‐crystal (SCSC) fashion. This motion led to the creation of channels in the product crystal and it absorbed water from the surroundings to fill these channels as H‐bonded water wire. The pseudoprotein undergo reversible hydration/dehydration in SCSC fashion many times under mild conditions: hydration at low relative humidity and dehydration at low temperature. Vapor sorption analyses suggest that this fully organic polymer might be useful as an energy‐efficient desiccant material for controlling indoor humidity.     

Photocyclization of 2,4,6-triethylbenzophenones in the solid state

Photocyclization of 2,4,6-triethylbenzophenones (TEBPs) into (E)- or (Z)-benzocyclobutenols ((E)- or (Z)-CBs) is E-selective in solution. By contrast, upon solid-state photolysis, TEBP-3,4-diCl and especially TEBP-4-t-Bu gave (Z)-CB relative to (E)-CB with a much higher proportion than that in solution. For TEBP-4-t-Bu, the most major product in the solid state is an indanol derivative (Inol) (E/Z/Inol=1:3.9:10.3 at 9% conversion). On the basis of the X-ray crystallographic analysis, Inol and (Z)-CB are both topochemical products. Notably, the relative proportion of (E)-CB increased with increased conversion, namely with increased disruption of the crystal lattice. The DFT calculation of highly hindered 2,6-diisopropylbenzophenone (DIBP) was also conducted. These results in conjunction with the previous results on 2,4,6-triisopropylbenzophenones (TIBPs) indicate that CB is formed either via trans-enol followed by its conrotatory ring-closure (paths a and d) or through direct cyclization of biradical (BR) (path b) as shown in Scheme 1. Normally the former route is faster. However, in the crystalline state or in the case of sterically hindered phenyl ketones, path b tends to be adopted.     

Molecular structure of a D-homoandrostanyl steroid derivative: single crystal and powder diffraction analyses

The knowledge of the structure of a molecular crystal is frequently a prerequisite for the understanding of its solid state properties. Even though single-crystal diffractometry is the method of choice when it comes to crystal structure determination, methods using powder diffraction data become more and more competitive. There has been much recent interest in the development of a new generation of "direct-space" approaches that are particularly suited for molecular crystals. The crystallographic structure of a steroid derivative molecule (17,17-di-n-propyl-17a-aza-D-homo-5alpha-androstan-3beta-ol) was obtained in two independent ways: from a single crystal by laboratory X-rays and from a polycrystalline powder by high-resolution synchrotron powder diffraction. The molecule crystallizes in the orthorhombic space group P2(1)2(1)2(1) (a = 6.5346, b = 17.6006 and c = 19.6978 A). Hydrogen bonds form infinite chains of molecules parallel to the c axis.     

Ab initio molecular orbital study of ground and low-lying electronic states of CoCN

The ground and low-lying excited states of CoCN have been studied by ab initio multireference single and double excitation configuration interaction (MR-SDCI) calculations with Davidson's correction Q and Cowan-Griffin's relativistic corrections. The electronic ground state of CoCN is (3)Phi(i) and the equilibrium geometry is linear with bond lengths of r(e)(Co-C)=1.8540 A and r(e)(C-N)=1.1677 A, substantially different from the experimentally derived values of r(0)(Co-C)=1.8827(7) A and r(0)(C-N)=1.1313(10) A. The first excited state is (3)Delta(i), separated from the ground state by 727 cm(-1). Larger dynamical electron correlation energy for the low-spin (3)Phi state than for the high-spin (5)Phi state makes the (3)Phi state to be the ground state, which is discussed in terms of the differences in natural orbitals. A new spin-orbit interaction scheme between the X (3)Phi(i) and 1 (3)Delta(i) states is proposed.     

Steps to demarcate the effects of chromophore aggregation and planarization in poly(phenyleneethynylene)s. 2. The photophysics of 1,4-diethynyl-2-fluorobenzene in solution and in crystals

Crystals of 1,4-bis(2-hydroxy-2-methyl-3-butynyl)-2-fluorobenzene 4 have a rich packing structure with four distinct molecules in the unit cell. A complex hydrogen bonding network results in the formation of cofacial trimers, cofacial dimers, and monomers within the same unit cell. Given a remarkable opportunity to investigate the effect of aggregation on the photophysics of 1,4-diethynylbenzenes, we analyzed the absorption, diffuse reflectance, and emission spectra of compound 4 in solutions and in crystals. Diffuse reflectance and fluorescence excitation revealed a red-shifted absorption that is absent in dilute solution but becomes observable at high concentrations and low temperatures. The fluorescence emission in the solid state is dual with components assigned to monomers and aggregates. The excitation and emission assigned to the monomer are nearly identical in crystals and dilute solutions. The absorption and emission bands assigned to aggregates are broad and red-shifted by 60--80 nm. As expected for a sample with absorbers and emitters with different energies and incomplete equilibration, efficient monomer-to-aggregate energy transfer was observed by a proper selection of excitation wavelengths. The fluorescence quantum yield of 4 in solution is relatively low (Phi(F) = 0.15) and the singlet lifetime short (tau(F) = 3.8 ns). A lower limit for the triplet yield of Phi(T) = 0.64 was determined indirectly in solution by (1)O(2) sensitization, and a relatively strong and long-lived phosphorescence was observed in low-temperature glasses and in crystals at 77 K.     

Programmable photoresponsive materials based on a single molecule via distinct topochemical reactions

Engineering the preorganization of photoactive units remains a big challenge in solid-state photochemistry research. It is of not only theoretical importance in the construction of topochemical reactions but also technological significance in the fabrication of advanced materials. Here, a cyanostilbene derivative, (Z)-2-(3,5-bis(trifluoromethyl)phenyl)-3-(naphthalen-2-yl) acrylonitrile (BNA), was crystallized into two polymorphs under different conditions. The two crystals, BNA-α and BNA-β, have totally different intra-π-dimer and inter-π-dimer hierarchical architectures on the basis of a very simple monomer, which provides them with distinct reactivities, functions and photoresponsive properties. Firstly, two different types of solid-state [2 + 2] photocycloaddition reaction: (i) a typical olefin–olefin cycloaddition reaction within the symmetric π-dimers of BNA-α and (ii) an unusual olefin-aromatic ring cycloaddition reaction within the offset π-dimers of BNA-β have been observed, respectively. Secondly, the crystal of BNA-α can be bent to 90° without any fracture, exhibiting outstanding flexibility upon UV irradiation, while the reversible photocycloaddition/thermal cleavage process (below 100 °C) accompanied by unique fluorescence changes can be achieved in the crystal of BNA-β. Finally, micro-scale photoactuators and light-writable anti-counterfeiting materials have been successfully fabricated. This work paves a simple way to construct smart materials through a bottom-up way that is realized by manipulating hierarchical architectures in the solid state.

Two crystals with different hierarchical architectures are observed based on a single molecule, achieving different types of [2 + 2] photocycloaddition. Finally, controllable photoactuators and light-writable materials are successfully fabricated.     

The importance of spin-orbit coupling and electron correlation in the rationalization of the ground state of the CUO molecule

We present calculations at the relativistic coupled cluster theory that predict the (1)Sigma(0) (+) ground state of CUO to lie 58.2 kJ/mol below the first excited state, (3)Phi(2). This can be contrasted with the outcome of earlier density functional theory and complete active space second order perturbation theory (CASPT2) calculations that both predicted a (3)Phi(2) ground state upon inclusion of spin-orbit coupling in the calculations. Our result gives further justification to the interpretation of the measured frequency shifts of this species in various noble gas matrices as being caused by significant interaction between the uranium and the heavier noble gas atoms.     

Ultrafast excitation relaxation dynamics of lutein in solution and in the light-harvesting complexes II isolated from Arabidopsis thaliana

Ultrafast excitation relaxation dynamics and energy-transfer processes in the light-harvesting complex II (LHC II) of Arabidopsis thaliana were examined at physiological temperature using femtosecond time-resolved fluorescence spectroscopy. Energy transfer from lutein to Chl a proceeded with a rate constant of k(ET) = 1.8-1.9 x 10(13) s(-1) and a yield of approximately Phi(ET) = 0.70, whereas that from neoxanthin to Chl a had a rate constant of k(ET) = 6.5 x 10(11) s(-1) and a yield at the most of Phi(ET) = 0.09. Fluorescence anisotropic decay of lutein in LHC II showed a value larger than 0.4 at the initial state and decayed to approximately 0.1 in 0.3 ps, indicating that two lutein molecules interact with each other in LHC II. In solution, anisotropy of lutein remained constant (0.38) independent of time, and thus a new excited state inferred between the S(2) (1B(u)) state and the S(1) (2A(g)) state was not applicable for lutein in solution. Energy migration processes among Chl a or Chl b molecules were clearly resolved by kinetic analysis. On the basis of these results, relaxation processes and energy-transfer kinetics in LHC II of A. thaliana are discussed.     

Topochemical Polymerization of 1,3‐Diene Monomers and Features of Polymer Crystals as Organic Intercalation Materials

From the viewpoint of controlled polymer synthesis, topochemical polymerization based on crystal engineering is very useful for controlling not only the primary chain structures but also the higher‐order structures of the crystalline polymers. We found a new type of topochemical polymerization of muconic and sorbic acid derivatives to give stereoregular and high‐molecular weight polymers under photo‐, X‐ray, and γ‐ray irradiation of the monomer crystals. In this article, we describe detailed features and the mechanism of the topochemical polymerization of diethyl‐(Z,Z)‐muconate as well as of various alkylammonium derivatives of muconic and sorbic acids, which are 1,3‐diene mono‐ and dicarboxylic acid derivatives, to control the stereochemical structures of the polymers. The polymerization reactivity of these monomers in the crystalline state and the stereochemical structure of the polymers produced are discussed based on the concept of crystal engineering, which is a useful method to design and control the reactivity, structure, and properties of organic solids. The reactivity of the topochemical polymerization is determined by the monomer crystal structure, i.e. the monomer molecular arrangement in the crystals. Polymer crystals derived from topochemical polymerization have a high potential as new organic crystalline materials for various applications. Organic intercalation using the polymer crystals prepared from alkylammonium muconates and sorbates is also described.     

Spectrophotometric and analogue derivative spectrophotometric determination of ultramicro amounts of cadmium with cationic porphyrins

alpha,beta,gamma,delta-Tetrakis(1-methylpyridinium-3-yl)porphine [T(3-MPy)P] and alpha,beta,gamma,delta-tetrakis(1-methylpyridinium-4-yl)porphine [T(4-MPy)P] have been found to react rapidly with cadmium to give coloured complexes in weakly alkaline media at room temperature. Simple and practical methods for the determination of cadmium at ng/ml levels by conventional and analogue derivative spectrophotometry have been proposed. The analogue method gives higher sensitivity. T(3-MPy)P gives higher sensitivity than T(4-MPy)P. The interference of various foreign cations and anions has also been examined and in many cases eliminated or reduced. Adsorption of the porphyrins and their cadmium complexes onto the glassware, which is usually observed under the conditions of reaction and causes significant errors in the determination, can be suppressed almost completely by addition of fairly large amounts of a salt such as sodium chloride.     

Rotational energy barrier of 2-(2',6'-dihydroxyphenyl)benzoxazole: a case study by NMR

2-(2'-Hydroxyphenyl)benzoxazole (HBO) derivatives represent an important class of luminescent materials, as they can undergo excited state intramolecular proton transfer (ESIPT). The material's ESIPT properties are dependent on the ratio of two different rotamers, whose interconversion is poorly understood. By using HBO derivative 4, the rotational energy barrier of 2- (2',6'-hydroxyphenyl)benzoxazole is determined to be 10.5 kcal/mol by variable-temperature NMR. Although a HBO derivative typically exhibits two rotamers with O···H-O (e.g., 1a) and N···H-O bonding (e.g., 1b), correlation of NMR with fluorescence data reveals that the rotamer with N···H-O bonding is predominant in the solution.