Picosecond to Nanosecond Manipulation of Excited-State Lifetimes in Complexes with an FeII to TiIV Metal-to-Metal Charge Transfer: The Role of Ferrocene Centered Excited States.

Time-resolved transient absorption spectroscopy and computational analysis of D-π-A complexes comprising FeII donors and TiIV acceptors with the general formula RCp2Ti(C2Fc)2 (where RCp = Cp*, Cp, and MeOOCCp) and TMSCp2Ti(C2Fc)(C2R) (where R = Ph or CF3) are reported. The transient absorption spectra are consistent with an FeIII/TiIII metal-to-metal charge-transfer (MMCT) excited state for all complexes. Thus, excited-state decay is assigned to back-electron transfer (BET), the lifetime of which ranges from 18.8 to 41 ps. Though spectroscopic analysis suggests BET should fall into the Marcus inverted regime, the observed kinetics are not consistent with this assertion. TDDFT calculations reveal that the singlet metal-to-metal charge-transfer (1MMCT) excited state for the FeII/TiIV complexes is not purely MMCT in nature but is contaminated with the higher-energy 1Fc (d-d) state. For the diferrocenyl complexes, RCp2Ti(C2Fc)2, the ratio of MMCT to Fc centered character ranges from 57:43 for the Cp* complex to 85:15 for the MeOOCCp complex. For the diferrocenyl and monoferrocenyl complexes investigated herein, the excited-state lifetimes decrease with increased 1Fc character. The effect of CuI coordination was also analyzed by time-resolved transient absorption spectroscopy and reveals the elongation of the excited-state lifetime by 3 orders of magnitude to 63 ns. The transient spectra and TDDFT analysis suggest that the long-lived excited state in Cp2Ti(C2Fc)2·CuX (where X is Cl or Br) is a triplet iron species with an electron arrangement of TiIV-3FeII-CuI.

Boosting the triplet activity of heavy-atom-free difluoroboron dibenzoylmethane via sp3 oxygen-bridged electron donors.

Purely organic phosphors have important applications in imaging, sensing, informatics and illumination. Methoxy-substituted difluoroboron dibenzoylmethane (BF2dbm) complexes exhibit intense fluorescence with an almost unity quantum yield. Here we show that by simply introducing an sp3 oxygen-bridged methoxylphenyl group as a pendant to BF2dbm, the boron complex exhibits a triplet quantum yield of 0.16, a more than 100-fold increase compared to that of BF2dbm.

Proton-Activated "Off-On" Room-Temperature Phosphorescence from Purely Organic Thioethers.

Room-temperature phosphorescence (RTP)-based sensors have distinctive advantages over the fluorescence counterparts, such as larger Stokes shifts and longer lifetimes. Unfortunately, almost all RTP sensors are operated on quenching-based mechanisms given the sensitive nature of the emissive triplet state. Here we report a type of thioether RTP molecules that shows RTP "turn-on" when volatile acid vapors such as HCl are in contact. To elucidate the underlying mechanism, model thioethers containing different donor/acceptor combinations are investigated via fluorescence spectroscopy and theoretical calculations aided by molecular coordinates obtained from single-crystal X-ray diffraction. It is revealed that a charge-transfer character in the phosphorescence state is crucial. The "turn-on" design concept may significantly broaden the sensing application scope for organic RTP molecules.

Infrared spectroscopy of gas phase alpha hydroxy carboxylic acid homo and hetero dimers.

New gas phase infrared spectroscopy is reported for an aromatic alpha hydroxy carboxylic acid homo dimer of 9-hydroxy-9-fluorene carboxylic acid (9HFCA)2, and the hetero dimer of 9HFCA with glycolic acid. In terms of the 9-hydroxy stretching frequency, the 16 cm-1 blue-shift in the homo dimer and the 17 cm-1 blue-shift in the hetero dimer, relative to that in 9HFCA monomer, are attributed to collective effects with anti-cooperativity stronger than cooperativity. Furthermore, for the hetero dimer, the two alpha hydroxy groups' stretching frequencies are clearly resolved, and differ by 30 cm-1. This difference represents a modest, quantitative enhancement of the intramolecular H-bond by the fluorene moiety in 9HFCA monomer, as opposed to that in glycolic acid. Accurate vibrational frequencies of the alpha OH, 3568 cm-1 in the bare glycolic acid, and 3584 cm-1 in the glycolic acid homo dimer are determined for the first time by comparison to 9HFCA monomer, homo and hetero dimers. The quantitative studies by infrared spectroscopy reveal subtle interactions among intra- and intermolecular H-bonds in the alpha hydroxyl acid dimers, which are also uniquely extended to probe each monomer's subtle intramolecular interactions.

Phosphorescence Tuning through Heavy Atom Placement in Unsymmetrical Difluoroboron ß-Diketonate Materials.

Difluoroboron ß-diketonates (BF2 bdks) show both fluorescence (F) and room-temperature phosphorescence (RTP) when confined to a rigid matrix, such as poly(lactic acid). These materials have been utilized as optical oxygen sensors (e.g., in tumors, wounds, and cells). Spectral features include charge transfer (CT) from the major aromatic donor to the dioxaborine acceptor. A series of naphthyl-phenyl dyes (BF2 nbm) (1-6) were prepared to test heavy-atom placement effects. The BF2 nbm dye (1) was substituted with Br on naphthyl (2), phenyl (3), or both rings (4) to tailor the fluorescence/phosphorescence ratio and RTP lifetime-important features for designing O2 sensing dyes by means of the heavy atom effect. Computational studies identify the naphthyl ring as the major donor. Thus, Br substitution on the naphthyl ring produced greater effects on the optical properties, such as increased RTP intensity and decreased RTP lifetime compared to phenyl substitution. However, for electron-donating piperidyl-phenyl dyes (5), the phenyl aromatic is the major donor. As a result, Br substitution on the naphthyl ring (6) did not alter the optical properties significantly. Experimental data and computational modeling show the importance of Br position. The S1 and T1 states are described by two singly occupied MOs (SOMOs). When both of these SOMOs have substantial amplitude on the heavy atom, passage from S1 to T1 and emission from T1 to S0 are both favored. This shortens the excited-state lifetimes and enhances phosphorescence.

Unravelling hydrogen bonding interactions of tryptamine-water dimer from neutral to cation.

The neutral and cationic forms of tryptamine-water dimer present a variety of noncovalent interactions. To characterize these interactions, a series of complementary methods, including the quantum theory of atoms in molecules, noncovalent interaction plots, natural bond orbital analysis, and energy decomposition analysis, were used. For the first time, the existence of the three intermolecular H-bonds in the conformer-locked tryptamine-water dimer A-H2O are identified, highlighting a single water's role as one proton donor and two proton acceptors as it binds to tryptamine. Furthermore, upon threshold ionization of the A-H2O dimer, the network of the three intermolecular H-bonds is indeed preserved while the individual H-bonds' binding strengths are subject to change; this is attributed to the existence of the optically accessible minimum energy isomer A+-H2O in the cation. In addition, it is found that the global minimum energy isomer H+-H2O contains a single intermolecular H-bond, but is more stable, by ca. 3 kcal mol-1, than the local minimum energy isomer A+-H2O; this is due to the stronger intramolecular interaction of H+-H2O as opposed to A+-H2O.

Quantitative probing of subtle interactions among H-bonds in alpha hydroxy carboxylic acid complexes.

Hydrogen (H) bonds are of fundamental importance in a wide range of molecular sciences. While the study of two-center H-bonding AH is well advanced, much remains to be learned in a quantitative and definitive manner for complexes with multiple H-bonds. Exemplary cases are in the category of alpha hydroxy carboxylic acids, i.e., the complexes of glycolic acid with water and formic acid. In glycolic acid, an intramolecular H-bond forms between the carboxyl group and the alpha OH group. The alpha OH stretching frequency may be affected upon complexing with water or formic acid. Direct study of glycolic acid complexes is unfortunately very difficult. However, an aromatic analogue, 9-hydroxy-9-fluorene carboxylic acid (9HFCA), permits detailed and accurate gas phase spectroscopic studies. Since computational analysis establishes that H-bonding is very similar from glycolic acid complexes to 9HFCA complexes, direct studies on 9HFCA complexes by laser spectroscopy also deduce new and subtle information for glycolic acid complexes in terms of molecular structures, binding strength, and collective effects of multiple H-bonds associated with anti-cooperativity and cooperativity.

Dissection of H-bonding interactions in a glycolic acid-water dimer.

The binding strength and collective effects of multiple H-bonds in the glycolic acid-water dimer were studied in comparison to the aromatic analog, 9-hydroxy-9-fluorene carboxylic acid (9HFCA). Quantitative analysis by the generalized Kohn-Sham energy decomposition analysis shows that the energy difference in each specific physical interaction, from a glycolic acid-water dimer to a 9HFCA-water dimer, is small and amounts to less than 5% of the binding energy of the 9HFCA-water dimer. Extensive comparison of further, similar H-bonded complexes with widely varying binding strengths reinforces their excellent analogy in that the fluorene group acts as a non-interfering spectator for intermolecular H-bonding interactions. With reference to the spectroscopic measurement on the 9HFCA-water dimer (8.51 ± 0.09 kcal mol-1), the binding energy of the glycolic acid-water dimer is estimated to be 8.51 ± 0.31 kcal mol-1, a much better accuracy than previous reports. Furthermore, correlating the infrared spectra of 9HFCA H-bonded complexes provides a circumstantial probing of the existence and consequences of cooperative and anti-cooperative behaviors in the glycolic acid-water dimer. Our studies point to the interesting H-bonding phenomena in the glycolic acid-water dimer, which may inspire challenging experiments in future.

Communication: Physical origins of ionization potential shifts in mixed carboxylic acids and water complexes.

The ionization potential (IP) of the aromatic alpha hydroxy carboxylic acid, 9-hydroxy-9-fluorene carboxylic acid (9HFCA), is shifted by complexation with hydrogen bonding ligands such as water and formic acid. Generalized Kohn-Sham energy decomposition analysis decomposes the intermolecular binding energies into a frozen energy term, polarization, correlation, and/or dispersion energy terms, as well as terms of geometric relaxation and zero point energy. We observe that in each dimer the attractive polarization always increases upon ionization, enhancing binding in the cation and shifting the IP toward the red. For 9HFCA-H2O, a substantial decrease of the repulsive frozen energy in cation further shifts the IP toward red. For 9HFCA-HCOOH, the increase of the frozen energy actually occurs in the cation and shifts the IP toward blue. Consistent with the experimental measurements, our analysis provides new, non-intuitive perspectives on multiple hydrogen bonds interactions in carboxylic acids and water complexes.

Versatile Room-Temperature-Phosphorescent Materials Prepared from N-Substituted Naphthalimides: Emission Enhancement and Chemical Conjugation.

Purely organic materials with room-temperature phosphorescence (RTP) are currently under intense investigation because of their potential applications in sensing, imaging, and displaying. Inspired by certain organometallic systems, where ligand-localized phosphorescence ((3) π-π*) is mediated by ligand-to-metal or metal-to-ligand charge transfer (CT) states, we now show that donor-to-acceptor CT states from the same organic molecule can also mediate π-localized RTP. In the model system of N-substituted naphthalimides (NNIs), the relatively large energy gap between the NNI-localized (1) π-π* and (3) π-π* states of the aromatic ring can be bridged by intramolecular CT states when the NNI is chemically modified with an electron donor. These NNI-based RTP materials can be easily conjugated to both synthetic and natural macromolecules, which can be used for RTP microscopy.

External Heavy-Atom Effect via Orbital Interactions Revealed by Single-Crystal X-ray Diffraction.

Enhanced spin-orbit coupling through external heavy-atom effect (EHE) has been routinely used to induce room-temperature phosphorescence (RTP) for purely organic molecular materials. Therefore, understanding the nature of EHE, i.e., the specific orbital interactions between the external heavy atom and the luminophore, is of essential importance in molecular design. For organic systems, halogens (e.g., Cl, Br, and I) are the most commonly seen heavy atoms serving to realize the EHE-related RTP. In this report, we conduct an investigation on how heavy-atom perturbers and aromatic luminophores interact on the basis of data obtained from crystallography. We synthesized two classes of molecular systems including N-haloalkyl-substituted carbazoles and quinolinium halides, where the luminescent molecules are considered as "base" or "acid" relative to the heavy-atom perturbers, respectively. We propose that electron donation from a π molecular orbital (MO) of the carbazole to the σ* MO of the C-X bond (π/σ*) and n electron donation to a π* MO of the quinolinium moiety (n/π*) are responsible for the EHE (RTP) in the solid state, respectively.

Complexes with Tunable Intramolecular Ferrocene to Ti(IV) Electronic Transitions: Models for Solid State Fe(II) to Ti(IV) Charge Transfer.

Iron(II)-to-titanium(IV) metal-to-metal-charge transfer (MMCT) is important in the photosensitization of TiO2 by ferrocyanide, charge transfer in solid-state metal-oxide photocatalysts, and has been invoked to explain the blue color of sapphire, blue kyanite, and some lunar material. Herein, a series of complexes with alkynyl linkages between ferrocene (Fc) and Ti(IV) has been prepared and characterized by UV-vis spectroscopy and electrochemistry. Complexes with two ferrocene substituents include Cp2Ti(C2Fc)2, Cp*2Ti(C2Fc)2, and Cp2Ti(C4Fc)2. Complexes with a single ferrocene utilize a titanocene with a trimethylsilyl derivatized Cp ring, (TMS)Cp, and comprise the complexes (TMS)Cp2Ti(C2Fc)(C2R), where R = C6H5, p-C6H4CF3, and CF3. The complexes are compared to Cp2Ti(C2Ph)2, which lacks the second metal. Cyclic voltammetry for all complexes reveals a reversible Ti(IV/III) reduction wave and an Fe(II/III) oxidation that is irreversible for all complexes except (TMS)Cp2Ti(C2Fc)(C2CF3). All of the complexes with both Fc and Ti show an intense absorption (4000 M(-1)cm(-1) < ε < 8000 M(-1)cm(-1)) between 540 and 630 nm that is absent in complexes lacking a ferrocene donor. The energy of the absorption tracks with the difference between the Ti(IV/III) and Fe(III/II) reduction potentials, shifting to lower energy as the difference in potentials decreases. Reorganization energies, λ, have been determined using band shape analysis (2600 cm(-1) < λ < 5300 cm(-1)) and are in the range observed for other donor-acceptor complexes that have a ferrocene donor. Marcus-Hush-type analysis of the electrochemical and spectroscopic data are consistent with the assignment of the low-energy absorption as a MMCT band. TD-DFT analysis also supports this assignment. Solvatochromism is apparent for the MMCT band of all complexes, there being a bathochromic shift upon increasing polarizability of the solvent. The magnitude of the shift is dependent on both the electron density at Ti(IV) and the identity of the linker between the titanocene and the Fc. Complexes with a MMCT are photochemically stable, whereas Cp2Ti(C2Ph)2 rapidly decomposes upon photolysis.

Aggregation-Induced Emission from Fluorophore-Quencher Dyads with Long-Lived Luminescence.

Aggregation-induced emission (AIE) is an important photophysical phenomenon in molecular materials and has found broad applications in optoelectronics, bioimaging, and chemosensing. Currently, the majority of reported AIE-active molecules are based on either propeller-shaped rotamers or donor-acceptor molecules with strong intramolecular charge-transfer states. Here, we report a new design motif, where a fluorophore is covalently tethered to a quencher, to expand the scope of AIE-active materials. The fluorophore-quencher dyad (FQD) is nonemissive in solutions due to photoinduced electron-transfer quenching but becomes luminescent in the solid state. The intrinsic emission lifetimes are found to be within the microseconds domain at both room and low temperatures. We performed single-crystal X-ray diffraction measurement for each of the FQDs as well as theoretical calculations to account for the possible origin of the long-lived AIE. These FQDs represent a new class of AIE-active molecules with potential applications in organic optoelectronics.

Influences of the propyl group on the van der Waals structures of 4-propylaniline complexes with one and two argon atoms studied by electronic and cationic spectroscopy.

4-propylaniline complexes with one and two argon atoms formed in the molecular beam were studied in the first excited electronic state, S1, using resonance enhanced two-photon ionization spectroscopy and in the cation ground state, D0, using mass analyzed threshold ionization spectroscopy. The combination of electronic and cationic spectra of the clusters allows two conformations to be identified in both aniline-Ar1 and aniline-Ar2, which are assigned to either the gauche configuration or anti-configuration of 4-propylaniline. The gauche isomer exhibits complex bands shifted 29 cm(-1) and 89 cm(-1) from the S1 origin bands and 83 cm(-1) and 148 cm(-1) from the ionization potential assigned to the Ar1 and Ar2 complexes, respectively. For the anti-rotamer, the corresponding shifts actually become nearly additive, 53 cm(-1) and 109 cm(-1) for the S1 origin bands, and 61 cm(-1) and 125 cm(-1) for the ionization potentials. Ab initio calculations provide insights into the influences of the propyl and amino groups on the positions of the argon atoms within the clusters. In addition, the binding energy of one argon with the gauche isomer of 4-propylaniline has been measured to be 550 ± 5 cm(-1) in the D0 state, 496 ± 5 cm(-1) in the S1 state, and 467 ± 5 cm(-1) in the neutral ground state, S0.

Waterborne Polyurethanes with Tunable Fluorescence and Room-Temperature Phosphorescence.

Single-component materials with both fluorescence and room-temperature phosphorescence (RTP) are useful for ratiometric sensing and imaging applications. On the basis of a general design principle, an amino-substituted benzophenone is covalently incorporated into waterborne polyurethanes (WPU) and results in fluorescence and RTP single-component dual-emissive materials (SDMs). At different aminobenzophenone concentrations, the statistical, thermal, and optical properties of these SDMs are characterized. Despite their similar thermal behaviors, the luminescence properties as a function of the chromophore concentration are quite different: increasing concentrations led to progressively narrowed singlet-triplet energy gaps. The tunability of fluorescence and RTP via chromophore concentration is explained by a previously proposed model, polymerization-enhanced intersystem crossing (PEX). The proposal of PEX is based on Kasha's molecular exciton theory with a specific application in polymeric systems, where the polymerization of luminophores results in excitonic coupling and enhanced forward and reverse intersystem crossing. The mechanism of PEX is also examined by theoretical calculations for the WPU system. It is found that the presence of K1 aggregates indeed enhances the crossover from singlet excited states to triplet ones.

Fluorescent recognition of Hg(2+) by a 1,1'-binaphthyl-based macrocycle: a highly selective off-on-off response.

A 1,1'-bi-2-naphthol-based macrocyclic compound was found to show large fluorescent enhancement at a greatly red-shifted wavelength in the presence of one equiv. Hg(2+) but not with any other metal ions. This change was visually observable from blue-greenish emission to bright yellow emission. The fluorescence was quenched when more than 2.6 equiv. Hg(2+) was added.

Electronic and cationic spectroscopy of 9-hydroxy-9-fluorene carboxylic acid.

Resonance-enhanced multiphoton ionization spectroscopy of supersonically cooled gas-phase 9-hydroxy-9-fluorene carboxylic acid (9HFCA) is reported for its first electronic excited state, S1. The UV-UV hole-burning experiment identifies a single conformer in the molecular beam, stabilized by an intramolecular hydrogen bond. For this Cs symmetric conformer, two low frequencies in the S1 spectrum are assigned: an in-plane rocking mode of the carboxylic acid side chain lies at 58 cm(-1), and an in-plane fluorene bending mode appears at 183 cm(-1). The corresponding mode frequencies in the cation, 58 and 196 cm(-1), are measured by zero electron kinetic energy (ZEKE) spectroscopy upon pumping the S1 vibronic states. The adiabatic ionization potential is measured to be 64 923 ± 5 cm(-1). In addition, a feature established by ZEKE spectroscopy upon pumping the hot band is found at 67 cm(-1). This is assigned as a hot band of the HO-C9-COOH rocking mode in the neutral ground state.

Communication: The ionization spectroscopy of mixed carboxylic acid dimers.

We report mass analyzed threshold ionization spectroscopy of supersonically cooled gas phase carboxylic complexes with 9-hydroxy-9-fluorenecarboxylic acid (9HFCA), an analog of glycolic acid. The vibrationally resolved cation spectrum for the 9HFCA complex with formic acid allows accurate determination of its ionization potential (IP), 64,374 ± 8 cm(-1). This is 545 cm(-1) smaller than the IP of 9HFCA monomer. The IPs of 9HFCA complexes with acetic acid and benzoic acid shift by -1133 cm(-1) and -1438 cm(-1), respectively. Density functional calculations confirm that Cs symmetry is maintained upon ionization of the 9HFCA monomer and its acid complexes, in contrast to the drastic geometric rearrangement attending ionization in complexes of 9-fluorene carboxylic acid. We suggest that the marginal geometry changes and small IP shifts are primarily due to the collective interactions among one intramolecular and two intermolecular hydrogen bonds in the dimer.

Aromatic difluoroboron ß-diketonate complexes: effects of π-conjugation and media on optical properties.

Aromatic difluoroboron ß-diketonate complexes (BF2bdks) are classic fluorescent molecules that have been explored as photochemical reagents, two-photon dyes, and oxygen sensors. To gain a better understanding of their emissive properties in both solution and polymer matrices, BF2bdks with varying aromatic groups were synthesized and their photophysical properties were investigated in both methylene chloride and poly(lactic acid) (PLA). Absorption spectra showed systematic variations that are well correlated with structural features, including the size of the aryl substituent and the presence of a para electron-donating methoxy substituent. Computational modeling of the absorption spectra with the TD-B3LYP/6-311+G(d)//B3LYP/6-31G(d) formulation of density functional theory and a polarizable continuum model of dichloromethane solvent shows that all systems show intense π-π* one-electron excitations, usually from one of the highest occupied molecular orbitals (HOMO - k, k = 0, 1, 2) to the lowest unoccupied molecular orbital (LUMO). Emission properties are sensitive to the dye structure and medium. Based on spectroscopic and lifetime studies, BF2bdks exhibit comparable fluorescence properties in both solutions and polymers when the diketonate group is functionalized with smaller aromatic ring systems such as benzene. For BF2bdks with larger arene ring systems, such as anthracene, emission from a strong intramolecular charge-transfer (ICT) state was also noted in both solution and in PLA. There are differences in relative intensities of peaks arising from π-π* and ICT excitations depending upon dye loading in PLA. Substituent effects were also observed. Electron-donating methoxyl groups on the aromatic rings lead to enhanced fluorescence quantum yields. For certain dyes, phosphorescence is detected at low temperature or under a nitrogen atmosphere in PLA matrices.

Photophysical and analyte sensing properties of cyclometalated Ir(III) complexes.

The synthesis of some heteroleptic, cyclometalated iridium(III) complexes is described. The utility of these [Ir(ppy)(2)(N-N)]Cl (ppy = 2-phenylpyridine and N-N = substituted bipyridine, biquinoline, or phenanthroline) complexes as luminescence-based sensors is assessed. The emission intensity of an Ir(III) complex featuring the 3,3'-H(n)dcbpy ligand (H(n)dcbpy = dicarboxylic acid-2,2'-bipyridine; n = 0,1,2 to indicate deprotonated, mono- and diprotonated species, respectively) is seen to increase in the presence of Pb(II). Insight into the structure and analyte-sensing capability is achieved by X-ray crystallography in conjunction with computational modeling. Complexes incorporating carboxylic acid-functionalized bipyridine and biquinoline as the polypyridyl ligand show pH sensitivity while similar phenanthroline complexes do not.