Excited-state intramolecular proton transfer in the kinetic-control regime.

A new series of molecules bearing a 2,11-dihydro-1H-cyclopenta[de]indeno[1,2-b]quinoline (CPIQ) chromophore with the N-HN type of intramolecular hydrogen bond are strategically designed and synthesized, among which CPIQ-OH, CPIQ-NHAc and CPIQ-NHTs in solution exhibit a single emission band with an anomalously large Stokes shift, whereas CPIQ-NH2 and CPIQ-NHMe show apparent dual-emission property. This, in combination with time-resolved spectroscopy and the computational approach, leads us to conclude that CPIQ-OH, CPIQ-NHAc and CPIQ-NHTs undergo ultrafast, highly exergonic excited-state intramolecular proton transfer (ESIPT), while a finite rate of ESIPT is observed for CPIQ-NH2 and CPIQ-NHMe with a time constant of 117 ps and 39 ps, respectively, in acetonitrile at room-temperature. Further temperature-dependent studies deduce an appreciable ESIPT barrier for CPIQ-NH2 and CPIQ-NHMe. Different from most of the barrier associated ESIPT molecules that are commonly in the thermodynamic-control regime, i.e. found in the thermal pre-equilibrium between excited normal and proton-transfer tautomer states, CPIQ-NH2 and CPIQ-NHMe cases are in the kinetic-control regime where ESIPT is irreversible with a significant barrier. The barrier is able to be tuned by the electronic properties of the -R group in the NR-H proton donor site, resulting in ratiometric fluorescence for normal versus tautomer emission.

Sulfur-Based Intramolecular Hydrogen-Bond: Excited-State Hydrogen-Bond On/Off Switch with Dual Room-Temperature Phosphorescence.

We report O-H----S hydrogen-bond (H-bond) formation and its excited-state intramolecular H-bond on/off reaction unveiled by room-temperature phosphorescence (RTP). In this seminal work, this phenomenon is demonstrated with 7-hydroxy-2,2-dimethyl-2,3-dihydro-1 H-indene-1-thione (DM-7HIT), which possesses a strong polar (hydroxy)-dispersive (thione) type H-bond. Upon excitation, DM-7HIT exhibits anomalous dual RTP with maxima at 550 and 685 nm. This study found that the lowest lying excited state (S1) of DM-7HIT is a sulfur nonbonding (n) to π* transition, which undergoes O-H bond flipping from S1(nπ*) to the non-H-bonded S'1(nπ*) state, followed by intersystem crossing and internal conversion to populate the T'1(nπ*) state. Fast H-bond on/off switching then takes place between T'1(nπ*) and T1(nπ*), forming a pre-equilibrium that affords both the T'1(nπ*, 685 nm) and T1(nπ*, 550 nm) RTP. The generality of the sulfur H-bond on/off switching mechanism, dubbed a molecule wiper, was rigorously evaluated with a variety of other H-bonded thiones, and these results open a new chapter in the chemistry of hydrogen bonds.

Ratiometric fluorescent/colorimetric cyanide-selective sensor based on excited-state intramolecular charge transfer-excited-state intramolecular proton transfer switching.

A novel salicylideneaniline-based fluorescent sensor, SB1, with a unique excited-state intramolecular charge transfer-excited-state intramolecular proton transfer (ESICT-ESIPT) coupled system was synthesized and demonstrated to fluorescently sense CN(-) with specific selectivity and high sensitivity in aqueous media based on ESICT-ESIPT switching. A large blue shift (96 nm) was also observed in the absorption spectra in response to CN(-). The bleaching of the color could be clearly observed by the naked eye. Moreover, SB1-based test strips were easily fabricated and low-cost, and could be used in practical and efficient CN(-) test kits. Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations further support the cyanide-induced ESICT-ESIPT switching mechanism. The results provide the proof of concept that the colorimetric and ratiometric fluorescent cyanide-selective chemodosimeter can be created based on an ESICT-ESIPT coupled system.

Acenes generated from precursors and their semiconducting properties.

Acenes are a class of aromatic hydrocarbons composed of linearly fused benzene rings. Noteworthy features of these molecules include their extended flat structure and the narrow gap between the HOMO and LUMO energy levels. However, the preparation of larger acenes, those that are larger than pentacene, has been challenging. These molecules are relatively unstable and have low solubility in typical solvents. Recently researchers have developed a new synthesis route for higher acenes using stable and soluble "precursors," which generate these structures on demand by either heating or irradiation of light. Using this method, nonsubstituted hexacene, heptacene, octacene, and nonacene were successfully prepared. In this Account, we summarize the preparation of nonsubstituted acenes from corresponding precursors, describe their physical properties, and discuss potential applications including potential usage in organic semiconductor devices. We first introduced the concept of using a precursor in the work with pentacene. Overall, we divide this methodology into two categories: masking pentacene itself with a dienophile to form a cycloadduct and the construction of higher acenes through conventional synthetic procedures. For the first category, a diverse array of dienophiles could be chosen, depending on the processing needs, especially for use in field-effect transistors (FETs). For the second category, researchers synthesized the pentacene precursor molecules using a multistep procedure. Upon proper activation, these molecules expel small fragments to generate pentacene readily. This strategy enabled the production of pentacene andunprepared higher acenes ranging from hexacene to nonacene. This new method provides a way to unravel the fascinating chemistry of higher acenes.

Synthesis, X-ray structure, spectroscopic properties and DFT studies of a novel Schiff base.

A series of Schiff bases, salicylideneaniline derivatives 1-4, was synthesized under mild conditions and characterized by 1H NMR, HRMS, UV-Vis and fluorescence spectra, and single-crystal X-ray diffraction. In solid and aprotic solvents 1-4 exist mainly as E conformers that possess an intramolecular six-membered-ring hydrogen bond. A weak intramolecular C-H···F hydrogen bond is also observed in fluoro-functionalized Schiff base 4, which generates another S(6) ring motif. The C-H···F hydrogen bond further stabilizes its structure and leads it to form a planar configuration. Compounds 1-3 exhibit solely a long-wavelength proton-transfer tautomer emission, while dipole-functionalized Schiff base 4 shows remarkable dual emission originated from the excited-state intramolecular charge transfer (ESICT) and excited-state intramolecular proton transfer (ESIPT) states. Furthermore, the geometric structures, frontier molecular orbitals (MOs) and the potential energy curves for 1-4 in the ground and the first singlet excited state were fully rationalized by density functional theory (DFT) and time-dependent DFT calculations.

Highly Soluble Monoamino-Substituted Perylene Tetracarboxylic Dianhydrides: Synthesis, Optical and Electrochemical Properties.

Three dialkylamino-substituted perylene tetracarboxylic dianhydrides with different n-alkyl chain lengths (n = 6, 12 or 18), 1a-1c, were synthesized under mild conditions in high yields and were characterized by 1H NMR, 13C NMR and high resolution mass spectroscopy. Their optical and electrochemical properties were measured using UV-Vis and emission spectroscopic techniques, as well as cyclic voltammetry (CV). This is the first time that the structures and the properties of monoamino-substituted perylene tetracarboxylic dianhydrides have been reported. These molecules show a deep green color in both solution and the solid state and are soluble in most organic solvents. They all show a unique charge transfer emission in the near-infrared region, and the associated peaks exhibit solvatochromism. The dipole moments of the compounds have been estimated using the Lippert-Mataga equation, and upon excitation, they show slightly larger dipole moment changes than those of corresponding perylene diimides, 2a-2c. Additionally, Compounds 1a-1c undergo two quasi-reversible one-electron oxidations and two quasi-reversible one-electron reductions in dichloromethane at modest potentials. Complementary density functional theory calculations performed on these chromophores are reported in order to gain more insight into their molecular structures and optical properties.

1,6- and 1,7-regioisomers of asymmetric and symmetric perylene bisimides: synthesis, characterization and optical properties.

The 1,6- and 1,7-regioisomers of dinitro- (1,6-A and 1,7-A) and diamino-substituted perylene bisimides (1,6-B and 1,7-B), and 1-amino-6-nitro- and 1-amino-7-nitroperylene bisimides (1,6-C and 1,7-C) were synthesized. The 1,6-A and 1,7-A regioisomers were successfully separated by high performance liquid chromatography and characterized by 500 MHz 1H-NMR spectroscopy, and subsequently, their reduction which afforded the corresponding diaminoperylene bisimides 1,6-B and 1,7-B, respectively. On the other hand, the monoreduction of 1,6-A and 1,7-A, giving the asymmetric 1-amino-6-nitro (1,6-C) and 1-amino-7-nitroperylene bisimides (1,7-C), respectively, can be performed by shortening the reaction time from 6 h to 1 h. This is the first time the asymmetric 1,6-disubstituted perylene bisimide 1,6-C is obtained in pure form. The photophysical properties of 1,6-A and 1,7-A were found to be almost the same. However, the regioisomers 1,6-C and 1,7-C, as well as 1,6-B and 1,7-B, exhibit significant differences in their optical characteristics. Time-dependent density functional theory calculations performed on these dyes are reported in order to rationalize their electronic structure and absorption spectra.

1-Meth-oxy-11H-benzo[b]fluoren-11-one.

In the title compound, C18H12O2, the non-H atoms are nearly coplanar, the maximum atomic deviation being 0.113 (2) Å. π-π stacking is observed in the crystal structure, the shortest centroid-centroid distance being 3.5983 (19) Å. The mol-ecular packing is further stabilized by weak C-H⋯O hydrogen bonds, forming an infinite chain along [100] and generating a C(6) motif.

Low-Molecular-Weight J-Aggregate Solid Red Emitter for Selective and Quantitative Detection of Cyanide.

A dimethylaniline (donor)-indanedione (acceptor) conjugate (sensor 1) with a very low molecular weight of 277 g mol-1 and intramolecular charge transfer (ICT) characteristics was synthesized. Sensor 1 shows weak ICT fluorescence in solution, but strong emission (Φ=16%) in the solid state owing to intramolecular and intermolecular C-H⋅⋅⋅O hydrogen bonds that inhibit the free rotation of the exocyclic C-C single bond. Compared to yellow emitter 1Y, which has a similar donor-acceptor structure, sensor 1 shows red fluorescence in the solid state owing to J-aggregate formation. The colorimetric and fluorometric responses of sensor 1 to cyanide in both solution and solid state are due to the nucleophilic addition of cyanide to the ß-conjugated carbon of the indanedione group, which prohibits ICT. Additionally, inexpensive portable paper-based test kits based on sensor 1 were easily prepared and could be used for fast and quantitative naked-eye cyanide detection in real time.

7-Meth-oxy-indan-1-one.

In the title compound, C(10)H(10)O(2), the 1-indanone unit is essentially planar (r.m.s. deviation = 0.028 Å). In the crystal, molecules are linked via C-H⋯O hydrogen bonds, forming layers lying parallel to the ab plane. This two-dimensional structure is stabilized by a weak C-H⋯π inter-action. A second weak C-H⋯π inter-action links the layers, forming a three-dimensional structure.

2-(Dibromo-meth-yl)benzoic acid.

In the crystal structure of the title compound, C(8)H(6)Br(2)O(2), the carboxyl groups are involved in pairs of O-H⋯O hydrogen bonds, which link the mol-ecules into inversion dimers.

1-Hy-droxy-11H-benzo[b]fluoren-11-one.

The title compound, C(17)H(10)O(2), is nearly planar, the maximum atomic deviation being 0.053 (2) Å. In the mol-ecule, an intra-molecular O-H⋯O hydrogen bond generates an S(6) ring motif. In the crystal, inversion-related mol-ecules are linked by pairs of weak C-H⋯O hydrogen bonds, forming dimers. π-π stacking is observed in the crystal structure, the closest centroid-centroid distance being 3.7846 (16) Å.

1,2-Bis(dibromo-meth-yl)benzene.

In the title compound, C(8)H(6)Br(4), intra-molecular C-H⋯Br hydrogen bonds generate two S(6) rings. The two geminal bromine-atom substituents point to opposite sides of the aromatic ring system. In the crystal, mol-ecules are linked by inter-molecular π-π inter-actions with centroid-centroid distances of 3.727 (9) and 3.858 (9) Å.

Penta-cyclo-[8.2.1.1(4,7).0(2,9).0(3,8)]tetra-deca-5,11-diene.

The title compound, C(14)H(16), was prepared through [2 + 2] cyclo-addition of norbornadiene. There are two independent mol-ecules in the asymmetric unit: each is centrosymmetric with the centroid of the four-membered ring located about an inversion center. Each mol-ecule possesses an exo-trans-exo conformation.

3-Oxo-2,3-dihydro-1H-inden-4-yl acetate.

In the title compound, C(11)H(10)O(3), the 1-indanone unit is essentially planar (r.m.s. deviation = 0.036 Å). In the crystal, mol-ecules are linked by non-classical C-H⋯O hydrogen bonds, forming a C(6) chain along [010].

4-Hy-droxy-indan-1-one.

The mol-ecule of the title compound, C(9)H(8)O(2), is essentially planar except for the methyl-ene H atoms [maximum deviation = 0.028 (1) Å]. In the crystal, the mol-ecules are linked by classical O-H⋯O hydrogen bonds and weak C-H⋯O inter-actions into chains along [110] and [1-10].

2-(4-Nitro-benzyl-idene)malononitrile.

In the title compound, C(10)H(5)N(3)O(2), the benzyl-idene-malono-nitrile unit is nearly planar, with a maximum deviation of 0.129 (2) Šfor a terminal N atom; the nitro group is approximately coplanar with the benzene ring [dihedral angle = 8.8 (3)°]. An intra-molecular C-H⋯N hydrogen bond stabilizes the mol-ecular conformation.

3-{[(4Z)-1,2-Dimethyl-5-oxoimidazol-4-yl-idene]meth-yl}-4-hy-droxy-benzonitrile.

In the title compound, C(13)H(11)N(3)O(2), an intra-molecular O-H⋯N hydrogen bond generates an S(7) ring. The dihedral angle between the mean plane of the benzene ring and the imidazolidinone ring is 3.05 (2)°. In the crystal, inversion-related mol-ecules are linked by dual C-H⋯O(carbon-yl) hydrogen bonds to form a dimer with an R(2) (2)(14) graph-set motif. A C-H⋯O(hy-droxy) inter-action links pairs of mol-ecules into another type of cyclic dimer with an R(2) (2)(18) motif. The mol-ecules are further linked by C-H⋯N inter-actions to form layers parallel to (001). Offset π-π stacking [3.3877 (8) Å] is observed in the crystal structure, with an inter-planar spacing between the planes of neighboring benzene rings of 3.444 (1) Å.

(Z)-4-(2-Hy-droxy-benzyl-idene)-1-methyl-2-phenyl-1H-imidazol-5(4H)-one.

In the title compound, C(17)H(14)N(2)O(2), the asymmetric unit comprises two mol-ecules that are comformationally similar [the dihedral angles between the phenyl rings in each are 46.35 (2) and 48.04 (3)°], with the conformation stabilized by intra-molecular O-H⋯N hydrogen bonds, which generate S(7) rings. In the crystal, inversion-related mol-ecules are linked by pairs of weak C-H⋯O hydrogen bonds, forming dimers with an R(2) (2)(16) graph-set motif. Weak inter-ring π-π stacking is observed in the structure, the shortest centroid-to-centroid distance being 3.7480 (13) Å.

(E)-4-[(4-Diethyl-amino-2-hy-droxy-benzyl-idene)amino]-benzonitrile.

The title compound, C(18)H(19)N(3)O, displays an E conformation with respect to the C=N double bond. The dihedral angle between the mean planes of the two benzene rings is 24.49 (3)°. An intra-molecular O-H⋯N hydrogen bond generates an S(6) ring. In the crystal, mol-ecules are linked by nonclassical inter-molecular C-H⋯O hydrogen bonds to form an infinite one-dimensional chain along [010], generating a C(8) motif.