Deactivation of the fluorescent state of 9-tert-butylanthracene. 9-tert-Butyl-9,10(Dewar anthracene)

Incubation of protonated NADH or NADPH with ortho-aminobenzaldehyde generates a novel fluorescent nicotinamide dihydroquinazoline condensate

Incubation of reduced nicotinamide adenine dinucleotide (NADH) but not oxidized NAD+ with ortho-aminobenzaldehyde (oABA) generated an uncharacterized chromophore with an absorption peak characteristic of a dihydroquinazoline condensate. This chromophore is responsible for a non-specific signal in a diamine oxidase (DAO) activity assay based on the generation of fluorescent dihydroquinazoline structures directly from DAO substrates. Herein we show that at pH values below 3.0 the glycosidic bond of NADH/NADPH is broken releasing double protonated dihydro-nicotinamide (dihydro-NAM), which consequently condensates with oABA to a novel dihydroquinazoline chromophore and fluorophore, namely the 6- or 8-carbamoyl-5H,7H,8H,9H-10λ⁵-pyrido[2,1-b]quinazolin-10-ylium isomer (CMPQ). The second protonation event closely correlates with the pKa of the N1 nitrogen of C5-protonated dihydro-NAM and fluorophore stability. The fusion partner of oABA is likely the iminium of the primary acid product of dihydro-NAM after glycosidic bond hydrolysis and before irreversible cyclization. Trapping of protonated dihydro-NAM from NADH or NADPH with oABA allows quantification of these dinucleotides. Despite almost a century of research studying acid-catalyzed molecular rearrangements of NADH and NADPH, new and surprising details can be discovered.

Aggregation-Induced Emission: From Small Molecules to Polymers-Historical Background, Mechanisms and Photophysics

The enhancement of photoluminescence through formation of molecular aggregates in organic oligomers and conjugated organic polymers is reviewed. A historical contextualization of aggregation-induced emission (AIE) phenomena is presented. This includes the loose bolt or free rotor effect and J-aggregation phenomena, and discusses their characteristic features, including structures and mechanisms. The basis of both effects is examined in key molecules, with a particular emphasis on the AIE effect occurring in conjugated organic polymers with a polythiophene (PT) skeleton with triphenylethylene (TPE) units. Rigidification of the excited state structure is one of the defining conditions required to obtain AIE, and thus, by changing from a flexible ground state to rigid (quinoidal-like) structures, oligo and PTs are among the most promising emerging molecules alongside with the more extensively used TPE derivatives. Molecular structures moving away from the domination of aggregation-caused quenching to AIE are presented. Future perspectives for the rational design of AIEgen structures are discussed.

The mechanisms of boronate ester formation and fluorescent turn-on in ortho-aminomethylphenylboronic acids

ortho-Aminomethylphenylboronic acids are used in receptors for carbohydrates and various other compounds containing vicinal diols. The presence of the o-aminomethyl group enhances the affinity towards diols at neutral pH, and the manner in which this group plays this role has been a topic of debate. Further, the aminomethyl group is believed to be involved in the turn-on of the emission properties of appended fluorophores upon diol binding. In this treatise, a uniform picture emerges for the role of this group: it primarily acts as an electron-withdrawing group that lowers the pKa of the neighbouring boronic acid thereby facilitating diol binding at neutral pH. The amine appears to play no role in the modulation of the fluorescence of appended fluorophores in the protic-solvent-inserted form of the boronic acid/boronate ester. Instead, fluorescence turn-on can be consistently tied to vibrational-coupled excited-state relaxation (a loose-bolt effect). Overall, this Review unifies and discusses the existing data as of 2019 whilst also highlighting why o-aminomethyl groups are so widely used, and the role they play in carbohydrate sensing using phenylboronic acids.

Synthesis and Photodimerization of 2- and 2,3-Disubstituted Anthracenes: Influence of Steric Interactions and London Dispersion on Diastereoselectivity

There is increased evidence that the effect of bulky groups in organic, organometallic, and inorganic chemistry is not only repulsive but can be attractive because of London dispersion interactions. The influence of the size of primary alkyl substituents in 2- and 2,3-positions of anthracenes on the diastereoselectivity (anti vs syn dimer) of the [π_4s + π_4s] photoinduced dimerization is investigated. The synthesis of the anthracene derivatives was achieved by Suzuki-Miyaura reaction of 2,3-dibromoanthracene with alkylboronic acids as well as by reduction of anthraquinones that were obtained from 2,3-disubstituted 1,3-butadienes and naphthoquinone followed by dehydrogenation. The mixtures of dianthracene isomers were analyzed with respect to the anti/syn-ratio of the products by X-ray crystallography and nuclear Overhauser effect spectroscopy. While for the 2,3-dimethylanthracene the anti and syn isomers were formed in equal amounts, the anti dimers are the major products in all other cases. A linear correlation (R² = 0.98) between the steric size (Charton parameter) and the isomeric ratio suggests that the selectivity is dominated by classical repulsive steric effects. An exception is the iso-butyl substituent that produces an increased amount of the syn isomer. It is suggested that this is due to an exalted effect of London dispersion interactions.

Pressure dependence of the forward and backward rates of 9-tert-butylanthracene Dewar isomerization

9-tert-Butylanthracene undergoes a photochemical reaction to form its strained Dewar isomer, which thermally back-reacts to reform the original molecule. When 9-tert-butylanthracene is dissolved in a polymer host, we find that both the forward and reverse isomerization rates are pressure-dependent. The forward photoreaction rate, which reflects the sum of contributions from photoperoxidation and Dewar isomerization, decreases by a factor of 1000 at high pressure (1.5 GPa). The back-reaction rate, on the other hand, increases by a factor of ∼3 at high pressure. Despite being highly strained and higher volume, the back-reaction reaction rate of the Dewar isomer is at least 100× less sensitive to pressure than that of the bi(anthracene-9,10-dimethylene) photodimer studied previously by our group. These results suggest that the high pressure sensitivity of the bi(anthracene-9,10-dimethylene) photodimer reaction is not just due to the presence of strained four-membered rings but instead relies on the unique molecular geometry of this molecule.

Excited state structural changes of 10-cyano-9-tert-butyl-anthracene (CTBA) in polymer matrices

Two main deactivation processes are suggested to be present in the electronic excited state of 10-cayano-9-tert-butyl-anthracene (CTBA): one leading to the Dewar strucure resulted from photochemical effect, similar to the case of 9-tert-butyl-anthracene (TBA), which is enhanced in solution, and another leads to a more planar structure leading to a Charge Transfer (CT) state and is enhanced in polymers. The presence of donor (tert-butyl group) and an acceptor (cyano group) along the line of 9,10 positions in CTBA confirm our hypothesis of CT state. Our suggestions were supported by steady state and time resolved fluorescence spectra results.

Photochemical and thermal reactions of a kinetically stabilized 9-silaanthracene: the first spectroscopic observation of a 9,10-Dewar-9-silaanthracene isomer

A kinetically stabilized 9-silaanthracene (1) underwent unique photochemical and thermal reactions to afford 9,10-Dewar-9-silaanthracene 2a and the head-to-tail [4 + 4] dimer 3, respectively. The structure of 2a was confirmed by 1H, 13C, and 29Si NMR spectra, and the kinetic parameters for the thermal reversion of 2a to 1 were obtained by the measurement of UV/vis spectra. The dimer 3 was thermally stable, and the molecular structure of 3 was determined by X-ray crystallographic analysis. It was experimentally demonstrated for the first time that 9-silaanthracene, as well as anthracene, can afford either the Dewar isomer or [4 + 4] dimer, depending on the reaction conditions.

Photoprotodesilylation of 9-trimethylsilylanthracene in alcohols

It was found that the anomalous fluorescence quenching in methanol (as compared to other solvents) of 9-trimethylsilylanthracene (2) is accompanied by a chemical reaction leading to anthracene. This ipso substitution is due to a hydrogen bonding formation in the excited singlet state preceding the carbon-silicon bond cleavage. The mechanism was studied using stationary and dynamic fluorescence, laser flash photolysis and reactivity kinetics as a function of temperature. The kinetic parameters were determined for the formation of an intermediate "X" (A(X) approximately 2.5 x 10(11) s(-1) Ea(X) approximately 19.5 kJ mol(-1)) and for the global reaction (A(R) approximately 4 x 10(10) s(-1) and Ea(R) approximately 19.2 kJ mol(-1)), indicating that, after the formation of X, the reaction does not involve any step with an activation energy larger than 19 kJ mol(-1). The intermediate X undergoes partitioning between the product (a approximately 0.18) and the starting material. The isotopic effect for the quenching by methanol was found to be kX(H)/kX(D) approximately 1.9, in keeping with the proposed protonation in the excited singlet state (S1). The reactivity ratio between the S1 state and the ground state, kR(S1)/ kR(S0) approximately 10(12) is in line with those observed by other authors. This unusual photoreaction can proceed for 2 with other alcohols. ipsoProtodesilylation of aromatic silanes are known to occur in the ground state but in acidic media.