Four types of non-starch polysaccharides including cellulose, guar gum, locust bean gum and xanthan gum were modified by carboxymethylation. The degree of substitution (DS) was determined by a ...colorimetric method. Raman and Fourier transform infrared spectra of the native and modified polysaccharides were acquired and analysed. Representative marker bands, with intensities and/or integrated areas affected by carboxymethylation, were selected at 1607
cm
−1 (Raman) and 1315
cm
−1 or 1605
cm
−1 (IR), attributed to C
O carbonyl stretching vibration. The ratios of intensities (or areas) of the marker bands to that of an internal standard band, corresponding to the skeletal configuration and linkages (850–950
cm
−1), were plotted against DS. Linear fits were obtained with high correlation coefficients,
r
>
0.96 (
p
<
0.01), suggesting a strong correlation between the spectroscopic data and DS determined by wet chemistry. Some structural changes were also observed from the spectral data.
The 3',5'-dimethoxybenzoin (DMB) system has been widely investigated as a photoremovable protecting group (PRPG) for the elimination of various functional groups and has been applied in many fields. ...The photolysis of DMB fluoride leads to a highly efficient photocyclization-deprotection reaction, resulting in a high yield of 3',5'-dimethoxybenzofuran (DMBF) in a MeCN solution, while there is a competitive reaction that produces DMB in an aqueous solution. The yield of DMB increased as the volume ratio of water increased. To understand the solvent effect of the photolysis of selected DMB-based compounds, a combination of femtosecond to nanosecond transient absorption spectroscopies (fs-TA and ns-TA), nanosecond time-resolved resonance Raman spectroscopy (ns-TR
) and quantum chemical calculation was employed to study the photophysical and photochemical reaction mechanisms of DMB fluoride in different solutions. Facilitated by the bichromophoric nature of DMB fluoride with electron-donating and -withdrawing chromophores, the cyclized intermediates could be found in a pure MeCN solution. The deprotection of a cyclic biradical intermediate results in the simultaneous formation of DMBF and a cyclic cation species. On the other hand, in aqueous solution, fs-TA experiments revealed that α-keto cations could be observed after excitation directly, which could easily produce the DMB through the addition of a hydroxyl within 8.7 ps. This work provides comprehensive photo-deactivation mechanisms of DMB fluoride in MeCN and aqueous conditions and provides critical insights regarding the biomedical application of DMB-based PRPG compounds.
ABSTRACT
The photolysis reactions of (8‐cyano‐7‐hydroxyquinolin‐2‐yl)methyl (CyHQ)‐caged amines have been investigated using time‐resolved spectroscopy methods. Unexpectedly, an unconventional ...Hofmann‐Martius rearrangement reaction with high yield and regioselectivity occurred during the photolysis of some CyHQ‐protected dialkylanilines (such as compounds 1a and 2a). To have more insights into the mechanism of this unexpected photorearrangement reaction, we characterized the reaction intermediates directly using time‐resolved spectroscopy. Our new results showed that the anionic form of compound 1a was photoexcited to the singlet excited state, then a heterolytic cleavage of the C‐N bond took place to give CyHQ+ and the corresponding aniline. Thereafter, the recombined intermediate 6 was found to appear in about 19.7 and 44.3 ps for 1a (A) and 2a (A), respectively, before the generation of an ortho‐substituted aniline (1b and 2b) via the excited‐state deprotonation of 6. Thus, a logical photodynamic mechanism of this photoinduced Hofmann‐Martius rearrangement reaction was deduced. This new insight into the reaction mechanisms may be helpful for the design of novel related photoactivatable aniline molecules and for understanding other similar photorearrangement reaction mechanisms.
Femtosecond and nanosecond time‐resolved transient absorption (fs‐TA and ns‐TA) spectra were obtained to investigate the reaction mechanism of an intriguing Hofmann‐Martius photorearrangement reaction that occurred after photoexcitation of some (8‐cyano‐7‐hydroxyquinolin‐2‐yl)methyl (CyHQ)‐protected dialkylanilines. The heterolysis of the C‐N bond was found to take place in around 2.5 ps to generate the CyHQ+ and the corresponding aniline in their singlet excited states. Then, they recombined in around 19 ps to produce the singlet excited state of the recombined species as observed in the fs‐TA experiments and supported by results from quantum chemical calculations.
Ultrasmall copper nanoclusters have recently emerged as promising photocatalysts for organic synthesis, owing to their exceptional light absorption ability and large surface areas for efficient ...interactions with substrates. Despite significant advances in cluster-based visible-light photocatalysis, the types of organic transformations that copper nanoclusters can catalyze remain limited to date. Herein, we report a structurally well-defined anionic Cu
nanocluster that emits in the second near-infrared region (NIR-II, 1000-1700 nm) after photoexcitation and can conduct single-electron transfer with fluoroalkyl iodides without the need for external ligand activation. This photoredox-active copper nanocluster efficiently catalyzes the three-component radical couplings of alkenes, fluoroalkyl iodides, and trimethylsilyl cyanide under blue-LED irradiation at room temperature. A variety of fluorine-containing electrophiles and a cyanide nucleophile can be added onto an array of alkenes, including styrenes and aliphatic olefins. Our current work demonstrates the viability of using readily accessible metal nanoclusters to establish photocatalytic systems with a high degree of practicality and reaction complexity.
Cocrystal engineering is an efficient and simple strategy to construct functional materials, especially for the exploitation of novel and multifunctional materials. Herein, we report two kinds of ...nucleic-acid-base cocrystal systems that imitate the strong hydrogen bond interactions constructed in the form of complementary base pairing. The two cocrystals studied exhibit different colors of phosphorescence from their monomeric counterparts and show the feature of rare high-temperature phosphorescence. Mechanistic studies reveal that the strong hydrogen bond network stabilizes the triplet state and suppresses non-radiative transitions, resulting in phosphorescence even at 425 K. Moreover, the isolation effects of the hydrogen bond network regulate the interactions between the phosphor groups, realizing the manipulation from aggregation to single-molecule phosphorescence. Benefiting from the long-lived triplet state with a high quantum yield, the generation of reactive oxygen species by energy transfer is also available to utilize for some applications such as in photodynamic therapy and broad-spectrum microbicidal effects. In vitro experiments show that the cocrystals efficiently kill bacteria on a tooth surface and significantly help prevent dental caries. This work not only provides deep insight into the relationship of the structure-properties of cocrystal systems, but also facilitates the design of multifunctional cocrystal materials and enriches their potential applications.
The self‐assembled fluorogen activating protein (FAP)‐malachite green (MG) complex is a well‐established protein‐ligand system, which can realize binding‐caused fluorescence turn‐on of MG and singlet ...oxygen (1O2) generation by MG iodination. To clarify the mechanism of fluorescence activation and 1O2 generation, the photodynamics of different halogen‐substituted MG derivatives and their corresponding FAP‐MG complexes were studied by femtosecond transient absorption spectroscopy and theoretical computations. The results show that the rotation of MG is restricted by FAP binding, which prevents a rapid internal conversion to allow a longer lifetime for the excited MG to undergo fluorescence emission and intersystem crossing. Moreover, these FAP‐MG complexes exhibit notably varied fluorescence quantum yields (ΦFL) and 1O2 yields. The study on the decay pathways indicates that such an anti‐heavy atom effect predominately stems from the lifetimes of the excited‐state species. The photodynamic mechanism study here will lead to more advanced FAP‐MG systems with high spatiotemporal resolution.
Malachite green will rapidly decay to ground state via internal conversion after excitation, since the free rotation significantly decreases the energy level of the excited state. However, after binding with the fluorogen activating protein, the interaction will greatly restrict the excited‐state rotation and allow the fluorescence turn‐on and 1O2 generation.
In this review, noncovalent functionalization of single-wall carbon nanotubes (SWCNTs) is briefly reviewed. The functional materials summarized here include metalloporphyrin derivatives, biomolecules ...and conjugated polymers. Notably, time-resolved spectroscopic techniques such as time-resolved fluorescence and transient absorption were employed to directly investigate the electron transfer and recombination processes between the functionalities and the SWCNTs. In addition, Raman spectroscopy is also useful to identify the interaction and the electron transfer direction between both the functionalities and the SWCNTs. An improved understanding of the mechanisms of these SWCNT-based nanohybrids in terms of their structural and photophysical properties can provide more insights into the design of new electronic materials.
The realization of photocatalysis for practical synthetic application hinges on the development of inexpensive photocatalysts which can be prepared on a large scale. Herein an air-stable, ...visible-light-absorbing photoluminescent tungsten(
vi
) complex which can be conveniently prepared at the gram-scale is described. This complex could catalyse photochemical organic transformation reactions including borylation of aryl halides, such as aryl chloride, reductive coupling of benzyl bromides for C-C bond formation, reductive coupling of phenacyl bromides, and decarboxylative coupling of redox-active esters of alkyl carboxylic acid with high product yields and broad functional group tolerance.
A luminescent tungsten(
vi
) complex catalyses a broad spectrum of light-driven organic transformation reactions with high product yields and good functional group tolerance.
By employing broadband femtosecond Kerr-gated time-resolved fluorescence (KTRF) and transient absorption (TA) techniques, we report the first (to our knowledge) femtosecond combined time- and ...wavelength-resolved study on an ultraviolet-excited nucleoside and a single-stranded oligonucleotide (namely adenosine (Ado) and single-stranded adenine oligomer (dA)20) in aqueous solution. With the advantages of the ultrafast time resolution, the broad spectral and temporal probe window, and a high sensitivity, our KTRF and TA results enable the real time monitoring and spectral characterization of the excited-state relaxation processes of the Ado nucleoside and (dA)20 oligonucleotide investigated. The temporal evolution of the 267 nm excited Ado KTRF spectra indicates there are two emitting components with lifetimes of ∼0.13 ps and ∼0.45 ps associated with the L a and L b ππ* excited states, respectively. These Ado results reveal no obvious evidence for the involvement of the nπ* state along the irradiative internal conversion pathway. A distinct mechanism involving only the two ππ* states has been proposed for the ultrafast Ado deactivation dynamics in aqueous solution. The time dependence of the 267 nm excited (dA)20 KTRF and TA spectra reveals temporal evolution from an ultrafast “A-like” state (with a ∼0.39 ps decay time) to a relatively long-lived E1 “excimer” (∼4.3 ps decay time) and an E2 “excimer-like” (∼182 ps decay time) state. The “A-like” state has a spectral character closely resembling the excited state of Ado. Comparison of the spectral evolution between the results for Ado and (dA)20 provides unequivocal evidence for the local excitation character of the initially photoexcited (dA)20. The rapid transformation of the locally excited (dA)20 component into the delocalized E1 “excimer” state which then further evolves into the E2 “excimer-like” state indicates that base stacking has a high ability to modify the excited-state deactivation pathway. This modification appears to occur by suppressing the internal conversion pathway of an individually excited base component where the stacking interaction mediates efficient interbase energy transfer and promotes formation of the collective excited states. This feature of the local excitation that is subsequently followed by rapid energy delocalization into nearby bases may occur in many base multimer systems. Our results provide an important new contribution to better understanding DNA photophysics.
Non‐alternant topologies have attracted considerable attention due to their unique physiochemical characteristics in recent years. Here, three novel topological nanographenes molecular models of ...nitrogen‐doped Stone–Thrower–Wales (S–T–W) defects were achieved through intramolecular direct arylation. Their chemical structures were unambiguously elucidated by single‐crystal analysis. Among them, threefold intramolecular direct arylation compound (C42H21N) is the largest nanographene bearing a N‐doped non‐alternant topology to date, in which the non‐benzenoid rings account for 83 % of the total molecular skeleton. The absorption maxima of this compound was located in the near‐infrared region with a long tail up to 900 nm, which was much longer than those reported for similarly sized N‐doped nanographene with six‐membered rings (C40H15N). In addition, the electronic energy gaps of these series compounds clearly decreased with the introduction of non‐alternant topologies (from 2.27 eV to 1.50 eV). It is noteworthy that C42H21N possesses such a low energy gap (Egopt=1.40 eV; Egcv=1.50 eV), yet is highly stable under ambient conditions. Our work reported herein demonstrates that the non‐alternant topology could significantly influence the electronic configurations of nanocarbons, where the introduction of a non‐alternanting topology may be an effective way to narrow the energy gap without extending the molecular π‐conjugation.
Saddle‐shaped nanographenes containing N‐doped Stone‐Thrower‐Wales topological defects have been synthesized that possess much narrower energy gaps than similar‐sized N‐doped nanographenes with six‐membered rings. This work indicates that the introduction of non‐alternant topologies is an efficient way to narrow the energy gap without extending the molecular size.
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