Conjugated polymers composed of tricoordinate boron and π-conjugated units possess extended conjugation with relatively low-lying LUMOs arising from p B –π interactions. However, donor–acceptor (D–A) polymers that feature triorganoboranes alternating with highly electron-rich donors remain scarce. We present here a new class of hybrid D–A polymers that combine electron-rich dithienosiloles or dithienogermoles with highly robust tricoordinate borane acceptors. Polymers of modest to high molecular weight are readily prepared by Pd-catalyzed Stille coupling reaction of bis(halothienyl)boranes and distannyldithienosiloles or -germoles. The polymers are obtained as dark red solids that are stable in air and soluble in common organic solvents. Long wavelength UV-vis absorptions at ca. 500–550 nm indicate effective π-conjugation and pronounced D–A interactions along the backbone. The emission maxima occur at wavelengths longer than 600 nm in solution and experience further shifts to lower energy with increasing solvent polarity, indicative of strong intramolecular charge transfer (ICT) character of the excited state. The powerful acceptor character of the borane comonomer units in the polymer structures is also evident from cyclic voltammetry (CV) analyses that reveal relatively low-lying LUMO levels of the polymers, enhancing the D–A interaction. Density functional theory (DFT) calculations on model oligomers further support these experimental observations.
more »
« less
Donor acceptor fluorophores: synthesis, optical properties, TD-DFT and cytotoxicity studies
Donor–π-acceptor (D–π-A) fluorophores consisting of a donor unit, a π linker, and an acceptor moiety have attracted attention in the last decade. In this study, we report the synthesis, characterization, optical properties, TD-DFT, and cytotoxicity studies of 17 near infrared (NIR) D–π-A analogs which have not been reported so far to the best of our knowledge. These fluorophores have chloroacrylic acid as the acceptor unit and various donor units such as indole, benzothiazole, benzo[ e ]indole, and quinoline. The fluorophores showed strong absorption in the NIR (700–970 nm) region due to their enhanced intramolecular charge transfer (ICT) between chloroacrylic acid and the donor moieties connected with the Vilsmeier–Haack linker. The emission wavelength maxima of the fluorophores were in between 798 and 870 nm. Compound 20 with a 4-quinoline donor moiety showed an emission wavelength above 1000 nm in the NIR II window. The synthesized fluorophores were characterized by 1 H NMR and 13 C NMR, and their optical properties were studied. Time dependent density functional theory (TD-DFT) calculations showed that the charge transfer occurs from the donor groups (indole, benzothiazole, benzo[ e ]indole, and quinoline) to the acceptor chloroacrylic acid moiety. Fluorophores with [HOMO] to [LUMO+1] transitions were shown to possess a charge separation character. The cytotoxicity of selected fluorophores, 4 , 7 , 10 and 12 was investigated against breast cancer cell lines and they showed better activity than the anti-cancer agent docetaxel.
more »
« less
- Award ID(s):
- 2018144
- NSF-PAR ID:
- 10230172
- Date Published:
- Journal Name:
- Organic & Biomolecular Chemistry
- Volume:
- 19
- Issue:
- 8
- ISSN:
- 1477-0520
- Page Range / eLocation ID:
- 1835 to 1846
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
more » « less
Abstract The synthesis and reactivity of 3,8‐dibromo‐dodecafluoro‐benzo‐fused BOPHY
2 are reported, via SNAr with O‐, N‐ S‐ and C‐nucleophiles, and in Pd(0)‐catalyzed cross‐coupling reactions (Suzuki and Stille). The resulting perfluoro‐BOPHY derivatives were investigated for their reactivity in the presence of various nucleophiles. BOPHY3 displays reversible color change and fluorescence quenching in the presence of bases (Et3N, DBU), whereas BOPHY7 reacts preferentially at the α‐pyrrolic positions, and BOPHY8 undergoes regioselective fluorine substitution in the presence of thiols. The structural and electronic features of the fluorinated BOPHYs were studied by TD‐DFT computations. In addition, their spectroscopic and cellular properties were investigated; BOPHY10 shows the most red‐shifted absorption/emission (λ max659/699 nm) and7 the highest fluorescence (Φ f=0.95), while all compounds studied showed low cytotoxicity toward human HEp2 cells and were efficiently internalized. -
Hydrogen-rich cation radicals (GATT + 2H) + ˙ and (AGTT + 2H) + ˙ represent oligonucleotide models of charged hydrogen atom adducts to DNA. These tetranucleotide cation radicals were generated in the gas phase by one-electron reduction of the respective (GATT + 2H) 2+ and (AGTT + 2H) 2+ dications in which the charging protons were placed on the guanine and adenine nucleobases. We used wavelength-dependent UV/Vis photodissociation in the valence-electron excitation region of 210–700 nm to produce action spectra of (GATT + 2H) + ˙ and (AGTT + 2H) + ˙ that showed radical-associated absorption bands in the near-UV (330 nm) and visible (400–440 nm) regions. Born–Oppenheimer molecular dynamics and density-functional theory calculations were used to obtain and rank by energy multiple (GATT + 2H) dication and cation-radical structures. Time-dependent density functional theory (TD-DFT) calculations of excited-state energies and electronic transitions in (GATT + 2H) + ˙ were augmented by vibronic spectra calculations at 310 K for selected low-energy cation radicals to provide a match with the action spectrum. The stable product of one-electron reduction was identified as having a 7,8-dihydroguanine cation radical moiety, formed by intramolecular hydrogen atom migration from adenine N-1–H. The hydrogen migration was calculated to have a transition state with a low activation energy, E a = 96.5 kJ mol −1 , and positive activation entropy, Δ S ‡ = 75 J mol −1 K −1 . This allowed for a fast isomerization of the primary reduction products on the ion-trap time scale of 150 ms that was substantially accelerated by highly exothermic electron transfer.more » « less
-
more » « less
Abstract A panchromatic triad, consisting of benzothiazole (BTZ) and BF2‐chelated boron‐dipyrromethene (BODIPY) moieties covalently linked to a zinc porphyrin (ZnP) core, has been synthesized and systematically characterized by using1H NMR spectroscopy, ESI‐MS, UV‐visible, steady‐state fluorescence, electrochemical, and femtosecond transient absorption techniques. The absorption band of the triad, BTZ‐BODIPY‐ZnP, and dyads, BTZ‐BODIPY and BODIPY‐ZnP, along with the reference compounds BTZ‐OMe, BODIPY‐OMe, and ZnP‐OMe exhibited characteristic bands corresponding to individual chromophores. Electrochemical measurements on BTZ‐BODIPY‐ZnP exhibited redox behavior similar to that of the reference compounds. Upon selective excitation of BTZ (≈290 nm) in the BTZ‐BODIPY‐ZnP triad, the fluorescence of the BTZ moiety is quenched, due to photoinduced energy transfer (PEnT) from1BTZ*to the BODIPY moiety, followed by quenching of the BODIPY emission due to sequential PEnT from the1BODIPY* moiety to ZnP, resulting in the appearance of the ZnP emission, indicating the occurrence of a two‐step singlet–singlet energy transfer. Further, a supramolecular tetrad, BTZ‐BODIPY‐ZnP:ImC60, was formed by axially coordinating the triad with imidazole‐appended fulleropyrrolidine (ImC60), and parallel steady‐state measurements displayed the diminished emission of ZnP, which clearly indicated the occurrence of photoinduced electron transfer (PET) from1ZnP* to ImC60. Finally, femtosecond transient absorption spectral studies provided evidence for the sequential occurrence of PEnT and PET events, namely,1BTZ*‐BODIPY‐ZnP:ImC60→BTZ‐1BODIPY*‐ZnP:ImC60→BTZ‐BODIPY‐1ZnP*:ImC60→BTZ‐BODIPY‐ZnP.+:ImC60.−in the supramolecular tetrad. The evaluated rate of energy transfer,
k EnT, was found to be 3–5×1010 s−1, which was slightly faster than that observed in the case of BODIPY‐ZnP and BTZ‐BODIPY‐ZnP, lacking the coordinated ImC60. The rate constants for charge separation and recombination,k CSandk CR, respectively, calculated by monitoring the rise and decay of C60.−were found to be 5.5×1010and 4.4×108 s−1, respectively, for the BODIPY‐ZnP:ImC60triad, and 3.1×1010and 4.9×108 s−1, respectively, for the BTZ‐BODIPY‐ZnP:ImC60tetrad. Initial excitation of the tetrad, promoting two‐step energy transfer and a final electron‐transfer event, has been successfully demonstrated in the present study. -
Electron transfer (ET) in donor–bridge–acceptor (DBA) compounds depends strongly on the structural and electronic properties of the bridge. Among the bridges that support donor–acceptor conjugation, alkyne bridges have attractive and unique properties: they are compact, possess linear structure permitting access to high symmetry DBA molecules, and allow torsional motion of D and A, especially for longer bridges. We report conformation dependent electron transfer dynamics in a set of novel DBA compounds featuring butadiyne (C4) bridge, N -isopropyl-1,8-napthalimide (NAP) acceptors, and donors that span a range of reduction potentials (trimethyl silane (Si-C4-NAP), phenyl (Ph-C4-NAP), and dimethyl aniline (D-C4-NAP)). Transient mid-IR absorption spectra of the CC bridge stretching modes, transient spectra in the visible range, and TD-DFT calculations were used to decipher the ET mechanisms. We found that the electronic excited state energies and, especially, the transition dipoles (S 0 → S n ) depend strongly on the dihedral angle ( θ ) between D and A and the frontier orbital symmetry, offering an opportunity to photo-select particular excited states with specific ranges of dihedral angles by exciting at chosen wavelengths. For example, excitation of D-C4-NAP at 400 nm predominantly prepares an S 1 excited state in the planar conformations ( θ ∼ 0) but selects an S 2 state with θ ∼ 90°, indicating the dominant role of the molecular symmetry in the photophysics. Moreover, the symmetry of the frontier orbitals of such DBA compounds not only defines the photo-selection outcome, but also determines the rate of the S 2 → S 1 charge separation reaction. Unprecedented variation of the S 2 –S 1 electronic coupling with θ by over four orders of magnitude results in slow ET at θ ca. 0° and 90° but extremely fast ET at θ of 20–60°. The unique features of high-symmetry alkyne bridged DBA structures enable frequency dependent ET rate selection and make this family of compounds promising targets for the vibrational excitation control of ET kinetics.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/D0OB02313B
