Bromination of the widely used triplet sensitizer thioxanthone extends the absorption spectrum into the visible range with only minor loss of lowest triplet state energy (3 kcal/mol for di-bromination). Because of bromine substitution, a slight increase in triplet quantum yield was observed. The di-brominated derivative was effective as organo-photocatalyst in performing [2 + 2] cross-photocycloaddition of acrylimide-based compounds under visible light irradiation. | INTRODUCTIONThioxanthone 1 is a popular triplet sensitizer in photoinitiated reactions because of its high molar absorptivity in the near-UV spectral region (300-400 nm), high triplet quantum yield and relatively high triplet energy. [1,2] Thioxanthone and its derivatives are frequently used in organo-photocatalytic reactions [3,4] and as initiator in photopolymerization formulations. [5] Although 1 absorbs strongly in the near-UV spectral region (λ max = 381 nm), its absorption shows only a minor tailing in the visible region above 400 nm, which makes it only poorly suited as a sensitizer/catalyst for visible light illumination. We envisioned a simple structural modification of the thioxanthone scaffold that would result in a bathochromic shift without altering the triplet energy significantly. [6] A wide variety of thioxanthone derivatives have been reported where substituents not only shift the absorption bathochromically but also reduce the triplet energy significantly. [7][8][9][10][11] This prompted us to investigate the effect of brominating thioxanthone on its photochemical and photophysical properties, as we expected the heavy atom to facilitate a more-efficient intersystem crossing. [12][13][14][15][16][17] Unlike other substitutions (eg, amino substitution) that alters the triplet excited-state characteristics of thioxanthone, we believed that substituting bromine will be advantageous because of its dual nature of both electron donation (by resonance) and electron withdrawal (by induction) that will minimally alter the triplet state energy, while providing us an avenue to excite the molecule effectively with visible light. | RESULTS AND DISCUSSIONBrominated thioxanthones were synthesized by electrophilic bromination of parent thioxanthone 1. We found that the use of bromine in glacial acetic acid with catalytic amounts of iodine (Scheme 1) resulted in a mixture of mono-brominated thioxanthone 2 and di-brominated thioxanthone 3. The brominated derivatives were chromatographically separated and characterized by nuclear magnetic resonance (NMR) spectroscopy and mass spectrometry. The regioselectivity of bromination was ascertained by single crystal X-ray powder diffraction (XRD) analysis.[18] † This article is published as part of a special issue to celebrate the 80 th birthday of Professor Waldemar Adam

A novel fluorinated quinoxaline-based acceptor has been synthesized and then copolymerized with an electron-rich benzodithiophene derivative to yield a low band gap polymer (PBDT-QxF). A non-fluorinated analogue of the same polymer (PBDT-Qx) has also been synthesized in order to disclose the effect of fluorination on polymer properties. PBDT-QxF exhibits better thermal and oxidative stability compared to non-fluorinated analogue. Fluorine atoms induce crystalline domains in solid statepossibly as a result of favorable C−F•••H interactionswhereas such ordering is absent in PBDT-Qx. Principal component analysis on variable temperature absorption data collected in solution revealed a stabilization energy of ∼0.5 kcal mol −1 per repeat unit upon fluorination. Theoretical calculations predict higher oxidation potential for PBDT-QxF, which is confirmed by experimental data. Theoretical calculations also suggest inductive effect of fluorine atoms on electronic structure. The hole mobility of PBDT-QxF is also higher than that of PBDT-Qx. Overall, the studies show promising photovoltaic properties of this novel monomer if used in low band gap polymers for organic solar cell applications.

Photochemical transformations are a powerful tool in organic synthesis to access structurally complex and diverse synthetic building blocks. However, this great potential remains untapped in the mainstream synthetic community due to the challenges associated with stereocontrol originating from excited state(s). The finite lifetime of an excited state and nearly barrierless subsequent processes present significant challenges in manipulating the stereochemical outcome of a photochemical reaction. Several methodologies were developed to address this bottleneck including photoreactions in confined media and preorganization through noncovalent interactions resulting in stereoenhancement. Yet, stereocontrol in photochemical reactions that happen in solution in the absence of organized assemblies remained largely unaddressed. In an effort to develop a general and reliable methodology, our lab has been exploring non-biaryl atropisomers as an avenue to perform asymmetric phototransformations. Atropisomers are chiral molecules that arise due to the restricted rotation around a single bond (chiral axis) whose energy barrier to rotation is determined by nonbonding interactions (most often by steric hindrance) with appropriate substituents. Thus, atropisomeric substrates are chirally preorganized during the photochemical transformation and translate their chiral information to the expected photoproducts. This strategy, where "axial to point chirality transfer" occurs during the photochemical reaction, is a hybrid of the successful Curran's prochiral auxiliary approach involving atropisomers in thermal reactions and the Havinga's NEER principle (nonequilibrating excited-state rotamers) for photochemical transformations. We have investigated this strategy in order to probe various aspects such as regio-, enantio-, diastereo-, and chemoselectivity in several synthetically useful phototransformations including 6π-photocyclization, 4π-ring closure, Norrish-Yang photoreactions, Paternò-Büchi reaction, and [2 + 2]- and [5 + 2]-photocycloaddition. The investigations detailed in this Account clearly signify the scope of our strategy in accessing chirally enriched products during phototransformations. Simple design modifications such as tailoring the steric handle in atropisomers to hold reactive units resulted in permanently locked/traceless axial chirality in addition to incorporating multiple stereocenters in already complex scaffolds obtained from phototransformation. Further improvements allowed us to employ low energy visible light rather than high energy UV light without compromising the stereoenrichment in the photoproducts. Continued investigations on atropisomeric scaffolds have unraveled new design features, with outcomes that are unique and unprecedented for excited state reactivity. For example, we have established that reactive spin states (singlet or triplet excited state) profoundly influence the stereochemical outcome of an atropselective phototransformation. In general, the photochemistry and photophysics of atropisom...

Four novel donor-acceptor (D-A) alternating copolymers were designed and successfully synthesized by the palladium-catalyzed Stille coupling and Suzuki coupling reactions. Utilizing thieno [3,4-c]pyrrole-4,6-dione (TPD) as an acceptor comonomer coupled with dialkoxybithiophene or cyclopentadithiophene as the donor gave polymers PTBT and PTCT. Employing carbazole as the donor and the dithiophene-substituted TPD serving as the acceptor monomers yielded polymers PTC1 and PTC2. Owing to the various strengths of electronic coupling between the donors and the acceptor unit, the band gaps of these polymers can be adjusted from 1.57 to 1.90 eV. Due to the different electron-donor ability of dialkoxybithiophene, cyclopentadithiophene, and carbazole, the HOMO energy levels of polymers were tuned from 25.34 to 25.67 eV, while LUMO levels remained relatively unchanged. The theoretical calculations provided insight to the observed photophysical properties of these polymers. Theoretically estimated band gaps and oxidation potentials correlate well with the experimental data. Carrier mobility and photovoltaic properties of TPD polymers were also investigated for which 1.3% power conversion efficiency was obtained from a blend of PTCT:PC 71 BM (1 : 2) bulk-heterojunction device.

Nonbiaryl atropisomeric acrylimides underwent facile [2 + 2] photocycloaddition leading to cross-cyclobutane adducts with very high stereospecificity (enantiomeric excess (ee): 99% and diastereomeric excess (de): 99%). The photoreactions proceeded smoothly in isotropic media for both direct and triplet sensitized irradiations. The reactions were also found to be very efficient in the solid state where the same cross-cyclobutane adduct was observed. Photophysical studies enabled us to understand the excited-state photochemistry of acrylimides. The triplet energy was found to be ∼63 kcal/mol. The reactions proceeded predominantly via a singlet excited state upon direct irradiation with very poor intersystem crossing that was ascertained by quantification of the generated singlet oxygen. The reactions progressed smoothly with triplet sensitization with UV or visible-light irradiations. Laser flash photolysis experiments established the triplet transient of atropisomeric acrylimides with a triplet lifetime at room temperature of ∼40 ns.

Atropisomeric α-oxoamides were synthesized and employed for intramolecular Paternò-Büchi reaction leading to very high enantio- and diastereoselectivity in the bicyclic oxetane photoproduct. A reversal of product selectivity was observed in solution and in the solid-state.

Most traditional photoreactions require UV light to yield the desired product. To address this issue, photoreaction of hydrazide based chromophores was evaluated with visible light using a metal free photocatalyst to afford photoproducts in high yields. This hydrazide functionality itself may be removed/modified after the photoreaction, highlighting its role as a "photo-auxiliary". A preliminary mechanistic model based on photophysical experiments is provided to highlight the generality of the strategy.

Conjugate addition occurs efficiently from excited hydrazide based acrylanilides under both UV and metal free visible light irradiations. The reaction proceeds via an excited state encounter complex that bifurcates either via an electron or energy transfer pathway. The generality of excited state conjugate addition is demonstrated using chloromethylation and by thiol addition.