NBN- and BNB-Phenalenyls: the Yin and Yang of Heteroatom-doped π Systems.

NBN- and BNB-doped phenalenyls are isoelectronic to phenalenyl anions and cations, respectively. They represent a pair of complementary molecules that have essentially identical structures but opposite properties as electron donors and acceptors. The NBN-phenalenyls 1-4 considered here were prepared from N,N'-dimethyl-1,8-diaminonaphthalene and readily available boron-containing building blocks (i. e., BH3⋅SMe2 (1), p-CF3-C6H4B(OH)2 (2), C6H5B(OH)2 (3), or MesBCl2/iPr2NEt (4)). Treatment of 1 with 4-Me2N-2,6-Me2-C6H2Li gave the corresponding NBN derivative 5. The BNB-phenalenyl 6 was synthesized from 1,8-naphthalenediyl-bridged diborane(6), PhNH2, and MesMgBr. A computational study reveals that the photoemission of 1, 4, and 5 originates from locally excited (LE) states at the NBN-phenalenyl fragments, while that of 2 is dominated by charge transfer (CT) from the NBN-phenalenyl to the p-CF3-C6H4 fragment. Depending on the dihedral angle θ between its Ph and NBN planes, compound 3 emits mainly from a less polar LE (θ >55°) or more polar CT state (θ <55°). In turn, the energetic preference for either state is governed by the polarity of the solvent used. An equimolar aggregate of the NBN- and BNB-phenalenyls 3 and 6 (in THF/H2O) shows a distinct red-shifted emission compared to that of the individual components, which originates from an intermolecular CT state.

BNB-Doped Phenalenyls: Modular Synthesis, Optoelectronic Properties, and One-Electron Reduction.

A highly modular synthesis of BNB- and BOB-doped phenalenyls is presented. Treatment of the 1,8-naphthalenediyl-bridged boronic acid anhydride 1 with LiAlH4/Me3SiCl afforded the corresponding 1,8-naphthalenediyl-supported diborane(6) 2, which served as the starting material for all subsequent transformations. Upon addition of MesMgBr/Me3SiCl, 2 was readily converted to the tetraorganyl diborane(6) 5. The further heteroatoms were finally introduced through the reaction of 2 with (Me3Si)2NR' or 5 with H2NR' or H2O (R' = H, Me, p-Tol). A helically twisted, fully BNB-embedded PAH 11 was prepared by combining 2 with a dibrominated m-terphenylamine, followed by a Grignard-mediated double ring-closure reaction. All compounds devoid of B-H bonds show favorable optoelectronic properties, such as luminescence and reversible reduction behavior. In the case of the BNB-phenalenyl 7 (BMes, NMe), the radical-anion salt K[7•] was generated through chemical reduction with K metal and characterized by EPR spectroscopy. K[7•] is not long-term stable in a THF/c-hexane solution, but abstracts an H atom with formation of the diamagnetic BNB-doped 1H-phenalene K[7H].

On the molecular mechanism of non-radiative decay of nitrobenzene and the unforeseen challenges this simple molecule holds for electronic structure theory.

In this work, we present a complete mechanistic picture of the non-radiative decay of the mono-substituted aromatic compound nitrobenzene from the bright singlet state to the electronic ground state. This mechanism involves internal conversion (IC) and inter-system crossing (ISC) along three dominating internal coordinates of the nitro group and consistently explains the experimental findings. Relaxation from the lowest triplet state via ISC occurs along the out-of-plane bending coordinate of the nitro group, while initial IC as well as ultrafast ISC into the triplet manifold take place along symmetric NO stretching and ONO bending modes that have not been considered yet. The proposed mechanism is based on high-level single- and multi-reference electronic structure calculations employing ADC3, MOM-CCSD(T), EOM-CCSD, DFT/MRCI and CAS-SCF/NEVPT2 levels of theory, which is, as we will demonstrate, absolutely necessary to assure a reliable and sufficiently accurate theoretical description of nitrobenzene. The need for third-order methods will be traced back to the large double-excitation character of about 50% of the second excited singlet state of nitrobenzene. As a result, second-order methods like approximate coupled-cluster of second order (CC2) and partially even (EOM-)CCSD yield a qualitatively wrong picture of the excited states. Surprisingly, already the description of the ground state geometries is problematic at the CC2 and partially also CCSD level of theory.

Combined theoretical and experimental investigation of the photodecarboxylation of nitrophenylacetates and its implications for the design of improved ortho-nitrobenzylic caging groups.

The photodecarboxylation mechanism of different structural isomers of nitrophenylacetate (NPA) has been investigated using quantum chemical calculations as well as time-resolved UV-pump VIS-probe spectroscopy. It is shown that for a proper theoretical description of the excited states of anionic NPA in aqueous solution a careful consideration of the influence of the solvent is indispensable. In this sense, NPA is an example that demonstrates how character and lifetime of the involved excited states affect the results of equilibrium and nonequilibrium solvation approaches. An ultrafast decay channel via a repulsive singlet state has been found to be responsible for observed ultrafast CO(2) release, while another very efficient but slower CO(2) release channel is found to proceed via intersystem crossing and subsequent decay via a repulsive triplet state. After all, differences and similarities in the observed excited state dynamics of the isomers are conclusively explained. Most notably, the much smaller quantum yield of CO(2) release from the ortho-isomer is due to an alternative excited-state hydrogen-transfer channel, which occurs along a triplet and singlet pathway. On the basis of theoretical and experimental evidence suggesting that the multiplicity of the route taken determines the photoproduct yield, we provide guidelines for the design of ortho-nitrobenzylic caging groups with improved uncaging yield.

Excitation energy transfer and carotenoid radical cation formation in light harvesting complexes - a theoretical perspective.

Light harvesting complexes have been identified in all chlorophyll-based photosynthetic organisms. Their major function is the absorption of light and its transport to the reaction centers, however, they are also involved in excess energy quenching, the so-called non-photochemical quenching (NPQ). In particular, electron transfer and the resulting formation of carotenoid radical cations have recently been discovered to play an important role during NPQ in green plants. Here, the results of our theoretical investigations of carotenoid radical cation formation in the major light harvesting complex LHC-II of green plants are reported. The carotenoids violaxanthin, zeaxanthin and lutein are considered as potential quenchers. In agreement with experimental results, it is shown that zeaxanthin cannot quench isolated LHC-II complexes. Furthermore, subtle structural differences in the two lutein binding pockets lead to substantial differences in the excited state properties of the two luteins. In addition, the formation mechanism of carotenoid radical cations in light harvesting complexes LH2 and LH1 of purple bacteria is studied. Here, the energetic position of the S(1) state of the involved carotenoids neurosporene, spheroidene, spheroidenone and spirilloxanthin seems to determine the occurrence of radical cations in these LHCs upon photo-excitation. An elaborate pump-deplete-probe experiment is suggested to challenge the proposed mechanism.