Modular Synthesis of Bipyridinium Oligomers and Corresponding Donor-Acceptor Oligorotaxanes with Crown Ethers

Donor-acceptor [4]- and [6]rotaxanes have been prepared from bipyridinium (BIPY2+) oligomers and 1,5-dinaphtho[38]crown-10 (DN38C10) by a threading-followed-by-stoppering protocol employing click chemistry. An efficient, straightforward route to the BIPY2+ oligomers has been developed that requires little to no chromatographic purification. Unlike most donor-acceptor oligorotaxanes that have been reported to date, 100% of the recognition sites on the dumbbells are occupied by rings

Estimating chemical reactivity and cross-influence from collective chemical knowledge

Although modern chemical databases store a great wealth of structural and reactivity data, this vast "universe'' of chemical information has not yet been systematically analyzed. Here, we use computers to derive from the entire body of organic-chemical knowledge the indices that estimate the reactivity and cross influence of functional groups. The major premise of our approach is that in sufficiently large and diverse collections of reactions (as the entire "history'' of organic chemistry is), the frequencies with which transformations of certain groups occur, reflect their reactivities. Illustrative examples spanning several classes of reactions demonstrate that our knowledge-based indices capture the well-known reactivity trends. A free-access software is also developed with which other trends can be analyzed for various combinations of functional groups

Parallel Optimization of Synthetic Pathways within the Network of Organic Chemistry

Finding a needle in a haystack : The number of possible synthetic pathways leading to the desired target of a synthesis can be astronomical (1019 within five synthetic steps). Algorithms are described that navigate through the entire known chemical???synthetic knowledge to identify optimal synthetic pathways. Examples are provided to illustrate single???target optimization and parallel optimization of syntheses leading to multiple targets

Strong Steric Hindrance Effect on Excited State Structural Dynamics of Cu(I) Diimine Complexes

The metal-to-ligand-charge-transfer (MLCT) excited state of Cu­(I) diimine complexes is known to undergo structural reorganization, transforming from a pseudotetrahedral <i>D</i>_2<i>d</i> symmetry in the ground state to a flattened <i>D</i>₂ symmetry in the MLCT state, which allows ligation with a solvent molecule, forming an exciplex intermediate. Therefore, the structural factors that influence the coordination geometry change and the solvent accessibility to the copper center in the MLCT state could be used to control the excited state properties. In this study, we investigated an extreme case of the steric hindrance caused by attaching bulky <i>tert</i>-butyl groups in bis­(2,9-di-<i>tert</i>-butyl-1,10-phenanthroline)­copper­(I), [Cu^I(dtbp)₂]⁺. The two bulky <i>tert</i>-butyl groups on the dtbp ligand lock the MLCT state into the pseudotetrahedral coordination geometry and completely block the solvent access to the copper center in the MLCT state of [Cu^I(dtbp)₂]⁺. Using ultrafast transient absorption spectroscopy and time-resolved emission spectroscopy, we investigated the MLCT state property changes due to the steric hindrance and demonstrated that [Cu^I(dtbp)₂]⁺ exhibited a long-lived emission but no subpicosecond component that was previously assigned as the flattening of the pseudotetrahedral coordination geometry. This suggests the retention of its pseudotetrahedral <i>D</i>_2<i>d</i> symmetry and the blockage of the solvent accessibility. We made a comparison between the excited state dynamics of [Cu^I(dtbp)₂]⁺ with its mono-<i>tert</i>-butyl counterpart, bis­(2-<i>tert</i>-butyl-1,10-phenanthroline)­copper­(I) [Cu^I(tbp)₂]⁺. The subpicosecond component assigned to the flattening of the <i>D</i>_2<i>d</i> coordination geometry in the MLCT excited state was again present in the latter because the absence of a <i>tert</i>-butyl on the phenanthroline allows flattening to the pseudotetrahedral coordination geometry. Unlike the [Cu^I(dtbp)₂]⁺, [Cu^I(tbp)₂]⁺ exhibited no detectable emission at room temperature in solution. These results provide new insights into the manipulation of various excited state properties in Cu diimine complexes by certain key structural factors, enabling optimization of these systems for solar energy conversion applications

Effects of Electronic and Nuclear Interactions on the Excited-State Properties and Structural Dynamics of Copper(I) Diimine Complexes

The effects of structural constraints on the metal-to-ligand charge transfer (MLCT) excited state structural dynamics of cuprous bis-2,9-diphenyl-phenanthroline ([Cu­(I)­(dpp)₂]⁺) in both coordinating acetonitrile and noncoordinating toluene were studied using X-ray transient absorption (XTA) spectroscopy and density functional theory (DFT) calculations. The phenyl groups attached to the phenanthroline ligands not only effectively shield the Cu­(I) center from solvent molecules, but also force a flattened tetrahedral coordination geometry of the Cu­(I) center. Consequently, the MLCT state lifetime in [Cu­(I)­(dpp)₂]⁺ is solvent-independent, unlike the previously studied 2,9-methyl substituted bis-phenanthroline Cu­(I) complex. The MLCT state of [Cu­(I)­(dpp)₂]⁺ still undergoes a “pseudo Jahn-Teller distortion,” with the angle between the two phenanthroline ligand planes decreased further by 7°. The XTA results indicate that, in the MLCT excited state of [Cu­(I)­(dpp)₂]⁺, the phenyls at the 2, 9 positions of the phenanthroline rotate, breaking the π–π interaction with the phenanthroline ligands without ever rotating in-plane with the phenanthroline ligands. Hence, the transferred electron density from the Cu­(I) center is localized on the phenanthroline moiety with no charge density present on the phenyl rings. The insight about the effect of the structural constraints on the MLCT state properties will guide the design of Cu­(I) diimine complexes with suitable excited-state properties to function as earth-abundant dye sensitizers for solar electricity generation