Acid/base-triggered photophysical and chiroptical switching in a series of helicenoid compounds

International audienceA series of molecules that possess two quinolines, benzoquinolines, or phenanthrolines connected in a chiral fashion by a biaryl junction along with their water-soluble derivatives was developed and characterized. The influence of the structure on the basicity of the nitrogen atoms in two heterocycles was examined and the photophysical and chiroptical switching activity of the compounds upon protonation was studied both experimentally and computationally. The results demonstrated that changes in the electronic structure of the protonated vs. neutral species, promoting a bathochromic shift of dominant electronic transitions and alternation of their character from π-to-π* to charge-transfer-type, when additionally accompanied by the high structural flexibility of a system, leading to changes in conformational preferences upon proton binding, produce particularly pronounced modifications of the spectral properties in acidic medium. The latter combined with reversibility of the read-out make some of the molecules in this series very promising multifunctional pH probes

Theoretical characterisation of the excited states of pyromellitic diimide and modelling of its UV-VIS and MCD spectra

Dokonano charakterystyki stanu podstawowego i nisko leżących stanów wzbudzonych cząsteczki diimidu kwasu 1,2,4,5-benzenotetrakarboksylowego (1) oraz jego N,N’ dicykloheksylowej pochodnej (2) w oparciu o wyniki obliczeń TDDFT i CC2. Zbadano wpływ szeregu czynników: konfiguracji podwójnie wzbudzonych, oddziaływań z rozpuszczalnikiem oraz stanów rydbergowskich na dokładność obliczeń. Na potrzeby pracy zsyntezowano diimid 2 oraz wykonano pomiary widm absorpcyjnych i magnetycznego dichroizmu kołowego (MCD) dla 1 i 2. Modelowanie widm absorpcyjnych oparte o wyniki obliczeń CC2 doprowadziło do niemal ilościowej reprodukcji widma doświadczalnego dla 1. W przypadku parametryzacji opartej o wyniki DFT zgodność z eksperymentem była niewiele gorsza. Dla 2, ze względu na większe rozmiary i niższą symetrię cząsteczki, jakość reprodukcji widma doświadczalnego była nieco niższa. Modelowania widm MCD dokonano jedynie dla 2 ze względu na wątpliwe dane eksperymentalne dla 1. Zgodność widma teoretycznego (obliczonego dla modelu 4 stanów elektronowych i 4 drgań) z eksperymentem jest bardzo dobra. Na podstawie porównania teoretycznego i eksperymentalnego widma MCD dokonano weryfikacji przewidywań teoretycznych dotyczących intensywności dwóch najniżej leżących stanów wzbudzonych oraz wprowadzono poprawki do uzyskanych wcześniej energii wzbudzeń wynikających wprost z obliczeń kwantowochemicznych.The low-lying excited states of 1,2,4,5-benzenotetracarboxy diimide (1) and its N,N’-dicyclohexyl derivative (2) were characterized by means of TDDFT and CC2 calculations. The influence of the doubly excited configurations on the excitation energies, the solvent effects, and the possibility of the Rydberg-valence mixing were carefully examined. For the purpose of this work diimide 2 was synthesized and the absorption and magnetic circular dichroism spectra (MCD) of 1 and 2 were obtained. The absorption spectrum based on CC2 calculations is in nearly quantitative agreement with experiment for 1. In case of TDDFT parametrization, this agreement is only slightly worse. For 2, the theoretical spectra still contains all the important features of their experimental counterparts, even though quantitatively the quality of reproduction is somewhat lower due to larger size and lower symmetry of the molecule. The simulation of MCD spectra were performed only for 2, owing to the lack of credible experimental results for 1. The agreement between the theoretical (calculated for 4 states and 4 vibrations model) spectrum and the experimental one is very good. Their comparison led to verification of the theoretical predictions concerning the intensity of two lowest optically active transitions and introduction of small corrections to the calculated energies of the higher excited states

Excited states manifold of 2,2'-bithiophene : basis set dependence study

Aromaticity is a concept invented to account for the unusual stability of an important class of organic molecules: the aromatic compounds. Here we report experimental and theoretical evidence of aromaticity in all-metal systems. A series of bimetallic clusters with chemical composition MAl4- (M=Li, Na or Cu) was created and studied with photoelectron spectroscopy and ab initio calculations. All the MAl4- species posses a pyramidal structure containing an M+ cation interacting with a square Al42- unit. Ab initio studies indicate that Al42- exhibits characteristics of aromaticity with two delocalized p electrons (thus following the (4n+2) p electron counting rule) and a square planar structure and maintains its structural and electronic features in all the MAl4-complexes. These findings expand the aromaticity concept into the arena of all-metal species

The role of the long-range exchange corrections in the description of electron delocalization in aromatic species

In this article, we address the role of the long-range exchange corrections in description of the cyclic delocalization of electrons in aromatic systems at the density functional theory level. A test set of diversified monocyclic and polycyclic aromatics is used in benchmark calculations involving various exchange-correlation functionals. A special emphasis is given to the problem of local aromaticity in acenes, which has been a subject of long-standing debate in the literature. The presented results indicate that the noncorrected exchange-correlation functionals significantly overestimate cyclic delocalization of electrons in heteroaromatics and aromatic systems with fused rings, which in the case of acenes leads to conflicting local aromaticity predictions from different criteriaThe research was supported in part by the Foundation for Polish Science (FNP START 2015, stipend 103.2015, DS), National Science Centre, Poland (NCN SONATA, grant 2015/17/D/ST4/ 00558, DS) as well as the PL-Grid Infrastructure of the Academic Computer Centre CYFRONET, with the calculations performed on cluster platforms Zeus and Prometheus. MS thanks for the support of the Ministerio de Economía y Competitividad of Spain (Project CTQ2014-54306-P), Generalitat de Catalunya (project number 2014SGR931, Xarxa de Referència en Química Teòrica i Computacional, and ICREA Academia prize), and European Fund for Regional Development (FEDER grant UNGI10-4E-801