Experimental and calculated circular dichroism spectra of monoaza[5]helicenes

Abbate, Sergio; Bazzini, Cristina; Caronna, Tullio; Fontana, Francesca; Gangemi, Fabrizio; Lebon, France; Longhi, Giovanna; Sora, Isabella Natali

doi:10.1016/j.ica.2006.06.041

Cited by 19 publications

(22 citation statements)

References 17 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…Surprisingly,t he experimental data show the opposite (vide infra). These calculations concerning compounds 1, 2,a nd 8 corroborate the findings recently published in an extensive study on pyrido [5]-and pyrido [6]helicene by Stary et al [19] In particular,t he calculated activation barriers DG ¼ 6 298 K agree very well with the remarkablyl ow barrier calculated for compound 1 and geometrical parameters are comparable, including the intramolecular improper blue-shifting hydrogen bond established in the TS of compound 1. [16,20,21] Some key structural parameters of the transition-state structures obtained by DFT calculations at the B3LYP/6-311G(d,p) level, describing the distortion of the fused rings system from planarity,a re summarized in Ta ble 2.…”

Section: Quantumc Hemical Calculations(dft)supporting

confidence: 86%

“…As am odel for understanding the unexpected differenceb etween experimental DHPLC and calculated values in the gas phase for compound 1 (see Ta ble 1), the influence of the presence of an explicit solvent molecule [22] wasi nvestigated. Hydrogen bonding and protonation were also considered in related studies to explain the dimerization and proton affinities of 1-aza [6]helicene and 2-aza [6]helicene determined by mass spectrometry. [23,24] The formation of ac omplex between 1 in its minimum-energy conformation GS ando ne 2-propanol molecule via an OH···N hydrogen bond ( Figure 3) was simulated, resultingi na nO ( ÀH)···N distance of 2.95 ( shortert han the sum of Oa nd Nv an der Waals radii)a nd an OÀH···N angle of about 1508.B 3LYP/ 6-311G(d,p) and wB97X-D/6-311G(d,p) calculations estimate intermoleculari nteraction energies of 28.5 and 29.0 kJ mol À1 ,r espectively,t hus explaining the gap between the calculated and experimental energy barrier for compound 1.I nf act, the formation of TS implies the disruption of this hydrogen bond because the nonbonding pair of the Na tom oc- Chem.E ur.J.2015, 21,13919 -13924 www.chemeurj.org cupies as p 2 -hybridized orbital strongly coordinated with the HÀC (14) moiety.T he nonbonding Np air becomes ap oor hydrogen bond acceptor and it is no longera ble to coordinate with isopropanol molecules thus accounting for the energy differencec orresponding to the breakage of this hydrogen bond.…”

Section: Quantumc Hemical Calculations(dft)mentioning

confidence: 99%

“…As paramount examples, we have already reportedt he dependenceo ft he lifetime of the singlet and the triplet electronic states, ranging between 3.6 and 4.5 ns and 0.9 to 2.5 s, respectively, [4] as well as the half-life t of racemization [5] ranging from 7t o5 0min depending on the position of the nitrogen atom. [6] The preliminarily estimated interconversion barriers [5] and the importance to understands tructurala nd electronic properties demands ad etailed investigation of the stereodynamics of interconversion for the complete set of monoaza [5]helicenes 1-7 (Scheme 1) to be carried out both experimentally and by quantum chemical calculations. Theoreticalc alculations predict ac onsiderably lower racemization barrier for monoaza [5]helicene 1 compared to monoaza [5]helicenes 2-7 and [5]helicene 8.…”

Section: Introductionmentioning

confidence: 99%

See 2 more Smart Citations

A Combined Experimental and Theoretical Study on the Stereodynamics of Monoaza[5]helicenes: Solvent‐Induced Increase of the Enantiomerization Barrier in 1‐Aza‐[5]helicene

Caronna

Famulari

Mendola

et al. 2015

Chemistry A European J

View full text Add to dashboard Cite

Helicenes and heterohelicenes are attractive compounds with great potential in materials sciences to be used in optoelectronics as ligand backbones in enantioselective catalysis and as chiral sensors. The properties of these materials are related to the stereodynamics of these helical chiral compounds. However, little is known about features controlling stereodynamics in helicenes; in particular, for heterohelicenes the position of the heteroatom could be relevant in this respect. Herein the complete stereodynamic characterization of monoaza[5]helicenes is shown by enantioselective dynamic HPLC and DFT calculations. At variance with previous theoretical calculations, 1-aza[5]helicene shows a surprisingly high enantiomerization barrier, which is triggered by specific solvent interactions.

show abstract

Section: Quantumc Hemical Calculations(dft)supporting

confidence: 86%

Section: Quantumc Hemical Calculations(dft)mentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

A Combined Experimental and Theoretical Study on the Stereodynamics of Monoaza[5]helicenes: Solvent‐Induced Increase of the Enantiomerization Barrier in 1‐Aza‐[5]helicene

Caronna

Famulari

Mendola

et al. 2015

Chemistry A European J

View full text Add to dashboard Cite

show abstract

“…The half-life times were completed in 2007 for the all series of aza [5]helicenes. 85 Scheme 9. Synthesis of mono-and bis-aza- [5]helicenes investigated by oxidative photocyclization, except 35b 2 which was prepared by an acid cyclization process.…”

Section: Oxidative Photocyclizationmentioning

confidence: 99%

Enantioenriched Helicenes and Helicenoids Containing Main-Group Elements (B, Si, N, P)

2019

View full text Add to dashboard Cite

In this review, we discuss the rich chemistry of helicenes and helicenoids containing maingroup elements. Enantioenriched helicenic derivatives containing main-group elements B, Si, N and P, either incorporated within the helical backbone or grafted to it, will be thoroughly presented. We will describe their synthesis, resolution, and asymmetric synthesis, their structural features, electronic and chiroptical properties, emission, together with other photochemical properties and applications. OUTLINE 14 The absorption spectrum of 19c recorded in CH 2 Cl 2 solutions exhibited a maximum at 376 nm which is significantly red shifted compared with that of double [5]helicene 19a (310 nm) due to the more extended π-conjugation in 19c. According to TD-DFT calculations, the low-energy absorption band from 400 to 500 nm is assignable to the HOMO→LUMO transition corresponding to a π−π* transition of the fully conjugated bis-helical system. DFT calculations revealed a very high theoretical isomerization barrier of 45.1 kcal/mol for the (P,P)/(M,M)-19c stereoisomer, i.e. 4.4 kcal/mol higher than the (P,M) stereoisomer (not observed experimentally). The high configurational stability of 19c enabled to obtain the pure (P,P)and (M,M)-19c by chiral HPLC (Daicel Chiralcel IE column; EtOAc/MeOH = 9:1 as eluent) and to study their chiroptical properties. The ECD spectrum of (P,P)-19c in CH 2 Cl 2 displayed two strong positive bands at 275 and 320 nm, a strong negative one at 375 nm and a broad and less intense one between 425-475 nm. 52 Note that the isomerization barrier of the OBO-fused double [7]helicene is much higher than for oxabora[6]helicene 9 (vide supra) and higher than carbo[7]helicene (42.0 kcal/ mol), indicating the advantage of double helicenes over monohelicenes in terms of conformational stability. Indeed no racemization was observed upon heating enantiomer (P,P) and (M,M)-19c for 24 h up to 200 °C. 52 Note that in 2017 a longer double OBO-helicene analogue was deposited on a Au(111) under UHV conditions and its surface-assisted cyclodehydrogenation into a planar OBO-perihexacene was observed by STM. 54 Similarly to 6 and 9, the strong bond alternation found in the BOC4 rings of 19a 51 may account for the small NICS value of 1.3 while the surrounding C6 rings, including the distorted central benzene ring, show large negative NICS values (Scheme 3c). For comparison, in the all-carbon analogue tetrabenzo-[a,f,j,o]perylene shown in Scheme 3c, the central C6 ring shows a positive NICS value, indicating substantial antiaromatic character. Notably, molecular orbital calculations of 19a indicate that the HOMO and LUMO are spread over the C6 rings rather than on the boron and oxygen atoms, accounting for the substantial stability of 19a (Scheme 3c). Borahelicenes are efficient blue fluorophores. Indeed, azabora[6]helicene 6 displays blue fluorescence at 447 nm (Table 2), while smaller achiral [4]helicenic systems were successfully used as host materials in phosphorescent OLEDs with efficiencies better than the classical 4,4'...

show abstract

“…Although an increase in the number of ortho annulated rings generally leads to an increase in the racemization time and conformational stability, some tetracyclic compounds exhibit an unusually high thermal conformational stability. 52,[55][56][57] A comprehensive study of the circular dichroism properties of monoaza[5]helicenes was conducted by Caronna et al 61,62 It was observed that although the studied compounds showed similar features to the parent [5]helicene, all of them had shorter racemization time and showed distinct features due to the presence of the nitrogen atom. Ihmels and coworkers 63 showed that diazonia derivatives 46, 47 and 48 have a high binding selectivity for triple-helical DNA, most probably due to their double charge and helical molecular shape.…”

Section: Propertiesmentioning

confidence: 99%