Conformational isomerism in squaraines: saturation transfer NMR studies on hydroxy squaraines

Abstract

Can. J. Chem. 68, 530 (1990). The proton and I3C NMR of azulene and dialkylaminohydroxyphenyl squaraines have been investigated. The four-membered ring carbon resonances occur in the 180-190 ppm range and are attributed to the cyclobutenediylium character of this structure. Some charge delocalization from the central ring to the pendent aromatic substituents was also observed, indicative of a substituent effect of the squaraine moiety on aminoaromatic systems equivalent to that of a formyl or acetyl group. Hydroxy squaraines 10 and 11 show the presence of two isomeric squaraine species in the proton NMR in deuterochloroform. Saturation transfer experiments confirm that these species are readily interconverted, and cisltrans isomerism (9a 9b) is consistent with these observations. The free energy difference between the two isomers at 318 K is calculated to be 3.0 and 3.2 kJ/mol for hydroxysquaraines 10 and 11 respectively.Key words: squaraines, saturation transfer, NMR. On a examink les spectres RMN du proton et du I3c de l'azulkne et des dialkylaminohydroxyphCnyl squara'ines. Les resonances des carbones du cycle 5 quatre chainons sont observkes dans la plage allant de 180 a 190 ppm et elles sont attribukes au caractere cyclobutenediylium de cette structure. On a aussi observe une certaine dClocalisation de la charge du noyau central vers les substituants attaches au noyau aromatique; ces observations suggerent qu'il existe un effet de substituant de la portion squaraine sur les systkmes aminoaromatiques equivalent a celui d'un groupement formyle ou acyle. Dans les spectres RMN du proton des hydroxy squarai'nes 10 et 11 en solution dans le deutkrochloroforme, on observe la prksence de deux especes isomkres. Des expkriences de transfert de saturation confirment que ces especes se transforment facilement l'une dans I'autre; une isomkrie cislrrans (90 S 9b) est en accord avec ces observations. On a calcule que, a 318 K, les diffkrences d'knergie libre entre les deux isomeres sont de 3,O et de 3,2kJ/mol respectivement pour les hydroxysquarai'nes 10 et 11.

Squaraines as Reporter Units: Insights into their Photophysics, Protonation, and Metal‐Ion Coordination Behaviour

“…[15,18,19,24,31,32] The chemical structures and nomenclature of the prepared compounds are shown here; namely SNO 3 , [24] SNO 4 , [24] SNO 5 , [24] SNS 2 O 2 , [15,19] SBu, [31] SAb, [18] SJu, [32] SCar [18] and SJuOH. [32] All the derivatives were characterised by standard techniques, including HRMS and NMR spectroscopy. In the latter case, the characteristic doublets in the aromatic region at approximately 6.7 and 8.2 ppm were detected (except for SJu and SJuOH).…”

“…16-Phenyl-16-aza-1,4,7,10,13-pentaoxacyclooctadecane, 13-phenyl-13-aza-1,4,7,10-tetraoxacyclopentadecane, 10-phenyl-10-aza-1,4,7-trioxacyclododecane, N,N-diA C H T U N G T R E N N U N G [ethyl-2-(2 methoxy)]aniline, 10-phenyl-10-aza-1,4-dioxa-7,13-dithiacyclopentadecane and 2-[(2-butylcarbamoiloxiethyl)phenylamine]ethyl ester were prepared following known procedures. [15,18,19,24,31,32] Physical measurements and instrumentation: The NMR spectra were recorded with a Varian Gemini 300 spectrometer. Chemical shifts are reported in ppm downfield from the TMS signal.…”

“…General synthesis: SNO 3 , [24] SNO 4 , [24] SNO 5 , [24] SBu, [31] SJu [32] and SJuOH [32] were prepared according to literature procedures. For SAb, SCar and SNS 2 O 2 , we employed the general synthetic procedure for squaraines.…”

Pyrrolyl Squaraines–Fifty Golden Years

“…Up to this point in time all papers on pyrrolyl squaraines came from researchers based in German institutions and three (of the five) of these early papers were written in German. Moving forward, apart from a mention in the introduction section of a paper in 1990 [4], the only paper from 1970-1990 to report any new research on pyrrolyl squaraines was for the polysquaraine derived from 1-methylpyrrole [poly(1-methylpyrrolylsquaraine) or PMPS] in 1990. In this paper, PMPS was prepared by refluxing in butanol and was studied as a dark blue insoluble precipitate with one water molecule per squarate unit [5].…”

Squaraines based on 2-arylpyrroles

“…with dialkylaminohydroxyphenyl squaraines). 25 Recent studies in non-protic solvents on semisquaraines with o-diethylaminophenyl substitution have been interpreted in terms of intramolecular hydrogen bonding which is disrupted in protic solvents. 31 …”