| Title
| - Diarylamino Groups as Photostable Auxofluors in2-Benzoxazolylfluorene, 2,5-Diphenyloxazoles, 1,3,5-Hexatrienes,1,4-Distyrylbenzenes, and 2,7-Distyrylfluorenes
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| Abstract
| - The relationship of structure to optical spectral properties was determined for five types of fluorsin a search for an optimum-wavelength shifter to be used as part of the detection systems for high-energy particles from accelerators. In a search for photostable fluors to serve as waveshifters inplastic fibers it was found that the wavelengths of interest, absorption max 410 ± 10 nm andfluorescence emission max 480 ± 20 nm, along with other properties, such as high solubility andshort fluorescence decay time, could be obtained from fluorophors composed of aromatic rings andvinyl groups only by using amino groups as auxochromes to give bathochromic shifts of wavelengths.Since primary, monoalkyl, and dialkylamino groups were not sufficiently photostable, a number offluorophores bearing diarylamino groups were investigated. Syntheses of the fluors made use ofthe Buchwald amination, an improved version of the Emmons−Horner reaction, and other commonreactions. The fluor types were the following: a 2-benzoxazolyl-7-(4-diarylamino)fluorene 7, 2-(4-cyanophenyl)-5-(4-aminophenyl)oxazoles 14 and 20, 1,3,5-hexatrienes 24a−d and 26a−c, 1,4-distyrylbenzenes 31d−g and 32a−e, and 2,7-distyrylfluorenes 40a,d−e. The unsymmetrical fluors7, 14, and 20 were not as bright as the best hexatrienes, distyrylbenzenes, and distyrylfluorenes,which were all symmetrical. Where the 1,6-diaryl-1,3,5-hexatrienes 24a−d had high fluorescencequantum yield (Φf), the 1,1,6,6-tetraryl-1,3,5-hexatrienes 26a−c had both lower ε and Φf. Wherethe 1,4-distyrylbenzenes 31d−g had high Φf, the 1,4-bis(2-phenylstyryl)benzenes 32a−e had Φf =0. Diarylamino groups as auxofluors conferred higher photochemical stability than dialkylaminogroups on similar fluorophores. The 1,4-distyrylbenzenes 31d,e and the 2,7-distyrylfluorenes 40d,ehad the most desirable properties overall, which included fast decay times of 2 ns. Computersimulations predicted absorption and emission wavelengths fairly well, but were of little help forthe prediction of brightness, stability, Φf, or decay time.
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