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Regioselective photoaddition of amine to styrylthiophenes

T. I. Ho,* C. S. Ho, S. M. Shin and K. Pa

Department of Chemistry,National Taiwan University,Taipei, Taiwan

Abstract

The photochemistry of styrylthiophene(ST) and its derivatives with amines is investigated. Exciplex emission for tertiary amines and 3-ST systems have been observed. Photochemical addition of tertiary and secondary amines to 2-ST are non-regioselective. Photoaddition of ammonia to 2-ST sensitized by dicyano benzene is regioselective. The difference in the photochemical behaviour is compared.

Styrylthiophene-Amine Exciplexes

The reductive fluorescence quenching of para-substituted trans-3-styrylthioplienes [X=OCH3, CH3, CH(CH3)2, H, F, Cl, Br, CN] by tertiary amines (triethylamine TEA, diisopropylmethylamine DIPEA, diisopropylmethylamine DIPMA) were studied.

Exciplexes emission are observed in non-polar solvents. Figure 1a shows the fluorescence spectra of 3-styrylthiophenes and triethylamine (TEA) exciplexes in n-hexane. Figure 1b shows the fluorescence spectra of 3-styrylthiophenes and diisopropylmethylamine (DIPMA) exciplexes in benzene. Figure 1c shows the fluorescence spectra of 3-styrylthiophenes and diisopropylethylamine (DIPEA) exciplexes in benzene. The data are summarized in Table 1.

Table 1 Maximum emission of 3-(4'-X-)StT - amine exciplexes in benzenea and n-hexaneb

DIPEAa DIPMAa TEAb
3-(4'-X-)StT l(max) l(max) lmax
X=OCH3 414.5- -
X=CH3 433.0 436.2 426.2
X=CH(CH3)2 434.0 437.5 427.0
X=H 442.0 450.5 438.7
X=F 453.0 446.5 427.5
X=Cl 439.0 459.3 441.0
X=Br 499.2 - 449.0
X=CN 2.48 436.2 480.0

Photochemical addition of tertiary amines

Amine adduct 3 : 4 ratio 1 : 1 nonregioselective

3 + 4 27% yield

Photochemical addition of secondary amines

7 : 8 = 4.5 : 1

10 : 11 = 3.7 : 1 (~5% yield)

Photochemical addition of ammonia

Explanation:

Nucleophilic addition of NH3 only occurs at 15.