R. Kannan et al., Diphenylaminofluorene-based two-photon-absorbing chromophores with variouspi-electron acceptors, CHEM MATER, 13(5), 2001, pp. 1896-1904
A new series of linear, asymmetrical diphenylaminofluorene-based chromophor
es (AFX) with various strong pi -electron accepters were synthesized and ev
aluated for two-photon absorptivity. These chromophores were studied to det
ermine a suitable replacement for 2-(4-vinyl)pyridine, the it acceptor for
our previously reported AFX series, which contains a photochemically and th
ermo-oxidatively unstable olefinic unit. In addition to the benzoyl group (
AF-370), these pi -electron accepters include 2-benzothiazolyl (AF-240), 2-
benzoxazolyl (AF-390), 2-(N-phenyl)benzimidazolyl (AF-386), and 2-(3,4-diph
enyl)imidazolyl (AF-385) moieties five-membered heterocycles) and the 2-qui
noxalinyl (AF-260) group (six-membered heterocycle). From nanosecond nonlin
ear transmission measurements, these new chromophores have effective two-ph
oton cross sections (sigma (2)') at 800 nm, spanning from 3.87 x 10(-48) cm
(4) s/(photon molecule) for AF-385 to 97.46 x 10(-48) Cm-4 s/(photon molecu
le) for AF-240. Two of them, 2-benzothiazolyl-end-capped AF-240 and benzoyl
-containing AF-370 [sigma (2)' = 84.32 x 10(-48) cm4 s/(photon molecule)] s
tand out as having relatively good, albeit lower, values of two-photon cros
s sections, as compared to that of previously reported N,N-diphenyl-7-[2-(4
-pyridinyl)ethenyl] -9,9-didecyl-fluorene-2-amine, AF-50 [sigma (2)' = 115.
6 x 10-48 Cm4 s/(photon molecule)]. However, we observed that AF-240 was mo
re photochemically robust than AF-50 when their THF solutions were subjecte
d to repetitive and prolonged exposure to nanosecond laser radiation. Final
ly, on the basis of our nanosecond TPA cross-section data (sigma (2)'/MW va
lues), the general trend for pi -electron accepting ability, i.e., ability
to accept charge transferred from diphenylamine appears to be as follows: 2
-(4-vinyl)pyridine > 2-benzothiazolyl > benzoyl > 2-(N-phenylbenzimidazolyl
> 2-quinoxalinyl > 2-benzoxazolyl > 2-(4,5-diphenyl)imidazoyl.