Abstract:Substituent effects are quite important in fine tuning the photonic properties of conjugated molecules. In designing new chromophores, electron-donating or withdrawing substituents affect the electron distribution in the conjugation sequence, and previous studies to establish structureproperty relationships have noted that when second row elements replace first row elements in the structure (e.g. S for O in donor groups), large enhancements of both the second and third order optical nonlinearity are observed. … Show more
“…More recently, we have demonstrated the concept of multidimensional conjugation as an approach to further enhancing the cross sections of TPA molecules , 1 Molecular-structure motifs for highly active two-photon absorbing chromophores. …”
A new series of linear, asymmetrical diphenylaminofluorene-based chromophores (AFX)
with various strong π-electron acceptors were synthesized and evaluated for two-photon
absorptivity. These chromophores were studied to determine a suitable replacement for 2-(4-vinyl)pyridine, the π acceptor for our previously reported AFX series, which contains a
photochemically and thermo-oxidatively unstable olefinic unit. In addition to the benzoyl
group (AF-370), these π-electron acceptors include 2-benzothiazolyl (AF-240), 2-benzoxazolyl
(AF-390), 2-(N-phenyl)benzimidazolyl (AF-386), and 2-(3,4-diphenyl)imidazolyl (AF-385)
moieties (five-membered heterocycles) and the 2-quinoxalinyl (AF-260) group (six-membered
heterocycle). From nanosecond nonlinear transmission measurements, these new chromophores have effective two-photon cross sections (σ2‘) at 800 nm, spanning from 3.87 ×
10-48 cm4 s/(photon molecule) for AF-385 to 97.46 × 10-48 cm4 s/(photon molecule) for AF-240. Two of them, 2-benzothiazolyl-end-capped AF-240 and benzoyl-containing AF-370 [σ2‘
= 84.32 × 10-48 cm4 s/(photon molecule)] stand out as having relatively good, albeit lower,
values of two-photon cross sections, as compared to that of previously reported N,N-diphenyl-7-[2-(4-pyridinyl)ethenyl]-9,9-didecyl-fluorene-2-amine, AF-50 [σ2‘ = 115.6 × 10-48 cm4
s/(photon molecule)]. However, we observed that AF-240 was more photochemically robust
than AF-50 when their THF solutions were subjected to repetitive and prolonged exposure
to nanosecond laser radiation. Finally, on the basis of our nanosecond TPA cross-section
data (σ2‘/MW values), the general trend for π-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.
“…More recently, we have demonstrated the concept of multidimensional conjugation as an approach to further enhancing the cross sections of TPA molecules , 1 Molecular-structure motifs for highly active two-photon absorbing chromophores. …”
A new series of linear, asymmetrical diphenylaminofluorene-based chromophores (AFX)
with various strong π-electron acceptors were synthesized and evaluated for two-photon
absorptivity. These chromophores were studied to determine a suitable replacement for 2-(4-vinyl)pyridine, the π acceptor for our previously reported AFX series, which contains a
photochemically and thermo-oxidatively unstable olefinic unit. In addition to the benzoyl
group (AF-370), these π-electron acceptors include 2-benzothiazolyl (AF-240), 2-benzoxazolyl
(AF-390), 2-(N-phenyl)benzimidazolyl (AF-386), and 2-(3,4-diphenyl)imidazolyl (AF-385)
moieties (five-membered heterocycles) and the 2-quinoxalinyl (AF-260) group (six-membered
heterocycle). From nanosecond nonlinear transmission measurements, these new chromophores have effective two-photon cross sections (σ2‘) at 800 nm, spanning from 3.87 ×
10-48 cm4 s/(photon molecule) for AF-385 to 97.46 × 10-48 cm4 s/(photon molecule) for AF-240. Two of them, 2-benzothiazolyl-end-capped AF-240 and benzoyl-containing AF-370 [σ2‘
= 84.32 × 10-48 cm4 s/(photon molecule)] stand out as having relatively good, albeit lower,
values of two-photon cross sections, as compared to that of previously reported N,N-diphenyl-7-[2-(4-pyridinyl)ethenyl]-9,9-didecyl-fluorene-2-amine, AF-50 [σ2‘ = 115.6 × 10-48 cm4
s/(photon molecule)]. However, we observed that AF-240 was more photochemically robust
than AF-50 when their THF solutions were subjected to repetitive and prolonged exposure
to nanosecond laser radiation. Finally, on the basis of our nanosecond TPA cross-section
data (σ2‘/MW values), the general trend for π-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.
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