DUFFY ET AL.
5 of 6
SCHEME 2 Gibbs free energy
difference (ΔG) of 9‐substituted Fluorenyls
TABLE 2 Free energy difference of Fluorenyl C‐H bond dissoci-
ation and for the reaction in Scheme 2 (kcal Mol )
4
| CONCLUSIONS
‐1
In summary, photoexcitation of 9,9′‐bifluorenyls gener-
ates the corresponding fluorenyl radicals in methanol
solutions as evident from the laser flash photolysis of
both bifluorenyls and the corresponding fluorenols.
These transients show peaks in the region λ 500‐540 nm
and decay by second order kinetics. Their spectral
features are similar to the corresponding carbocations,
with the latter showing red shifts of 15‐20 nm. These rad-
icals can undergo hydrogen abstraction from solvent to
produce the reduced 9‐substituted fluorenes, or dimerize
to the bifluorenyls as was evident from the photolysis of
monosubstituted bifluorenyl 3e, which results in both
fluorenyl and 9‐cyanofluorenyl radicals. Computed reso-
nance stabilization energies indicate an inherent destabi-
lization of fluorenyl radicals except for the 9‐CF3
analogue, which is rationalized in terms of delocalization
of spin density into the planar benzene rings. This
also accounts for the formation of the unusual dimeriza-
tion to the unsymmetrical bifluorenyl 4c. Whereas
the fluorenyl radicals are destabilized (except for
9‐trifluoromethylfluorenyl) relative to their nonaromatic
analogues, their kinetic stabilities increase with substitu-
tion at C‐9 due to stereoelectronic factors.
9
H‐Fluorene
9,10‐Dihydroanthracene
(ΔG)
R
(ΔG)
(ΔGrxn)
H
76.4
72.1
77.4
67.0
71.2
72.0
78.2
63.7
‐5.2
‐0.1
CH
3
CF
3
+1.2
‐3.3
CN
steric factors. The relative thermodynamic stabilities of
‐substituted fluorenyl radicals may be computed by
comparison of the C‐H bond dissociation energies of
the C(9)‐H bond of fluorene with that of the benzylic
C‐H bond of the nonaromatic dihydroanthracene
according to Scheme 2. The computed values are tabu-
lated in Table 2.
As can be seen, the fluorenyls are all destabilized
relative to the nonaromatic model except for 9‐CF . Of
note is the nonplanarity of the comparative model system
dihydroanthracene) relative to fluorenyl, which dimin-
9
3
(
ishes the extent of electron delocalization into the
benzene ring of the former. The relative free energy dif-
ference values point to an inherent destabilization of
fluorenyls. In regard to the 9‐CF substituted fluorenyl,
3
ORCID
the strong electron‐withdrawing substituent may induce
greater spin delocalization into the conjugated benzene
rings, which can be better accommodated in the more
planar fluorene ring than the model system. This observa-
tion can also account for the unusual formation of 4c
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(
R = CF ) on photoexcitation of either 1c or 3c where
3
the corresponding fluorenyl radical has been shown to
be directly involved in product formation. The spin
density of this radical is predominantly localized at C‐1
and C‐3 (0.156). With the peri‐steric effect recombination
of the fluorenyl radicals takes place selectively at C‐3
accounting for the regiochemistry of the product. It is
interesting to note that no evidence of a similar
photoproduct is observed in the photolysis of bifluorenyls
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[
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3
d and 3e, which also has an electron‐withdrawing
CN group. The 9‐cyanofluorenyl radical has most of the
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