Full Paper
acidic media leads to the formation of the corresponding
acetal 34, emulating the reactivity of dihydropyran, a very
commonly used protective group for alcohols.[41] As an exam-
ple, a terminal triple bond, that could be later elongated
through click chemistry was attached to the fluorescence spi-
robinaphthyl to yield the functionalized derivative 34b
(Scheme 16). This very simple reaction might enable the modi-
fication of these structures to tune their physical properties,
such as solubility or aggregation in solid phase, or to attach
the fluorophore structures to polymers, solid supports or mon-
olayers, as well as open the door to the employment of these
molecules as fluorescence tags that can be introduced and
cleaved under the same conditions as the THP protective
group.
From our point of view, the data obtained from spirodiben-
zofluorenes 9 and 34 are quite appealing. Their absorption
spectra in CH2Cl2 highly dilute solution feature p!p* transi-
tions at about lmax =353 and 370 nm, showing a large batho-
chromic shift when compared with the absorption spectra re-
ported for 1,1’-binaphthyl (lmax =280 and 295 nm).[42] Interest-
ingly, whereas in typical 2,2’-binaphthyls there is no conjuga-
tion between the two napthyl rings because of the steric re-
pulsions between the substituents at positions 8 and 8’, the
geometrical distortion created by the formation of the five-
membered ring enforces a rigid structure in which both naph-
thyl rings are arranged in a situation closer to planarity, there-
fore promoting effective conjugation between the rings. In
fact, DFT molecular modeling studies predict a structure in
which both naphthalene rings are slightly bent (torsion angle
of 218) to avoid the steric interaction between the hydrogens
at the internal positions (Figure 3). Nevertheless, despite the
distortion from planarity, conjugation between both naphtha-
lene fragments occurs: the graphical representations of HOMO
and LUMO orbitals clearly show the extension of the p-conju-
gation along both aromatic structures.[43] Interestingly, these
compounds turned out to be highly fluorescent, showing an
emission band at about 403 nm with also high quantum
yields.
Scheme 16. Attachment of alcohols to fluorescence spirobinaphthyl 9 taking
advantage of the enol ether functionality.
Regarding the thiophene-containing spiroderivatives, the
simplest 4H-cyclopenta[2,1-b:3,4-b’]dithiophene 18 showed an
absorption band at lmax =318 nm, but no significant emission
bands. The non-symmetric compound 25a presented an emis-
sion band at 357 nm, although with a modest quantum yield.
However, the benzocondensed analogues 19a and 19c fea-
tured significant emission bands at 407 and 383 nm, respec-
tively. These results indicate the potential interest of these pre-
viously unknown structures in the design of new electrolumi-
nescent materials by modifying the aromatic backbones at-
tached at the spirocyclic central core.
Photophysical properties
Most of the molecules described along this work feature un-
precedented structures with extended p-conjugation. To evalu-
ate the potential interest of these classes of molecules in the
development of functional organic materials and luminescent
devices, we carried out a study of the photophysical properties
of some selected compounds by means of their UV/Vis absorp-
tion and fluorescence spectra. Thus, thiophene- and benzothio-
phene-containing spirocompounds (18, 19, and 25), spiro di-
benzofluorenes (9 and 34) and bisfluorenes (33) were studied.
A summary of the results is presented in Table 1.
Finally, the UV/Vis spectra of bisfluorenes 33 show an ab-
sorption band at lmax =367 nm. The importance of the pres-
ence of the fluorene structure to enforce the extended p-con-
jugation is shown by comparing
these data with the spectra of
the tetrabromide precursor 28,
which features absorption at
Table 1. Absorption and emission spectral data for selected compounds.[e]
labs
[nm][a]
(eÆs[b])104
lem
[nm][a]
F[d] Æs[b]
lmax =320 nm (Figure 4). More-
[LmolÀ1 cmÀ1
]
[%]
over, compounds 33 are strongly
9e
250
250
250
318 (315)
316 (305 sh, 315)
315
355 (350)
334, 351 (333 347)
326
366
366
366
4.1Æ0.1
385 sh, 403
385 sh, 401
385 sh, 402
399 (372)
357
356
407 (404)
383, 403 (377 398)
379
399–422
60Æ10
fluorescence in CH2Cl2 solution,
34a
34b
18b
25a[c]
25c[c]
19a[c]
19c[c]
28
2.15Æ0.1
2.15Æ0.06
2.8Æ0.1
110 Æ20
with sharp emission bands at
60Æ10
399 and 423 nm, and very high
1.0Æ0.1 (0.3)
quantum yields (Figure 4 and
1.7Æ0.1 (0.97Æ0.01)
1.0Æ0.2
16Æ2 (11.8Æ0.4)
17Æ2
Table 1). The type of substitution
1.2Æ0.1 (0.28Æ0.01)
0.1 (0.77Æ0.3)
2.3Æ0.2
24Æ3 (49Æ3)
at C9 on the fluorene moieties
18Æ3 (25Æ1)
has no significant influence on
the position of the absorption
and emission bands, neverthe-
less, the determined quantum
yields differ slightly (Table 1) but
consistently maintaining very
high values.
33a
33b
33c
3.18Æ0.08
68Æ9
0.98Æ0.06
399–423
399–422
98Æ6
1.0Æ0.1
100Æ10
[a] Measured in CH2Cl2 (110À5 m) at RT. [b] Standard deviation. [c] Measurements carried out in hexanes are in-
dicated in brackets. [d] Fluorescence quantum yields relative to quinine sulfate 0.1 N in H2SO4 (F=0.55 at
360 nm). [e] More detail is provided in the Supporting Information.
Chem. Eur. J. 2015, 21, 16463 – 16473
16470
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim