K.R. Idzik et al. / Dyes and Pigments 103 (2014) 55e61
59
1.0
0.8
0.6
0.4
0.2
0.0
additional furyl substituent results in a red shift from 16 nm to
29 nm.
F1
F2c
F2t
F3
Careful analysis of the absorption spectra of 2Fc and 2Ft leads
to the conclusion, that for different solvents the peak position is
very similar (252 nm, 298 nm and 389 nm in CH2Cl2). Moreover,
the peak areas differ insignificantly, when compared to each
other. This indicates that the type of substitution of pyrene core
by two furyl groups influences the absorption properties only
slightly. The absorption spectra with three well resolved peaks
are characteristic for pyrene derivatives [18]. The lowest energy
F4
350
400
450
500
550
600
Wavelenght [nm]
1.2
1.0
0.8
0.6
0.4
0.2
1F
2Fc
2Ft
3F
peak is assigned to the S0/S1 (
p-p*) transition lmax and shifted
from 360 nm (1F) to 432 nm (4F). The second weaker one is
assigned to S0/S2 (p-p*) transition and shifted from 288 nm (1)
to 329 nm (4F) [16].
All the TDDFT-derived spectra are red-shifted by approximately
0.45 eV (equivalent to 65 nm in the studied spectral range) with
respect to the experimental data, which is common for DFT
methods. The relative absorption energies along the series of 5
studied compounds are very well reproducible by theoretical re-
sults (see Fig. 2). For all compounds three distinct absorption bands
% (5)
400
450
500
550
600
Wavelenght [nm]
Fig. 4. Fluorescence spectra: compounds in CH2Cl2 solution (top); results of DFT cal-
culations (bottom).
can be distinguished. The first one in the visible region (lIII
)
following TDDFT calculations (see Table 2) can be attributed to the
HOMO e LUMO transition. The lII absorption peaks represent
mainly transitions form HOMOꢁ3, ꢁ2 and ꢁ1 to LUMO and from
HOMO to LUMO þ1, þ2 and þ3. The highest energy absorption
bands lI can be attributed to numerous transitions between more
separated orbitals (see Table in Supporting materials).
All compounds show only weak solvatochromic effects. The
replacement of the polar DMF with less polar CH3CN or non-polar
CH2Cl2 solution results in a red solvatochromic shift in UVeVis
absorption. Fig. 3 presents UVeVis spectra of 3F in different sol-
vents. The values of shifts are smaller e below 7 nm. The largest
shift is observed for 4F reaches almost 11 nm. The absorption
maximum lIII of 4F in CH3CN is equal to 422 nm, in CH2Cl2
lIII ¼ 432 nm and in the case of DMF solution lIII ¼ 433 nm. Similar
results were obtained for lII (313 nm in CH3CN, 328 nm in CH2Cl2
and 313 nm in DMF).
[17], respectively. Mono- and dibromopyrene were prepared in
DCM, while in the case of tri- and tetrabromopyrene we used
nitrotoluene as a solvent. Cross-coupling of these bromopyrenes
with 2-(tributylstannyl)furan under the conditions of the Stille re-
action afforded mono-, bis-, tris-, and tetrakis(furyl) pyrenes 7e11,
respectively. For preparation of di(furyl)pyrenes we used a mixture
of 1,6-dibromopyrene (3) and 1,4-dibromopyrene (4). We success-
fully isolated 1,6- di(furyl)pyrene (8) from 1,4- di(furyl)pyrene (9).
These latter derivatives were purified through a chromatography
column. The structures of compounds 7e11 were confirmed by
NMR and elemental analysis. The reactions were conducted under
easy to perform, mild conditions in moderate to good yields. This
synthesis allows the production of heterocyclic compounds of po-
tential interest for optical and electronical applications.
In all of the studied compounds HOMO and LUMO orbitals are
delocalized in the whole molecular system, including both the
pyrene core and furyl substituents. The large spatial overlap of
these orbitals is in agreement with the observed strong absorption
coefficient of the first absorption band, which results from HOMO
to LUMO transition (Table 3).
3.2. UVeVis spectra analysis and DFT calculation (density
functional theory, quantum mechanical modeling method)
The basic optical properties of studied compounds such as the
position of absorption peaks in different solvents and molar ab-
sorption coefficients are collected in Table 1. Obtained compounds
are soluble in common organic solvents. Their solubility decrease
with increasing molecular weight. 4F shows the lowest solubility.
UVeVis spectra recorded in CH2Cl2 are shown in Fig. 2. All
compounds present absorption with at least three strong transi-
3.3. Fluorescence and TDDFT calculation (time dependent density
functional theory)
All of the studied compounds exhibit a fluorescence as vibronic
bands with a peak energy dependent on the number of furyl sub-
stituents. The fluorescence maximum in 1F (402 nm) is red shifted
in comparison to the unsubstituted pyrene (378 nm) [19]. The po-
sition of substitution does not affect the fluorescence. Both 2Ft and
2Fc have equal fluorescence maxima (432 nm) and similar shape. In
the case of 3F and 4F a further shift is observed (453 nm and
477 nm) (Fig. 4). Batochromic shift is observed in both absorbance
and fluorescence spectra with increasing electron delocalization,
which is related to an increase in the effective conjugation length
[19,20].
tions. Molar absorption coefficients
ε
are greater than
20,000 molꢁ1 cmꢁ1 for all compounds. The highest energy peak lI
of 1F is located at approximately 242 nm. Other compounds present
similar peaks but slightly red shifted in the range from 244 nm to
252 nm. The second peak consists of at least two components. A
batochromic shift is observed according to the increasing number
of furyl substituents. The maximum of the second peak lII of 1F can
be found at 288 nm and is shifted up to 328 nm. The third signif-
icant peak is red-shifted by more than 60 nm. The presence of each
Comparing with the DFT calculations, the experimental excited
state geometry is more planar in case of all studied compounds. An
average dihedral angle between the pyrene core and its furyl sub-
stituents is decreased from approximately 30ꢀ in a ground state to
15 in an excited state (Table 4).
Table 4
Average dihedral angle between pyrene core and furyl substituents in the ground
and excited (marked with asterisk) states in DFT-optimized geometries.
Compound
1F
1F* 2Fc 2Fc* 2Ft 2Ft* 3F
3F* 4F 4F*
The fluorescence spectra recorded for different concentrations
of 1F in CH2Cl2 are presented in Fig. 5. For low concentrations
Dihedralangles [ꢀ] 29.6 12.4 30.8 12.5 30 16.5 28.6 17.6 26 17.7