18
B. Xu et al. / Journal of Molecular Structure 917 (2009) 15–20
in Table 1, phenyl substituted in amino group has little effect on
nitrogen atoms and oxygen atoms. Fluorine atoms substituted in
methyl decrease the negative charges in oxygen atoms. Especially,
the negative charges in carbonyl oxygen atoms are decreased evi-
dently. It is also attributed to the electron-attracting effect of fluo-
rine atoms.
4.4. UV absorption spectra
Experimentally, UV absorption spectra of PAC, PAFC and C151
were measured in acetonitrile and the UV absorption spectra of
C120 were obtained from literature [21]. In order to find out the
method which can predict absorption spectra of coumarins more
accurately, the experimental data were compared with the calcu-
lated data by ZINDO, TD-B3LYP/6-31G(d) and TD-B3LYP/6-
31+G(d) methods. The comparisons of results in gas phase were
4.2. Electronic structures of the ground states
Molecular orbital (MO) calculations were performed on the base
of the geometrical structures optimized by B3LYP/6-31G(d) meth-
od. Shapes and types of molecular orbitals (MOs) of PAC, PAFC,
C120 and C151 were presented in Fig. 2. The lowest unoccupied
molecular orbital (LUMO), the second unoccupied molecular orbi-
tal (LUMO+1) and the third unoccupied molecular orbital
listed in Table 4 and the peaks with the largest wavelength (kmax)
were selected to be compared. There are 10–30 nm differences be-
tween the calculated kmax by ZINDO method and the experimen-
tally measured kmax. The predict results using TD-DFT method is
more accurate than ZINDO method for PAC and PAFC, but less
accurate than ZINDO method for C151 and C120. Furthermore,
using dispersion basis set such as 6-31+G(d) can improve the accu-
racy of the calculations for absorption spectra.
(LUMO+2) of four compounds are
pied molecular orbital (HOMO) and the second occupied molecular
p
*-type MO. The highest occu-
orbital (HOMOꢁ1) of four compounds and the third occupied
The calculations above-mentioned are all in gas phase, not in
solvent. The experimental data are obtained in ACN. So the calcu-
lations by TD-DFT in solvent were done using SCIPCM model. The
results are also listed in Table 5. As shown in Table 5, the solvent
effect induces the red shifts of absorption wavelengths of couma-
rins and makes the calculated values nearer to the experimental
data. For ACN solvent, the maximal absorption wavelengths of
PAFC are red-shifted compared with those of PAC which is because
that the substitution of hydrogen atoms of the 4-methyl group by
fluorine atoms in PAFC has a better electron delocalization than
that in PAC. And the maximal absorption wavelengths of PAC and
PAFC are red-shifted compared with those of C120 and C151 which
is because of the substitution of one hydrogen atom in the amino
group by one phenyl group in PAC and PAFC.
molecular orbital (HOMOꢁ2) of PAC and PAFC are
p-type MO.
The third occupied molecular orbital (HOMOꢁ2) of C151 and
C120 are n-type MO.
Different orbital energies and HOMO–LUMO energy gaps of the
four coumarin compounds are presented in Table 2. As shown in
Table 2, phenyl substituted in amino group raise energies of HOMO
and HOMOꢁn (n = 1,2), and lower energies of LUMO and LUMO+n
(n = 1, 2). So the difference of energy between HOMO and LUMO is
reduced, and the transition from HOMO to LUMO becomes easier
for PAC and PAFC. Fluorine atoms substituted in methyl also makes
the transition from HOMO to LUMO easier because the energy gap
of PAFC is lower than that of PAC and the energy gap of C151 is
lower than that of C120.
Phenyl substituted in amino group make PAC and PAFC have
The average deviations (nm) for four coumarin compounds by
five methods are listed in Table 6. It is indicated that TD-B3LYP/
6-31+G(d) with SCIPCM model is the most accurate method for
coumarins in this work.
more
p
and p* type MOs. LUMO+2 of PAC and HOMOꢁ2 of PAFC
are mostly localized on benzene ring. Furthermore, phenyl substi-
tuted in amino group activates the whole conjugated system and
raises energies of
p
-orbitals.
UV absorption strengths f can be expressed by extinction coef-
ficients ecal. The extinction coefficients were calculated by the for-
4.3. Ionization potential
mula [25]:
ecal ¼ f ꢀ 5:398 ꢀ 104
ð1Þ
Ionization potential indicates the ability of losing electron.
Table 3 lists the calculated ionization potentials (IP) of the four cou-
marins PAC, PAFC, C120 and C151 using B3LYP/6-31G(d) level. In
Table 3 there are two parameters including vertical ionization po-
tential (IPV) and adiabatic ionization potential (IPa). Vertical ioniza-
tion potential (IPV) denotes the energy difference between cation
and molecule on the base of the optimized structure of neutral
molecule. Adiabatic ionization potential (IPa) means the energy dif-
ference between molecule and cation on the base of the optimized
structures of neutral molecule and cation, respectively. From the
data we can see that the vertical ionization potential and adiabatic
ionization potential from PAC to C151 is increased in turn, i.e. the
ability of losing electron is reduced gradually (PAC <
PAFC < C120 < C151). This means PAC and PAFC can act as good
electron-donating materials.
Table 3
Ionization potential (Ip) of compounds PAC, PAFC, C120 and C151 obtained by B3LYP/
6-31G(d) method
Molecules
IPV (eV)
IPa (eV)
PAC
6.90
7.17
7.44
7.79
6.84
7.09
7.22
7.57
PAFC
C120
C151
Table 4
Comparison between the experimental and calculated max absorption wavelength
(kab) and molar extinction coefficients (L/(mol cm)) of PAC, PAFC, C151 and C120 in
gas phasea
Molecules ZINDO
TD-B3LYP/6-31G(d) TD-B3LYP/6-31+G(d) Exp.
Table 2
PAC
344.48
344.89
352.72
360
Energies (eV) of frontier molecular orbitals obtained by B3LYP/6-31G(d) method
3.21 ꢀ 104 2.51 ꢀ 104
2.60 ꢀ 104
384.03
2.24 ꢀ 104
385
Orbitals
PAC
PAFC
C120
0.479
ꢁ0.120
ꢁ1.372
ꢁ5.664
ꢁ6.589
ꢁ7.095
4.292
C151
0.008
ꢁ0.412
ꢁ1.995
ꢁ5.997
ꢁ6.907
ꢁ7.544
4.002
PAFC
C120
C151
356.89
372.19
3.33 ꢀ 104 2.21 ꢀ 104
2.22 ꢀ 104
313.93
2.08 ꢀ 104
LUMO+2
LUMO+1
LUMO
ꢁ0.218
ꢁ0.278
ꢁ1.483
ꢁ5.414
ꢁ6.454
ꢁ6.924
3.931
ꢁ0.463
ꢁ0.515
ꢁ2.054
ꢁ5.680
ꢁ6.790
ꢁ7.163
3.626
331.71
307.97
342b
2.61 ꢀ 104 1.75 ꢀ 104
1.88 ꢀ 104
337.87
2.53 ꢀ 104
367
342.45
329.20
HOMO
2.78 ꢀ 104 1.67 ꢀ 104
1.74 ꢀ 104
1.49 ꢀ 104
HOMOꢁ1
HOMOꢁ2
Energy gap
a
The italic values are the molar extinction coefficients (
Ref. [21].
ecal).
b