J.-H. Ho et al. / Tetrahedron Letters 55 (2014) 5727–5731
5729
Table 1
Photophysical and voltammeric data
a
c
En
Compd
kabs (nm)
mabs (cmÀ1
)
kem (nm)
mem (cmÀ1
)
D
mStokes (cmÀ1
)
D
kStokes (nm)
E0,0 (nm)
E0,0 (cmÀ1
)
Eox (V)
1
2
3
4
14DPN
14DTN
14DFN
14DPA
302
327
345
33113
30581
28986
26667
377
432
427
26525
23148
23419
22779
6588
7433
75
105
82
340
378
389
406
29412
26455
25707
24631
1.77
1.40
1.14
1.24
5567
3888 (1684)b
64
337(sh),357,375,394
395
407
422(sh),439
474
477
5
6
7
14DTA
14DFA
910DPA
25316
24570
26667
21097
20964
23148
4219
79
70
57
433
447
402
23095
22371
24876
1.23
0.94
1.28
3606
3519 (1044)b
339,357,375,395
412,432,460(sh)
449
8
9
910DTA
910DFA
26385
25381
22272
21645
4113 (2791)b
70
68
414
423
24155
23641
1.28
1.07
343(sh),360,379,399
394
462
3736
a
b
c
The middle peak of the absorption band (with vibrational structures) is presented with an underline.
The energy difference of the longest absorption peak and the shortest emission peak.
Measured in dichloromethane with 0.1 M nBu4NClO4 as electrolyte, a Pt working electrode, an Ag wire reference electrode and a Pt counter electrode.
the naphthalene and aryl groups (phenyl > 2-thienyl > 2-furyl
groups) in their ground states. The emission maxima are in the
order 14DPN (377 nm) < 14DTN (432 nm) ꢀ 14DFN (427 nm).
The shortest emission maximum of 14DPN means it has larger
dihedral angles in the fluorescing excited state, and the similar
emission of 14DFN and 14DTN would represent the similar dihe-
dral angles of their fluorescing excited states. The Stokes shifts dis-
play the order 14DTN (105 nm) > 14DFN (82 nm) > 14DPN
(75 nm), which reasonably explains that 14DTN has the largest
geometrical difference between its ground state and fluorescing
excited state.
ground state and in the 910DTA and 910DFA fluorescing excited
states delocalized between anthracene and diaryl groups.
As expected, the absorption maxima are in the order 910DPA
(375 nm) ꢀ 910DTA (379 nm) < 910DFA (394 nm) which means
910DPA and 910DTA have very large dihedral angles in the ground
state, and only 910DFA has a smaller one. The emission maxima
are in the order 910DPA (432 nm) < 910DTA (449 nm) < 910DFA
(462 nm) which represents 910DTA and 910DFA would have more
planar geometry in the fluorescing excited states. The Stokes shifts
display the order 910DTA (70 nm) > 910DFA (68 nm) > 910DPA
(57 nm), which is in consistent with the structural difference
between their ground states and fluorescing excited states.
1,4-Diarylanthracenes
Theoretical calculation
The absorption and emission spectra of 1,4-diarylanthracenes
show two different situations (Fig. 2). First, 14DPA displays struc-
tured absorption and emission bands (kabs at 337sh, 357, 375, and
394 nm; kem at 422sh and 439 nm in dichloromethane), which are
very similar to the spectra of anthracene (kabs at 326, 342, 359, and
378 nm9a; kem at 385, 405, 430, and 455 nm9b in chloroform), but
with about 16 nm redshift. The alike vibrational structures may
The optimal ground state structures and corresponding physical
properties were predicted by density functional theory (DFT) cal-
culation at the B3LYP/6-31G⁄ level with the Spartan’10 program.10
The computed HOMOs and LUMOs are shown in Figure 4. The dihe-
dral angles between the side aryl groups and the central benze-
noids (h), the energy difference between HOMO and LUMO (DE),
calculated kabs from the energy difference (kcalÀ E) and kabs from
deduce the
p-conjugation of 14DPA mainly localized on the
D
the computed UV–vis spectrum (kcalÀspec) are reported in Table 2.
These computation results deduced good explanations for the
experimental observations.
anthracene core in the ground state and fluorescing excited state.
Second, 14DTA and 14DFA, compared to anthracene, show the
structureless and more redshifted absorption and emission bands,
which interpret the more
p-conjugation between anthracene and
The redshifted trend of the experimental absorption maxima
(kexp) matching the degree of the dihedral angles
diaryl groups in both of ground and fluorescing excited states.
The absorption maxima are increasing in the order 14DPA
(375 nm) < 14DTA (395 nm) < 14DFA (407 nm) matches the theo-
retically dihedral angles in the ground states, and the emission
maxima are in the order 14DPA (439 nm) < 14DTA
(474 nm) ꢀ 14DFA (477 nm) which means 14DPA has larger dihe-
dral angle but 14DTA and 14DFA have smaller and similar dihedral
angles in the fluorescing excited state. The Stokes shifts display the
order 14DTA (79 nm) > 14DFA (70 nm) > 14DPA (64 nm), and this
shows that 14DTA has the largest geometrical difference between
its ground state and fluorescing excited state.
In the 1,4-diarylnaphthalene and 1,4-diarylanthracene systems,
the calculated dihedral angles between the side aryl groups and
the central benzenoids (phenyl: 57°, 2-thienyl: 48°, 2-furyl: 32°)
were consistent with the expected repulsion resulted from differ-
ent aryl groups (phenyl > 2-thienyl > 2-furyl) and also showed a
good agreement to the order of the absorption maxima
(14DFN > 14DTN > 14DPN and 14DFA > 14DTA > 14DPA). In the
9,10-diarylanthracene system, the dihedral angles were larger
(phenyl: 83.7°, 2-thienyl: 87.9°, 2-furyl: 59.1°) because of the
stronger steric repulsion on 9 and 10 positions of anthracene than
that on 1 and 4 positions. Therefore, only 910DFA showed an obvi-
ous redshift in the absorption spectrum.
9,10-Diarylanthracenes
The absorption and emission spectra show three different situ-
ations (Fig. 3). First, 910DPA, just like 14DPA, has anthracene-like
structured absorption and emission bands (kabs at 339, 357, 375,
and 395 nm; kem at 412, 432, and 460sh nm in dichloromethane).
Second, 910DTA has an anthracene-like structured absorption
band (kabs at 343sh, 360, 379, and 399 nm) but a structureless
and redshifted emission band. Third, 910DFA has structureless
and redshifted absorption and emission bands. These spectra inter-
Good prediction for the absorption maxima
Although the calculated energy differences between the HOMOs
and LUMOs did not exactly match the energy of the experimental
absorption maximum, the trend of the kcalÀ E is consistent with that
D
of the kexp (14DFN > 14DTN > 14DPN, 14DFA > 14DTA > 14DPA and
910DFA > 910DTA > 910DPA). However, the kcalÀspec from the
computed UV-Vis spectrum in Spartan’1011 were close to the kexp
within 10 nm difference for all the compounds, except of 14DFA
pret that the
states and in the 910DPA fluorescing excited state mainly localized
on the anthracene moiety, but the -conjugation in the 910DFA
p-conjugation in the 910DPA and 910DTA ground
p