TICT fluorescence of N-borylated 2,5-diarylpyrroles: A gear like dual motion in the excited state

Article abstract of DOI:10.1039/c2dt32134c

A significantly red-shifted fluorescence and a high fluorescence quantum yield for the emission from the twisted intramolecular charge transfer (TICT) state are attained in new aminoboranes, N-borylated 2,5-diarylpyrroles. A gear like dual structural motion in the excited state is responsible for their large Stokes shifts.

Full text of DOI:10.1039/c2dt32134c

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TICT fluorescence of N-borylated 2,5-diarylpyrroles:
a gear like dual motion in the excited state†
Cite this: Dalton Trans., 2013, 42, 620
Received 15th September 2012,
Accepted 8th October 2012
Takuhiro Taniguchi,a Jian Wang,a Stephan Irle*a,b and Shigehiro Yamaguchi*a,b
DOI: 10.1039/c2dt32134c
www.rsc.org/dalton
A significantly red-shifted fluorescence and a high fluorescence
quantum yield for the emission from the twisted intramolecular
charge transfer (TICT) state are attained in new aminoboranes,
N-borylated 2,5-diarylpyrroles. A gear like dual structural motion
in the excited state is responsible for their large Stokes shifts.
In the design of new π-electron materials, incorporation of
main group elements into the π-conjugated carbon skeleton is
a powerful strategy for creating intriguing photophysical and
electronic properties.1,2 In particular, an attractive strategy is
to make use of the synergistic effects of two or more main
group elements, which enables the construction of unprece-
dented π-conjugated skeletons with unusual electronic
structures.2–8 In this regard, the combination of boron and
nitrogen, complementary to each other in terms of their elec-
tron-accepting/donating abilities, is of particular interest.
Recent intensive efforts have produced various kinds of fasci-
nating N–B bond-containing π-conjugated compounds.5–12
Concerning the N–B bond-containing π-electron systems, a
notable property is the unusual fluorescence of simple
aminoboranes.9–12 For example, Ph2N–BMes2 shows absorp-
tion (λabs) and emission maxima (λem) at 282 and 456 nm,
Fig. 1 Borylpyrroles: (a) known derivatives and (b) a gear like structural motion
in the excited state.
reduced value of the transition moment, resulting from a
small overlap of the π orbitals in the highly twisted molecular
structure.15 A rational design to attain the TICT emission with
a large Stokes shift and a high quantum yield at the same time
remains a challenge in this field.16,17
As a clue to solving this issue, we now focus on the fact that
a benzene-fused derivative, N-borylindole 2, can show fluo-
rescence with a relatively high ΦF of 0.27 as well as an associ-
ated large Stokes shift of 10 600 cm−1 in CH2Cl2.12 This fact
suggests that appropriate structural modification should
enable the improvement of the fluorescence properties. There-
fore, we decided to study the structure–property relationship
for a series of new aminoboranes, including N-boryl-substi-
tuted carbazoles 3 and pyrroles 4–7. Our idea for the design of
these molecules was to extend the π-conjugation in the excited
state to enhance the ICT character. We found that some of
these molecules exhibit intriguing fluorescence based on a
gear like dual structural change in the excited state (Fig. 1).
N-Borylcarbazoles 3a and 3b with different diarylboryl
groups were synthesized by lithiation of carbazole with nBuLi
in THF followed by treatment with the corresponding diaryl-
fluoroboranes, as shown in Scheme 1. The synthesis of
N-boryl-2,5-diarylpyrroles 4–7 was carried out in refluxing
toluene or xylene by the reaction of the corresponding
N-lithiated diarylpyrroles with dimesitylfluoroborane. High
temperature conditions were necessary to complete the reac-
tion, presumably due to the bulkiness of the 2,5-diarylpyrrole
respectively, in which the Stokes shift exceeds 13 500 cm−1 9
N-Dimesitylborylpyrrole 1 (Fig. 1) also exhibits an emission
with a significantly large Stokes shift of 13 100 cm−1 11
Theor-
.
.
etical studies suggested that these fluorescences can be ration-
alized by considering the emission from the twisted
intramolecular charge transfer (TICT) state.11b,13,14 The emis-
sion properties with such large Stokes shifts are highly attrac-
tive as the fluorescent dyes. However, as is usual for the TICT
system, their quantum yields are low. This is due to the
aDepartment of Chemistry, Graduate School of Science, Nagoya University, Furo,
Chikusa, Nagoya, 464-8602, Japan. E-mail: yamaguchi@chem.nagoya-u.ac.jp,
sirle@chem.nagoya-u.ac.jp
bCREST, Japan Science and Technology Agency (JST), Japan
†Electronic supplementary information (ESI) available: Experimental details for
new compounds, crystallographic data, and calculation results. CCDC 794958
and 794959 for 3a and 4. For ESI and crystallographic data in CIF or other elec-
tronic format see DOI: 10.1039/c2dt32134c
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Table 1 Photophysical data for borylated carbazoles and pyrroles 3–7
Absorption
Fluorescence Stokes shift
c
Compd Solvent
λabs/nma (log ε)b λem/nm (Φf
)
Δλ/cm−1
3a
3b
4
Cyclohexane 320 (4.12)
THF 320 (4.13)
Cyclohexane 369 (3.75)
THF 367 (3.77)
Cyclohexane 351(sh) (3.26)
THF 349(sh) (3.29)
Cyclohexane 342(sh) (3.34)
THF 337(sh) (3.35)
Cyclohexane 379(sh) (3.27)
THF 383(sh) (3.32)
Cyclohexane 357(sh) (3.29)
THF 356(sh) (3.30)
439 (0.13)
8500
9300
455 (0.33)
603 (0.03)
646 (<0.01)
626 (0.07)
657 (0.02)
514 (0.59)
550 (0.37)
641 (0.06)
686 (<0.01)
502 (0.45)
525 (0.37)
10 500
11 800
12 500
13 400
9800
11 500
10 800
11 500
8100
5
6
7
9000
a Only the longest absorption maximum wavelength is given. The peak
top of the shoulder band is determined by a Peak Fit program. b Molar
extinction coefficient at the λabs.
c Absolute fluorescence quantum yield
determined by an integrating sphere system.
Scheme 1 Synthesis of N-borylated carbazoles and pyrroles.
Fig. 2 ORTEP drawings of (a) 3a and (b) 4 (50% probability for thermal ellip-
soids). Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and
torsion angles (°): (a) B1–N1 1.442(3), C1–N1–B1–C7 23.14(8). (b) B1–N1 1.472
(7), C4–N1–B1–C16 28.6(5), C1–N1–B1–C25 41.1(5), N1–C1–C5–C6 76.1(6), N1–
C4–C9–C10A 58.3(12).
Fig. 3 UV-vis absorption (dashed lines) and fluorescence spectra (solid lines) of
4 (orange), 5 (green), 6 (red), and 7 (blue) in cyclohexane.
frameworks. These compounds were isolated by silica gel
column chromatography in air.
Both series of aminoboranes indeed exhibited interesting
photophysical properties, which are summarized in Table 1.
As a representative example, the absorption and fluorescence
spectra of the N-borylpyrroles 4–7 in cyclohexane are shown in
Fig. 3.
Among the produced compounds, the structures of 3a and
4 were determined by X-ray crystallographic analysis, as shown
in Fig. 2. Whereas the N–B bond moiety generally prefers to
take a planar conformation due to its partial zwitterionic
double bond character, these compounds take a slightly
twisted conformation. The dihedral angles between the boron
plane and the carbazole or pyrrole rings are 23.2 and 36.6° for
3a and 4, respectively. Compound 4 has a more twisted confor-
mation due to the heavier steric congestion between the
mesityl groups on the boron atom and the thienyl groups on
the pyrrole. In conjunction with this, the dithienylpyrrole
moiety takes a highly twisted conformation, in which the dihe-
dral angles between the thiophene and pyrrole rings are 60.3
and 77.5°. The N–B bond lengths in 3a and 4 (1.442(3) and
1.472(7) Å, respectively) are slightly elongated compared to a
typical N–B bond length (e.g. Me2N–BMe2, 1.403(1) Å),18
indicative of the weaker N–B double bond character in these
sterically congested molecules.
In the carbazole series, the Mes2B derivative 3a showed the
λabs and λem values at 320 and 439 nm in cyclohexane with the
Stokes shift of 8500 cm−1 19
This value is smaller than that of
.
Ph2NBMes2 (13 500 cm−1 in cyclohexane), but still relatively
large among compounds exhibiting TICT emissions. The
incorporation of the more electron-withdrawing [2,4,6-
(CF3)3C6H2]2B group as the boron moiety resulted in a more
significant red shift in the fluorescence maximum. Thus, 3b
showed an orange fluorescence with the maximum at 603 nm,
which is more than 160 nm longer than that of 3a. Its Stokes
shift reached 10 500 cm−1, although the fluorescence quantum
yield is lower than that of 3a. While 3a does not show a signifi-
cant solvent effect in both the absorption and fluorescence
spectra, compound 3b exhibited a red shift in λem by 43 nm
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from cyclohexane to THF, indicative of its more polar character
in the excited state.
A series of the N-borylpyrroles showed more characteristic
photophysical properties. Thus, 2,5-dithienylpyrrole 4 pos-
sesses a weak absorption band at 351 nm as a shoulder. Upon
excitation at this band, 4 exhibited a large Stokes shift of
12 500 cm−1, and showed an orange emission with λem at
626 nm in cyclohexane. In more polar solvents, such as THF,
its emission band further shifted to longer wavelengths with
increased Stokes shifts (13 400 cm−1 in THF). However, its
quantum yield is only moderate even in cyclohexane. In con-
trast, the 2,5-diphenylpyrrole derivative 5 showed intense
green fluorescence with a ΦF of 0.59 in cyclohexane, while
maintaining a reasonably large Stokes shift (9800 cm−1) for
the TICT emission. We performed time-resolved fluorescence
spectroscopy to determine the fluorescence lifetimes for 4 and
5, which were 8.7 and 64 ns in cyclohexane, respectively. On
the basis of these values and the ΦF values, we determined the
radiative (kr) and nonradiative (knr) decay rate constants from
the singlet excited state: kr = 8.1 × 106 s−1 and 9.2 × 106 s−1
and knr = 1.1 × 108 s−1 and 6.4 × 106 s−1 for 4 and 5, respect-
ively. The low kr values for these compounds are consistent
with the low molar extinction coefficients for their longest
absorption bands. While the knr value for 4 is high, similar to
other molecules possessing TICT emissions, the knr value for 5
is significantly low, demonstrating that the intense emission
of 5 is attributable to the suppressed nonradiative decay
process.
In order to further improve the TICT emission for the N-
borylpyrroles, we synthesized two other derivatives 6 and 7. In
the former compound, we introduced the p-(dimethylamino)
phenyl group as an electron-donating group with a steric bulki-
ness similar to the phenyl group in 5. The latter compound
has a sterically more congested substitution pattern with four
phenyl groups at the periphery. Both compounds have the
longest absorption maxima around 355–380 nm with low
molar extinction coefficients. In the fluorescence spectra, the
aminophenyl-substituted 6 has λem values at 641 and 686 nm
in cyclohexane and THF, respectively, which are even longer
wavelengths than those of the thienyl derivative 4. However,
unfortunately, the ΦF of 6 is as low as 4. Besides, the peripher-
ally all-phenylated 7 possesses a high ΦF of ∼0.4, while the
extent of its Stokes shift (8000–9000 cm−1) is slightly smaller
than that of 5.
Fig. 4 Optimized structures for the S0 (S0) and S1 (S1’) electronic states and an
energy diagram for 3a, calculated at the B3LYP/SV(P) level of theory.
Fig. 5 Optimized structures for the S0 (S0) and S1 (S1’) electronic states and an
energy diagram for 4, calculated at the B3LYP/SV(P) level of theory.
Electronic structure calculations were conducted using the
TURBOMOLE program package at the B3LYP/SV(P) level of
theory. Excited states were treated using time-dependent
density functional theory, which was shown to work well in the
description of TICT states of N–B-containing compounds.14
Fig. 4 shows the optimized structures of 3a in the ground
state (S0) and excited state (S1′) along with a schematic energy
profile for both states. After vertical excitation from the S0
Franck–Condon region to the S1 state, a large structural
change occurs upon relaxation from S1 to the minimum of the
excited state at S1′. The major geometric change during this
relaxation is the rotation around the N–B bond, combined
with an increase in its bond length. Thus, the dihedral angle
between the carbazole plane and the boron plane increases
from 28.8° to 62.4°. In addition, the N–B bond is significantly
elongated from 1.454 Å to 1.598 Å. These structural changes
are similar to those already discussed for other aminobora-
nes.11b We also independently reported the detailed theoretical
analysis of the aminoboranes.14
These results provide the following notable findings: (1)
Both borylcarbazoles and borylpyrroles exhibit TICT emissions
with large Stokes shifts.19 In particular, (2) the electron-donat-
ing aryl-substituted N-borylpyrroles 4 and 6 showed reddish
orange emissions with significantly large Stokes shifts. (3) The
sterically more congested diarylpyrrole derivatives 5 and 7,
compared to 4, have high ΦF, but their Stokes shifts become
somewhat smaller compared to the other derivatives. To eluci-
date these observations and precisely understand the behavior
in the excited state, we next conducted structural optimiz-
ations of the singlet excited state for borylcarbazole 3a and
In contrast, the dithienylpyrrole derivative 4 shows a more
complicated structural relaxation in the excited state from S1
to S1′, as shown in Fig. 5. According to the X-ray structural
analysis, 4 has a more twisted conformation of the N–B bond
in the ground state, in comparison to 3a. These structural fea-
tures are well reproduced in the optimized ground state S0
boryldithienylpyrrole
4
as the representative examples.
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structure. The calculated N–B distance and the dihedral angle
along the N–B bond are 1.482 Å and 38.5°, respectively. The
calculated dihedral angles between the central pyrrole ring
and the two thiophene rings are 65.9° and 66.2°. After the
vertical excitation to the S1 excited singlet state, structural
relaxation occurs from the Franck–Condon geometry at S1 to
the minimum structure of the excited state at S1′. The N–B
bond is significantly elongated to 1.591 Å and twisted with a
dihedral angle between the pyrrole ring and the boron
plane of 57.8°. The degrees of the changes are comparable to
those for the carbazole analogue 3a. However, it is worth
noting that, in addition to these changes, the highly twisted
dithienylpyrrole moiety rotates into a planar conformation.
The dihedral angles between the central pyrrole and the two
thiophene rings are 13.6° and 14.2°. This gear like dual
motion of the twisting/stretching of the B–N bond and the pla-
narization of the diarylpyrrole moiety stabilizes the S1′ energy
level by 1.04 eV and destabilizes the S0′ energy by 0.74 eV.
Notably, the degrees of these energy changes are greater com-
pared to those for the carbazole analogue 3a. Consequently,
the boryl-substituted dithienylpyrrole 4 does show the larger
Stokes shift.
These quantum chemical calculations demonstrate that the
fluorescences of the boryl-substituted carbazoles and 2,5-dia-
rylpyrroles are indeed derived from the TICT state.19 In the 2,5-
diarylpyrroles, the dual structural motion to form the TICT
state is the origin of the significant Stokes shift. In particular,
the twisted conformation of the diarylpyrrole moiety in the
ground state is crucial to attain the present fluorescence,
because if the 2,5-diarylpyrrole skeleton has a coplanar confor-
mation in the ground state, these compounds would only
show an emission by the π–π* transition for the diarylpyrrole
moiety, instead of the TICT emission of the N–B moiety. The
planarization of the 2,5-diaryl moiety in the excited state at S1′
increases the electron-donating ability, since the positive
charge on the N atom becomes stabilized by greater π delocali-
zation. Therefore, the more electron-donating aryl groups, like
the thienyl or p-(amino)phenyl groups, enhance the ICT char-
acter, resulting in the more red-shifted emission. Besides, the
degree of the planarization of the 2,5-diarylpyrrole moiety in
the excited state at S1′ is the other factor to attain the signifi-
cant Stokes shift. The sterically congested diphenyl- or tetra-
phenyl-substituted compounds 5 and 7 exhibited a less
significant Stokes shift. However, worth noting is that these
structures realize high fluorescence quantum yields of 0.4–0.6,
which is among the highest for the TICT emission. These
results not only provide examples of the emissive TICT mol-
ecular systems, but also important knowledge for further mol-
ecular designs of more elaborate fluorescent molecular
systems. In particular, precise control of the structural change
in the excited state will be the key for further designs.
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19 The fluorescence maxima of 3a and 3b in the crystalline
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624 | Dalton Trans., 2013, 42, 620–624
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