Indole-Based Aza[n]helicenes (n=5, 6) with Violet-Blue Fluorescence and Two-Photon Absorption (TPA)

Article abstract of DOI:10.1002/cplu.202000178

A series of indole-based helicenes, namely, 15-hexyl-15H-tetraphenyl[1,2-e]indole (HTPI), 14-hexyl-14H-benzo[4’,5’]thieno [2’,3’:7,8]naphtha[1,2-e]indole (HBTNI), 7-hexyl-7H-indolo[5,4-k] phenanthridine (HIPD) and 3-hexyl-3H-phenanthro[4,3-e]indole (HPI)

Full text of DOI:10.1002/cplu.202000178

Accepted Article
Title: Indole-Based Aza[n]helicenes (n = 5, 6) with Violet-Blue
Fluorescence and Two-Photon Absorption (TPA)
Authors: Zhenhao Hu, Li Li, Zhi Liu, Zhiqiang Liu, Dingchao Zhang,
and Kunlun Li
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Indole-Based Aza[n]helicenes (n = 5, 6) with Violet-Blue
Fluorescence and Two-Photon Absorption (TPA)
Zhenhao Hu+, Li Li+, Zhi Liu,* Zhiqiang Liu, Dingchao Zhang, and Kunlun Li[a]
[a]
Z. H. Hu,[+] L. Li,[+] Prof. Dr. Z. Liu, Prof. Dr. Z. Q. Liu, D. C. Zhang, K. L. Li
State Key Laboratory of Crystal Materials
Shandong University
Jinan 250100 (P. R. China)
E-mail: lz@sdu.edu.cn
These authors contributed equally to this work.
[+]
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/cplu.202000178.
Abstract: New indole-based helicenes, namely, 15-hexyl-15H-
tetraphenyl[1,2-e]indole (HTPI), 14-hexyl-14H-benzo[4’,5’]thieno
[2’,3’:7,8]naphtha[1,2-e]indole (HBTNI), 7-hexyl-7H-indolo[5,4-k]
phenanthridine (HIPD) and 3-hexyl-3H-phenanthro [4,3-e]indole (HPI)
were successfully synthesized by a three-step or four-step reaction.
They exhibited good solubility and high thermal stability of Td = 247,
388, 294 and 251°C, respectively. These compounds emitted violet-
blue light with maximum emission peaks at 415, 397, 397 and 391
nm in hexane. Among them, HBTNI had excellent thermal stability,
narrow and sharp emission peaks, and highest photoluminescence
quantum yields. Thus, HBTNI was an ideal candidate for the violet-
blue emitters of OLEDs. Furthermore, four compounds had two-
photon absorption and two-photon excited fluorescence. HTPI
achieved the maximum TPA with the TPA cross-section (δ) of 171.5
GM at 770 nm. They were rare examples of helicenes with both
violet-blue emission and TPA.
[4,3-e]indole (HPI), were prepared from commercially available
1H-indole-5-carbaldehyde (ICD). Up to date, it is not reported
that helicenes have both violet-blue fluorescence and TPA.
As for violet-blue luminescence materials, violet-blue-colored
(<450 nm) light-emitting diodes (LEDs) are few[10] and costly to
fabricated, but suitable for applications in optical detectors,[11]
fluorescence-based chemical and biological sensors,[12] and
wide-color gamut, full-color displays.[13] Nowadays, violet InGaN-
based LEDs are made by way of expensive high-temperature
and high-vacuum thin-film growth techniques.[14] Therefore, it is
of vital importance to seek inexpensive and easily processed
organic materials as the emitters in violet-blue LEDs. Likewise,
helicenes with TPA and Two-photon excited fluorescence (TPEF)
are also rare. Grafting of one or two tetracyanobutadienes in
positions
2
and 15 on carbo[6]helicenes achieved the
experimental δ in the range 5 - 40 GM for mono-substituted
1b,[4b] similar to those of 2-cyano-carbo[6]helicene,[4a] and 1-6
GM for bis-substituted helicene 1c.[4b] Instead of the substitution
by electron-accepting groups in the helical peripheral, the title
compounds were constructed via combinating indole unit with
anthracene unit, and so on in donor–acceptor helical cores.
They had TPA, TPEF, and HTPI achieved the maximum TPA
with δ = 171.5 GM at 770 nm, which amounted to approximately
5-fold compared with 1b and 2-cyano-carbo[6]helicene. The
occurrence of an undecabenzo[7]carbohelicene[4c] demonstrated
the huge potential of helicenes for TPA materials once again.
Introduction
Helicenes have attracted considerable attention in the last
decades owing to their intriguing structural features and physical
properties,[1] and various applications in the fields of non-linear
optics (NLO),[2] circularly polarized luminescence (CPL),[3] two-
photon absorption (TPA),[4] electronic and optoelctronic
devices.[5] Heterohelicenes can be regarded as heteroatom-
substituted helicenes at no less than one carbon position in the
screw skeleton of carbohelicenes. They constitute an important
class of helicenes with characteristics properties and diverse
biological activities.[6] Small core or peripheral modifications of
helical structures might give rise to a great difference in the
physicochemical properties, which prompts us to explore new
heterohelicenes through structural modifications.
Indole is easily modified and a fundamental structural unit in
a myriad of natural products, pharmaceutical agents.[7] It is not
only an electron-rich molecule but also a potential hydrogen
bond donor.[8] Indole units have been utilized as good building
blocks for red electroluminescent materials.[9] Incorporation of
indole unit with anthracene, dibenzothiophene, quinoline or
naphthalene unit into aza[n]helicenes (n = 5, 6) generates the
title compounds HTPI, HBTNI, HIPD and HPI (Figure 1). Herein,
four novel indole-based aza[n]helicenes (n = 5, 6), that is, 15-
hexyl-15H-tetraphenyl[1,2-e]indole (HTPI), 14-hexyl-14H-benzo
[4’,5’]thieno[2’,3’:7,8]naphtha[1,2-e]indole (HBTNI), 7-hexyl-7H-
indolo[5,4-k]phenanthridine (HIPD) and 3-hexyl-3H-phenanthro
Figure 1. Chemical structures of HTPI, HBTNI, HIPD and HPI.
1
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Scheme 1. Synthesis of HTPI, HBTNI, HIPD and HPI. a) PPh3, PhMe, 90°C , 12 h. b) 1-Bromohexane, KI, K2CO3, Acetone, 100°C , 48 h. c) [Ph3PCH3]+I-, KOtBu,
THF, -10°C , 10 min. d) Pd(OAc)2, K2CO3, 2-Bromoanthracene, Tri(o-tolyl)phosphine, DMF, 100°C , 48 h. e) I2, PO, Benzene, uv, 30 min. f) Pd(OAc)2, K2CO3, 2-
Bromodibenzothiophene, Tri(o-tolyl)phosphine, DMF, 100°C , 48 h. g) Pd(OAc)2, K2CO3, 3-Bromoquinolin, Tri(o-tolyl)phosphine, DMF, 80°C , 48 h. h) KOtBu, THF,
-10°C , 15min. i) I2, PO, Benzene, uv, 90 min.
The thermal behavior of HTPI, HBTNI, HIPD and HPI was
examined by differential scanning calorimetry (DSC) and
Results and Discussion
thermogravimetric analysis (TGA). Their decomposition
temperatures (Td, corresponding to 5% weight loss) were
determined at 247°C, 388°C, 294°C and 251°C , respectively
(Figure 2). Especially, HBTNI showed a high melting point (Tm)
of 136°C, close to Tm = 130.9°C of the carbazole-based
monoaza[6]helicene,[23] and a high Td of nearly 400°C, which
was favorable to form uniform films upon thermal evaporation.[24]
Synthesis and characterization
Over the past two decades, various approaches to synthesizing
heterohelicenes have been further developed, such as
photocyclization,[15] substitution reaction,[16] metal-catalyzed
cyclization,[17] the Diels-Alder reaction,[18] the Ramberg-Bäcklund
rearrangement,[19] homolytic aromatic substitution reaction.[20]
Herein, HTPI, HBTNI, HIPD and HPI, were facilely prepared
through a three-step or four-step reaction including the key step
photocyclization. HIC was synthesized by introducing the hexyl
group to ICD at first, and then HVI and HNVI were obtained
through Wittig reaction within 10 and 15 minutes.[21] Then, HVI
reacted with 2-Bromoanthracene, 2-Bromodibenzothiophene or
3-Bromoquinoline through Mizoroki-Heck reaction to afford AVHI,
DTVHI and HIVQ, respectively.[22]At last, HTPI, HBTNI, HIPD
and HPI were harvested through photocyclization reaction using
the Hanovia high-pressure mercury lamp in 30 or 90 minutes
(Scheme 1). 1H NMR, 13C NMR and HRMS of synthetic
compounds, and single crystal structure data for HIVQ (CCDC
1961630) were contained in the Figures S1-S24, Table S7 and
Figure S29.
Thermal properties
Figure 2. TG and DSC curves of (a) HTPI, (b) HBTNI, (c) HIPD and (d) HPI.
2
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These results manifested that HBTNI was most thermally stable
and suitable for vacuum thermal evaporation for device
fabrication among them.
HPI in hexane and in thin-films. The photophysical parameters
were presented in Table S1 and Table S2. HTPI, HBTNI, HIPD
and HPI had absorption maximum peaks at 299, 298, 228 and
229 nm in hexane, which were mainly due to the HOMO – 1 →
LUMO + 2, HOMO – 1 → LUMO + 1, HOMO – 5 → LUMO and
HOMO – 1 → LUMO + 3 transitions, respectively (Figure 5 and
Tables S3-S6). The HOMO energy level of HTPI by theoretical
calculations was almost identical to that from CV, while HOMOs
energy levels of HBTNI, HIPD and HPI were a little lower than
those from CV (Figures 3 and S28). The considerably blue-
shifted absorption maxima of HIPD and HPI compared with
HTPI and HBTNI was attributed to the decreased π-conjugation
length. In addition, they all exhibited several weak absorptions in
the range of 370 - 390 nm, which were characteristic absorption
of helicenes.[26] As for HTPI and HIPD, the weak absorption
bands in the range of 400 - 420 nm were assigned to the
intramolecular charge transfer (ICT) transition from indole group
to anthracene group or quinoline group.[27] Redshifts for HTPI,
HBTNI, HIPD and HPI in films compared to those in hexane
were found.
Electrochemical properties
Cyclic voltammetry (CV) was used to identify the electrochemical
behavior of HTPI, HBTNI, HIPD and HPI (Figure 3). The
oxidation peak of HTPI was located at 1.54 V with the onset
oxidation potential of 0.96 V while that of HBTNI was at 1.10 V
with the onset oxidation potential of 0.80 V. Introducing electron-
donating atom S to the HTBNI resulted in the lower oxidation
potential and onset oxidation potential compared with HTPI. This
exhibited that HTPI had more tolerance to oxidation and air-
stable than HTBNI.[3b] In contrast, the onset oxidation potential of
HPI was at 0.76 V with the oxidation peak of 1.30 V and the
reduction peak of -0.04 V while that of HIPD was determined at
0.75 V. Thus, the redox processes of HBTNI, HIPD were
thought to be irreversible and HTPI, HPI were quasi-reversible.
The HOMOs (highest occupied molecular orbital) energy levels
of HTPI, HBTNI, HIPD and HPI were calculated to be -5.20 eV, -
5.04 eV, -5.00 eV and -4.99 eV, respectively. Correspondingly,
the LUMOs (lowest unoccupied molecular orbital) energy levels
of HTPI, HBTNI, HIPD and HPI were determined from the
optical band gaps and energy levels of HOMOs, whose values
were -2.34 eV, -2.03 eV, -2.12 eV and -2.07 eV, respectively. In
addition, energy levels of HOMOs and LUMOs for these
[6]helicenes were in the order HTPI  the carbazole-based
monoaza[6]helicene[23]  HBTNI, indicating that HTPI had
excellent electrochemical stablity.[25]
As shown in Figure 4, HTPI, HBTNI, HIPD and HPI displayed
well-defined vibronic structures in PL spectra of hexane
solutions, indicating that they had a rigid backbone structure.[28]
These compounds exhibited maximum emission peaks at 415
nm, 397 nm, 397 nm and 391 nm, respectively, which were rare
violet light. The majority of helicenes emitted blue light up to
date.[29] These compounds revealed two high fluorescence
peaks at the range of 390 - 440 nm, corresponding to 0 → 0 and
0
→ 1 radiative transitions, respectively. The emission
bathochromic shifts and broadening of HTPI, HIPD and HPI in
films as relative to those of hexane solutions were pronounced.
It might be attributed to two causes: (i) relatively strong π-π
interactions in the excited solid state and (ii) self-absorption
phenomenon.[30] From solution to film, the ignorable emission
bathochromic shift of HBTNI demonstrated the suppressed
intermolecular aggregation. In addition, the strong peak
weakened and sub-strong emission peak became the main
fluorescence peak in thin-film for HBTNI, because the PL
spectrum overlapped with absorption spectrum, resulting in the
Ultraviolet-visible (UV-vis) absorption and single-photon
excited fluorescence (SPEF).
HTPI, HBTNI, HIPD and HPI were soluble in the common
solvents such as anhydrous hexane, dichloromethane, ethyl
acetate, trichloromethane and methanol. Figure 4 showed the
absorption and fluorescence spectra of HTPI, HBTNI, HIPD and
Figure 4. Normalized absorption and PL spectra of (a) HTPI, (b) HBTNI, (c)
HIPD, and (d) HPI.
Figure 3. Cyclic voltammograms of (a) HTPI, (b) HBTNI, (c) HIPD and (d) HPI.
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excitation wavelength of 770 nm was shown in Figure 6b. HTPI
had emission band in the region of 405 - 600 nm, and the
maximum emission peak was at 450 nm. The TPEF spectrum
and SPEF spectrum of HTPI had almost similar patterns,
revealing the same excited state for the radiative decay
processes.[34] Values of HTPI two-photon absorption cross-
sections at different excitation wavelengths were shown in
Figure 6c. HTPI achieved maximum two-photon absorption at
excitation wavelengths of 730 - 870 nm, and the maximum δ
was 171.5 GM at 770 nm. In comparison, HBTNI, HIPD and HPI
had maximum δ of 4.2 GM at 750 nm, 2.5 GM at 750 nm, and
3.4 GM at 730 nm, respectively (Figure S27).
Although the absorption spectra of HTPI, HBTNI, HIPD and
HPI, which located at 298 - 420, 298 - 397, 228 - 410 and 229 -
389 nm, overlapped with the two-photon absorption band, HTPI
had the strongest absorption and others had weak absorption in
the region of 370 - 430 nm (Figure S25). Therefore, HTPI had
more opportunities to undergo two-photon absorption.[35] HTPI,
HBTNI, HIPD and HPI presented a screw-shaped nonplanar
structure, which resulted in conjugation blocking.[23] However,
HTPI had a larger π-conjugated system compared with HBTNI,
Figure 5. TD-DFT calculated absorption spectra of (a) HTPI, (b) HBTNI, (c)
HIPD and (d) HPI compared with the experimental absorption spectra of HTPI,
HBTNI, HIPD and HPI in methanol.
self-absorption
phenomenon
at
short
wavelengths.[31]
Photoluminescence quantum yields (PLQY) of HTPI, HBTNI,
HIPD and HPI in solvents were measured ranging from 8% -
21%, 12% - 32%, 3% - 15%, and 6% - 13%, respectively (Tables
S1 and S2). The PLQY of HIPD and HPI were lower than those
of HTPI and HBTNI, due to reduced π-conjugation of HIPD and
HPI. In addition, HBTNI had the highest oscillator strength of
nearly 0.1 for electronic transitions from HOMOs to LUMOs
(Tables S3-S6). This demonstrated that HBTNI possessed more
characteristic of allowed transition, which might be favorable for
efficient fluorescence. Especially, the maximum emission peaks
of HBTNI in solvents and thin-film were blue-shifed by ca. 24 -
26 nm and 22 nm compared with the carbazole-based
monoaza[6]helicene.[23] Meanwhile, the PLQY of HBTNI were
higher than those of the carbazole-based monoaza[6]helicene
(17% - 21%). Therefore, HBTNI exhibited better fluorescence
performance than the carbazole-based monoaza[6]helicene.
Furthermore, Td of HBTNI was 141°C higher than that of HTPI
while Td of HIPD was ca. 43°C higher than that of HPI. This
indicated more heteroatoms in the screw skeleton could give
rise to increase of Td (Figure 2). These results indicated that
heteroatoms such as sulfur and nitrogen of HBTNI might avail to
increase thermal stability and fluorescence quantum yield.[32]
TPA and TPEF
Coumarin 307 of 1  10-4 M in MeOH was selected as reference
for δ calculations.[33] Both the SPEF parameters and the TPEF
properties of compounds HTPI, HBTNI, HIPD, HPI, Fluorescein
and Coumarin 307 were displayed in Table 1. Power-squared
dependence of two-photon-excited fluorescence for HTPI at the
excitation wavelength of 800 nm was investigated. As show in
Figure 6a, when the laser power was lower than 350 mW,
power-squared dependence of two-photon excited fluorescence
was linear. Thus, laser power was set as 235 mW in the
measurement to assure that all the fluorescence came from two-
photon excitation. The test results revealed that four compounds
had two-photon absorption and TPEF at excitation wavelengths
of 730 - 870 nm. Among them, HTPI achieved the maximum
TPA and TPEF (Figure S26). The TPEF spectrum of HTPI at the
Figure 6. (a) Power-squared dependence of two-photon-excited fluorescence
for HTPI at the excitation wavelength of 800 nm. (b) The TPEF spectrum of
HTPI at the excitation wavelength of 770 nm and the SPEF spectrum of HTPI
at the excitation wavelength of 340 nm in dichloromethane. (c) δ values of
HTPI at different excitation wavelengths.
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Table 1. Comparison of the photophysical properties of compounds with commercial dyes.
Compounds
HTPI
ε[a] [M-1 cm-1]
149323
87067
f[b]
0.18
0.32
0.15
0.13
0.95
0.56
/
δ[c] [GM]
171.5
4.2
δ  f[d] [GM]
Solvents
30.9
1.3
0.4
0.4
37
11[f]
/
Dichloromethane
Dichloromethane
Acetonitrile
HBTNI
HIPD
104950
927261
89350
2.5
HPI
3.4
Dichloromethane
PBS[e]
Fluorescein
Coumarin 307
1b[g]
38
17600
19[f]
MeOH
42000[g]
65000[g]
33[g]
5.5[g]
Dichloromethane
Dichloromethane
1c[g]
/
/
[a] ε (M-1 cm-1): molar absorptivity at maximum absorption wavelengths (HTPI: 302 nm, HBTNI: 301 nm, HIPD: 228 nm, HPI: 233 nm, Fluorescein: 490 nm,
Coumarin 307: 396 nm, 1b: 280 nm, 1c: 280 nm ). [b] f: compounds quantum yield at the maximum absorption wavelengths determined using quinine sulfate (
= 0.55) as reference. [c] δ (GM): (HTPI: λex = 770 nm, HBTNI: λex = 750 nm, HIPD: λex = 750 nm, HPI: λex = 730 nm, Fluorescein: λex = 780 nm, Coumarin 307: λex
= 776 nm, 1b and 1c: λex = 690 nm ). [d] δ  f (GM): TPEF action cross-sections (the excitation wavelengths are consistent with those of δ). Concentration of
HTPI, HBTNI, HIPD, HPI and Coumarin 307 for TPEF: 1  10-4 M. [e] PH = 13. [f] Ref.[33] [g] 1b and 1c were reported in Ref.[4b]
HIPD and HPI due to its linear component anthracene.
Theoretical studies have shown that the large π-conjugated
system can effectively increase the probability of two-photon
absorption.[36] In addition, HTPI also possessed a relatively
strong electron-accepting group anthracene (Figure S28). Hence
HTPI possessed the TPA cross-section of 171.5 GM at 770 nm.
The two-photon absorption parameters of HTPI were much
higher than those of Coumarin 307. This confirmed that core
structural changes of heterohelicenes could enhance two-photon
absorption performance.
helicenes with both violet-blue fluorescence and TPA. The study
disclosed that helicenes with characteristic properties could be
acquired through core modifications of helical structures. This
class of helicenes was promising for potential applications in
two-photon excited fluorescence probes, optoelectronics, and
other fields.
Experimental Section
This work aimed to synthesize new indole-based helicenes
with special properties. We found that these compounds had
violet-blue fluorescence and two-photon absorption properties.
As is known, the geometry of a helicene results in π-conjugation
blocking for wide band gaps and the nonplanar twist structure
which reduces molecular aggregations.[23, 37] Therefore, indole-
embedded helicenes could exhibit more fluorescence of the
indole unit and so on. Fluorescence peaks of N-Alkyl-5-
arylindole-3-carboxaldehydes mainly located within 325-425
nm.[38] Thus, indole-embedded helicenes can emit violet-blue
light due to π-conjugation blocking. In addition, HTPI had a
larger π-conjugated system due to its linear component and
relatively strong electron-accepting group, anthracene.
Consequently, HTPI possessed the largest TPA cross-section
among them.
Materials and Methods
Reagents were used as purchased without further purification unless
otherwise stated. DMF was dried with molecular sieves. THF was
refluxed with sodium in the presence of benzophenone. HTPI, HBTNI,
HPI and HIPD were synthesized by photocyclization using the Hanovia
high-pressure mercury lamp (500 W). 1H NMR and 13C NMR spectra
were recorded on a Bruker Advance 400 spectrometer. Matrix-assisted
laser-desorption/ionization time-of-flight (MALDI-TOF) mass spectrum
(MS) was recorded on a Bruker Biflex III mass spectrometer that was
equipped with a 337 nm nitrogen laser. High resolution mass spectra
(HRMS) were recorded on a Q-TOF6510 spectrograph (Agilent). X-ray
diffraction intensity data were collected on a Bruker APEX3 (Bruker, 2017)
area-detector diffractometer equipped with graphite-monochromated Cu-
Kα radiation (λ = 1.54178 Å) at the temperature of 120 K. The intensity
data were processed by using the Bruker SMART routine and the
structure was solved by using Bruker APEX3 (Bruker, 2017) and refined
by a full-matrix least-squares technique based on F2 with the SHELXL-
2014/7 (Sheldrick, 2014) program.
Conclusion
Thermogravimetric analysis (TGA) and Differential scanning calorimetry
(DSC) curves were obtained on a SDT Q600 V8.3 Differential Scanning
Calorimeter under nitrogen atmosphere with heating and cooling rates of
10°C min-1 from room temperature to 600°C.
In summary, HTPI, HBTNI, HIPD and HPI were synthesized
successfully. Their thermal, electrochemical, UV-vis absorption,
SPEF, TPA properties, and DFT calculations were investigated.
HTPI, HBTNI, HIPD and HPI emitted violet-blue light with
maximum emission peaks at 415 nm, 397 nm, 397 nm and 391
nm in hexane. Among them, HBTNI was most thermally stable
and had best fluorescence performance. Furthermore, the four
compounds had TPA and TPEF, and HTPI achieved the
maximum δ = 171.5 GM at 770 nm. They were rare examples of
Cyclic voltammetry (CV) measurements of HTPI, HBTNI, HIPD and HPI
were run on a CHI660D electrochemical workstation. A three-electrode
cell with a glassy carbon working electrode, a platinum auxiliary electrode
and a saturated Ag/AgCl reference electrode was used.[23] The potential
of the Ag/AgCl reference electrode in CH2Cl2 (0.1 M tetrabutylammonium
perchlorate) was calibrated by using the ferrocene/ferrocenium (Fc/Fc+)
5
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redox system.[39] In this arrangement, the half-wave potential of Fc/Fc+
added dropwise into the reaction mixture. After addition, the mixture was
stirred for 10 min. A few drops of water were added to quench the
reaction. The mixture was extracted by CH2Cl2 (3  100 mL). The organic
layer was washed with water for three times (3  100 mL) and dried over
anhydrous Na2SO4. The solvent was removed in vacuo. The residue was
purified by flash chromatography on silica gel (petroleum ether) to give a
(E1/2,FOC) was measured to be 0.56 V (vs. Ag/AgCl). EHOMO = -e(Eonset(ox)
-
E1/2,FOC) - 4.8 eV.[40] The optical band gaps were estimated from the
onset wavelength of the Ultraviolet-visible (UV-vis) absorptions.[41]
Ultraviolet-visible (UV-vis) absorption spectroscopy was performed on a
TU-1800 spectrophotometer (Hitachi). PL spectra were recorded on a
Hitachi F-4500 fluorescence spectrophotometer. The neat films were
prepared by spin-coating on SiO2 glass slides. Fluorescence quantum
yields were calculated by using quinine sulfate in 0.1 M H2SO4 solution
as reference.[42]
1
cololess liquid HVI (310 mg). Yield: 42 %. H NMR (400 MHz, CDCl3): δ
= 7.55 (d, J = 1.2 Hz, 1H; Ar-H), 7.27 (dd, J = 5.0 Hz, 1H; Ar-H), 7.19 (t, J
= 8.8 Hz, 1H; Ar-H), 6.99 (d, J = 3.2 Hz, 1H; Ar-H), 6.76 (dd, J = 14.4 Hz,
1H; Ar-H), 6.39-6.37 (m, 1H; Ar-H), 5.62 (dd, J = 9.2 Hz, 1H; Ar-H), 5.05
(dd, J = 5.8 Hz, 1H; Ar-H), 4.01 (t, J = 7.2 Hz, 2H; hexyl-H), 1.74 (t, J =
7.2 Hz, 2H; hexyl-H), 1.21 (t, J = 6.4 Hz, 6H; hexyl-H), 081-0.77 (m, 3H;
hexyl-H) ppm. 13C NMR (400 MHz, CDCl3): δ = 136.91, 134.80, 128.13,
127.64, 127.29, 118.47, 118.37, 109.60, 108.41, 100.18, 45.47, 30.39,
29.20, 25.61, 21.50, 12.98 ppm. HRMS: calcd for C16H21N [M+H]+:
228.1746; found: 228.1702.
Two-photon excited fluorescence (TPEF) was measured using
a
SpectroPro300i and the pump laser beam came from a mode-locked
Ti:sapphire laser system at the pulse duration of 220 fs and a repetition
rate of 76 MHz (Coherent Mira900-D). The excitation source of the TPEF
spectrometer had wavelengths of ca. 730 - 870 nm. TPEF of compounds
HTPI, HBTNI, HIPD and HPI at different excitation wavelengths under
the room temperature was systematically investigated in CH2Cl2 or
acetonitrile. TPA cross-sections have been measured using the two-
photon induced fluorescence method. Coumarin 307 in MeOH was
selected as reference.[33] The cross-sections can be calculated using
Equation (1):
(E)-5-(2-(anthracen-2-yl)vinyl)-1-hexyl-1H-indole (AVHI): K2CO3 (691
mg, 5.00 mmol), 2-Bromoanthracene (1286 mg, 5.00 mmol), Tri(o-
tolyl)phosphine (1522 mg, 5.00 mmol), Palladium acetate (112 mg, 0.50
mmol) and HVI (1136 mg, 5.00 mmol) were added into anhydrous DMF
(30 mL) under argon atmosphere. After bubbling argon for 30 min, the
reaction mixture was then stirred at 100°C for 48 h under argon
atmosphere. A few drops of water were added to quench the reaction.
The mixture was extracted by CH2Cl2 (3  100 mL). The organic layer
was washed with water for three times (3  100 mL) and dried over
anhydrous Na2SO4. The solvent was removed in vacuo, the residue was
purified by flash chromatography on silica gel (petroleum ether/EtOAc,
r cr nr F
s
(1)
s =
r s cs ns F
r
Where the subscripts s and r refer to the sample and the reference
materials, respectively. δ is the TPA cross-section value, c is the
concentration of the solution, n is the refractive index of the solution, F is
the two-photon excited fluorescence integral intensity and  is the
fluorescence quantum yield.
1
40:1 v/v) to give a yellow solid AVHI (585 mg). Yield: 29%. H NMR (400
MHz, CDCl3): δ = 8.35 (d, J = 2.4 Hz, 2H; Ar-H), 7.97-7.92 (m, 4H; Ar-H),
7.79 (d, J = 8.4 Hz, 2H; Ar-H), 7.51-7.22 (m, 6H; Ar-H), 7.08 (d, J = 2.0
Hz, 1H; Ar-H), 6.51 (d, J = 2.4 Hz, 1H; Ar-H), 4.08 (t, J = 7.2 Hz, 2H;
hexyl-H), 1.83 (t, J = 6.8 Hz, 2H; hexyl-H), 1.30 (s, 6 H; hexyl-H), 0.87 (t,
J = 6.8 Hz, 3H; hexyl-H) ppm. 13C NMR (400 MHz, CDCl3): δ = 135.98,
135.10, 132.18, 132.15, 131.64, 131.15, 130.68, 129.00, 128.98, 128.50,
128.48, 128.24, 128.12, 126.13, 126.04, 125.44, 125.20, 123.22, 120.11,
119.94, 109.77, 101.42, 46.57, 31.45, 30.29, 26.69, 22.56, 14.02 ppm.
HRMS: calcd for C30H29N [M+H]+: 404.2372; found: 404.2329.
Theoretical calculations were carried out using the Gaussian 09
program.[43] The molecular geometry was optimized by density functional
theory (DFT) methods at the B3LYP function and the 6-31 G(d) basis
sets. Vertical electronic transitions were obtained by time-dependent
density functional theory (TD-DFT) methods at the B3LYP/6-31 G(d)
level. The UV-vis absorption spectra were calculated by the Origin
program, revision 9.1, at the Gaussian model.
15-hexyl-15H-tetraphenyl[1,2-e]indole (HTPI): Argon was bubbled
through a solution of AVHI (182 mg, 0.45 mmol) in benzene (500 mL)
with stirring for 30 min. After I2 (117 mg, 0.46 mmol) and propylene oxide
(18 mL, 258.00 mmol) were added into the solution, the reaction mixture
was irradiated by a 500 W high pressure mercury lamp through a quartz
filter. The reaction was stopped 30 min later. The residue was dissolved
in CH2Cl2 (20 mL) when the solvent had been removed in vacuo. The
solution was then washed with aqueous Na2S2O3 (60 mL, 15%) and
water sequentially. The organic layer was purified by flash
chromatography on silica gel (petroleum ether/EtOAc, 25:1 v/v) to give a
yellow solid HTPI (18 mg). Yield: 10%. 1H NMR (400 MHz, CDCl3): δ =
8.37 (s, 2H; Ar-H), 7.98 (d, J = 9.2 Hz, 4H; Ar-H), 7.82-7.79 (m, 1H; Ar-H),
7.59 (s, 1H; Ar- H), 7.56-7.53 (m, 1H; Ar-H), 7.48-7.40 (m, 2H; Ar- H),
7.35 (s, 1H; Ar-H), 7.32- 7.24 (m, 1H; Ar-H), 7.18 (s, 1H; Ar-H), 4.10 (t, J
= 7.2 Hz, 2H; hexyl-H), 1.82 (t, J = 6.4 Hz, 2H; hexyl-H), 1.34-1.25 (m,
6H; hexyl-H), 0.88 (t, J = 7.2 Hz, 3H; hexyl-H) ppm. 13C NMR (400 MHz,
CDCl3): δ = 136.02, 134.83, 132.34, 132.14, 132.10, 131.69, 131.18,
130.85, 130.09, 129.99, 128.57, 128.24, 128.12, 126.91, 126.47, 126.13,
126.08, 125.48, 125.28, 123.10, 121.24, 119.93, 110.04, 55.51, 46.95,
31.39, 30.33, 26.60, 22.51, 14.00 ppm. MS (MALDI-TOF): calcd for
C30H27N [M+H]+: 403.22; found: 403.02. HRMS: calcd for C30H27N [M+H]+:
403.2177; found: 403.2256.
Synthesis
Bromo(naphthalen-2-ylmethyl)triphenylphosphorane (BNTP): BNTP
was synthesized referring to the literature by our research group.[23]
1-hexyl-1H-indole-5-carbaldehyde (HIC): Potassium iodide (200 mg,
1.20 mmol), 1H-indole-5-carbaldehyde (ICD) (1462 mg, 10.00 mmol) and
K2CO3 (2073 mg, 15.00 mmol) were added into acetone (30 mL) under
argon atmosphere. Then, 1-Bromohexane (C6H13Br, 2 mL, 15.00 mmol)
dissolved in acetone (15 mL) was slowly added dropwise into the
reaction mixture. After addition, the reaction mixture was then stirred at
100°C for 48 h under argon atmosphere. A few drops of water were
added to quench the reaction. The mixture was extracted by CH2Cl2 (3 
100 mL). The organic layer was washed with water for three times (3 
100 mL) and dried over anhydrous Na2SO4. The solvent was removed in
vacuo, the residue was purified by flash chromatography on silica gel
(petroleum ether/EtOAc, 40:1 v/v) to give a yellow liquid HIC (894 mg).
Yield: 39%. HRMS: calcd for C15H19NO [M+H]+: 230.1539; found:
230.1513.
(E)-5-(2-(dibenzo[b,d]thiophen-2-yl)vinyl)-1-hexyl-1H-indole (DTVHI):
2-Bromodibenzothiophene (1316 mg, 5.00 mmol), K2CO3 (691 mg, 5.00
mmol), Tri(o-tolyl)phosphine (1522 mg, 5.00 mmol), Palladium acetate
(112 mg, 0.50 mmol) and HVI (1136 mg, 5.00 mmol) were added into
anhydrous DMF (30 mL) under argon atmosphere. After bubbling argon
for 30 min, the reaction mixture was then stirred at 100°C for 48 h under
1-hexyl-5-vinyl-1H-indole (HVI): Methyltriphenylphosphonium iodide
([Ph3PCH3]+I-, 1314 mg, 3.25 mmol) was dissolved in dry THF (10 mL)
under argon atmosphere and the solution was cooled to -10°C.
Potassium t-butoxide (KOtBu, 480 mg, 4.28 mmol) was added and stirred
for 1 min. A solution of HIC (745 mg, 3.25 mmol) in dry THF (10 mL) was
6
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10.1002/cplu.202000178
ChemPlusChem
FULL PAPER
argon atmosphere. A few drops of water were added to quench the
reaction. The mixture was extracted by CH2Cl2 (3  100 mL). The organic
layer was washed with water for three times (3  100 mL) and dried over
anhydrous Na2SO4. The solvent was removed in vacuo, the residue was
purified by flash chromatography on silica gel (petroleum ether) to give a
white solid DTVHI (593 mg). Yield: 29%. 1H NMR (400 MHz, CDCl3): δ =
8.25 (d, J = 1.2 Hz, 1H; Ar-H), 8.23-8.19 (m, 1H; Ar-H), 7.87-7.82 (m, 1H;
Ar-H), 7.79 (t, J = 3.6 Hz, 2H; Ar-H), 7.67 (dd, J = 4.2 Hz, 1H; Ar-H),
7.51-7.43 (m, 3H, Ar-H), 7.35 (t, J = 8.0 Hz, 2H, Ar-H), 7.25 (t, J = 4.8 Hz,
1H, Ar-H), 7.10 (s, 1H, Ar-H), 6.51 (d, J = 2.0 Hz, 1H; Ar-H), 4.11 (t, J =
7.2 Hz, 2H; hexyl-H), 1.84 (m, 2H; hexyl-H), 1.5-1.27 (m, 6H; hexyl-H),
0.88 (t, J = 6.8 Hz, 3H; hexyl-H) ppm. 13C NMR (400 MHz, CDCl3): δ =
139.94, 137.90, 136.04, 135.91, 135.56, 134.87, 130.11, 128.97, 128.92,
128.46, 126.73, 125.70, 124.96, 124.39, 122.91, 122.81, 121.64, 120.01,
119.78, 119.21, 109.74, 101.37, 46.58, 31.44, 30.29, 26.68, 22.55, 14.01
ppm. HRMS: calcd for C28H27NS [M+H]+: 410.1936; found: 410.1899.
in CH2Cl2 (20 mL) when the solvent had been removed in vacuo. The
solution was then washed with aqueous Na2S2O3 (60 mL, 15%) and
water sequentially. The organic layer was purified by flash
chromatography on silica gel (petroleum ether/EtOAc, 50:1 v/v) to give a
1
yellow liquid HIPD (90 mg). Yield: 57%. H NMR (400 MHz, CDCl3): δ =
9.36 (s, 1H; Ar-H), 9.15 (d, J = 9.2 Hz, 1H; Ar-H), 8.31 (d, J = 7.6 Hz, 1H;
Ar-H), 8.06 (d, J = 8.4 Hz, 1H; Ar-H), 7.81-7.73 (m, 4H; Ar-H), 7.54 (t, J =
7.2 Hz, 1H; Ar-H), 7.16-7.10 (m, 2H; Ar-H), 4.26 (t, J = 7.2 Hz, 2H; Hexyl-
H), 1.93 (t, J = 7.2 Hz, 2H; Hexyl-H), 1.35-1.31 (m, 6H; Hexyl-H), 0.89 (t,
J = 6.8 Hz, 3H; Hexyl-H) ppm. 13C NMR (400 MHz, CDCl3): δ = 152.32,
145.31, 134.65, 131.29, 131.00, 129.54, 128.80, 128.32, 127.81, 125.71,
125.47, 124.58, 124.52, 124.34, 123.45, 122.70, 121.97, 113.10, 105.41,
46.79, 31.43, 30.56, 26.70, 22.55, 14.01 ppm. HRMS: calcd for C25H24N2
[M+H]+: 353.2011; found: 353.2016.
(E)-1-hexyl-5-(2-(naphthalen-2-yl)vinyl)-1H-indole
(HNVI):
BNTP
(1138 mg, 2.36 mmol) was dissolved in dry THF (10 mL) under argon
atmosphere and the solution was cooled to -10°C. Potassium t-butoxide
(KOtBu, 557 mg, 4.96 mmol) was added and stirred for 1 min. A solution
of HIC (541 mg, 2.36 mmol) in dry THF (10 mL) was added dropwise into
the reaction mixture. After addition, the mixture was stirred for 15 min. A
few drops of water were added to quench the reaction. The mixture was
extracted by CH2Cl2 (3  100 mL). The organic layer was washed with
water for three times (3  100 mL) and dried over anhydrous Na2SO4.
The solvent was removed in vacuo, the residue was purified by flash
chromatography on silica gel (petroleum ether/EtOAc, 50:1 v/v) to give a
14-hexyl-14H-benzo[4’,5’]thieno[2’,3’:7,8]naphtha[1,2-e]indole
(HBTNI): Argon was bubbled through a solution of DTVHI (184 mg, 0.45
mmol) in benzene (500 mL) with stirring for 30 min. After I2 (117 mg, 0.46
mmol) and propylene oxide (18 mL, 258.00 mmol) were added into the
solution, the reaction mixture was irradiated by a 500 W high pressure
mercury lamp through a quartz filter. The reaction was stopped 30 min
later. The residue was dissolved in CH2Cl2 (20 mL) when the solvent had
been removed in vacuo. The solution was then washed with aqueous
Na2S2O3 (60 mL, 15%) and water sequentially. The organic layer was
purified by flash chromatography on silica gel (petroleum ether/EtOAc,
1
white solid HNVI (325 mg). Yield: 39%. H NMR (400 MHz, CDCl3): δ =
1
25:1 v/v) to give a white solid HBTNI (26 mg). Yield: 14%. H NMR (400
7.89-7.82 (m, 6H; Ar-H), 7.56-7.52 (m, 1H; Ar-H), 7.51-7.44 (m, 2H; Ar-H),
7.39 (t, J = 4.8 Hz, 2H; Ar-H), 7.29 (d, J = 6 Hz, 1H; Ar-H), 7.13 (d, J =
2.8 Hz, 1H; Ar-H), 6.55 (d, J = 2.8 Hz, 1H; Ar-H), 4.15 (t, J = 7.2 Hz, 2H;
Hexyl-H), 1.88 (t, J = 7.2 Hz, 2H; Hexyl-H), 1.32 (d, J = 19.6 Hz, 6H;
Hexyl-H), 0.92 (t, J = 6.8 Hz, 3H; Hexyl-H) ppm. 13C NMR (400 MHz,
CDCl3): δ = 135.93, 135.67, 133.88, 132.76, 130.59, 128.96, 128.93,
128.48, 128.19, 127.89, 127.70, 126.22, 125.88, 125.84, 125.50, 123.64,
120.04, 119.88, 109.75, 101.38, 46.58, 31.46, 30.30, 26.69, 22.57, 14.04
ppm. HRMS: calcd for C26H27N [M+H]+: 354.2215; found: 354.2219.
MHz, CDCl3): δ = 9.69 (s, 1H; Ar-H), 8.69 (s, 1H; Ar-H), 8.33-8.30 (m, 1H;
Ar-H), 7.91-7.88 (m, 3H; Ar-H), 7.72 (d, J = 19.2 Hz, 1H; Ar-H), 7.68 (d, J
= 2.4 Hz, 2H; Ar- H), 7.52-7.49 (m, 2H; Ar-H), 7.41 (d, J = 3.2 Hz, 1H; Ar-
H), 4.30 (t, J = 7.2 Hz, 2H; hexyl-H), 1.93 (t, J = 7.2 Hz, 2H; hexyl-H),
1.34-1.31 (m, 6H; hexyl-H), 0.88 (t, J = 6.8 Hz, 3H; hexyl-H) ppm. 13
C
NMR (400 MHz, CDCl3): δ = 140.25, 137.87, 135.42, 134.95, 134.00,
130.66, 130.37, 127.68, 127.63, 127.57, 127.22, 124.59, 124.48, 124.23,
123.19, 123.16, 122.88, 121.90, 120.54, 119.31, 110.93, 103.28, 46.86,
31.45, 30.51, 26.70, 22.55, 14.00 ppm. HRMS: calcd for C28H25NS
[M+H]+: 408.1780; found: 408.1743.
3-hexyl-3H-phenanthro[4,3-e]indole (HPI): Argon was bubbled through
a solution of HNVI (159 mg, 0.45 mmol) in benzene (500 mL) with stirring
for 30 min. After I2 (117 mg, 0.46 mmol) and propylene oxide (18 mL,
258.00 mmol) were added into the solution, the reaction mixture was
irradiated by a 500 W high pressure mercury lamp through a quartz filter.
The reaction was stopped 90 min later. The residue was dissolved in
CH2Cl2 (20 mL) when the solvent had been removed in vacuo. The
solution was then washed with aqueous Na2S2O3 (60 mL, 15%) and
water sequentially. The organic layer was purified by flash
chromatography on silica gel (petroleum ether/EtOAc, 50:1 v/v) to give a
brown liquid HPI (90 mg). Yield: 57%. 1H NMR (400 MHz, CDCl3): δ =
9.12 (d, J = 8.4 Hz, 1H; Ar-H), 7.93 (t, J = 9.6 Hz, 2H; Ar-H), 7.86 (d, J =
8.4 Hz, 1H; Ar-H), 7.80 (d, J = 8.4 Hz, 1H; Ar-H), 7.75 (d, J = 8.8 Hz, 1H;
Ar-H), 7.67-7.61 (m, 2H; Ar-H), 7.54 (t, J = 7.2 Hz, 1H; Ar-H), 7.43 (t, J =
8 Hz, 1H; Ar-H), 7.01 (d, J = 2.8 Hz, 1H; Hexyl-H), 6.98 (d, J = 3.2 Hz, 1H;
Ar-H), 4.15 (t, J = 7.2 Hz, 2H; Hexyl-H), 1.85 (t, J = 7.2 Hz, 2H; Hexyl-H),
1.29 (s, 6H; Hexyl-H), 0.86 (t, J = 6.8 Hz, 3H; Hexyl-H) ppm. 13C NMR
(400 MHz, CDCl3): δ = 134.43, 132.79, 131.76, 130.33, 129.10, 129.07,
128.36, 127.62, 127.24, 127.03, 126.70, 125.94, 125.05, 124.73, 124.55,
123.74, 123.62, 122.82, 111.10, 105.54, 46.76, 31.53, 30.60, 26.79,
22.64, 14.12 ppm. HRMS: calcd for C26H25N [M+H]+: 352.2059; found:
352.2507.
(E)-3-(2-(1-hexyl-1H-indol-5-yl)vinyl)quinoline (HIVQ): HVI (454 mg,
2.00 mmol), 3-Bromoquinoline (416 mg, 2.00 mmol), K2CO3 (276 mg,
2.00 mmol), Tri(o-tolyl)phosphine (609 mg, 2.00 mmol) and Palladium
acetate (45 mg, 0.20 mmol) were added into anhydrous DMF (30 mL)
under argon atmosphere. After bubbling argon for 30 min, the reaction
mixture was then stirred at 80°C for 48 h under argon atmosphere. A few
drops of water were added to quench the reaction. The mixture was
extracted by CH2Cl2 (3  100 mL). The organic layer was washed with
water for three times (3  100 mL) and dried over anhydrous Na2SO4.
The solvent was removed in vacuo, the residue was purified by flash
chromatography on silica gel (petroleum ether/EtOAc, 25:1 v/v) to give a
yellow solid HIVQ (298 mg). Yield: 42%. 1H NMR (400 MHz, CDCl3): δ =
9.18 (d, J = 2.4 Hz, 1H; Ar-H), 8.18 (t, J = 8.4 Hz, 2H; Ar-H), 7.86 (t, J =
3.6 Hz, 2H; Ar-H), 7.72-7.69 (m, 1H; Ar-H), 7.61-7.48 (m, 3H; Ar-H), 7.39
(d, J = 8.8 Hz, 1H; Ar-H), 7.22 (d, J = 16.4 Hz, 1H; Ar-H), 7.14 (d, J = 2.8
Hz, 1H; Ar-H), 6.55 (d, J = 3.2 Hz, 1H; Ar-H), 4.15 (t, J = 7.2 Hz, 2H;
Hexyl-H), 1.88 (t, J = 7.2 Hz, 2H; Hexyl-H), 1.34 (t, J = 6.8 Hz, 6H; Hexyl-
H), 0.91 (d, J = 14 Hz, 3H; Hexyl-H) ppm. 13C NMR (400 MHz, CDCl3): δ
= 149.37, 146.77, 136.16, 132.64, 131.65, 131.19, 128.94, 128.93,
128.90, 128.66, 128.34, 128.24, 127.72, 126.97, 121.99, 120.28, 120.01,
109.84, 101.51, 46.57, 31.42, 30.27, 26.65, 22.53, 14.00 ppm. HRMS:
calcd for C25H26N2 [M+H]+: 355.2168; found: 355.2176.
Acknowledgements
7-hexyl-7H-indolo[5,4-k]phenanthridine (HIPD): Argon was bubbled
through a solution of HIVQ (159 mg, 0.45 mmol) in benzene (500 mL)
with stirring for 30 min. After I2 (117 mg, 0.46 mmol) and propylene oxide
(18 mL, 258 mmol) were added into the solution, the reaction mixture
was irradiated by a 500 W high pressure mercury lamp through a quartz
filter. The reaction was stopped 90 min later. The residue was dissolved
The research was supported by the Natural Science Foundation
of Shandong Province (no. ZR2015EM006) and the National
Natural Science Foundation of China (no. 21672130).
7
This article is protected by copyright. All rights reserved.

10.1002/cplu.202000178
ChemPlusChem
FULL PAPER
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Keywords: fluorescence • helicenes • indoles • photophysics •
two-photon absorption
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Zhenhao Hu+, Li Li+, Zhi Liu,* Zhiqiang
Liu, Dingchao Zhang, and Kunlun Li
Novel indole-based helicenes HTPI,
HBTNI, HIPD and HPI were facilely
prepared through a three-step or four-
step reaction. They emitted violet-blue
light with maximum peaks ranging
from 391 nm to 415 nm in hexane.
Furthermore, they all possessed TPA
and TPEF, and HTPI achieved the
maximum TPA with the TPA cross-
section of 171.5 GM at 770 nm.
Page No. – Page No.
Indole-Based Aza[n]helicenes (n =
5, 6) with Violet-Blue Fluorescence
and Two-Photon Absorption (TPA)
10
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