Synthesis and Photophysical Properties of Naphtho[b]- and Indeno[b]-fused BODIPYs

Article abstract of DOI:10.1016/j.dyepig.2020.108676

A new synthetic method to construct [b]-fused BODIPY derivatives through PtCl₂-catalyzed cycloisomerization reaction of peripherally phenylacetylene substituted BODIPYs is reported. Electron donating methoxy group facilitate both the 6-endo and

Full text of DOI:10.1016/j.dyepig.2020.108676

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Synthesis and Photophysical Properties of Naphtho[b]- and Indeno[b]-fused BODIPYs
Baogeng Wu, Zhikuan Zhou, Lei Chen, Aihua Yuan
PII:
S0143-7208(20)31373-5
DOI:
https://doi.org/10.1016/j.dyepig.2020.108676
DYPI 108676
Reference:
To appear in: Dyes and Pigments
Received Date: 30 May 2020
Revised Date: 27 June 2020
Accepted Date: 27 June 2020
Please cite this article as: Wu B, Zhou Z, Chen L, Yuan A, Synthesis and Photophysical Properties
of Naphtho[b]- and Indeno[b]-fused BODIPYs, Dyes and Pigments, https://doi.org/10.1016/
j.dyepig.2020.108676.
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Baogeng Wu: Investigation, Formal analysis, Data curation. Lei Chen: Formal analysis. Aihua Yuan:
Supervision, Writing - original draft. Zhikuan Zhou: Supervision, Writing - original draft, Writing -
review & editing.

Synthesis and Photophysical Properties of Naphtho[b]- and Indeno[b]-fused
BODIPYs
Baogeng Wu,ab Zhikuan Zhou,*b Lei Chen,a Aihua Yuan*a
a. School of Environmental and Chemical Engineering, Jiangsu University of Science
and Technology, Zhenjiang 212003, China.
b. Key Laboratory of Organosilicon Chemistry and Material Technology of Ministry
of Education, Key Laboratory of Organosilicon Material Technology of Zhejiang
Province, Hangzhou Normal University, No. 2318, Yuhangtang Road, Hangzhou,
311121, China.
Corresponding Authors: zkzhou@hznu.edu.cn; aihua.yuan@just.edu.cn
Abstract
A new synthetic method to construct [b]-fused BODIPY derivatives through
PtCl2-catalyzed cycloisomerization reaction of peripherally phenylacetylene
substituted BODIPYs is reported. Electron donating methoxy group facilitate both the
6-endo and 5-exo cyclization procedure to produce naphtho[b]- and indeno[b]-fused
BODIPYs in one pot reaction. Naphtho[b]-fused BODIPY exhibits redshifted
absorption and emission (591 nm/621 nm) with higher quantum yield (0.56 in DCM),
while indeno[b]-fused BODIPY shows very low quantum yield (0.01 in DCM) with
shorter absorption and emission wavelength (571 nm/600 nm) and stronger absorption
intensity. Quantum-chemical calculation revealed the antiaromatic nature of
cyclopentadienyl ring in indeno[b]-fused BODIPY, which accounts for its inferior
photophyscal properties.
Keywords: BODIPY; [b]-fusion; photophysical properties; cyclization; TD-DFT
calculation
1. Introduction
Due to the divers modification sites and excellent properties,
4-bora-3a,4a-diaza-s-indacene (BODIPY) dyes and their derivatives have attracted
tremendous research interest in recent years.[1-4] BODIPYs possess rigid core

structure and conjugated π systems, which can be further extended through
phostfunctionalization to meet wider applications.[4-6] π-extended BODIPYs with
near-infrared (NIR) absorbtion and emission are promising to be utilized for
bioimaging,[3, 7-12] photodynamic therapy (PDT),[13-25] organic field electron
transfer (OFET) materials,[26, 27] and other advanced electronic and optical
materials.[28-32]
Various π-extended BODIPYs have been synthesized through substitution or
ring fusion of aromatic groups at peripheral positions, and construction of conjugated
oligomers.[33-39] Among them, aromatic ring-fused BODIPYs are particularly
promising with extension of rigid π skeleton and excellent electronic properties.
Depending on the position on which the aromatic ring fused, annulated BODIPYs can
be classified into [a]-,[40, 41] [b]-,[35, 42, 43] zig-zag-,[44, 45] and boron-fused[46]
patterns. The [b]-fusion could effectively decrease the LUMO level, which is prone to
obtain air stable NIR dyes.[47] To achieve annulated BODIPYs, postfunctionalization
of BODIPY core is more versatile than the de novo synthesis utilizing aromatic
ring-fused pyrroles as starting materials, which are difficult to obtain and easy to be
oxidized under ambient condition.[48, 49] The key procedure for
postfunctionalization is to rationally preinstall the aromatic ring on the proper
peripheral site of BODIPY core for further annulation. Acid-induced intramolecular
cyclization are commonly employed for the efficient fabrication of aromatic ring
[b]-fused BODIPYs.[26, 42, 50, 51] However, in some cases, strong acid could lead
to spontaneous deboronation of BODIPY core structures, which made the direct
annulation reaction frustrated.[52, 53] As alternative, transition metal catalyzed C-H
functionalization seems to be an easy-to-operate way to rationally control the fusion
mode of aromatic rings.[40, 54, 55]
Dienylalkyne derivatives are versatile building blocks for the synthesis of carbo
and hetero polycyclic aromatics.[56-58] With the assistance of acid, base, or
metal-based catalyst, straightforward 6-endo or 5-exo cyclization may occur, which
results in the formation of benzo[b]- or methylenecyclopenta[b]-fused derivatives
(Scheme 1). PtCl2 has been widely used as catalyst for carbocyclization of

alkynylated biaryl derivatives to fabricate aromatic fused molecules.[59, 60] In the
BODIPY precursor A, through the rational substitution at the β-position of pyrrole
unit, an intramolecular dienylalkyne structure is fabricated, which should be prone to
cyclize under PtCl2 catalyzation to produce two possible isomers. In pursuit of
aromatic ring [b]-fused BODIPY, and based on our previous investigations,[43, 61]
we now report the synthesis and photophysical properties of mono-naphtho[b]-fused
and indeno[b]-fused BODIPYs via PdCl2-catalysed intramolecular cycloisomerization
reaction.
2. Results and Discussion
2.1. Synthesis
1-bromo-2-(4-R-phenylethynyl)benzene 1a-1c were synthesized through
Sonogashira cross-coupling of substituted iodobenzene with substituted
phenylacetylene in the yield of 70% to 76% (See details in Supplementary data).
Borate compounds 2a-2c were synthesized through the substitution of bromine atom
by pinacolatoboron (Bpin) group, which was carried out by lithium halogen exchange
reaction
and
subsequent
reaction
with
2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane in the yield of 73% to 86%.
Standard Suzuki-Miyaura cross-coupling reactions employing 2 and 2-bromo
BODIPY 6[62] afford the key BODIPY precursors 3. The final intramolecular
cycloaromatization procedure was conducted under the catalyzing of PtCl2 without
any extra adducts in reflux toluene for 12 h. Lower reaction temperature results in
lower yield, while longer reaction time did not improve the yield but lead to the
decomposition of the target compounds. Interestingly, it was found that the major
ring-fused products varied according to the substitution on the para position of
phenylacetylene. Electron-donating methoxy group facilitated the formation of both
6-endo and 5-exo products, namely naphtho[b]-fused BODIPY 4b and
indeno[b]-fused BODIPY 5b, respectively (Scheme 2). Phenyl and
electron-withdrawing trifluoromethyl groups substituted precursors prompted the

6-endo reaction against the 5-exo pathway. The same substituent effect has been
reported in PtCl2-catalyzed cycloisomerization reactions for phenanthrenes.[59, 60]
The aromatic ring fused BODIPYs 4a, 4b, 4c, and 5b were characterized by
high-resolution mass spectroscopy (HR-MS) and NMR spectroscopy. HR-ESI-MS
showed the peaks of the molecular ion [M + Na]+ at m/z = 539.2070 and m/z =
539.2080 for 4b and 5b, respectively, clarifying that they are isomeric compounds.
In principle, the cycloaromatization reaction of BODIPY 3 could occur between
alkynyl group and the adjacent α or γ position of pyrrole unit to give either [a]- or
1
[b]-fused species. The H NMR spectra of 4b and 5b clearly showed the absence of
the singlet peak of α proton of BODIPY 3b at 8.59 ppm,[51] while the γ proton still
exist after the cycloaromatization reaction at 7.40 and 7.31 ppm, respectively (Fig. 1).
The major structural difference between 4b and 5b is the position of ω protons. In
naphtho[b]-fused BODIPY 4b, the ω proton is fixed on the phenyl group in the
aromatic BODIPY skeleton, thus it appears at lower field (8.03 ppm) due to the lower
electronic density on the neighboring carbon.[43] While in indeno[b]-fused BODIPY
5b, the ω proton is located on a typical vinyl group, it appears at upper field (7.53
ppm).[63]
2.2. X-ray single crystal structures
Single crystals of 3b and 3c were obtained from the slow evaporation of n-hexane into
their dichloromethane solutions at room temperature. The diphenylacetylene fragment
rotate around the 2-position of BODIPY core with the OCH3 and CF3 “tail” oriented
upward to the meso-mesityl group (Fig. 2). In 3b and 3c, the dihedral angles between
phenyl group A and BODIPY core C are 18.6o and 34.5o, respectively. The dihedral
angels between phenyl rings A and B in 3b and 3c are 13.5o and 17.7o, respectively.
These results indicated the twisted conformation and the existence of considerable
repulsion between the diphenylacetylene unit and BODIPY core, which suggested the
difficulty of intramolecular cyclization in the [a]-fused fashion.
2.3. Photophysical properties

The absorption and emission spectra of the aromatic ring fused BODIPYs 4a-4c,
and 5b in CH2Cl2 are shown in Fig. 3 and the photophysical properties are
summarized in Table 1. The absorption and fluorescence emission spectra in
n-hexane, toluene, THF, and acetonitrile are plotted in Fig. S1-S7, the detailed
photophysical data are summarized in Table S1. The precursor BODIPYs 3a-3c
exhibit large stocks shifts ranging from 50 nm to 56 nm with quantum yield around
0.2. Upon cyclization to form [b]-fused BODIPYs 4a-4c, and 5b, the absorption
maxima redshifted 31 nm to 53 nm, indicating the efficient extension of π systems.
The observed absorption and emission band maxima of the same compound
fluctuated within 23 nm in different solvents. However, no obvious trend as function
of polarity of the solvent was found (Fig. S4-S7). Different from BODIPYs 4a-4c, the
molar extinction coefficient and emission intensity of BODIPY 5b differed largely in
various solvents, indicating the unique photophysical properties brought by its
indeno[b]-fused structure. For naphtho[b]-fused BODIPY 4a-4c, it is interesting that
4b absorb and emit at longer wavelength than both 4a and 4c with quantum yield of 3
times higher than the later ones. It can be deduced that the methoxy group on the
unfused phenyl unit at BODIPY 3 position aid in the extention of π system. The
indeno[b]-fused BODIPY 5b exhibits almost the same absorption and emission band
maxima as 4a and 4c, while the molar absorption coefficient increased nearly 2 times
(91000 M-1cm-1). The indeno[b]-fusion mode leads to dramatic decrease of
fluorescence quantum yield of 5a (0.01 in DCM).
2.4. TD-DFT calculation
DFT calculations with B3LYP functional and 6-31G(d) basis set were carried out
to get depth insight into the structure-spectroscopy relationship. The optimized
conformation of 3b and 3c were the same with their crystal structures. In principle,
cis- and trans- isomers of BODIPY 5b may be generated during the formation of
vinyl compound. The comparison of the electronic energy of these two isomers
revealed that the cis-isomer shown in Fig. 4 is more stable (Fig. S8), which is chosen
to be studied in the following part. The 6-endo fusion compounds 4a and 4c possessed

more stabilized LUMO and destabilized HOMO energy compared with their
precursors 3a and 3c (Fig. S9 and Fig. S11), which consequently decreased their
HOMO-LUMO gaps and resulted in the redshifted absorption. When compared with
3b, the 6-endo fusion only stabilized the LUMO energy of 4b, while the 5-exo fusion
destabilized both the HOMO and LUMO energies of 5b. As a result, the
HOMO-LUMO gaps of 4b was smaller than that of 5b, which corresponded with the
observed red-shifts in the absorption maxima of 4b. When carefully check the frontier
MOs of 4b and 5b, it is clearly to observe that the electron density on their HOMO
spread all over the BODIPY core, fused phenyl and methoxyphenyl group. However,
the electron density on the LUMO of 5b not only located on the BODIPY core and
fused phenyl, but also spread over the methoxyphenyl group, which differed from 4b
and largely destabilized the LUMO of 5b. We could conclude that the methoxy group
aid in the delocalization of electron density over the cycloarmatization products.
To better understand the origin of difference in UV-Vis spectra of isomers 4b
and 5b, TD-DFT calculations were performed for them and the results were plotted
and summarized in Fig. 5 and Table S2. The calculated maximum absorption peaks
of both 4b and 5b mainly composed of HOMO to LUMO transition and HOMO-1 to
LUMO transition. The calculated absorption maxima of 4b is larger than 5b, but the
oscillator strength is much smaller, which is in accordance with the experimental data.
The nucleus-independent chemical shift (NICS)(0) values of selected five or
six-membered rings were calculated to examine their aromaticity (Fig. 5). NICS(0)
values of the selected pyrrole ring in BODIPYs 4b and 5b were -7.66 and -4.53 ppm,
respectively, indicating the more aromatic nature of the BODIPY core of 4b.[42] This
is mainly affected by the character of [b] fused rings. The fused phenyl ring in 4b still
retain the aromatic nature with a NICS(0) value of -4.47 ppm, however, the fused
cyclopentadienyl ring in 5b became antiaromatic with a NICS(0) value of 2.69 ppm,
which is totally different from the analogous five-membered heterocycles such as
thiophene.[27, 53] Thus, the global aromaticity of 5b is weakened, which also have
inferior extension of π-conjugation after ring fusion.

3. Conclusion
In summary, we have synthesized a series of aromatic ring [b]-fused BODIPYs
through PtCl2-catalyzed intramolecular cycloisomerization reaction between
peripherally preinstalled alkynyl group and pyrrole unit of BODIPY core. No
substituent or electron withdrawing CF3 on the para position of phenylethynyl group
result in the exclusive formation of 6-endo product, while electron donation methoxy
group facilitate the formation of both 6-endo and 5-exo products in one pot. The
photophysical properties of all the aromatic ring [b]-fused BODIPYs were
investigated. Naphtho[b]-fused BODIPY with methoxy substitution exhibits the most
redshifted absorption and emission (591 nm/621 nm) and the highest quantum yield
(0.56 in DCM), while indeno[b]-fused BODIPY shows the lowest quantum yield
(0.01 in DCM) with shorter absorption and emission wavelength (571 nm/600 nm)
and the strongest absorption intensity (91000 M-1cm-1). Quantum calculation revealed
the aromatic nature of the [b]-fused phenyl ring in naphtho[b]-fused BODIPY, on the
contrary, the fused cyclopentadienyl ring in indeno[b]-fused BODIPY exhibited
antiaromatic character, which accounts for the dramatic difference in their
photophysical properties.
4. Materials and methods
4.1 Instruments and reagents
All reagents were obtained from commercial suppliers and used without further
purification unless otherwise indicated. All air and moisture-sensitive reactions were
carried out under nitrogen atmosphere in oven-dried glassware. Glassware was dried
in an oven at 100°C and cooled under a stream of inert gas before use. Both
dichloromethane and triethylamine were distilled over calcium hydride before use.
1H-NMR spectra were recorded on a Bruker DRX400 spectrometer and referenced to
the residual proton signals of the solvent. HR-MS were recorded on a Bruker
Daltonics microTOF-Q II spectrometer. All the solvents employed for the
spectroscopic measurements were of UV spectroscopic grade (Aldrich).
4.2. Synthesis

4.2.1 Synthesis of compound 1, 2, and BODIPY 6
The detailed synthetic procedures of 1 and 2 were summarized in the
supplementary data. 2-Bromo-BODIPY 6 was synthesized according to the literature
[62].
4.2.2 Synthesis of 3a, 3b, and 3c
A solution of BODIPY-Br 6 (380 mg, 1 mmol), 2a (335 mg, 1.1 mmol) in THF (20
mL) were added into 100 mL flask filled with nitrogen. Pd(PPh3)4 (116 mg, 0.1 mmol)
was added under nitrogen, then 3 mL 2 mol/L Na2CO3 was added. The reaction was
heated to reflux for 4 h. The reaction was monitored by TLC. After completion, the
solvent was removed and the residue was purified by silica gel column
chromatography eluting with 20% to 40% dichloromethane / petroleum ether. Red
1
solid, 400 mg, yield 83 %. H NMR (400 MHz, CDCl3) δ 8.55 (s, 1H), 7.93 (s, 1H),
7.62 – 7.56 (m, 1H), 7.47 (d, J = 7.7 Hz, 1H), 7.38 (dd, J = 7.1, 2.7 Hz, 2H), 7.33 (dd,
J = 6.5, 1.6 Hz, 4H), 7.27 (s, 1H), 7.23 (d, J = 3.7 Hz, 1H), 6.91 (s, 2H), 6.68 (d, J =
4.0 Hz, 1H), 6.48 (d, J = 2.7 Hz, 1H), 2.36 (s, 3H), 2.11 (s, 6H). 13C NMR (100 MHz,
CDCl3) δ 144.38, 144.18, 138.76, 136.37, 134.14, 133.80, 131.43, 130.02, 129.68,
128.70, 128.34, 127.58, 127.29, 122.92, 120.43, 93.68, 89.19, 21.20, 20.11 ppm.
HRMS-ESI: m/z: calcd. for [C32H25BF2N2+Na]+:509.1977, found:509.1966.
The synthesis of compounds 3b and 3c were obtained by following a similar
procedure to that of 3a.
Compound 3b was obtained in 58% yield as red solid. 1H NMR (400 MHz,
CDCl3) δ 8.59 (s, 1H), 7.93 (s, 1H), 7.56 (d, J = 8.6 Hz, 1H), 7.45 (d, J = 7.6 Hz, 1H),
7.34 (s, 1H), 7.32 (s, 1H), 7.28 (d, J = 9.0 Hz, 1H), 7.22 (d, J = 15.4 Hz, 2H), 6.93 (s,
2H), 6.87 (d, J = 8.8 Hz, 2H), 6.68 (d, J = 4.0 Hz, 1H), 6.48 (d, J = 3.9 Hz, 1H), 3.85
(s, 3H), 2.37 (s, 3H), 2.12 (s, 6H). 13C NMR (100 MHz, CDCl3) δ:159.87, 138.86,
136.51, 133.95, 133.66, 133.03, 129.82, 128.44, 128.36, 127.66, 127.33, 93.98, 88.12,
55.46, 21.34, 20.26 ppm. HRMS-ESI: m/z: calcd. for [C32H25BF2N2+Na]+: 539.2082,
found: 539.2085.
1
Compound 3c was obtained in 69% yield as red solid. H NMR (400 MHz,

CDCl3) δ 8.54 (s, 1H), 7.95 (s, 1H), 7.60 (d, J = 8.6 Hz, 3H), 7.49 (d, J = 7.9 Hz, 3H),
7.36 (td, J = 7.7, 1.4 Hz, 1H), 7.29 – 7.27 (m, 1H), 7.17 (s, 1H), 6.91 (s, 2H), 6.70 (d,
J = 4.1 Hz, 1H), 6.52 – 6.48 (m, 1H), 2.35 (s, 3H), 2.12 (s, 6H). 13C NMR (100 MHz,
CDCl3) δ 144.62, 143.99, 136.35, 133.84, 131.62, 130.35, 129.29, 128.27, 127.69,
127.18, 125.28, 21.11, 20.12 ppm. HRMS-ESI: m/z: calcd. for [C33H24BF5N2+Na]+:
577.1851, found: 577.1840.
4.2.3 Synthesis of 4a and 4c
3a (200 mg, 0.41 mmol) was added into 100 mL flask filled with nitrogen. Dry
toluene (10 mL) was added. PtCl2 (15 mg, 0.05 mmol) was then added under nitrogen.
The mixture was heated under 110 overnight before cooled to room temperature.
The solvent was removed and the residue was purified by silica gel column
chromatography eluting with 40% dichloromethane / petroleum ether. 4a, Black solid,
1
50 mg, yield 25 %. H NMR (400 MHz, CDCl3) δ 8.03 (d, J = 7.8 Hz, 1H), 7.88 (s,
1H), 7.73 (d, J = 7.6 Hz, 1H), 7.59 (d, J = 3.0 Hz, 2H), 7.55 (s, 1H), 7.52 – 7.46 (m,
4H), 7.46 – 7.42 (m, 1H), 7.41 (s, 1H), 7.03 (s, 2H), 6.72 (d, J = 4.2 Hz, 1H), 6.49 (d,
J = 3.1 Hz, 1H), 2.43 (s, 3H), 2.16 (s, 6H). 13C NMR (100 MHz, CDCl3) δ 146.35,
135.75, 135.07, 130.73, 129.30, 128.20, 127.81, 127.23, 126.91, 126.46, 125.97,
124.96, 122.17, 120.37, 28.68, 19.15 ppm. Elemental analysis calcd. (%) for
C32H25BF2N2: C 79.02, H 5.18, N 5.76. Found: C 79.37, H 4.95, N 5.44. HRMS-ESI:
m/z: calcd. for [C32H25BF2N2+Na]+: 509.1977, found: 509.1966.
The synthesis of BODIPY 4c were obtained by following a similar procedure to
that of 4a.
Compound 4c was obtained in 40% yield as black solid. 1H NMR (400 MHz,
CDCl3) δ 8.04 (d, J = 7.9 Hz, 1H), 7.86 (s, 1H), 7.74 (dd, J = 8.1, 4.1 Hz, 3H), 7.69 (d,
J = 8.2 Hz, 2H), 7.55 – 7.48 (m, 2H), 7.46 (d, J = 8.0 Hz, 1H), 7.41 (s, 1H), 7.04 (s,
2H), 6.75 (d, J = 4.2 Hz, 1H), 6.52 (d, J = 3.2 Hz, 1H), 2.43 (s, 3H), 2.16 (s, 6H). 13C
NMR (100 MHz, CDCl3) δ 147.78, 139.14, 136.75, 135.74, 131.62, 130.82, 130.35,
130.11, 129.30, 129.02, 128.95, 128.32, 128.27, 128.11, 127.89, 126.12, 124.04,
123.26, 121.15, 21.24, 20.19, 20.12 ppm. Elemental analysis calcd. (%) for

C33H24BF5N2: C 71.50, H 4.36, N 5.05. Found: C 71.72, H 4.67, N 5.39. HRMS-ESI:
m/z: calcd. for [C33H24BF5N2+Na]+: 577.1851, found: 577.1840.
4.2.4 Synthesis of 4b and 5b
3b (200 mg, 0.38 mmol) was added into 100 mL flask filled with nitrogen. Dry
toluene (10 mL) was added. Then, PtCl2 (15 mg, 0.05 mmol) was added under
nitrogen. The mixture was heated under 110
overnight before cooled to room
temperature. The solvent was removed and the residue was purified by silica gel
column chromatography eluting with 40% dichloromethane / petroleum ether. Black
solid 4b, 30 mg, yield 20%; Black solid 5b, 50 mg, yield 25%.
1
4b H NMR (400 MHz, CDCl3) δ 8.04 (d, J = 7.3 Hz, 2H), 7.84 (d, J = 7.9 Hz, 1H),
7.77 (s, 1H), 7.49 (d, J = 8.6 Hz, 3H), 7.37 (s, 1H), 7.31 (s, 1H), 7.07 – 7.01 (m, 4H),
6.75 (d, J = 4.1 Hz, 1H), 6.56 (dd, J = 4.3, 1.7 Hz, 1H), 3.91 (s, 3H), 2.43 (s, 3H),
2.17 (s, 6H). 13C NMR (100 MHz, CDCl3) δ 167.73, 159.56, 151.54, 146.51, 145.69,
139.02, 135.77, 132.39, 130.09,129.05, 128.87, 127.71, 125.84, 124.92, 123.99,
120.87, 119.88, 116.58, 113.81, 55.44, 34.25, 30.34, 20.20, 14,17 ppm. Elemental
analysis calcd. (%) for C33H27BF2N2O: C 76.76, H 5.27, N 5.42, O 3.10. Found: C
76.54, H 5.03, N 5.27. HRMS-ESI: m/z: calcd. for [C33H27BF2N2O+Na]+: 539.2082,
found: 539.2070.
1
5b H NMR (400 MHz, CDCl3) δ 8.02 (d, J = 7.9 Hz, 1H), 7.88 (s, 1H), 7.72 (d, J =
7.7 Hz, 1H), 7.50 (dd, J = 17.9, 10.0 Hz, 4H), 7.45 – 7.37 (m, 2H), 7.02 (d, J = 6.5 Hz,
4H), 6.72 (d, J = 3.6 Hz, 1H), 6.49 (d, J = 3.3 Hz, 1H), 3.92 (s, 3H), 2.43 (s, 3H), 2.15
(s, 6H). 13C NMR (100 MHz, CDCl3) δ 159.06, 147.35, 139.12, 136.90, 136.67,
131.58, 130.63, 130.39, 129.45, 128.92, 128.38, 128.02, 127.63, 126.10, 123.31,
121.57, 120.84, 112.52, 55.33, 21.37, 20.32 ppm. Elemental analysis calcd. (%) for
C33H27BF2N2O: C 76.76, H 5.27, N 5.42, O 3.10. Found: C 76.49, H 5.46, N 5.30.
HRMS-ESI: m/z: calcd. for [C33H27BF2N2O+Na]+: 539.2082, found: 539.2070.
Conflicts of interest
There are no conflicts to declare.

Acknowledgements
We thank the National Natural Science Foundation of China (No. 21501073), the
organosilicon chemistry innovation team, the research funding project of Hangzhou
Normal University (2019QDL019) for financial support. Theoretical calculations
were carried out at the Computational Centre for Molecular Design of Organosilicon
Compounds, Hangzhou Normal University.
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Captions of Tables, Fig.s and Schemes
Scheme 1. Two possible cycloaromatization pathways for dienylalkyne derivatives.
Scheme 2. Synthetic route to BODIPYs 4a, 4b, 4c, and 5b.
Fig. 1. 1H NMR superposition of 3b, 4b, and 5b (400 MHz, CDCl3).
Fig. 2. X-ray structures of 3b and 3c. Hydrogen atoms were omitted for clarity. The thermal
ellipsoids were set at 50% probability.
Fig. 3. UV/Vis absorption (left) and normalized fluorescence (right) spectra of 4a, 4b, 4c, and 5b
in DCM.
Table 1. Spectroscopic and photophysical properties of 3a-5b in CH2Cl2 at 298K.
Fig. 4. DFT calculated frontier molecular orbital profiles and energy levels.
Fig. 5. The overlaid plots of the calculated electronic transition (gray bars) and experimental
UV-Vis spectra (colored lines) of 4b and 5b. The NICS(0) values (ppm) at the geometric center
points of the corresponding five-/six-membered rings.

Scheme 1. Two possible cycloaromatization pathways for dienylalkyne derivatives.

Br
O
B
1) n-BuLi, THF, -78 o
C
O
Br
N
N
B
2)
F
F
O
R
B
6
O
O
R
2a
2b
2c
R = H, 78%
R = OCH3, 86%
R = CF3, 73%
1a
1b
1c
R = H, 70%
R = OCH3, 70%
R = CF3, 76%
Pd(PPh3)4, THF
Na2CO3 (a.q.)
N
N
PtCl2
B
N
N
N
N
B
B
F
F
F
F
F
F
Toluene, reflux
R
R
R
4a
4b
4c
5a
5b
5c
R = H, 25%
R = OCH3, 15%
R = CF3, 40%
R = H, not obtain
R = OCH3, 25%
R = CF3, not obtain
3a R = H, 83%
3b R = OCH3, 58%
3c R = CF3, 69%
Scheme 2. Synthetic route to BODIPYs 4a, 4b, 4c, and 5b.
Fig. 1. 1H NMR superposition of 3b, 4b, and 5b (400 MHz, CDCl3).

Fig. 2. X-ray structures of 3b (CCDC No. 2006582) and 3c (CCDC No. 2006583).
Hydrogen atoms were omitted for clarity. The thermal ellipsoids were set at 50%
probability. C gray, N blue, F yellowish green, B red, and O deep green.
4a
4b
4c
5b
4a
4b
4c
5b
8
6
4
2
0
550
600
650
700
750
300 400 500 600 700
Wavelength / nm
Wavelength / nm
Fig. 3. UV/Vis absorption (left) and normalized fluorescence (right) spectra of 4a, 4b,
4c, and 5b in DCM. (For 4a and 4c, λex = 470 nm; for 4b and 4d, λex = 550 nm. The
emission plot of 5b was not normalized due to low intensity.)
Table 1. Spectroscopic and photophysical properties of 3a-5b in CH2Cl2 at 298K.
max
[a]
max
[b]
[f]
λabs
εabs
λem
Δvem-abs
τ[d]
κr[e]
κnr
[c]
ΦF
[nm] [M-1 cm-1] [nm]
[cm-1]
1759
1893
1609
793
[ns]
6.27
3.76
6.38
3.74
[108 s‒1] [108 s‒1]
3a
3b
3c
4a
537
538
536
570
33000
28000
38300
56200
593
599
586
597
0.20
0.25
0.20
0.18
0.32
0.66
0.31
0.48
1.28
1.99
1.25
2.19

4b
4c
5b
591
567
571
41000
50000
91000
621
594
600
817
817
846
0.56
0.19
0.01
4.78
4.95
—
1.17
0.38
—
0.92
1.64
—
[a] Molar absorption coefficients of the maximum absorption peak. [b] Stokes-shift
value. [c] The fluorescence quantum yields were calculated using fluorescein as the
standard (ΦF = 0.95 in 0.1 M NaOH) for 3a, 3b, and 3c and Rhodamine 6G as the
standard (ΦF = 0.95 in ethanol) for 4a, 4b, 4c, and 5b. The standard errors are less
than 5%. [d] Fluorescence liftime. [e] The radiative decay rate, kr = ΦF/τ. [f] The
nonradiative decay rate, knr = (1-ΦF)/τ.
Fig. 4. DFT calculated frontier molecular orbital profiles and energy levels.

Fig. 5. The overlaid plots of the calculated electronic transition (gray bars) and
experimental UV-Vis spectra (colored lines) of 4b and 5b. The NICS(0) values (ppm)
at the geometric center points of the corresponding five-/six-membered rings.

Highlights
A facile method for aromatic ring [b]-fused BODIPYs was reported
Methoxy group facilitate the formation of two isomers with different photophysical
properties in one pot reaction.
Naphtho[b]-fused BODIPY exhibits higher quantum yield (0.56 in DCM), while the
isomeric indeno[b]-fused BODIPY shows stronger absorption intensity.
Theoretical calculations revealed the structure-property relationships of BODIPYs
with different conformations.

Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships
that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered
as potential competing interests: