Regulation of an Ambient-Light-Induced Photocyclization Pathway (Norrish–Yang Versus 6π) by Substituent Choice

Article abstract of DOI:10.1002/chem.202000990

Photocyclization, irrespective of whether multiple steps (e.g., Norrish–Yang cyclization) or a single concerted step (e.g., 6π photocyclization) are involved, is an intramolecular photochemical process resulting in the formation of one new single bond to afford a ring system. In particular, visible-light-induced photocyclization offers a green and sustainable route to organic cyclic compounds that are difficult to access by thermal reactions. Herein, we describe the ambient light-induced intramolecular photocyclization of a series of donor/acceptor chromophores 1 d–3 d containing two types of photoresponsive motifs, namely an electron-deficient BF₂-chelated ketone fused with an electron-rich thiophene, and probe the solution-phase and solid-state photochromic performance of these compounds. The results reveal that simple variation of R substituents on the diaryl moiety allows one to control the intramolecular photocyclization mechanism with high photochemical selectivity, e.g., under ambient light, methyl-substituted 1 d and 2 d undergo reversible 6π photocyclization, whereas ethyl-substituted 3 d exclusively undergoes irreversible Norrish–Yang photocyclization. Single-crystal X-ray analysis of Norrish–Yang cyclization products reveals the formation of four pairs of conformational enantiomers differing in the dihedral angle between benzothiophene and the BF₂ core, namely (±)N-3 d@68°, (±)N-3 d@-77°, (±)N-3 d@-78°, and (±)N-3 d@-102°. The UV/Vis absorption spectra of 1 d–3 d cover a broad visible-light region (380–572 nm), while DFT and TD-DFT calculations reveal that absorption in this region is dominated by the charge-transfer (CT) transition from the thiophene-centered HOMO to the LUMO of the electron-deficient π-conjugated BF₂-chelated unit and the n→π* and π→π* transitions within the latter unit. The spatial separation of the HOMO and LUMO of these dyes promotes triplet-state generation and self-photosensitizes intramolecular photocyclization in the visible-light region. Three-dimensional time-resolved and steady-state emission spectra of 3 d show that the Norrish–Yang photocyclization takes place within milliseconds with excellent conversion efficiency (96 %).

Full text of DOI:10.1002/chem.202000990

Accepted Article
Accepted Article
Title: Regulation of ambient light–induced photocyclization pathway
(Norrish-Yang vs. 6π) by substituent choice
Authors: Hui Liu, Hu Gao, Sisi Wang, Shengxin Yao, Fan Wu, Yue
Zhao, Kin Shing Chan, and Zhen Shen
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.202000990
Link to VoR: https://doi.org/10.1002/chem.202000990
0
1/2020

Chemistry - A European Journal
10.1002/chem.202000990
FULL PAPER
Regulation of ambient light–induced photocyclization pathway
Norrish-Yang vs. 6π) by substituent choice
(
Hui Liu, [a] Hu Gao, [a] Sisi Wang, [a] Shengxin Yao, [a] Fan Wu, [a] Yue Zhao, [a] Kin Shing Chan, [a] and
Zhen Shen*[a]
[
a]
Title(s), H. Liu, H. Gao, S. Wang, S. Yao, Dr. F. Wu, Dr. Y. Zhao, Prof. K. S. Chan, Prof. Z. Shen
State Key Laboratory of Coordination Chemistry
Collaborative Innovation Center of Advanced Microstructures
School of Chemistry and Chemical Engineering
Nanjing University
Nanjing 210046, China
E-mail: zshen@nju.edu.cn
Supporting information for this article is provided via a link at the end of the document.
Dedicated to Professor Jörg Daub on the occasion of his 80th birthday.
Abstract: Photocyclization, irrespective of whether multiple steps
perfumes, and dyes.[1] The ongoing search for enhanced physical,
biological, chemical and optoelectronic properties has motivated
the construction of highly functionalized defined-configuration
carbo- and heterocycles with unique properties.[2] Hence, visible
light–induced photocyclizations have drawn much attention, as
they offer a green and sustainable strategy of obtaining highly
valuable molecular structures that may be difficult to fabricate
using thermal reactions.[3]
(e.g., Norrish-Yang cyclization) or a single concerted step (e.g., 6π
photocyclization) are involved, is an intramolecular photochemical
process resulting in the formation of one new single bond to afford a
ring system. In particular, visible light–induced photocyclization offers
a green and sustainable route to organic cyclic compounds that are
difficult to access by thermal reactions. Herein, we describe the
ambient light–induced intramolecular photocyclization of a series of
donor/acceptor chromophores 1d–3d containing two types of
2
photoresponsive motifs, namely an electron-deficient BF -chelated
ketone fused with an electron-rich thiophene, and probe the solution-
phase and solid-state photochromic performance of these
compounds. The results reveal that simple variation of R substituents
on the diaryl moiety allows one to control the intramolecular
photocyclization mechanism with high photochemical selectivity, e.g.,
under ambient light, methyl-substituted 1d and 2d undergo reversible
6π photocyclization, whereas ethyl-substituted 3d exclusively
undergoes irreversible Norrish-Yang photocyclization. Single-crystal
X-ray analysis of Norrish-Yang cyclization products reveals the
formation of four pairs of conformational enantiomers differing in the
dihedral angle between benzothiophene and the BF
2
core, namely
(
±)N-3d@68°, (±)N-3d@-77°, (±)N-3d@-78°, and (±)N-3d@-102°.
2
Figure 1. Substituent-regulated 6π and Norrish-Yang photocyclization of BF -
chelated dyes bearing two types of photoresponsive motifs (diarylethene and
ketone).
The UV-vis absorption spectra of 1d–3d cover a broad visible light
region (380–572 nm), while DFT and TD-DFT calculations reveal that
absorption in this region is dominated by the charge transfer (CT)
transition from the thiophene-centered HOMO to the LUMO of the
2
electron-deficient π-conjugated BF -chelated unit and the n→π* and
Among intramolecular photocyclizations, the Norrish type-II
cyclization and the photochemically allowed conrotatory 6π
photocyclization are particularly versatile and valuable, offering
the advantages of atom economy and absence of toxic transition
metal catalysts, as reflected by their widespread use in the
π→π* transitions within the latter unit. The spatial separation of the
HOMO and LUMO of these dyes promotes triplet state generation and
self-photosensitizes intramolecular photocyclization in the visible light
region. Three-dimensional time-resolved and steady-state emission
spectra of 3d show that the Norrish-Yang photocyclization takes place
within milliseconds with excellent conversion efficiency (96%).
[4]
syntheses of natural products and other complex molecules.
Typically, these two types of photochemical reactions differ both
in their mechanism and substrate structural requirements. Norrish
type-II cyclization requires a γ-hydrogen-containing ketone and
proceeds via γ-hydrogen abstraction by the electron-deficient
oxygen of the photoexcited carbonyl nπ* state to afford a 1,4-
hydroxy biradical intermediate, which then cyclizes to form a
cyclobutanol (Norrish-Yang cyclization) and/or decomposes to
produce an alkene and an enol (Norrish type-II reaction).[
Moreover, photoinduced cyclopentanol formation can occur in
Introduction
Organic cyclic compounds possess unique molecular shapes,
properties, and bioactivities, and have been extensively studied
because of their widespread occurrence in many natural products,
biologically active structures, medicinally relevant compounds,
5]
1
This article is protected by copyright. All rights reserved.

Chemistry - A European Journal
10.1002/chem.202000990
FULL PAPER
cases of ketones with no γ-hydrogens and very reactive δ-C‒H
bonds.[6] In contrast, the photocyclic reactions of 6π electronic
systems are concerted processes that obey Woodward–
Hoffmann rules and proceed in a conrotatory manner with an s-
cis–cis–s-cis configuration of the hexatriene moiety to yield a
cyclohexadiene skeleton with two stereogenic sp3 carbons.[7]
Importantly, the reactive hexatriene moiety must have an s-cis–
cis–s-cis configuration with an antiparallel orientation of two
terminal substituents. So, both of the above reactions are rotation-
controlled intramolecular photocyclizations sensitive to ground-
state conformation owing to the short lifetimes of photoexcited
states, thus probably exhibiting low efficiency and poor
chemoselectivity.[3c, 8] The control of photoreaction product
selectivity can be improved via the use of organized assemblies,
which can be prepared using confined media, supramolecular
templates, and solid-state photoreactions.[9] Moreover, molecular
conformation can be constrained via the use of hydrogen bonding,
aromaticity chromophores is desirable.[14] Recently, we have
successfully demonstrated that ketone-modified BODIPYs
display lower aromaticity than traditional BODIPYs as a result of
the replacement of pyrrole by pyrrolidone.[15]
Herein, we report
chromophores 1d–3d containing two types of photoresponsive
motifs (Scheme1), namely an electron-deficient BF -chelated
a
series of donor-acceptor (D-A)
2
ketone and an electron-rich thiophene, and probe the
photochromic performances of these compounds in solution and
the solid state. Notably, the R substituent on the diaryl moiety is
shown to determine whether the intramolecular photocyclization
proceeds via the 6π or Norrish-Yang pathway with high
photochemical selectivity, e.g., under ambient light, methyl-
substituted 1d and 2d readily undergo reversible 6π
photocyclization, while ethyl-substituted 3d exclusively
undergoes irreversible Norrish-Yang cyclization. Three-
dimensional time-resolved and steady-state emission spectra
confirm that 3d undergoes highly efficient photocyclization, and
theoretical calculations suggest that the charge-transfer triplet
state (3CT) promotes intramolecular photocyclization under
visible light irradiation.
weak
intramolecular
interactions,
and
covalent
[
10]
bonding/intramolecular sterics.
The widely investigated photochromic diarylethenes undergo
6π photocyclization upon irradiation with ultraviolet light, with the
reverse process observed upon visible light irradiation.[11]
However, to the best of our knowledge, although the Norrish-Yang
cyclization of 1,2-diketones was reported to be triggered by light-
emitting diode blue light,[4b] most Norrish photoreactions occur
under ultraviolet mercury lamp irradiation,[9c, 12] which can lead to
side reactions and photodamage, especially in biological systems.
Furthermore, owing to the low molar extinction coefficient of
ketones in the UV-visible region, their Norrish-Yang
photocyclizations are always inefficient. Thus, highly selective
and efficient visible light–driven photoreactions are much more
amenable, as exemplified by visible light–induced cyclizations
under metal-free or organometallic photocatalysis conditions.[13]
To increase the efficiency of visible light–driven photoreactions,
we herein incorporate two photoresponsive motifs (diarylethene
and ketone) into one chromophore as part of a π-conjugated
system, thus pushing n→π*/π→π* transitions to the visible range
Results and Discussion
Molecular design
Diarylethene derivatives bearing 2-methylthiophene (1a–1d), 2-
methylbenzothiophene (2a–2d), and 2-ethylbenzothiophene (3a–
3d) units were prepared to investigate the effect of substituents
on the cyclization mechanism of the diarylethene moiety (Scheme
1 [
).
16]
cis-Dicyanoethene derivatives 1a–3a were synthesized
according to modified procedures (Supplementary Figure 1).
Reaction of 1a–3a with N,N'-diethylhydroxylamine in chloroform
afforded 1b–3b.[17] Diarylethene BF
-fused chromophores 1d–3d
were synthesized in good yields by heating equivalent amounts of
b–3b, respectively, and 2-aminobenzothiazole followed by
treatment with BF ·OEt (Scheme 1).
2
1
3
2
(
Figure 1). As the presence of highly aromatic structures is known
to reduce photocyclization efficiency, the introduction of lower-
3 3 3 2
Scheme 1. Syntheses of diarylethene derivatives. Reaction conditions: (i) N,N'-diethylhydroxylamine, CHCl , reflux; (ii) n-butanol, reflux; (iii) toluene, Et N, BF ·Et O,
reflux.
2
This article is protected by copyright. All rights reserved.

Chemistry - A European Journal
10.1002/chem.202000990
FULL PAPER
1
Figure 2. H NMR spectrum of 3d with signals of two conformational isomers, 3d-anti and 3d-para.
NMR spectrum of 3d
The UV-vis absorption data of 1b–3b and 1d–3d recorded in
solvents of different polarity are summarized in Supplementary
Table 1. Figure 3b presents absorption spectra recorded in 1,4-
dioxane, revealing that those of 1b–3b featured broad
structureless bands at 300–421, 300–438, and 300–446 nm,
respectively. The absorption spectra of 1d–3d were significantly
red-shifted and intensified compared with those of 1b–3b,
featuring broad bands peaking at ~390 nm and tails extending to
The impeded rotation of the two thiophene units resulted in the
observation of conformational isomers in the nuclear magnetic
resonance (NMR) spectra of 1b–3b and 1d–3d (Figure 2,
Supplementary Figures 2–18). As an example, the 1H NMR
spectrum of 3d in CD Cl exhibited four pairs of well-resolved
2 2
triplets at 0.81, 0.87, 1.14, and 1.17 ppm, as well as eight pairs of
quartets centered at 2.43, 2.48, 2.61, 2.66, 2.69, 2.73, 2.88, and
566, 567, and 572 nm, respectively.
2.91 ppm (Figure 2), indicating the presence of four chemically
To definitively assign the main spectral bands, geometry
non-equivalent ethyl substituents from two asymmetric
conformational isomers, named antiparallel 3d-anti and parallel
optimizations were carried out with B3LYP 6-31G(d) basis sets,
and TD-DFT calculations were carried out in a similar manner with
the CAM-B3LYP functional. As shown in Figure 3a, the HOMO
and HOMO-1 were largely localized on the electron-rich
thiophene unit, whereas the LUMO was predominantly localized
3
d-para, respectively, according to the orientation of the two
ethyl-substituted benzothiophenes.[18] The ratio of the two
atropisomers was calculated as 51:49 from the integral peak
areas. The methyl protons of 3d-para resonated at a higher field
than those of 3d-anti because of the shielding effect of the
neighboring benzothiophene, as has been previously reported for
other diarylethenes.[10d, 10e]
2
on the electron-deficient BF -chelated core for 1d–3d and on the
maleimide bridge for 1b–3b. Therefore, HOMO-1/HOMO →
LUMO transitions had a pronounced charge transfer (CT)
character with virtually complete charge localization in both
occupied and unoccupied orbitals, which accounted for the broad
absorption of 1b–3b and the absorption tail bands of 1d–3d. The
Photophysical properties and theoretical calculation results
of 1b–3b and 1d–3d
2
Figure 3. UV-vis absorption spectra and theoretical calculation results for 1b–3b, 1d–3d, and the BF core. a) Angular nodal patterns and energies of the frontier
π-MOs obtained by DFT calculations with the B3LYP functional. Occupied MOs are denoted by small black diamonds. b) TD-DFT spectra calculated for B3LYP-
optimized geometries using the CAM-B3LYP functional and 6-31G(d) basis sets. The experimental absorption spectra in 1,4-dioxane are plotted against a secondary
axis.
3
This article is protected by copyright. All rights reserved.

Chemistry - A European Journal
10.1002/chem.202000990
FULL PAPER
Scheme 2. Schematic representation of substituent-dependent photocyclizations of 1b–3b and 1d–3d.
LUMOs of 1d–3d were significantly stabilized compared to those
of 1b–3b, while the HOMOs-1 were slightly destabilized, and the
Substituent-dependent photocyclization
HOMOs were barely affected because of the incorporation of the
BF core. The above stabilization resulted in a decrease of the
2
HOMO-1/HOMO–LUMO gap, eventually leading to a largely red-
shifted CT absorption relative to those of 1b–3b. Besides, TD-
DFT calculations suggested that the main absorption bands of
The photochromic properties of 1b–3b and 1d–3d were
investigated upon irradiation with UV-visible light and visible light
only, respectively. Notably, 1b–3b, 1d, and 2d underwent
reversible 6π photocyclization, whereas 3d exclusively underwent
an irreversible Norrish-Yang cyclization (δ-hydrogen abstraction),
yielding two atropisomers N-3d-anti and N-3d-para in almost
equal amounts (Scheme 2).
1
d–3d peaking at 390 nm can be attributed to the n→π* and π→π*
transitions of the BF core (Figure 3a and Supplementary Table
).
2
2
Figure 4. Electrocyclization of 1b–3b and 1d in 1,4-dioxane upon irradiation with 405 nm light. a–d Time-dependent absorption spectral changes of 1b–1d,
respectively. Inset: photographs of 1b (6.2 × 10−4 mol L ), 2b (6.1 × 10 mol L ), 3b (4.3 × 10 mol L ), and 1d (2.5 × 10 mol L ) solutions before and after
−1
-4
−1
−4
−1
−5
−1
irradiation.
4
This article is protected by copyright. All rights reserved.

Chemistry - A European Journal
10.1002/chem.202000990
FULL PAPER
1
Figure 5. a) Molecular structures and b) H NMR spectra of N-3d-anti and N-3d-para.
and 13C- H HSQC). As only one ethyl substituent exists in the
structures of N-3d-anti and N-3d-para, the methyl protons of N-
3d-para resonated at a higher field than those of N-3d-anti
1
6
π photocyclization of 1b–3b, 1d, and 2d
The photochromic properties of 1b–3b and 1d were investigated
in 1,4-dioxane. Upon irradiation of 1b–3b with 405 nm light, new
absorption bands appeared at 504, 495, and 517 nm, respectively, because of the shielding effect of the neighboring benzothiophene
resulting in an evident color change from colorless (for 1b and 2b)
or yellowish (for 3b) to pink (Figures 4a–4c), as expected for the
typical ring-closed form of diarylethene derivatives.[11a] Upon
irradiation of 1d with visible light, the peak at 390 nm lost intensity,
while the lower-energy absorption band at 490 nm concomitantly
gained intensity (Figure 4d), which resulted in a color change from
yellowish to red. In contrast to the cyclization of 1d, which
proceeded with an excellent conversion of ˃90%, the
photocyclization of 2d was less efficient and resulted in less
pronounced absorption spectrum changes (Supplementary
Figure 19). The 6π photocyclization of 1b–3b, 1d, and 2d could
be reversed by subsequent irradiation with visible light.
(Figure 5b). The splitting of methylene protons suggested that
both N-3d-para and N-3d-anti are asymmetric molecules.
Besides, the spectra of N-3d-anti and N-3d-para featured almost
identical shifts of the hydroxyl proton (3.5 ppm), methyl protons
(1.8 ppm), and the proton of the newly formed cyclopentanol (3.85
ppm).
Crystal structures
The molecular structures of Norrish-Yang cyclization products
were further confirmed by single-crystal X-ray analysis (Figure 6).
N-3d-anti and N-3d-para were separated by column
chromatography after irradiation of 3d with visible light. Single
crystals suitable for X-ray structural analysis were obtained by
slow diffusion of hexane into a dichloromethane solution of either
Norrish-Yang photocyclization of 3d
In contrast to the reversible electrocyclization observed for 1b–3b, isomer. In principle, N-3d-anti and N-3d-para are conformational
d, and 2d, 3d underwent a Norrish-Yang photocyclization in both isomers differing by the dihedral angle between one of the
benzothiophene units and the BF core. The asymmetry of the
1
solution and the solid state under visible or ambient light. Although
the conformational isomers of 3d-anti and 3d-para could not be
isolated, the corresponding Norrish-Yang photocyclization
products N-3d-anti and N-3d-para were separated and
characterized by high-resolution mass spectrometry and NMR
2
molecular structure resulted in the presence of a pair of
enantiomers in the unit cell of (±)N-3d-anti (Figure 6a). In (±)N-
3d-anti, the dihedral angle between one of the benzothiophenes
and the BF
2
core equaled 67.6(1)°, and these enantiomers were
(
Figure 5, Supplementary Figures 20–29), with structure
therefore denoted as (±)N-3d@68°. In contrast, three pairs
1
1
elucidation performed by two-dimensional NMR ( H- H COSY
5
This article is protected by copyright. All rights reserved.

Chemistry - A European Journal
10.1002/chem.202000990
FULL PAPER
Figure 6. Single-crystal structures of N-3d-anti and N-3d-para. a) Structures of (±)N-3d-anti enantiomers in top and side views. b) Three pairs of conformational
enantiomers of (±)N-3d-para in top and side views. Red arrows indicate the rotational direction and angle of benzothiophene unit around BF core. c), d)
2
Molecular packing diagrams of (±)N-3d-anti and (±)N-3d-para, respectively, showing enantiomers in alternating arrangement. Dotted lines represent
intermolecular interactions.
of enantiomers with dihedral angles of −76.9(3)°, −78.2(2)°, and
102.4(3)° were found in the unit cell of (±)N-3d-para, namely (±)-
7°, (±)-78°, and (±)N-3d@-102°, respectively (Figure 6b). The
newly formed central cyclopentanol was fused with the
2
neighboring benzothiophene and the BF core to form a system
of seven fused rings. The cyclopentanol moiety adopted an
envelope conformation with the carbon substituted by the
−
7
Figure 7. Theoretical calculation results for N-3d-anti and N-3d-para. a) Angular nodal patterns and energies of the frontier π-MOs obtained by DFT calculations
with the B3LYP functional. Occupied MOs are denoted by small black diamonds. b) UV-vis absorption spectra and TD-DFT calculations for N-3d-anti and N-3d-
6
This article is protected by copyright. All rights reserved.

Chemistry - A European Journal
10.1002/chem.202000990
FULL PAPER
para. TD-DFT spectra calculated for B3LYP optimized geometries with the CAM-B3LYP functional and 6-31G(d) basis sets. The experimental absorption spectra
in hexane are plotted against a secondary axis.
Figure 8. Photoresponsive absorption properties of 3d in hexane (c = 2.0 × 10−5 mol L−1). a) Absorption spectrum changes under ambient light irradiation (light
intensity = 238 lux). Inset: digital illumination intensity meter. Absorption spectra of N-3d-anti and N-3d-para plotted in black short-dash and short-dot lines,
respectively. b) Time-dependent absorption spectrum changes monitored at 455 nm (red circles) and 375 nm (blue circles).
hydroxyl group being the visor (located 0.51–0.53 Å above the
mean envelope plane), and the other four carbon atoms lying
almost in the same plane (rms = 0.0086–0.013). The methyl and
hydroxyl substituents on the cyclopentanol ring adopted a cis-
conformation. The seven-fused-ring skeleton adopted distortion
angles of 41.4(3)°, 34.7(1)°, 30.5(1)°, and 18.9(1)°, respectively,
between the fused plane containing benzothiophene and the
envelope plane of cyclopentanol (rms = 0.0404–0.0579) and the
HOMO/HOMO-1 orbital (close-lying in energy) and the
benzothiophene and cyclopentanol ring fused chromophore–
centered LUMO orbital. Both have apparent CT character.
Norrish-Yang photocyclization under ambient light
Compared to 3d, Norrish-Yang photocyclization products N-3d-
anti and N-3d-para exhibited slightly blue-shifted absorptions
peaking at 375 nm (Figure 8a, black lines). Upon irradiation of 3d
with ambient light, the shoulder peak at 455 nm lost intensity,
while the peak at 375 nm gained intensity, and a well-defined
isosbestic point was observed at 412 nm, which was indicative of
a clean conversion of 3d into N-3d in oxygenated hexane (Figure
8a, grey lines). Upon irradiation, the originally pale-yellow solution
of 3d turned colorless, and the conversion yield reached 80%
(Figure 8b).
plane containing the visor carbon and the BF
2
core (rms =
0
.0459–0.1086). Weak (BF ) F···H (CH from cyclopentanol)
2
3
intramolecular hydrogen bonds were observed (2.53, 2.49, 2.62,
and 2.84 Å, dotted red line in Figures 6a and 6b). The two
enantiomers of (±)N-3d@68° were packed in an alternating
fashion with multiple intermolecular contacts, e.g., F1···H1–O1
(
2.08 Å, 156.1(1)°), F1···O1–C7 (2.87 Å, 110.2(1)°), F2···H23–
C23 (2.67 Å, 130.2(2)°), N2···H26–C26 (2.73 Å, 147.3(2)°)
Figure 6c). (±)N-3d@-77°, (±)N-3d@-78°, and (±)N-3d@-102°
Steady-state and time-resolved fluorescence spectra of 3d,
N-3d-anti, and N-3d-para
(
showed an alternating arrangement in the crystal packing
diagrams of N-3d-para, also with multiple intermolecular contacts
Upon irradiation with a xenon lamp at 370 nm, the fluorescence
spectrum of 3d shifted to lower wavelengths (e.g., the emission
maximum shifted from 600 to 529 nm), and the fluorescence
quantum yield significantly increased from 3 to 36% (Figure 9a).
Photocyclization was estimated to occur on a millisecond
timescale using a charge coupling device (CCD) fluorescent
detector. Kinetic time-correlated single photon counting (TCSPC)
lifetime measurements demonstrated an excellent conversion
efficiency of 96% comparable to the value calculated by the
steady-state method (Figure 9b). Three-dimensional excitation-
emission matrix (3D-EEM) fluorescence spectra of N-3d-anti and
N-3d-para were recorded in hexane at room temperature
(Figures 9c and 9d). Only one emission band was readily
identified for N-3d-anti (522 nm) and N-3d-para (530 nm)
irrespective of the excitation wavelength. Two major excitation
bands were consistently observed in 3D-EEM spectra at 274 nm
and 375 nm for N-3d-anti and N-3d-para, respectively, closely
paralleling the main bands in the corresponding absorption
spectra. Upon irradiation at 250–650 nm, the 3D-EEM spectrum
of 3d gradually changed into a superposition of N-3d-anti and N-
(
Figure 6d). These weak hydrogen bonds along with
intermolecular C–H···π (3.39 Å) and π–π interactions (2.37 and
.39 Å) resulted in the formation of a three-dimensional network.
3
Furthermore, the steric repulsion between the two benzothienyl
moieties (~3.10 Å) was indicative of impeded rotation of
benzothiophene linked to the BF core.
2
Photophysical properties and theoretical calculation results
of N-3d-anti and N-3d-para
The UV-vis absorption and emission spectra of N-3d-anti and N-
3d-para recorded in solvents of different polarity are shown in
Supplementary Figures 30, 31 and summarized in Supplementary
Table 1. Theoretical calculations revealed that the LUMOs of N-
3d-anti and N-3d-para were considerably destabilized, while the
corresponding HOMOs were only slightly destabilized in
comparison with those of 3d, which resulted in larger energy gap,
in line with the blue-shifted absorption spectra (Figure 7). The
main absorption bands of N-3d-anti and N-3d-para were
assigned to transition states (S
1 2
and S ) with much higher
oscillator strengths mainly involving the diary-centered
7
This article is protected by copyright. All rights reserved.

Chemistry - A European Journal
10.1002/chem.202000990
FULL PAPER
Figure 9. Photo-responsive fluorescent properties of 3d in hexane. a) Time-dependent emission spectrum changes upon irradiation with a 450-W CW xenon lamp
at 370 nm (before: red line, after: blue line), detected by a CCD detector through variation of integration time from 0.01 to 1 s. Inset: images of the solution before
and after irradiation. b) Kinetic TCSPC lifetime measurement of 3d at 600 nm upon irradiation with a DeltaDiode-405 picosecond pulsed laser. c), d) 3D-EEM
fluorescence spectra of N-3d-anti and N-3d-para in hexane, respectively. e) 3D-EEM spectra of 3d before and after 10-min irradiation. 3D-EEM spectra are provided
as elliptical contours. The x-axis represents emission spectra, while the y-axis is the excitation wavelength. The z-axis, as the third dimension, represents
luminescence intensity.
3
d-para spectra (Figure 9e), as reflected by the intensity gain of
observed under all conditions, with conversions reaching 70%
(Figures 10b and 10d). The observed rate constants linearly
decreased with increasing irradiation wavelength (Figure 10c),
thus being proportional to light intensity and density, which was
ascribed to the energy barrier on the potential energy surface in
the excited electronic state.[19] The reaction rate was remarkably
affected by solvent polarity, decreasing in the order of hexane >
toluene > 1,4-dioxane > tetrahydrofuran > dichloromethane >
acetonitrile, in accordance with the increasing order of solvent
dielectric constants (ε). This relationship was almost linear except
the blue-shifted emission band and the intensity loss of the longer
excited tail band above 450 nm.
Kinetics of the Norrish-Yang reaction
Irradiation of 3d at different wavelengths and in solvents with
various polarities resulted in similar photoresponses
(
a
Supplementary Figures 32–40). As the photocyclization of 3d is
unimolecular reaction, pseudo-first order in [N-3d] was
8
This article is protected by copyright. All rights reserved.

Chemistry - A European Journal
10.1002/chem.202000990
FULL PAPER
for hexane and acetonitrile (Figure 10e). In contrast to the
solvents,[20] the cyclization of 3d was suppressed by increasing
classical Norrish type-II reaction, which is favored by polar
solvent polarity. This behavior was attributed to the adjustment of
Figure 10. Pseudo-first-order kinetics for the photocyclization of 3d upon irradiation at different wavelengths in solvents with various polarities. a) Proposed
mechanism of self-sensitized photochemical reactions of 3d. b), d) First-order rate plots for N-3d production from 3d upon irradiation at different wavelengths in 1,4-
dioxane and at 488 nm in various solvents (c = 2.0 × 10−5 mol L ). c), e) Plots showing first-order rate constant κ as a function of dielectric constant ε and irradiation
wavelength λ, respectively.
−1
3
the energy level of CT involved in the promotion of the Norrish-
Yang photocyclization by switching of solvents with different
polarities (Figure 10a).
Supplementary Table 1). This suggests that in the solid state,
emission was significantly enhanced because of the asymmetry
of dye structure. Inspired by the above results, we explored the
possibility of using 3d as a photoactivatable fluorescent material.
In doing so, we wrote ‘‘NJU’’ with a dichloromethane solution of
3d (10−4 mol L ) on filter paper (Figure 11b) and completely dried
it. Prior to irradiation, the text appeared pale yellowish under room
light and exhibited red fluorescence. With increasing irradiation
Norrish-Yang photocyclization in the solid state
−1
Similar to the solution-phase case, 3d, N-3d-para, and N-3d-anti
displayed solid-state fluorescence at 632, 530, and 524 nm with
quantum yields of 0.5, 14, and 21%, respectively (Figure 11a,
9
This article is protected by copyright. All rights reserved.

Chemistry - A European Journal
10.1002/chem.202000990
FULL PAPER
time, the letters turned pale greenish and exhibited brighter
yellowish-green emission, in line with the colors observed for N-
3d-para and N-3d-anti. Thus, the above findings demonstrate
that 3d also undergoes the Norrish-Yang photocyclization in the
Figure 11. Fluorescent properties of 3d and N-3d and the photoresponse of 3d in the solid state. a) Emission spectra of 3d (red), N-3d-anti (green), and N-3d-para
yellow) in powder form (λex = 390 nm). Inset: photographs of 3d, N-3d-anti, and N-3d-para excited by a UV lamp at 365 nm. b) Photographs of a fluorescent text
(
−4
−1
written using a solution of 3d in dichloromethane (10 mol L ) before and after 30-min irradiation with ambient light.
solid state and can therefore be potentially employed for one-time
anti-counterfeiting protection, especially in cases of high-value
products.[21]
ambient temperature. Only cis form of cyclopentanol ring was
observed indicating the high stereoselectivity of Norrish-Yang
photocyclization, which was probably duo to the presence of
intramolecular CH···F hydrogen bonds between the ethyl
2
substituent and the central BF . In contrast to 6π photocyclization,
which led to fluorescence quenching, the remarkable change in
fluorescence wavelength and intensity upon Norrish-Yang
photocyclization either in solution or the solid state may find
promising applications in the production of anti-counterfeiting
materials or new cyclic organic molecules for medicinal
applications.
Conclusion
In summary, three diarylethene and ketone-modified BF
d–3d were synthesized and structurally characterized. The BF
chromophores exhibited appreciable visible light absorption at
80–572 nm and could be excited with ambient light to induce
2
dyes
1
2
3
efficient and selective intramolecular photocyclizations. 6π
photocyclization prevailed in precursors 1b–3b under 405-nm
light and in BF
Norrish-Yang photocyclization occurred for BF
2
dyes 1d and 2d under visible light, whereas Experimental Section
dye 3d in solution
2
and the solid state under ambient light. These unique phenomena
were explained by considering self-sensitization and C–H bond
strength.
General synthesis of 1b–3b
3,4-Diaryl-substituted 5-iminopyrrol-2-ones 1b–3b were prepared
[17]
according to a known procedure.
Briefly, a mixture of 1,2-diaryl-
2
Theoretical calculations on BF dyes 1d–3d revealed that their
lower-energy absorption profile was dominated by a CT transition
substituted cis-dicyanoethane (1 equiv.) and N,N'-diethylhydroxylamine (2
equiv.) in freshly distilled dry CHCl was placed in a round-bottom flask
3
from the diarylethene fragment (electron donor) to the π-
conjugated BF core (electron acceptor) with virtually complete
2
charge localization in both occupied and unoccupied orbitals, in
line with the appearance of a broad absorption tail at 450–572 nm.
The main absorption bands at 300–450 nm was ascribed to local
equipped with a magnetic stirrer and refluxed for 4 h, cooled to room
temperature, stirred for 1 h, and concentrated in vacuo, and the residue
was purified by silica gel column chromatography.
1
b was obtained as a pale-yellow solid in 89% yield (199 mg, 0.58 mmol).
1
2 2
H NMR (400 MHz, CD Cl ): δ = 8.46 (br, N-H), 2.27 (s, 3H), 2.25 (s, 9H),
2
3
3
.16 (s, 3H), 2.02 (s, 3H), 2.01 (s, 3H), 2.00 (s, 3H), 1.85 (s, 6H), 1.84 (s,
excitation within the π-conjugated BF
2
core as well as to the mixed
] +
H), 1.71 ppm (s, 3H); HRMS (ESI): m/z calcd. for [C18
H
21
N
2
OS
2
:
n→π* and π→π* transitions. The fluorescence spectrum of 3d was
sensitive to solvent polarity, which further demonstrated the CT
nature of excited states (Supplementary Figure 41). Thus, 3d was
concluded to be a photosensitizer for promoting the Norrish
photochemical reaction, probably via triplet-triplet energy
+
45.1017; found: 345.1090 [M+H ]; UV-vis (dioxane): λmax (ε) = 365 nm (1
3
-1
-1
981 dm mol cm ).
2b was obtained as a yellow solid in 85% yield (223 mg, 0.57 mmol). H
NMR (400 MHz, CD Cl ): δ = 7.77-7.75 (m, 1H), 7.74-7.70 (m, 2H), 7.67
d, J = 8.0 Hz, 1H), 7.53-7.51 (m, 2H), 7.42 (d, J = 7.9 Hz, 1H), 7.38-7.25
m, 5H), 7.23-7.15 (m, 3H), 6.98 (ddd, J = 8.1, 7.1, 1.1 Hz, 1H), 2.38 (s,
1
2
2
(
(
transfer.[13d] Besides, the C–H bond energies of PhCH
–H and
2
3
H), 2.25 (s, 3H), 2.06 (s, 3H), 2.01 ppm (s, 3H); HRMS (ESI): m/z calcd.
PhCH(Me)–H (88.5 and 85.4 kcal/mol, respectively) favored H
atom transfer of ethyl-substituted 3d, whereas the methyl-
substituted analogue exclusively underwent electrocyclization
because of its higher C–H bond energy.
The distribution of Norrish-Yang products (fully confirmed by
NMR and X-ray diffraction analyses) showed that the energy
barriers of single-bond rotation were sufficiently large to allow the
isolation of individual atropisomers N-3d-anti and N-3d-para at
+
+
for [C22
H
17
N
2
OS
2
] : 389.0704; found: 389.0782 [M+H ]; UV-vis (dioxane):
3
-1
-1
λmax (ε) = 364 nm (3 814 dm mol cm ).
1
3
b was obtained as a yellow solid in 82% yield (239 mg, 0.57 mmol). H
Cl ): δ = 8.17 (br, N-H), 7.79 (d, J = 7.7 Hz, 1H), 7.75
(d, J = 8.0 Hz, 2H), 7.69 (d, J = 8.0 Hz, 1H), 7.54-7.57 (m, 2H), 7.46 (d, J
NMR (400 MHz, CD
2
2
= 7.7 Hz, 1H), 7.41-7.24 (m, 6H), 7.22-7.14 (m, 2H), 6.98-6.94 (m, 1H),
2.92 (dq, J = 15.7, 7.5 Hz, 1H), 2.76 (dq, J = 15.7, 7.5 Hz, 1H), 2.70 (dq, J
= 15.7, 7.5 Hz, 1H), 2.61-2.48 (m, 3H), 2.32 (dq, J = 15.7, 7.5 Hz, 1H),
1
0
This article is protected by copyright. All rights reserved.

Chemistry - A European Journal
10.1002/chem.202000990
FULL PAPER
2
3
.26 (dq, J = 15.7, 7.5 Hz, 1H), 1.22 (t, J = 7.5 Hz, 3H), 1.06 (t, J = 7.5 Hz,
H), 0.70 (t, J = 7.5 Hz, 3H), 0.64 ppm (t, J = 7.5 Hz, 3H); 13C NMR (100
2H), 7.78–7.70 (m, 6H), 7.60–7.57 (m, 2H), 7.54–7.52 (m, 2H), 7.36 (m,
2H), 7.34–7.27 (m, 4H), 7.24 (m, 2H), 7.09 (ddd, J = 8.1, 7.2, 1.0 Hz, 2H),
2.91 (dq, J = 14.8, 7.6 Hz, 1H), 2.89 (dq, J = 14.8, 7.6 Hz, 1H), 2.77–2.57
(m, 4H), 2.48 (dq, J = 14.8, 7.6 Hz, 1H), 2.43 (dq, J = 14.8, 7.6 Hz, 1H),
1.17 (t, J = 7.6 Hz, 3H), 1.14 (t, J = 7.6 Hz, 3H), 0.87 (t, J = 7.6 Hz, 3H),
MHz, CD
2
Cl
2
): δ = 170.68, 170.58, 163.83, 163.79, 151.00, 150.95, 150.77,
1
1
1
2
50.21, 139.08, 138.87, 138.44, 138.22, 137.13, 136.51, 136.00, 125.37,
25.23, 124.87, 124.83, 124.78, 124.51, 124.42, 124.32, 122.84, 122.81,
22.71, 122.47, 122.31, 122.15, 121.32, 121.26, 120.59, 120.22, 24.02,
0.81 ppm (t, J = 7.6 Hz, 3H); 13C NMR (126 MHz, CD
Cl ): δ = 173.56,
2 2
3.64, 23.48, 23.36, 15.71, 15.51, 15.13, 15.05 ppm; HRMS (ESI): m/z
172.39, 166.63, 166.58, 152.34, 151.90, 151.34, 150.83, 141.34, 140.80,
138.84, 138.67, 138.48, 138.44, 138.17, 138.15, 137.68, 137.20, 129.32,
127.54, 124.90, 124.89, 124.83, 124.76, 124.65, 124.60, 124.58, 122.96,
] +: 417.1017; found: 417.1090 [M+H ]; UV-vis
+
calcd. for [C24
H N OS
21 2 2
3
-1
-1
(dioxane): λmax (ε) = 365 nm (4 192 dm mol cm ).
122.84, 122.83, 122.81, 122.54, 122.50, 122.46, 121.33, 121.18, 121.13,
1
21.06, 119.93, 24.32, 24.13, 23.94, 23.92, 15.67, 15.65, 15.32, 15.30
General synthesis of 1d–3d
+
ppm; HRMS (ESI): m/z calcd. for [C31
2 3 3
H22BF N OS Na] : 620.0884; found:
A solution of 3,4-diaryl-substituted 5-iminopyrrol-2-one 1b–3b (1 equiv.)
and heteroaromatic amine (1 equiv.) in n-butanol was heated under reflux
+
620.0887 [M+Na ]; UV-vis (dioxane): λmax (ε) = 387 (20 355), 457 nm (10
3
-1
-1
611 dm mol cm ).
for 48 h, which resulted in the release of NH
by adding water (5mL) and extracted with ethyl acetate (3×15mL). The
organic layer was dried over anhydrous Na SO , filtered, and evaporated
under vacuum. The crude product was dissolved in dry toluene and treated
with BF ·OEt (2.5 equiv.) in the presence of Et N (2.5 equiv.) at room
3
. The reaction was quenched
N-3d-anti was obtained as a yellow solid in 21% yield (11 mg, 18.4 μmol).
1H NMR (600 MHz, CD
2 2
Cl ): δ = 8.03 (d, J = 8.2 Hz, 1H), 7.82 (d, J = 8.1
2
4
Hz, 1H), 7.77 (d, J = 8.1 Hz, 1H), 7.71 (d, J = 7.8 Hz, 1H), 7.55 (ddd, J =
8
=
.4, 7.5, 1.1 Hz, 1H), 7.43–7.40 (m, 1H), 7.22–7.19 (m, 1H), 7.17 (ddd, J
8.1, 7.1, 1.1 Hz, 1H), 7.05 (d, J = 8.0 Hz, 1H), 7.03–7.00 (m, 1H), 6.96
3
2
3
temperature for 10 min. The resulting mixture was refluxed for 6 h, cooled,
and quenched with water. The organic layer was dried over anhydrous
(
1
ddd, J = 8.0, 7.1, 1.0 Hz, 1H), 6.79 (d, J = 8.0 Hz, 1H), 3.95 (q, J = 6.6 Hz,
H), 3.49 (br, 1H), 3.17 (dq, J = 15.0, 7.8 Hz, 1H), 3.12 (dq, J = 15.0, 7.8
Hz, 1H), 1.84 (d, J = 6.6 Hz, 3H), 1.46 ppm (t, J = 7.8 Hz, 3H); 13C NMR
151 MHz, CD Cl ): δ = 174.96, 170.56, 166.62, 163.33, 149.11, 143.68,
40.83, 140.19, 138.13, 132.70, 132.55, 128.05, 127.73, 125.98, 125.84,
25.32, 124.46, 124.31, 123.80, 123.20, 122.65, 122.61, 122.55, 122.39,
Na
purified by silica gel flash column chromatography and recrystallized from
CH Cl /hexane.
2 4
SO , filtered, and evaporated to dryness. The crude product was
(
1
1
2
2
2
2
Photolysis of 3d
118.28, 115.12, 106.04, 47.38, 23.79, 16.22, 12.31 ppm; HRMS (ESI): m/z
+
+
calcd. for [C31
dioxane): λmax (ε) = 385 nm (27 710 dm mol cm ).
N-3d-para was obtained as a yellow solid in 20% yield (10 mg, 16.7 μmol).
1H NMR (600 MHz, CD
Cl ): δ = 8.04 (d, J = 8.4 Hz, 1H), 7.94–7.91 (m,
H), 7.80 (d, J = 8.1 Hz, 1H), 7.78–7.76 (m, 1H), 7.72 (d, J = 7.8 Hz, 1H),
H23BF N OS ] : 598.1064; found: 598.1075 [M+H ]; UV-vis
2 3 3
A glass reaction bottle was charged with a solution of 3d (10 mg, 16.7
μmol) in 100 mL dichloromethane (1.67 × 10-4 mol·L−1), sealed with a
rubber plug, and the solution was degassed by bubbling argon (dried by
passing through Drierite) for 30 min. The solution was irradiated at 415 nm
using an HXGY-II instrument equipped with a 120-W HXP lamp. This
photoreaction trial was repeated for five times at room temperature. After
the irradiation, the reaction mixtures were combined and excess solvent
was evaporated under vacuum. Isomers N-3d-anti and N-3d-para were
obtained by purification of the resultant crude residue via silica gel
chromatography (hexane: dichloromethane = 4:1, v/v). The enantiomers
were not separated.
3
-1
-1
(
2
2
1
7
1
.57–7.54 (m, 1H), 7.43–7.39 (m, 3H), 7.27–7.25 (m, 1H), 7.06 - 7.03 (m,
H), 6.68 (d, J = 7.8 Hz, 1H), 3.87 (q, J = 6.6 Hz, 1H), 3.52 (br, 1H), 2.58
(
dq, J = 15.0, 7.8 Hz, 1H), 2.43 (dq, J = 15.0, 7.8 Hz, 1H), 1.84 (d, J = 6.6
Hz, 3H), 1.02 ppm (t, J = 7.8 Hz, 3H). 13C NMR (151 MHz, CD
Cl ): δ =
2
2
1
1
1
4
74.95, 170.60, 166.48, 163.81, 148.82, 143.74, 141.06, 140.82, 138.59,
32.65, 132.43, 128.05, 127.75, 126.07, 125.98, 125.46, 125.00, 124.65,
23.89, 123.17, 123.07, 122.80, 122.55, 122.17, 118.28, 115.36, 106.05,
7.31, 23.45, 15.67, 12.31, 12.30 ppm; HRMS (ESI): m/z calcd. for
Chemical characterization.
+
+
[
C
31
H
23BF
2 3 3
N OS ] : 598.1064; found: 598.1078 [M+H ]; UV-vis (dioxane):
3
-1
-1
λmax (ε) = 380 nm (21 740 dm mol cm ).
1d was obtained as a red solid in 45% yield (137 mg, 0.26 mmol) using
3
,4-bis(2,4,5-trimethylthiophen-3-yl)-5-iminopyrrol-2-one (200 mg, 0.58
X-ray crystallographic analysis. X-ray diffraction data were collected on
a Bruker APEX-II CCD diffractometer using graphite-monochromatized Mo
mmol) and benzo[d]thiazol-2-amine (87 mg, 0.58 mmol), and purified by
silica gel column chromatography (hexane: ethyl acetate = 4: 1, v/v). 1
NMR (400 MHz, CD
H
K
α
radiation (λ = 0.71073 Å) in ω-2θ scan mode. Structures were solved by
2 2
Cl ): δ = 8.20 (d, J = 8.2 Hz, 2H), 7.89 (d, J = 8.0 Hz,
2
direct methods (SHELX-2014) and refined on F by full-matrix least-
squares methods using SHELX-2018. Absorption correction was applied
based on multi-scan methods. All the non-hydrogen atoms were refined
anisotropically and all the hydrogen atoms were placed by using AFIX
instructions. The SHELX-2018 program package and Diamond were used
for calculations and molecular graphics drawing, respectively. Single
2H), 7.71–7.67 (m, 2H), 7.59–7.55 (m, 2H), 2.30 (s, 3H), 2.28 (s, 3H), 2.28
(
1
s, 3H), 2.26 (s, 3H), 2.12 (s, 3H), 2.10 (s, 3H), 1.97 (s, 3H), 1.96 (s, 3H),
.95 (s, 3H), 1.95 (s, 3H), 1.80 (s, 3H), 1.79 ppm (s, 3H); 13C NMR (100
MHz, CD
2
Cl
2
): δ = 173.32, 172.39, 166.70, 140.94, 137.72, 137.52, 137.08,
136.81, 136.16, 135.84, 132.21, 132.18, 131.97, 131.89, 130.74, 130.71,
128.80, 127.80, 127.78, 127.65, 126.99, 122.49, 119.45, 14.40, 14.26,
crystals were grown from a solution of CH
refinement details are provided in Supplementary Table 6 and 7.
2 2
Cl . Crystal data and structure
1
4.01, 13.41, 13.14, 13.01, 12.91, 12.89, 12.87, 12.85 ppm; HRMS (ESI):
+
+
2 3 3
m/z calcd. for [C25H22BF N OS Na] : 548.0884; found: 548.0919 [M+Na ];
UV-vis (dioxane): λmax (ε) = 388 nm (21 473 dm mol cm ).
3
-1
-1
N-3d-anti.
31 2 3 3
C H22BF N OS ;
a
yellow block-like single crystal with
3
approximate dimensions of 0.25 × 0.22 × 0.19 mm was used. Triclinic,
2d was obtained as a red solid in 40% yield (132 mg, 0.23 mmol) using
space group P-1, a = 10.1521(9) Å, b = 11.6719(11) Å, c = 12.8742(12) Å,
α = 100.438(2)°, β = 101.761(2)°, γ = 98.763(2)°, V = 1440.1(2) Å , Z = 2,
3
,4-bis(2-methylbenzo[b]thiophen-3-yl)-5-iminopyrrol-2-one (225 mg, 0.58
3
mmol) and benzo[d]thiazol-2-amine (87 mg, 0.58 mmol), and purified by
silica gel column chromatography (hexane: ethyl acetate = 9: 1, v/v). 1
3
F (000) = 616, ρ = 1.378 Mg/m , R
1
= 0.0644, wR
2
= 0.1081, GOF = 0.954,
H
3
residual electron density between 0.306 and -0.267 e/Å . CCDC: 1875294.
2 2
NMR (400 MHz, CD Cl ): δ = 8.24 (d, J = 8.3 Hz, 2H), 7.89 (d, J = 8.1 Hz,
N-3d-para.
C
31
H
22BF
2
N
3
OS
3
;
a yellow block-like single crystal with
2
7
2
H), 7.69–7.74 (m, 6H), 7.60–7.56 (m, 2H), 7.52–7.49 (m, 2H), 7.34 (t, J =
.8 Hz, 2H), 7.30–7.25 (m, 4H), 7.25–7.20 (m, 2H), 7.11–7.04 (m, 2H),
.34 (s, 3H), 2.33 (s, 3H), 2.17 (s, 3H), 2.12 ppm (s, 3H); HRMS (ESI): m/z
3
approximate dimensions of 0.19 × 0.11 × 0.09 mm was used. Triclinic,
space group P-1, a = 14.455(2) Å, b = 15.955(3) Å, c = 22.774(4) Å, α =
3
+
+
76.921(3)°, β = 73.428(3)°, γ = 66.996(3)°, V = 4594.5(13) Å , Z = 2, F
2 3 3
calcd. for [C29H18BF N OS Na] : 592.0571; found: 592.0566 [M+Na ]; UV-
vis (dioxane): λmax (ε) = 387 (16 418), 450 nm (9 316 dm mol cm ).
3
3
-1
-1
(000) = 1848, ρ = 1.296 Mg/m , R
1
= 0.2004, wR
2
= 0.1978, GOF = 0.959,
3
residual electron density between 0.983 and -0.585 e/Å . CCDC: 1919872.
3d was obtained as a red solid in 47% yield (163 mg, 0.27 mmol) using
3,4-bis(2-ethylbenzo[b]thiophen-3-yl)-5-iminopyrrol-2-one (242 mg, 0.58
mmol) and benzo[d]thiazol-2-amine (87 mg, 0.58 mmol), and purified by
silica gel column chromatography (hexane: ethyl acetate = 9: 1, v/v). 1
NMR (500 MHz, CD Cl ): δ = 8.25 (d, J = 8.4 Hz, 2H), 7.89 (d, J = 8.1 Hz,
Absorption and fluorescence spectroscopic studies. UV-visible
H
absorption spectra were recorded on
a
Shimadzu UV-2550
2
2
spectrophotometer. Fluorescence spectra were recorded on a Hitachi F-
1
1
This article is protected by copyright. All rights reserved.

Chemistry - A European Journal
10.1002/chem.202000990
FULL PAPER
4
600 luminescence spectrometer equipped with a 150-W xenon lamp.
N. Jaidane, S. Delbaere, A. K. Tiwari, J. Dubois, M. Takeshita, Y.
Wada, S. Aloise, Phys.Chem.Chem.Phys. 2016, 18, 28091-28100.
P. J. Wagner, Acc. Chem. Res. 1983, 16, 461-467.
Fluorescence lifetimes were determined using a Horiba Jobin Yvon
Fluorolog-3 spectrofluorometer. Absorption and emission measurements
were carried out in 1 × 1 cm quartz cuvettes. Drop-cast films were prepared
by evaporation of dye solutions (1 mM) in dichloromethane on a clean
silicon wafer. These films were stable, with no change in emission
observed over an extended period of time. The absolute quantum yields in
the solid state and in solutions were measured using a Horiba Jobin Yvon
Fluorolog-3 spectrofluorometer with a calibrated integrating sphere. For all
measurements, the temperature was kept constant at (298 ± 2) K.
[8]
[9]
a) V. Ramamurthy, D. R. Corbin, D. F. Eaton, J. Org. Chem. 1990, 55,
5269-5278; b) M. Yoshizawa, J. K. Klosterman, M. Fujita, Angew.
Chem. Int. Ed. 2009, 48, 3418-3438; c) S. Yamada, A. Iwaoka, Y.
Fujita, S. Tsuzuki, Org. Lett. 2013, 15, 5994-5997; d) V. Ramamurthy,
J. Sivaguru, Chem. Rev. 2016, 116, 9914-9993; e) S. Das, N.
Okamura, S. Yagi, A. Ajayaghosh, J. Am. Chem. Soc. 2019, 141, 5635-
5639.
[
10] a) A. J.-L. Ayitou, J. L. Jesuraj, N. Barooah, A. Ugrinov, J. Sivaguru, J.
Am. Chem. Soc. 2009, 131, 11314-11315; b) S. Aloise, M. Sliwa, Z.
Pawlowska, J. Rehault, J. Dubois, O. Poizat, G. Buntinx, A. Perrier, F.
Maurel, S. Yamaguchi, M. Takeshita, J. Am. Chem. Soc. 2010, 132,
Photoresponse studies. Photolysis of 3d at 415 nm was carried out using
an HXGY-II instrument equipped with
a 120-W HXP lamp, while
photoresponses under various conditions were probed using
HW405AD100-16GD, HW450AD100-16GD, HW488AD60-16GD, and
HW520AD50-16GD lasers. Time-dependent and 3D-EEM fluorescence
7
379-7390; c) E. Kumarasamy, R. Raghunathan, S. Jockusch, A.
Ugrinov, J. Sivaguru, J. Am. Chem. Soc. 2014, 136, 8729-8737; d) Y.
Wu, Y. Xie, Q. Zhang, H. Tian, W. Zhu, A. D. Q. Li, Angew. Chem. Int.
Ed. 2014, 53, 2090-2094; e) J. Yoon, A. P. de Silva, Angew. Chem. Int.
Ed. 2015, 54, 9754-9756; f) N. Vallavoju, A. Sreenithya, A. J.-L. Ayitou,
S. Jockusch, R. B. Sunoj, J. Sivaguru, Chem. Eur. J. 2016, 22, 11339-
spectra were recorded on
spectrofluorometer equipped with
a
Horiba Jobin Yvon Fluorolog-3
CCD detector. Time-resolved
a
fluorescence spectra were recorded using the TCSPC technique and a 1%
laser attenuator.
1
1348.
11] a) M. Irie, T. Fulcaminato, K. Matsuda, S. Kobatake, Chem. Rev. 2014,
14, 12174-12277; b) S.-Z. Pu, Q. Sun, C.-B. Fan, R.-J. Wang, G. Liu,
[
[
1
Acknowledgements
J. Mater. Chem. C 2016, 4, 3075-3093; c) J. Zhang, H. Tian, Adv.
Optical Mater. 2018, 6, 1701278.
12] a) Y. Kanaoka, K. Koyama, Tetrahedron Lett. 1972, 13, 4517-4520; b)
V. A. Ignatenko, G. P. Tochtrop, J. Org. Chem. 2013, 78, 3821-3831; c)
M. Jang, B. S. Park, Bull. Korean Chem. Soc. 2016, 37, 1509-1514; d)
J. L. Markley, T. L. Morse, N. P. Rath, T. A. Wencewicz, Tetrahedron
Financial support was provided by the National Natural Science
Foundation of China (No. 21771102 and 21911540069) to Z.S.
and Nanjing University Innovation and Creative Program for PhD
candidate (No. CXCY17-18) to H. L.
2018, 74, 2743-2753.
[
13] a) A. Fawcett, J. Pradeilles, Y. Wang, T. Mutsuga, E. L. Myers, V. K.
Aggarwal, Science 2017, 357, 283-286; b) M. Silvi, C. Verrier, Y. P.
Rey, L. Buzzetti, P. Melchiorre, Nat. Chem. 2017, 9, 868-873; c) C.
Ghiazza, V. Debrauwer, C. Monnereau, L. Khrouz, M. Médebielle, T.
Billard, A. Tlili, Angew. Chem. Int. Ed. 2018, 57, 11781-11785; d) J. Lu,
B. Pattengale, Q. Liu, S. Yang, W. Shi, S. Li, J. Huang, J. Zhang, J.
Am. Chem. Soc. 2018, 140, 13719-13725; e) Q.-Q. Zhou, Y.-Q. Zou,
L.-Q. Lu, W.-J. Xiao, Angew. Chem. Int. Ed. 2019, 58, 1586-1604.
14] a) C.-T. Poon, W. H. Lam, H.-L. Wong, V. W.-W. Yam, J. Am. Chem.
Soc. 2010, 132, 13992-13993; b) N. M.-W. Wu, H.-L. Wong, V. W.-W.
Yam, Chem. Sci. 2017, 8, 1309-1315.
Keywords: Norrish-Yang photocyclization • 6π photocyclization •
ambient light • atropisomers • enantiomers
[
1]
a) A. Kamkaew, S. H. Lim, H. B. Lee, L. V. Kiew, L. Y. Chung, K.
Burgess, Chem. Soc. Rev. 2013, 42, 77-88; b) J. W. Blunt, B. R. Copp,
R. A. Keyzers, M. H. G. Munro, M. R. Prinsep, Nat. Prod. Rep. 2014,
31, 160-258; c) M. Ishikura, T. Abe, T. Choshi, S. Hibino, Nat. Prod.
[
[
Rep. 2015, 32, 1389-1471; d) L. Zhang, Y. Cao, N. S. Colella, Y. Liang,
J.-L. Bredas, K. N. Houk, A. L. Briseno, Acc. Chem. Res. 2015, 48,
500-509; e) F. Yang, M. Zhu, J. Zhang, H. Zhou, Med. Chem. Commun.
15] a) H. Lu, J. Mack, Y. Yang, Z. Shen, Chem. Soc. Rev. 2014, 43, 4778-
2
018, 9, 201-211; f) M. Hirai, N. Tanaka, M. Sakai, S. Yamaguchi,
4
823; b) H. Liu, H. Lu, J. Xu, Z. Liu, Z. Li, J. Mack, Z. Shen, Chem.
Chem. Rev. 2019, 119, 8291-8331; g) M. J. Toda, P. Lodowski, A. Al
Mamun, M. Jaworska, P. M. Kozlowski, Coord. Chem. Rev. 2019, 385,
Commun. 2014, 50, 1074-1076; c) H. Liu, H. Lu, F. Wu, Z. Li, N.
Kobayashi, Z. Shen, Org. Biomol. Chem. 2014, 12, 8223-8229; d) H.
Liu, Y. Wu, Z. Li, H. Lu, J. Porphyrins Phthalocyanines 2014, 18, 679-
20-43.
[
[
2]
3]
a) L.-W. Ye, J. Zhou, Y. Tang, Chem. Soc. Rev. 2008, 37, 1140-1152;
b) L.-Q. Lu, J.-R. Chen, W.-J. Xiao, Acc. Chem. Res. 2012, 45, 1278-
685; e) H. Liu, H. Lu, Z. Zhou, S. Shimizu, Z. Li, N. Kobayashi, Z. Shen,
Chem. Commun. 2015, 51, 1713-1716; f) Y. Wu, H. Lu, S. Wang, Z.
Lia, Z. Shen, J. Mater. Chem. C 2015, 3, 12281-12289; g) Y. Wu, S.
Wang, Z. Li, Z. Shen, H. Lu, J. Mater. Chem. C 2016, 4, 4668-4674.
16] a) K. Morimitsu, K. Shibata, S. Kobatake, M. Irie, J. Org. Chem. 2002,
1293; c) C. C. McAtee, P. S. Riehl, C. S. Schindler, J. Am. Chem. Soc.
2017, 139, 2960-2963; d) J. Xuan, A. Studer, Chem. Soc. Rev. 2017,
46, 4329-4346; e) Q.-Q. Cheng, Y. Yu, J. Yedoyan, M. P. Doyle,
[
ChemCatChem 2018, 10, 488-496; f) Z. Xu, Q. Wang, J. Zhu, Chem.
Soc. Rev. 2018, 47, 7882-7898.
6
7, 4574-4578; b) Y. Takagi, T. Kunishi, T. Katayama, Y. Ishibashi, H.
Miyasaka, M. Morimoto, M. Irie, Photochem. Photobiol. Sci. 2012, 11,
661-1665.
J. A. Elvidge, R. P. Linstead, J. Chem. Soc. 1952, 5000-5007.
a) N. Hoffmann, Chem. Rev. 2008, 108, 1052-1103; b) T. Bach, J. P.
Hehn, Angew. Chem. Int. Ed. 2011, 50, 1000-1045; c) R. Brimioulle, D.
Lenhart, M. M. Maturi, T. Bach, Angew. Chem. Int. Ed. 2015, 54, 3872-
1
[
[
17]
18] a) S. Aloïse, M. Sliwa, G. Buntinx, S. Delbaere, A. Perrier, F. Maurel, D.
3
890; d) N. Hoffmann, Synthesis 2016, 48, 1782-1802; e) Q.-Q. Zhou,
Y.-Q. Zou, L.-Q. Lu, W.-J. Xiao, Angew. Chem. Int. Ed. 2019, 58, 1586-
604.
a) C. M. Beaudry, J. P. Malerich, D. Trauner, Chem. Rev. 2005, 105,
Jacquemin, M. Takeshita, Phys. Chem. Chem. Phys. 2013, 15, 6226-
6234; b) A. Goldberg, A. Murakami, K. Kanda, T. Kobayashi, S.
1
Nakamura, K. Uchida, H. Sekiya, T. Fukaminato, T. Kawai, S.
Kobatake, M. Irie, J. Phys. Chem. A 2003, 107, 4982-4988.
T. Sumi, Y. Takagi, A. Yagi, M. Morimoto, M. Irie, Chem. Commun.
[
[
4]
5]
4757-4778; b) S. Yoshioka, M. Nagatomo, M. Inoue, Org. Lett. 2015,
[
[
19]
17, 90-93; c) R. J. Rapf, R. J. Perkins, H. Yang, G. M. Miyake, B. K.
2014, 50, 3928-3930.
Carpenter, V. Vaida, J. Am. Chem. Soc. 2017, 139, 6946-6959.^
a) D. C. Neckers, R. M. Kellogg, W. L. Prins, B. Schoustra, J. Org.
Chem. 1971, 36, 1838-1840; b) P. J. Wagner, Acc. Chem. Res. 1971,
20] a) A. G. Griesbeck, A. Henz, W. Kramer, P. Wamser, K. Peters, E.-M.
Peters, Tetrahedron Lett. 1998, 39, 1549-1552; b) U. C. Yoon, S. W.
Oh, S. M. Lee, S. J. Cho, J. Gamlin, P. S. Mariano, J. Org. Chem.
4, 168-177.
1999, 64, 4411-4418.
[
[
6]
7]
P. J. Wagner, Acc. Chem. Res. 1989, 22, 83-91.
[
21] a) B. Yoon, J. Lee, I. S. Park, S. Jeon, J. Lee, J.-M. Kim, J. Mater.
Chem. C 2013, 1, 2388-2403; b) X. Hou, C. Ke, C. J. Bruns, P. R.
a) R. B. Woodward, R. Hoffmann, Angew. Chem. Int. Ed. 1969, 8, 781-
853; b) A. Perrier, S. Aloise, M. Olivucci, D. Jacquemin, J. Phys. Chem.
Lett. 2013, 4, 2190-2196; c) I. Hamdi, G. Buntinx, A. Perrier, O. Devos,
1
2
This article is protected by copyright. All rights reserved.

Chemistry - A European Journal
10.1002/chem.202000990
FULL PAPER
McGonigal, R. B. Pettman, J. F. Stoddart, Nat. Commun. 2015, 6 :
6884.
1
3
This article is protected by copyright. All rights reserved.

Chemistry - A European Journal
10.1002/chem.202000990
FULL PAPER
Entry for the Table of Contents
Ambient light–induced intramolecular photocyclization occur in a series of donor/acceptor chromophores 1d–3d containing an
electron-deficient BF -chelated ketone fused with an electron-rich thiophene. Simple variation of R substituents on the diaryl moiety
2
allows one to control the intramolecular photocyclization mechanism with high photochemical selectivity, e.g., 6π photocyclization or
Norrish-Yang cyclization. Atropisomerism and enantiotropy have been observed in Norrish-Yang products.
1
4
This article is protected by copyright. All rights reserved.