One-pot syntheses of blue-luminescent 4-aryl-1H-benzo[f]isoindole-1, 3(2H)-diones by T3P activation of 3-arylpropiolic acids

Article abstract of DOI:10.3762/bjoc.13.231

In situ activation of 3-arylpropiolic acids with T3P (n-propylphosphonic acid anhydride) initiates a domino reaction furnishing 4-arylnaphtho[2, 3-c]furan-1, 3-diones in excellent yields. Upon employing these anhydrides as reactive intermediates blue-lumi

Full text of DOI:10.3762/bjoc.13.231

One-pot syntheses of blue-luminescent
4-aryl-1H-benzo[f]isoindole-1,3(2H)-diones
by T3P® activation of 3-arylpropiolic acids
Melanie Denißen1, Alexander Kraus1, Guido J. Reiss2 and Thomas J. J. Müller*1
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2017, 13, 2340–2351.
1Institut für Organische Chemie und Makromolekulare Chemie,
Heinrich-Heine-Universität Düsseldorf, Universitätsstraße 1, D-40225
Düsseldorf, Germany and 2Institut für Anorganische Chemie und
Strukturchemie, Heinrich-Heine-Universität Düsseldorf,
Universitätsstraße 1, D-40225 Düsseldorf, Germany
doi:10.3762/bjoc.13.231
Received: 01 August 2017
Accepted: 28 September 2017
Published: 03 November 2017
Email:
Associate Editor: D. Spring
Thomas J. J. Müller* - ThomasJJ.Mueller@uni-duesseldorf.de
© 2017 Denißen et al.; licensee Beilstein-Institut.
License and terms: see end of document.
* Corresponding author
Keywords:
absorption; cycloaddition; diversity-oriented synthesis; fluorescence;
heterocycles; imidation
Abstract
In situ activation of 3-arylpropiolic acids with T3P® (n-propylphosphonic acid anhydride) initiates a domino reaction furnishing
4-arylnaphtho[2,3-c]furan-1,3-diones in excellent yields. Upon employing these anhydrides as reactive intermediates blue-lumines-
cent 4-aryl-1H-benzo[f]isoindole-1,3(2H)-diones are formed by consecutive pseudo three-component syntheses in a one-pot
fashion. The Stokes shifts correlate excellently with the Hammett–Taft σR parameter indicating an extended degree of resonance
stabilization in the vibrationally relaxed excited singlet state.
Introduction
Luminophores as functional π-electron systems [1] are crucial advantage of creating and transforming reactive functionalities
in modern illumination technologies, such as organic light-emit- in the same reaction vessel without intermediary work-up [30].
ting diodes (OLEDs) [2-5]. As a consequence efficient and effi- Syntheses of functional chromophores by MCR [31,32] have
cacious syntheses of fluorescent molecules can be most advan- indeed become a powerful tool in synthetic chemistry for
tageously addressed by diversity-oriented syntheses [6-8], convergently approaching substance libraries of luminescent
namely by multicomponent [9-21] and domino reactions [22- molecules. In particular, blue-emissive heterocyclic chro-
24] where fundamental organic reactions are combined in one- mophores, intensively requested in illumination research, are
pot sequences [25-29]. Multicomponent reactions (MCR) take equally accessible by MCR strategies [33].
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Beilstein J. Org. Chem. 2017, 13, 2340–2351.
Cyclic imides are often prepared by condensation of acid anhy- ploying bis(2-oxooxazolidin-3-yl)phosphinic chloride as an
drides and amines [34]. Their applications are widespread, activating agent represents a suitable access [44]. Alternative
ranging from pharmaceutically active compounds [35] to agro- approaches include the use of phenylpropiolic acid chloride and
chemicals [36] and fluorophores [37]. The characteristic photo- phenylpropiolic acid as starting materials [45], and as well oxi-
physical properties of 1,8- and 2,3-naphthalene imides render dative arene–alkyne cyclization with dichloro-5,6-dicyano-
the substance class particularly attractive for the development of benzoquinone (DDQ) [46].
novel sensors and fluorescent dyes [38], for instance 6-chloro-
2,3-naphthaleneimide derivatives were successfully used for Based upon our experience in using propylphosphonic acid an-
labeling amino acids, and for studying peptide protein interac- hydride (T3P®) [47] as a condensation agent for in situ activa-
tions [39]. Even the superficial attachment to a binding domain tion of benzyl alcohols in the synthesis of N-benzylphenothi-
can be detected and monitored by absorption and emission azine derivatives [48], we reasoned that T3P® might be equally
spectroscopy. Furthermore, the investigation of a substance well suited for furnishing 4-phenylnaphtho[2,3-c]furan-1,3-
library of various 2,3- and 1,8-naphthalene imides has shown diones, and thereby opening a straightforward entry to 4-aryl-
that the electronic nature of the ground and the excited state is 1H-benzo[f]isoindole-1,3(2H)-diones in a diversity-oriented
decisively influenced by variation of the substitution pattern of one-pot process. Here, we report the development of the one-
the naphthalene scaffold. Even the smallest polarity change in pot synthesis of these title compounds by a consecutive pseudo
the solvent system effects their absorption and emission behav- three-component approach and the investigation of the lumines-
ior [34,40]. Very recent investigations on naphthaleneimide de- cence behavior by absorption and emission spectroscopy.
rivatives revealed enormous phosphorescence lifetimes that are
particularly interesting for imaging, sensing and display appli-
Results and Discussion
cations [41].
Synthesis and structure of
4-arylnaphtho[2,3-c]furan-1,3-diones and
4-Phenylnaphtho[2,3-c]furan-1,3-diones can well serve as reac-
tive intermediates in multicomponent reactions, e.g., for synthe- 4-aryl-1H-benzo[f]isoindole-1,3(2H)-diones
sizing the corresponding imides. A particularly intriguing Starting from phenylpropiolic acids 1 we first set out to opti-
access to 4-phenylnaphtho[2,3-c]furan-1,3-diones is the intra- mize the reaction conditions for the domino synthesis of the
molecular [4 + 2]-cycloaddition of phenylpropiolic acid anhy- 4-arylnaphtho[2,3-c]furan-1,3-diones 2a and 2b with T3P® as
drides. Usually, this synthesis requires harsh reaction condi- an activating agent, varying temperature and reaction time
tions starting from phenylpropiolic acids [42,43]. However, em- (Scheme 1, Table 1).
Scheme 1: Mechanistic rationale and optimization of the domino synthesis of 4-arylnaphtho[2,3-c]furan-1,3-diones 2.
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Table 1: Optimization of the reaction conditions for the synthesis of 4-arylnaphtho[2,3-c]furan-1,3-diones 2.
entry
arylpropiolic acid 1
solvent
T [°C]
t [h]
4-arylnaphtho[2,3-c]furan-1,3-dione 2 (isolated yield) [%]
1
R = H 1a
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
THF
20
20
40
20
20
20
4
2a (81)
2a (81)
2a (21)
2a (–)
2
R = H 1a
20
20
20
20
20
3
R = H 1a
4a
5
R = H 1a
R = OCH3 1b
R = OCH3 1b
2b (3)
2b (2)
6
aWithout addition of T3P®.
T3P®, employed as a 50 wt % solution in 1,2-dichloroethane transferred to give the desired product 2b, which was only ob-
(DCE), is added to a solution of phenylpropiolic acid (1a) in tained in traces. This is probably caused by the poor solubility
dichloromethane at 0 °C over the course of 30 min. After of starting material 1b. This problem can be circumvented by
warming the reaction mixture to room temperature, stirring for the addition of triethylamine as a deprotonating solubilizer [44].
4 h and aqueous work-up the desired product 2a can be ob- Under these modified conditions four substituted 4-arylnaphtho-
tained in good yield (Table 1, entry 1). A prolonged reaction [2,3-c]furan-1,3-dione derivatives 2 can be synthesized in very
time indicates that product 2a is stable under the reaction condi- good to quantitative yields (Scheme 2, Table 2).
tions (Table 1, entry 2). However, a temperature increase to
40 °C already causes decomposition as indicated by a dimin- Interestingly, in this domino synthesis of 4-phenylnaphtho[2,3-
ished isolated yield (Table 1, entry 3). The addition of T3P® as c]furan-1,3-diones electron-rich and electro-neutral substrates
an activation agent is a prerequisite for achieving the transfor- are equally well tolerated (Table 2, entries 1–3), while the elec-
mation (Table 1, entry 4).
tron-poor derivative 1d results in a slightly decreased yield of
88% due to the increased formation of side products (Table 2,
With these conditions in hand, 3-(4-methoxyphenyl)propiolic entry 4). Expectedly, the products 2 are not stable under acidic
acid (1b) was also chosen as a model substrate (Table 1, entries aqueous conditions (e.g., hydrochloric acid, silica gel and satu-
5 and 6). The optimized reaction conditions for the synthesis of rated aqueous solution of ammonium chloride) [49]. After 48 h
4-phenylnaphtho[2,3-c]furan-1,3-dione (2a) cannot directly be in the presence of deuterated hydrochloric acid (36% in D2O),
Scheme 2: Domino synthesis of 4-arylnaphtho[2,3-c]furan-1,3-diones 2 via in situ activation of arylpropiolic acids 1.
Table 2: Synthesized 4-arylnaphtho[2,3-c]furan-1,3-diones 2.
entry
arylpropiolic acid 1
4-arylnaphtho[2,3-c]furan-1,3-dione 2 (isolated yield [%])
1
2
3
4
R = H (1a)
2a (100)
2b (94)
2c (91)
2d (88)
R = OCH3 (1b)
R = CH3 (1c)
R = Cl (1d)
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Beilstein J. Org. Chem. 2017, 13, 2340–2351.
complete conversion of compound 2a to the free deuterated
dicarboxylic acid was observed (20% conversion after 60 min)
by NMR spectroscopy. For verification of the structure by
NMR and IR spectroscopy, the hydrolysis was performed on a
preparative scale with ammonium chloride in a mixture of water
and acetone. After complete removal of water, the IR spectrum
shows the characteristic OH-stretching vibrations of the free
carboxylic acid ( 2901–3082 cm−1). The signals at δ 12.70
and 12.98 in the 1H NMR spectrum can be assigned to the two
carboxylic acid functionalities (see Supporting Information
File 1). The extraction of the crude anhydrides with a saturated
aqueous solution of sodium bicarbonate can be achieved
uneventfully, leaving the anhydride unimpaired.
Scheme 3: Optimization of the synthesis of 2,4-diphenyl-1H-
benzo[f]isoindole-1,3(2H)-dione (4a) by imidation of
4-phenylnaphtho[2,3-c]furan-1,3-dione (2a) with aniline (3a).
such as 160 °C, cause a significant drop in yield. Finally, at
For employing 4-phenylnaphtho[2,3-c]furan-1,3-diones 2 as 115 °C with two equivalents of aniline the highest yield can be
reactive intermediates for the en route conversion with primary achieved (Table 3, entry 6).
amines 3 into 4-aryl-1H-benzo[f]isoindole-1,3(2H)-diones 4 the
reaction conditions were optimized with 4-phenylnaphtho[2,3- These imidation conditions are well-suited for concatenating the
c]furan-1,3-dione (2a) and aniline (3a) as model substrates arylpropiolic anhydride formation, intramolecular cycloaddi-
(Scheme 3, Table 3).
tion and imidation in a one-pot fashion in the sense of a
consecutive pseudo three-component synthesis of 4-aryl-1H-
In dichloromethane at only slightly elevated temperatures imide benzo[f]isoindole-1,3(2H)-diones 4, which was first probed
4a is not formed (Table 3, entry 1). Upon addition of N,N- with an electroneutral (1a), an electron-rich (1b) and electron-
dimethylformamide as a cosolvent at 80 °C the desired product poor (1c) propiolic acid substrate (Scheme 4, Table 4). Al-
4a can be isolated in 15% yield (Table 3, entry 2). The yield of though the electroneutral and electron-rich substrates give good
4a can be increased to 47% upon raising the reaction tempera- to excellent yields in the sequence (Table 4, entries 1–3), even
ture to 115 °C (Table 3, entry 3), however, higher temperatures, higher (Table 4, entry 1) than for a stepwise synthesis
Table 3: Optimization of the reaction conditions for the synthesis of 2,4-diphenyl-1H-benzo[f]isoindole-1,3(2H)-dione (4a).
entry
equivalents
solvent c(3a) = 0.125 M
T [°C]
2,4-diphenyl-1H-benzo[f]isoindole-1,3(2H)-dione (4a)
of aniline (3a)
(isolated yield [%])
1
2
3
4
5
6
1.0
1.0
1.0
1.0
2.0
2.0
CH2Cl2
50
–
CH2Cl2/DMF 1:1 (v/v)
CH2Cl2/DMF 1:1 (v/v)
CH2Cl2/DMF 1:1 (v/v)
CH2Cl2/DMF 1:1 (v/v)
CH2Cl2/DMF 1:1 (v/v)
80
15
47
33
71
80
115
160
160
115
Scheme 4: Pseudo three-component synthesis of 4-aryl-1H-benzo[f]isoindole-1,3(2H)-diones 4.
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Table 4: Pseudo three-component synthesis of 1H-benzo[f]isoindole-1,3(2H)-diones 4.
entry
arylpropiolic acid 1
amine 3
4-aryl-1H-benzo[f]isoindole-1,3(2H)-dione 4 (isolated yield [%])
1
2
R1 = H (1a)
R2 = C6H5 (3a)
4a (95)
4b (79)
R1 = OCH3 (1b)
3a
R2 = CH2(10-methyl-10H-
phenothiazine-3-yl)∙HCl (3b)
3a
4
1a
4c (85)
4d (27)
R1 = CN (1c)
3a
aCompound 3b was employed as the corresponding hydrochloride with 202 mg (2.00 mmol) Et3N in CH2Cl2 (1.0 mL) at rt for 1 h.
furnishing an overall yield of 4a of 80%, the cyano-substituted transformed to the corresponding 1H-benzo[f]isoindole-
substrate 1c furnishes a significantly lower yield of 27% 1,3(2H)-diones 4d–f in moderate yields.
(Table 4, entry 4), originating from the lower reactivity in the
anhydride formation.
Upon lowering the amount of T3P® to 0.5 equiv, electron-
neutral and electron-donating phenylpropiolic acids 1 can be
Therefore, for accessing acceptor-substituted derivatives of 4, employed with comparable efficiency applying various amines
an intermediate extraction with bicarbonate after the anhydride 3 and giving rise to a series of 4-aryl-1H-benzo[f]isoindole-
formation–cycloaddition was attempted (Scheme 5). With this 1,3(2H)-diones 4 in moderate to good yield (Table 5). Varia-
variation acceptor-substituted arylpropiolic acids 1c–e can be tion of the amine nucleophile 3 allows for the introduction of
Scheme 5: Modified sequence for the synthesis of acceptor-substituted 4-aryl-1H-benzo[f]isoindole-1,3(2H)-diones 4.
Table 5: Variation of the amine 3 in pseudo three-component syntheses of 1H-benzo[f]isoindole-1,3(2H)-diones 4.
entry
arylpropiolic acid 1
amine 3
1H-benzo[f]isoindole-1,3(2H)-dione 4 (isolated yield [%])
1
1a
R2 = C6H5 (3a)
4a (92)
4g (48)
4h (55)
4i (65)
4j (51)
4k (57)
4l (69)
4m (56)
4n (64)
4o (41)
4p (26)
4q (53)
4r (61)
4s (57)
2
R1 = CH3 (1f)
3a
3
R1 = 1a
1a
R2 = p-FC6H4 (3c)
R2 = p-ClC6H4 (3d)
R2 = p-IC6H4 (3e)
R2 = p-EtO2CC6H4 (3f)
R2 = 3,5-Me2C6H3 (3g)
R2 = 2,6-Me2C6H3 (3h)
R2 = 3,5-(MeO)2C6H3 (3i)
R2 = CH2Ph (3j)
R2 = CH2CCH (3k)
R2 = n-hexyl (3l)
R2 = n-butyl (3m)
3h
4
5
1a
6
1a
7
1a
8
1a
9
1a
10
11a
12
13
14
1a
1a
1a
1a
1b
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Beilstein J. Org. Chem. 2017, 13, 2340–2351.
functional groups for consecutive transformations. For instance,
para-substituted halogenated anilines 3 (Table 5, entries 3–5)
and ethyl 4-aminobenzoate (Table 5, entry 6) furnish the desired
products in 51–65% yield. Interestingly, also sterically hindered
anilines, such as 2,6-dimethylaniline can be obtained in 56%
yield (Table 5, entry 8). In addition, aliphatic amines (Table 5,
entries 10–13) are readily tolerated in the established reaction
sequence.
Besides comprehensive NMR spectroscopic and mass spectro-
metric characterization the structures of the title compounds 4
were additionally corroborated by a crystal structure determina-
tion of compound 4b (Figure 1). The twist angle of the phenyl
substituent (ring A) and the 1H-pyrrole-2,5-dionyl moiety is
51.37(7)°, whereas the p-anisyl substituent (ring B) is consider-
ably twisted against the adjacent six-membered ring by
70.95(7)° (Figure 1) [50].
Figure 1: The ORTEP-style plot of crystal structure 4b (ellipsoids are
draw at the 40% probability level).
Upon slightly varying the reaction conditions the potential of 1H-benzo[f]isoindole-1,3(2H)-dione by electrophilic attack on
the sequence can be extended. An increase of the T3P® concen- the sterically easier accessible carbonyl group. As a conse-
tration to 2.0 equiv leads to the regioselective formation of (E)- quence the imine condensation proceeds also with thermo-
2,9-diphenyl-3-(phenylimino)-2,3-dihydro-1H-benzo[f]isoindol- dynamic control giving exclusively the formation of the
1-one (5) in 15% yield in the sense of a pseudo four-component E-configured product 5. Interestingly the corresponding reac-
reaction (Scheme 6).
tion with ortho-phenylenediamine (3n) gives rise to the regiose-
lective formation of the pentacyclic condensation product 6,
The increased amount of the condensation agent T3P® where the intramolecular imine formation formally occurred at
obviously enables further activation of the initially formed the sterically more biased carbonyl group (Scheme 7).
Scheme 6: Pseudo four-component synthesis of (E)-2,9-diphenyl-3-(phenylimino)-2,3-dihydro-1H-benzo[f]isoindol-1-one (5).
Scheme 7: Synthesis of 6-phenyl-12H-benzo[f]benzo[4,5]imidazo[2,1-a]isoindol-12-one (6).
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In addition to NMR spectroscopic and mass spectrometric char-
acterization the crystal structures of the imine condensation
products 5 and 6 were determined (Figure 2 and Figure 3) [50].
In similarity to 4b the twist angles of the phenyl substituents
(rings A and B) are 61.1(1)° and 66.8(1)°, respectively
(Figure 2, left part). Interestingly, the crystal structure of 5
shows centrosymmetric dimers, formed by a close π–π interac-
tion of the planar naphthyl moieties (Figure 2, right part). The
intermolecular distance of the naphthalene moieties of the two
molecules accounts to 3.435(7) Å.
This intermolecular plane distance of the two adjacent naphthyl
moieties (C2–C11) must be understood as a π–π interaction of
two fused aromatic systems [51]. The twist angle of the at-
tached phenyl substituent (ring B) of the asymmetric unit of the
crystal structure 6 were determined as 62.28(5)° (Figure 3). The
significant difference of the twist angles in 4b, 5, and 6 could be
a consequence of packing in the individual crystal structure.
The crystal structure of the pentacyclic molecules 6 shows
stacks with an antiparallel arrangement of the monomers. The
found interplanar distance is 3.437(4) Å. Consequently, π–π
interactions must be considered for this structure [51,52]. Bond
lengths and angles of the reported crystal structures 4b, 5, and 6
are in the expected range. The tricyclic 1H-benzo[f]isoindole-
1,3(2H)-dionyl moiety in 4b, 5 and the corresponding 3-imino-
1-oxo-2,3-dihydro-1H-benzo[f]isoindolyl moiety in 6 are
absolutely planar.
Figure 3: The ORTEP-type plot of the asymmetric unit of the crystal
structure 6 (top) and π-stacking interactions (bottom) (ellipsoids are
drawn at the 40% probability level).
Photophysical properties
tronically diverse substitution patterns and already upon
The pseudo three-component synthesis of 1H-benzo[f]isoin- eyesight several derivatives are intensively blue and greenish
dole-1,3(2H)-diones 4 furnishes a substance library with elec- luminescent in solution at low concentration (Figure 4). There-
Figure 2: The ORTEP-type plot of the crystal structure 5 (left) and a centrosymmetric dimer formation by π–π interactions (right) (ellipsoids are drawn
at the 40% probability level).
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Beilstein J. Org. Chem. 2017, 13, 2340–2351.
Figure 4: Emission properties of compounds 4a,b,d–f, 5, and 6 under handheld UV-lamp (λexc ≈ 350 nm).
fore, the 1H-benzo[f]isoindole-1,3(2H)-diones 4 and 5 and the 388.5 nm as a shoulder with a molar extinction coefficients ε of
pentacyclic compound 6 were investigated with absorption and 21600 L∙mol−1∙cm−1 (Table 6, entry 12).
static fluorescence spectroscopy (Table 6).
Although most of the 1H-benzo[f]isoindole-1,3(2H)-diones 4
All 1-aryl-2,3-naphthaleneimides 4 possess two characteristic fluorescence upon excitation with UV light, a closer inspection,
absorption maxima λmax,abs between 258.5 and 273.5 nm with by comparing the relative intensities of the emission maxima at
molar extinction coefficients ε of 55000 L∙mol−1∙cm−1 and be- identical concentrations, reveals that only the methoxy deriva-
tween 359.0 and 379.0 nm with molar extinction coefficients ε tive 4b (Table 6, entry 2) is substantially fluorescent (Figure 5).
of about 3500 L∙mol−1∙cm−1. While electron-withdrawing sub- With this exception all qualitatively determined emission spec-
stituents R1 in tendency shift the absorption maxima slightly tra reveal a broad unstructured maximum followed by a
hypsochromically (Table 6, entries 1, and 3–6) the electron-do- shoulder. This appearance might result from the free rotation of
nating methoxy group (Table 6, entry 2) clearly causes a red the N-phenyl substituents, furnishing emissive conformers that
shift. The absorption spectra of the imine condensation prod- arise from a coplanar (λmax,em at 468.0 nm as a shoulder for
ucts 5 and 6 essentially display a similar appearance; however, compound 4a) and torsional orientation (λmax,em at 408.5 nm as
the pentacycle 6 possesses a considerably more intense longest a maximum for compound 4a) of the N-phenyl substituent in
wavelength absorption, which appears in the spectrum at the corresponding excited Franck–Condon states [54]. Qualita-
Table 6: Selected photophysical properties (absorption and emission maxima,a,b fluorescence quantum yields (Φf [53]),c and Stokes shifts Δ d) of
compounds 4a–g,m,n,s, 5, and 6.
entry compound λmax,abs (ε) [nm] ([L∙(mol∙cm)−1])
λmax,em [nm] (Φf)
Stokes shift Δ [cm−1]
1
4a
4b
4c
4d
4e
4f
364.5 (3900), 264.0 (49500)
468.0 sh, 408.5 (0.01) 3000
2
379.0 (4000), 273.5 (55300)
444.0 (0.14)
525.0 sh, 450.0
512.5, 401.0 sh
499.5, 417.0 sh
500.0, 397.0 sh
415.0
3900
5300
2500
3300
2700
3000
2000
8200
4000
–
3
363.0 (5080), 300.5 (13510), 262.5 (77840)
365.0 (3600), 266.5 (57300)
4
5
366.0 (3000), 268.5 (52600)
6
359.0 (3100), 343.0 (2600), 258.5 (50700)
369.5 (4200), 270.5 (53000)
7
4g
4m
4n
4s
5
8
364.0 (5400), 263.5 (58000)
391.0
9
364.0 (5600), 264.5 (67000)
520.0
10
11
371.5 (4700), 271.5 (56300)
437.0 (0.17)
–
362.5 (5900), 350.0 (4800) 265.5 (52900), 261.0 (52500)
388.5 sh (21600), 360.0 (54900) 296.0 (53000), 255.5
(21500)
12
6
500.0 (0.10)d
5700
aRecorded in CH2Cl2 UVASOL® at T = 293 K. bRecorded in CH2Cl2 UVASOL® at T = 293 K with λexc = 360.0/380.0 nm. cQuantum yields Φf were
determined with coumarine 1 in ethanol as a standard, Φf = 0.73, at T = 293 K with λexc = 360.0 nm. dΔ = 1/λmax,abs – 1/λmax,em [cm−1]. eQuantum
yields Φf were determined with coumarine 30 in acetonitrile as a standard, Φf = 0.67, at T = 293 K with λexc = 380.0 nm.
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Beilstein J. Org. Chem. 2017, 13, 2340–2351.
tively, also a red shift of the emission maximum can be detected electronic ground and excited states (see Table S6 in Support-
upon increasing the electron-withdrawing character of the sub- ing Information File 1). Although the linear correlations of
stituent R1. In the consanguineous series of 1H-benzo[f]isoin- λmax,abs with all σ parameters are relatively poor, the correla-
dole-1,3(2H)-diones 4a,b,d–f the emission maxima λmax,em are tions of λmax,em with σR and σp+ indicate a strong influence of
found in a range from 408.5 to 512.5 nm with Stokes shifts resonance stabilization in the vibrationally relaxed excited state.
lying between 3900 and 2500 cm−1 (Table 6, entries 1, 2 and This is even more the case in the nearly perfect linear correla-
4–6, Figure 6).
tion of the Stokes shift with σR (r2 = 0.989) and can be inter-
preted as a significant structural change upon photonic excita-
tion and excited state relaxation resulting from a considerable
charge transfer character, as supported by the influence of the
remote polar substitution (Figure 7).
Figure 5: Relative emission intensities of compounds 4a,b,d–f (re-
corded in CH2Cl2 UVASOL® at T = 293 K; λexc = 350 nm.
Figure 7: Hammett–Taft correlations of the emission maxima (red
circles, lmax,em = 4274 · sR + 24495 [cm−1], R2 = 0.925) and Stokes
shifts (blue triangles, Δ = −2319 · sR + 2909 [cm−1], R2 = 0.989) of
compounds 4a,b,d–f with sR (black squares are the corresponding
absorption maxima).
Interestingly, compound 4a, which has a fluorescence quantum
yield Φf of less than 0.01 in dichloromethane solution, experi-
ences an over eightfold increase to 0.08 in the solid state emis-
sion as determined from the powder by an integrating sphere.
The emission maximum appears at 468.5 nm, i.e., at the same
longest wavelength band as in solution. Therefore, 1H-benzo[f]-
isoindole-1,3(2H)-diones 4 can be considered as AIE (aggrega-
tion induced emission) chromophores [56-58].
Figure 6: Absorption and emission properties of selected imides 4
measured in CH2Cl2 UVASOL® at 293 K with λexc = 360 nm.
In comparison to the 1-phenyl-[2,3-c]-naphthaleneimide 5,
which is only weakly luminescent, the pentacycle 6 displays a
relative enhancement of the greenish emission at 500 nm by a
This electronic substituent effect of R1 on the Stokes shifts was factor of 340 (relative to compound 4a, Figure 8) and can be
further corroborated by linear structure–property relationships quantified with a fluorescence quantum yield of 0.10 (Table 6,
based upon Hammett–Taft correlations with the consan- entry 12).
guineous series 4a,b,d–f. Correlation studies of the longest
wavelength absorption maxima λmax,abs, the shortest wave- Conclusion
length emission maxima λmax,em, and the Stokes shifts Δ with 3-Arylpropiolic acids can be readily activated with T3P®
the Hammett–Taft parameters σp, σR, σp+, and σp− [55] disclose (n-propylphosphonic acid anhydride) to initiate a domino reac-
an interesting insight on electronic substituent effects in the tion furnishing 4-arylnaphtho[2,3-c]furan-1,3-diones in excel-
2348

Beilstein J. Org. Chem. 2017, 13, 2340–2351.
Supporting Information
The Supporting Information contains all experimental
procedures, spectroscopic and analytical data as well as
copies of NMR spectra of compounds 2, 4, 5, and 6. X-ray
structure analyses of compounds 4b, 5, and 6, and
Hammett–Taft correlations of compounds 4a,b,d–f are also
given.
Supporting Information File 1
Experimental part.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-13-231-S1.pdf]
Acknowledgements
The authors cordially thank the Fonds der Chemischen Indus-
trie and the Deutsche Forschungsgemeinschaft (Mu 1088/9-1)
for financial support. The work primarily originates from the
doctoral thesis of Melanie Denißen (inaugural dissertation,
University of Düsseldorf, 2015) and the master thesis of
Alexander Kraus (master thesis, University of Düsseldorf,
2015), who are both first and second authors of this paper.
ORCID® iDs
Thomas J. J. Müller - https://orcid.org/0000-0001-9809-724X
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Figure 8: Relative emission intensities of the 1-phenyl-2,3-naph-
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