Nitrile-substituted 2-(oxazolinyl)-phenols: Minimalistic excited-state intramolecular proton transfer (ESIPT)-based fluorophores

Article abstract of DOI:10.1039/d0tc00776e

Herein, we present minimalistic single-benzene, excited-state intramolecular proton transfer (ESIPT)-based fluorophores as powerful solid-state emitters. The very simple synthesis gave access to all four regioisomers of nitrile-substituted 2-(oxazolinyl)-

Full text of DOI:10.1039/d0tc00776e

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JPolueransael odfoMnaotteraiadljsuCsht emmairsgtriynsC
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DOI: 10.1039/D0TC00776E
ARTICLE
Nitrile-Substituted 2-(Oxazolinyl)-Phenols: Minimalistic Excited-
State Intramolecular Proton Transfer (ESIPT)-Based Fluorophores
Dominik Göbel,a Daniel Duvinage,b Tim Stauch,*,c,d,e Boris J. Nachtsheim*,a
Received 00th January 20xx,
Accepted 00th January 20xx
Herein, we present minimalistic single-benzene, excited-state intramolecular proton transfer (ESIPT)-based fluorophores as
powerful solid-state emitters. The very simple synthesis gave access to all four regioisomers of nitrile-substituted
2-(oxazolinyl)-phenols (MW = 216.1). In respect of their emission properties they can be divided into aggregation-induced
emission enhancement (AIEE) luminophores (1-CN and 2-CN), dual state emission (DSE) emitters (3-CN) and aggregation-
caused quenching (ACQ) fluorophores (4-CN). Remarkably, with compound 1-CN we discovered a minimalistic ESIPT-based
fluorophore with extremely high quantum yield in the solid state ΦF = 87.3% at λem = 491 nm. Furthermore, quantum yields
in solution were determined up to ΦF = 63.0%, combined with Stokes shifts up till 11.310 cm–1. Temperature dependent
emission mapping, crystal structure analysis and time-dependent density functional theory (TDDFT) calculations gave deep
insight into the origin of the emission properties.
DOI: 10.1039/x0xx00000x
show no or solely weak emission in solution but strong
luminescence in the aggregated-state. With regard to biological
application of organic fluorophores various properties are
desirable. Small structures avoid impairing metabolite traffic
within cells,11 solid-state emission prevent fluorescent
quenching12 and a large Stokes´ shifted emission minimizes the
tendency of self-reabsorption.
Introduction
Highly emissive organic fluorophores have received great
attention due to their intriguing and multifarious properties in
the fields of biological imaging,1 fluorescent sensors2 and
organic light-emitting diodes (OLEDs).3 Commonly, three
general strategies for the design of organic fluorescent
frameworks are widely used – extended
donor-acceptor (D–A) systems5 and aggregation induced
emission (AIE).6,7 Extended
-conjugated systems based on a
π
-conjugated systems,4
π
rigid and flat scaffold suffer from decreased solubility in
conventional solvents. Furthermore, upon aggregation either in
highly concentrated solution or in the solid state, their tendency
of intermolecular π-π-stacking leads to radiationless quenching
processes lowering the emission efficiency which results in low
quantum yields. Contrarily, D–A systems are mostly constructed
as small π-conjugated scaffolds, which are extensively used as
fluorescent dyes and as luminophores in optoelectronic
devices. The design and application of AIE-based luminophores
have captured great attention in the recent two decades, since
such structures such as siloles,8 stilbenes9 or arylethylenes,10
a.Institute for Organic and Analytical Chemistry, University of Bremen, Leobener
Straße NW2, D-28359 Bremen, Germany.
b.Institute for Inorganic and Crystallographic Chemistry, University of Bremen,
Leobener Straße NW2, D-28359 Bremen, Germany.
c. Institute for Physical and Theoretical Chemistry, University of Bremen, Leobener
Straße NW2, D-28359 Bremen, Germany
d.Bremen Center for Computational Materials Science, University of Bremen, Am
Fallturm 1, D-28359 Bremen, Germany
e. MAPEX Center for Materials and Processes, University of Bremen,
Bibliothekstraße 1, D-28359 Bremen, Germany
Scheme 1 Schematic representation of the ESIPT-process.
†Electronic Supplementary Information (ESI) available: Detailed experimental
procedures, characterization data, photophysical data, X-ray crystallographic data,
and copies of 1H-, 13C-NMR spectra. CCDC 1908036, 1970920, 1970921. See
DOI: 10.1039/x0xx00000x
The latter property is remarkably strong for excited-state
intramolecular proton transfer (ESIPT)-based luminophores.13
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Such fluorophores are underlying to an enol-keto- If the proton transfer in the excited state does nVoietwoAcrtciculerOinnlinae
phototautomerism cycle consisting of four levels, which causes quantitative manner, dual emission15 caDnObI:e10o.1b0s3e9r/vDe0dTC, 0w0h77ic6hE
the bathochromic-shifted emission and dual emission behavior may result in white light generation.16 Also, ESIPT-based
(Scheme 1). Within this cycle, upon light absorption in the enol luminophores have been identified as polymorph-dependent
form,
a
phototautomerization on the subpicosecond emitters,17,18
color-tuning
by
substitution-effect
timescale,14 emission of the keto form and ground-state luminophores19 and amplified spontaneous emission
intramolecular proton transfer (GSIPT) takes place.
molecules.20,21
Benzene is the simplest
π-unit for the construction of
luminescent systems. In general, emissive organic structures
contain more than just one benzene ring either for conjugation
or as substituent. The design of minimalistic, single-benzene
based, organic molecules exhibiting bright solid-state
fluorescence is of fundamental interest and extremely
challenging. Despite the supposedly straightforward synthesis
of such simple structures, literature examples of low molecular
weight, highly emissive molecules are extremely rare.
Efficacious known single-benzene luminophores can be divided
into two categories – D–A based X-shaped structures and ESIPT-
based molecules (Fig. 1). The popular tetrasubstituted X-shaped
design of benzene with two electron-donating groups (EDGs)
and two electron-withdrawing groups (EWGs) was first
reported by Shimizu’s group with the development of 1,4-
bis(alkenyl)-2,5-dipiperidinobenzenes (Fig. 1a).22 Through the
judicious choice of EWGs on the alkenyl side arm, the solid-state
emission could be tuned across the full-color spectrum. In the
following, Katagiri’s group published 2,5-bis(methylsulfonyl)-
1,4-phenyldiamine, which displayed green pH-independent and
blue solid-state emission properties (Fig. 1b).23 In 2017, Zhang
and coworkers developed dimethyl 2,5-bis-(methylamino)
terephthalate, exhibiting a strong red emission (Fig. 1c).24 Very
recently, full color range tuning was also achieved by Yuan’s
group through fully substituted 2,5-bis(alkylamino)-3,6-
difluoroterephthalonitriles (Fig. 1d).25 X-shaped single-benzene
fluorophores suffer from drawbacks like strong intra- and
intermolecular hydrogen-bonding and the highly substituted
character, which strongly restrict further derivatization and
emission tuning. Potent low molecular weight ESIPT-based
luminophores are truly rare, since potent structures are
commonly constructed using π-extended proton acceptors such
as benzazoles.26 Chuo and co-workers reported hydroxylated
analogues of the green fluorescent protein chromophore,
which display rather weak emission in solution (up to ΦF = 12%)
but strong yellow emission in the solid state (Fig. 1e).27
(2-Hydroxyphenyl)propenone derivatives exhibiting no
emission in solution but efficient green crystal emission were
published by Zhang’s group (Fig. 1f).21 While all these
investigations were based on the search of efficient functional
groups achieving high solid-state quantum yields for the given
chromophore, the power of different regioisomers was never
examined in detail.
In the course of our investigation toward minimalistic and highly
emissive luminophores we observed phenol generation of
magnesiated 2-aryloxazolines upon exposure to air or
molecular oxygen as sustainable oxygen sources.28,29 The so
developed deprotonative hydroxylation protocol delivered full
colored ESIPT-based fluorophores as already shown in initial
examinations. Herein, we report nitrile-substituted ortho-
Fig. 1 Literature examples of single-benzene solid-state emitters.
2 | J. Name., 2012, 00, 1-3
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hydroxy-2-phenyloxazolines as extremely efficient, low Absorption properties
View Article Online
DOI: 10.1039/D0TC00776E
molecular weight, single-benzene fluorophores (Fig. 1; bottom).
With all regioisomers of nitrile-substituted phenols (1-CN, 2-CN,
All four possible regioisomers were synthesized and analyzed
comprehensively in terms of photophysical properties, crystal
structures and theoretical calculations.
3-CN and 4-CN) in hand, we started to investigate their
photophysical properties. UV-Vis absorption spectra were
measured in seven solvents exhibiting differing polarities, i.e.,
methanol (MeOH), acetonitrile (ACN), ethyl acetate (EtOAc),
Wavenumber (cm-1
)
Results and discussion
Syntheses
40.000
36.000
32.000
28.000
4,0
3,5
3,0
2,5
2,0
1,5
1,0
0,5
0,0
a)
1-CN
2-CN
3-CN
4-CN
Nitrile-substituted phenols were achieved starting from
formylbenzonitriles, which were transformed to the
corresponding 2-aryloxazolines (1-Oxa
very good yields via two steps sequence including
condensation with 2-amino-2-methylpropan-1-ol and
, 2-Oxa and 3-Oxa) in
a
subsequent oxidation using NBS (Scheme 2, entry 1).30 Phenol
incorporation was accomplished by ortho-directed metalation
using TMPMgCl·LiCl and ensuing smooth hydroxylation with
240
260
280
300
320
340
360
380
molecular oxygen providing the desired phenols (1-CN, 2-CN
Wavelength (nm)
and 3-CN) in excellent yields (Scheme 2, entries 2–4).28 To our
delight, hydroxylation of 2-Oxa delivered besides the major
isomer 2-CN also the minor regioisomer 4-CN, even though in
low yields. Summarizing, this synthetic route demonstrates the
simplicity of achieving these highly potent luminophores. To
further demonstrate the robustness of our synthetic approach
we performed the synthesis of 1-CN in a scale-up reaction and
achieved this structure on a decagram scale (see the supporting
information, chapter 2.4)
Wavenumber (cm-1
24.000
)
40.000
32.000
16.000
b)
2,5
2,0
1,5
1,0
0,5
0,0
Absorption
Excitation (Em: 468 nm)
Emission (Ex: 326 nm)
300
400
500
600
700
Wavelength (nm)
Fig. 2 (a) Normalized UV-Vis absorption spectra of 1-CN (blue line), 2-CN (green
line), 3-CN (yellow line) and 4-CN (red line) in CH2Cl2. (b) Normalized absorption
(black line), excitation (blue line) and emission spectra (red line) of 3-CN in CH2Cl2.
2-methyltetrahydrofuran (2-MeTHF), dichloromethane (DCM),
toluene (PhMe) and cyclohexane (CH). Generally, the
absorption properties of the respective luminophore are widely
solvent independent (Table 1; see the supporting information,
Fig. S22, S48, S72, S96 and Table S1), while slight differences are
detectable in the same solvent (Fig. 2a). In DCM solution,
intense absorption bands deriving from the single-benzene core
are visible below 280 nm, which are indicative of typical π-π*
transitions. Furthermore, broad and unstructured absorption
bands are detected with maxima λabs between 303 and 326 nm
and extinction coefficients ranging from 5300 to 8850 mol L–1
cm–1 (Fig. 2a; Table 1). The excitation spectrum of 3-CN in CH2Cl2
indicates a maximum population of the excited-state upon
irradiation in the absorption band with a maximum at 326 nm,
while bands below 280 nm show diminished transitions for
emission at 468 nm (Fig. 2b).
Fluorescence properties in solution
Scheme 2 a) 1) 2-amino-2-methylpropan-1-ol, 4 Å MS, CH2Cl2, 25 °C, 20 h; 2) NBS,
CH2Cl2, 25 °C, 4 h; b) 1) TMPMgCl·LiCl, THF, 25 °C, 1 h; 2) O2 (1 atm), 25 °C, 24 h.
NBS = N-bromosuccinimide, TMP = 2,2,6,6-tetramethylpiperidinyl.
Complementary to absorption measurements, fluorescence
properties 1-CN
,
2-CN
,
3-CN and 4-CN were studied in solvents
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
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with distinctive dipole moments – i.e., MeOH, ACN, EtOAc, 2- phenols, with lowest half lifetime values in CH, ranging from
View Article Online
MeTHF, DCM, PhMe, CH. The results are listed in Table 1. 1.13 ns (1-CN) to 6.02 ns (3-CN), and hiDgOheI:s1t0.1v0a3l9u/eDs0TinC0D07C7M6E,
Excitation in the respective absorption maxima resulted in blue ranging from 3.09 ns (1-CN) to 8.10 ns (3-CN). Fluorescence half
to cyanic emission maxima λem (445 nm to 487 nm) depending lifetimes are significant higher in PhMe, ranging from 7.09 ns (1-
on the structure and the utilized solvent (Fig. 3). Notably, 1-CN CN, major species of biexponential decay) to 8.95 ns (3-CN). To
and 3-CN emit in average 10 to 20 nm more red-shifted than 2- investigate the role of aerated vs. deaerated media, 2-MeTHF
CN and 4-CN. Dual emission behavior, which could be caused by solutions of 1-CN and 3-CN were bubbled with argon for 30
the enol-keto-phototautomerization, was never identified. minutes and measured again. For 1-CN no prolonged
Slight solvatochromism is observed as expected for ESIPT-based fluorescence half lifetimes were observed (see Fig. S13), while
structures. Protic, polar solvents such as methanol result in the 3-CN solution shows a significant half lifetime increase by a
blue-shifted emission due to phenol coordination, while with factor of 1.17 (6.53 ns to 7.67 ns) (see Fig. S63). With values of
decreasing solvent polarity a red-shifted emission can be ΦF and
τ in hand, the radiative (kr) and nonradiative (knr) decay
detected as displayed for 1-CN 2-CN 3-CN and 4-CN (Fig. 4). rate constants were determined according to the equations
,
,
Due to their ESIPT-based character, all phenols exhibit large kr = ΦF
Stokes´ shifted emission of up to 11.310 cm–1, which resulted in
baseline separated absorption and emission bands (Fig. 2b).
Hence, highly efficient proton transfer in the excited state takes
place for all phenols in all solvents, indicating that emission
exclusively originate from the keto tautomer. Quantum yields
/τ and knr = (1 – ΦF)/τ.
(
ΦF) in solution show a significant solvent-dependency as well.
Lowest quantum yields are usually observed for CH, while
highest values are observed for DCM, ACN and EtOAc. For 1-CN
ΦF ranges from 8.2% (CH) to 24.9% (DCM), while for 2-CN ΦF
ranges from 15.1% (CH) to 25.2% (DCM). Significantly higher
quantum yields in solution were measured for 3-CN ranging
from 39.7% (CH) to 55.6% (DCM) and for 4-CN from 40.5% (ACN)
to 63.0% (EtOAc). Fluorescence half lifetimes (τ) of the excited
state were determined by time correlated single photon
counting (TCSPC). Similar tendencies were observed for all
Fig. 3 Representative images of 1-CN, 2-CN, 3-CN and 4-CN in CH2Cl2 solution at c = 10–4
mol L–1 under 366 nm irradiation.
Wavenumber (cm-1
20.000
)
Wavenumber (cm-1
)
24.000
16.000
MeOH
24.000
20.000
16.000
MeOH
a)
b)
1,0
0,8
0,6
0,4
0,2
0,0
1,0
0,8
0,6
0,4
0,2
0,0
ACN
ACN
EtOAc
2-MeTHF
DCM
PhMe
CH
EtOAc
2-MeTHF
DCM
PhMe
CH
400
450
500
550
600
650
400
450
500
550
600
650
Wavelength (nm)
Wavelength (nm)
Wavenumber (cm-1
20.000
)
Wavenumber (cm-1
20.000
)
24.000
16.000
24.000
16.000
c)
d)
1,0
0,8
0,6
0,4
0,2
0,0
1,0
0,8
0,6
0,4
0,2
0,0
MeOH
ACN
EtOAc
2-MeTHF
DCM
MeOH
ACN
EtOAc
2-MeTHF
DCM
PhMe
CH
PhMe
CH
400
450
500
550
600
650
400
450
500
550
600
650
Wavelength (nm)
Wavelength (nm)
Fig. 4: Normalized emission spectra of 1-CN (a), 2-CN (b), 3-CN (c) and 4-CN (d) in various solvents.
4 | J. Name., 2012, 00, 1-3
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Table 1 Photophysical data of 1-CN, 2-CN, 3-CN and 4-CN measured in aerated solution at 295 K.
View Article Online
DOI: 10.1039/D0TC00776E
Compd
Solvent
MeOH
λ
abs [nm]
λ
em [nm]
Δ휈
̃
ΦF
τ [ns]
(Rel %)b
1.87 (71.6)
2.87 (28.4)
2.08
2.34
2.07
2.11d
kr
knr
(ε [mol L–1 cm–1])
[cm–1]
9,740
[%]a
13.7
[108 s–1]c
0.64
[108 s–1]c
4.00
1-CN
321
467
ACN
EtOAc
2-MeTHF
320
320
321
471
473
476
10,020
10,110
10,140
15.2
15.6
14.0
0.73
0.67
0.68
4.07
3.61
4.16
THF
321
477
10,190
17.2
2.38 (92.5)
5.10 (7.5)
3.09
2.31 (21.5)
7.09 (78.5)
1.13
0.67
3.21
DCM
PhMe
322 (5790)
322
471
477
9,820
10,090
24.9
18.1
0.81
0.30
2.43
1.35
CH
MeOH
ACN
EtOAc
2-MeTHF
DCM
PhMe
CH
MeOH
322
303
301
303
304
481
445
449
454
455
452
462
468
465
10,270
10,530
10,950
10,980
10,920
10,880
11,040
11,310
9,450
8.2
0.73
0.76
0.68
0.86
0.77
0.76
0.33
0.74
0.70
8.16
2.94
3.57
2.64
2.70
2.27
1.15
4.17
0.67
2-CN
3-CN
20.5
16.0
24.7
22.1
25.2
22.3
15.1
51.0
2.71
2.35
2.86
2.88
3.30
6.74
2.04
303 (5300)
306
306
323
2.53 (5.5)
7.60 (94.5)
7.29
6.75
6.53
7.67d
ACN
EtOAc
2-MeTHF
324
326
327
465
468
469
9,360
9,310
9,260
49.4
47.4
47.0
0.68
0.70
0.72
0.69
0.78
0.81
DCM
PhMe
CH
MeOH
ACN
EtOAc
2-MeTHF
DCM
PhMe
CH
326 (5790)
328
468
479
487
454
456
459
462
457
469
479
9,310
9,610
10,050
9,220
9,420
9,460
9,510
9,270
9,730
10,180
55.6
55.0
39.7
49.8
40.5
63.0
48.7
52.9
52.4
44.8
8.10
8.95
6.02
5.27
5.30
5.22
5.14
5.52
0.69
0.61
0.66
0.95
0.76
1.21
0.95
0.96
0.66
0.88
0.55
0.50
1.00
0.95
1.12
0.71
1.00
0.85
0.60
1.08
327
320
319
320
4-CN
321
321 (8850)
322
7.92
5.10
322
a Absolute quantum yields were determined by using an integration sphere. b Relative ratio of the species of a double exponential function are given in parentheses.
c kr (108 s–1) and knr (108 s–1) were calculated using the equations kr = ΦF/τ and knr = (1 – ΦF)/τ. d Half lifetimes were measured after bubbling with argon for 30 minutes.
For phenols showing high emission in solution radiative decay intensity is multiplied by the factor of 5.65 at 80 K. Additionally,
rates kr ranging from 0.68 × 108 s–1
(
3-CN in ACN) to 1.21 × 108 s– a significant blue-shifted emission was detected while cooling
1
(
4-CN in EtOAc) and nonradiative decay rates knr ranging from below 200 K with a wavelength of 462 nm at 80 K emission
0.50 × 108 s–1
(
3-CN in PhMe) to 1.12 × 108 s–1
(4-CN in ACN) maximum (Fig. 5c). Analyzing the temperature dependent
were observed. These values indicate the suppression of emission of 3-CN in 2-MeTHF indicates an increasing intensity
nonradiative decay pathways and preferred radiative decays for up to 160 K (factor 1.66 compared to 295 K) followed by a
3-CN and 4-CN in solution.
decrease in intensity (Fig. 5d & e). This behavior of a lowered
intensity below a certain temperature is in accordance with
known ESIPT-based emitters.17
Fluorescence at low temperatures
Table 2 Photophysical data of 1-CN, 2-CN, 3-CN and 4-CN measured in deaerated
2-MeTHF at 80 K.
To shed light on the temperature dependent emission behavior,
deaerated 2-MeTHF solutions of 1-CN, 2-CN, 3-CN and 4-CN
were examined and temperature maps were created by
measuring in 40 K intervals down to 80 K (Fig. 5 and Table 2). A
steady increase in photoluminescence intensity was observed
upon cooling of 1-CN solution with maximum intensity detected
at 80 K (Fig. 5a & b). This occurrence is unusual for ESIPT-based
luminophores due to the typical suppression of the emission
process at deep temperatures. Compared to 295 K the emission
Compd
1-CN
2-CN
3-CN
4-CN
λem [nm]
440, 462
444, (432)
456, 476
444, (462)
τ [ns] (Rel %)
2.22 (36.4), 8.17 (51.7), 44.0 (11.9)
1.22 (24.2), 5.58 (61.6), 23.0 (14.2)
1.95 (20.2), 7.29 (63.8), 24.1 (16.0)
5.43 (100)
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Wavenumber (cm-1
20.000
)
Wavenumber (cm-1
20.000
)
View Article Online
DOI: 10.1039/D0TC00776E
24.000
16.000
24.000
16.000
3,0x105
a)
d)
1,0x106
8,0x105
6,0x105
4,0x105
80 K
120 K
160 K
200 K
240 K
280 K
295 K
80 K
120 K
160 K
200 K
240 K
280 K
295 K
2,5x105
2,0x105
1,5x105
1,0x105
2,0x105
0,0
5,0x104
0,0
400
450
500
550
600
650
400
450
500
550
600
650
Wavelength (nm)
Wavelength (nm)
478
1x106
472
e)
f)
b)
c)
3,0x105
2,5x105
2,0x105
1,5x105
1,0x105
5,0x104
470
468
466
464
462
460
458
456
454
476
474
472
470
468
466
464
462
460
1x106
9x105
8x105
7x105
6x105
80
120
160
200
240
280
320
80
120
160
200
240
280
320
80
120
160
200
240
280
320
80
120
160
200
240
280
320
Temperature (K)
Temperature (K)
Temperature (K)
Temperature (K)
Fig. 5 Top: Temperature dependent emission spectra of 1-CN (a) and 3-CN (d) in deaerated 2-methyl tetrahydrofuran during heating from 80 K to 295 K with excitation
at 321 and 327 nm, respectively. Bottom: Temperature dependent trends of intensity (1-CN (b) and 3-CN (e)) and wavelength (1-CN (c) and 3-CN (f)).
4-CN show similar behavior as 3-CN (see the supporting
information, chapters 3.2.2 and 3.4.2).
Next, fluorescence half lifetimes (τ) of the excited states at 80 K
a)
10000
295 K
80 K
were determined for all phenols in deaerated 2-MeTHF by
TCSPC. Remarkably, half lifetimes were prolonged in average
and triple exponential decays were observed for 1-CN, 2-CN and
1000
100
10
3-CN (Fig. 6 and Table 2). Long living species with half lifetimes
ranging from 23.0 ns (2-CN) to 44.0 ns (1-CN) were detected in
lower population. Only, 4-CN showed a contrary behavior with
only one single emitting species, which exhibits a slightly
increased half lifetime compared to measurements at 295 K. It
is worth mentioning that long living triplet emission
(phosphorescence) or thermally activated delayed fluorescence
(TADF) was never observed during our investigations, neither at
rt nor at 80 K.
0
25
50
75
100
125
150
Time (ns)
b)
10000
1000
100
295 K
80 K
Aggregation Induced Emission Enhancement
Fluorophores which show no emissive or low emissive
properties in solution can become highly emissive in the solid
state. Several mechanisms for the emission enhancement like
the restriction of intramolecular vibrational and rotational
motions (RIM), intramolecular charge transfer (ICT)31 or twisted
intramolecular charge transfer (TICT) are well known and result
in the phenomenon called aggregation-induced emission (AIE)
and aggregation-induced emission enhancement (AIEE).6,7 We
chose 1-CN for further investigations towards aggregation-
induced emission and utilized tetrahydrofuran (THF) as a non-
aggregating solvent and water as the aggregating solvent.32 In
the course of the investigation, the concentration was kept at
4 × 10–5 mol L–1. Initially, as depicted in Fig. 7a, fluorescence is
already present at low water fractions (fw = 10%), as confirmed
by emission properties of pure THF solution (λem = 477 nm,
ΦF = 17.2% (Table 1)).
0
25
50
75
100
125
150
Time (ns)
Fig. 6 Time resolved emission decay curves of 1-CN (a) and 3-CN (b) in deaerated
2-MeTHF at 80 K and 295 K.
Again, linear blue-shifted emission was detected with a minimal
emission wavelength of 456 nm at 80 K (Fig. 5f). Importantly,
both temperature maps (Fig. 5a & d) display the disappearance
of the broad emission band at 295 K and splitting into at least
two definite bands. Owing to suppressions of intramolecular
degrees of freedom line broadening is significantly reduced,
resulting in the given pattern. Temperature maps of 2-CN and
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Wavenumber (cm-1
20.000
)
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DOI: 10.1039/D0TC00776E
24.000
16.000
5x106
5,0x106
a)
b)
10 %
20 %
30 %
4,5x106
4x106
3x106
2x106
40 %
50 %
60 %
70 %
80 %
90 %
4,0x106
3,5x106
95 %
99 %
3,0x106
2,5x106
2,0x106
1,5x106
1x106
0
400
450
500
550
600
650
10
20
30
40
50
60
70
80
90
100
Wavelength (nm)
Water Fraction (f
w
)
60
50
40
30
20
10
c)
d)
490
485
480
475
470
465
460
f w = 99%:
D = 706 ± 87.4 nm
100
1000
Nanoparticle Size (nm)
10000
10
20
30
40
50
60
70
80
90
100
Water Fraction (f
w
)
e)
Fig. 7 (a) Emission spectra of 1-CN in THF/water mixtures with differing water fraction (fw in %) at a concentration of 4 × 10–5 mol L–1 under excitation with 321 nm. (b) Plot of
maximum intensity versus water fraction (fw). (c) Plot of wavelength at maximum versus water fraction (fw). (d) DLS analysis of nanoparticle size distribution of fraction fw = 99%.
(e) Representative AIEE image of 1-CN in THF/water mixtures with given water content (in %) under 366 nm irradiation.
With increasing water fractions of up to fw = 90%, a moderate their size using dynamic light scattering (DLS), revealing an
emission enhancement is detected with a slight drop of average hydrodynamic radius of 706 ± 87.4 nm (Fig. 7b).
emission intensity (Fig. 7b) at fw = 50% (λem = 466 nm) and Additionally, the emission properties of doped PMMA films with
fw = 90% (λem = 460 nm) including a blue-shifted emission different weight concentration of 1-CN (1wt%, 10wt% and
(Fig. 7c). This hypsochromic shift, starting from fw = 10% to 50wt%) were examined (see the supporting Information,
fw = 90% can be explained by the increasing polarity of the chapter 3.1.3). While films with 1wt% and 10wt% of 1-CN
surrounding medium, which is in accordance with emission exhibit similar blue emission, PMMA film with 50wt% of 1-CN
wavelength tendencies for polar solvents like methanol, showed bright green emission.
reasoned by phenol coordination. Interestingly, further water
addition to fw = 95% resulted in a red-shifted emission
Solid-state emission
(λem = 478 nm) and an increase of intensity. Maximum emission
All phenols (1-CN, 2-CN, 3-CN and 4-CN) exhibit bright
boosting was obtained at fw = 99% with another significant red-
shift (λem = 488 nm) and almost doubling of fluorescence
intensity, which resulted in a quantum yield of ΦF = 41.3%.
Within the last two analyzed fractions visible nano-aggregates
have been formed, while massive nanoparticle formation has
only taken place at fw = 99% (Fig. 7; bottom). The nano-
aggregated particles in fw = 99% were analyzed in respect of
luminescence in the solid-state to the naked eye by simple
irradiation with a 366 nm UV-handlamp (Fig. 8; bottom).
Therefore, investigations of the photophysical properties were
conducted in the crystalline state for 1-CN, 2-CN, 3-CN and in
the microcrystalline state for 4-CN (Fig. 8a). Optical properties
of all phenols in the solid-state are listed in Table 3.
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Wavenumber (cm-1
20.000
)
CIE 1931
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DOI: 10.1039/D0TC00776E
24.000
16.000
0,9
b)
a)
520
1,0
0,8
0,6
0,4
0,2
0,0
1-CN
0,8
0,7
0,6
2-CN
3-CN
4-CN
540
560
500
0,5
580
3-CN
1-CN
4-CN
0,4
0,3
0,2
0,1
0,0
600
620
2-CN
480
460
0,1
400
450
500
550
600
650
0,0
0,2
0,3
0,4
x
0,5
0,6
0,7
0,8
Wavelength (nm)
c)
Fig. 8 (a) Normalized emission spectra of 1-CN, 2-CN, 3-CN and 4-CN in the solid-state. (b) CIE 1931 chromaticity plot with emission color coordinates of 1-CN, 2-CN, 3-CN and 4-CN.
(c) Representative images of 1-CN, 2-CN, 3-CN and 4-CN in the solid-state under 366 nm irradiation.
Table 3 Photophysical data of 1-CN, 2-CN, 3-CN and 4-CN measured in the solid-state at 295 K.
Compd
1-CN
2-CN
3-CN
4-CN
λexc [nm]
347
326
348
λem [nm]
491
463
494
Δ휈
̃
[cm–1]
ΦF [%]a
87.3
τ [ns]
kr [108 s–1]c
0.66
knr [108 s–1]c
0.10
x; y (CIE 1931)
0.186; 0.387
0.153; 0.168
0.186; 0.417
0.142; 0.254
8.450
9.080
8.490
7.760
13.3
7.35
10.8
7.76
68.0
74.1
14.7
0.92
0.69
0.19
0.44
0.24
1.10
346
473
a Absolute quantum yields were determined by using an integration sphere. b kr (108 s–1) and knr (108 s–1) were calculated using the equations kr = ΦF/τ and knr = (1 – ΦF)/τ.
The brightest luminophore was 1-CN, which showed a strong λem = 473 nm and ΦF = 14.7%. In consideration of the
green-cyan emission with λem = 491 nm and ΦF = 87.3% in the remarkable emission properties in solution, 4-CN demonstrates
crystalline state. The fluorescence half lifetime was determined aggregation-caused quenching (ACQ) character. As in solution,
to
knr = 0.10 × 108 s–1) revealed that the negligible nonradiative proton transfer in the excited state, resulting in exclusive
decay pathways yielded the high ΦF Noteworthily, emission from the keto tautomer. CIE 1931 chromaticity
τ
= 13.3 ns. Dynamics studies (kr = 0.66 × 108 s–1 and the large Stokes shifted emission is caused by an efficient
.
semicrystalline powder of 1-CN showed the same emission coordinates (Fig. 8b) were calculated from the corresponding
properties, indicating that no crystallization-induced emission photoluminescence spectra, giving the following (x; y) values for
enhancement (CIEE) has taken place (see the supporting 1-CN (0.186; 0.387), 2-CN (0.153; 0.168), 3-CN (0.186; 0.417)
information, Fig. S21). To the best of our knowledge 1-CN and 4-CN (0.142; 0.254). In the course of photophysical
represents the lowest molecular weight ESIPT-based investigations, none of these analyzed emitters showed the
fluorophore with a quantum yield above 85% in the solid-state. tendency for photobleaching, neither in solution nor in the
Also, 2-CN displays strong blue emission with λem = 463 nm and solid-state.
ΦF = 68.0% in the crystalline state and a high Stokes shift of
9.080 cm–1. Based on these powerful solid-state emission
properties and the weak emission in solution, both, 1-CN and 2-
CN show a typical AIEE. Powerful green-cyan emission is
observed for 3-CN with λem = 494 nm and ΦF = 74.1% in the
Single crystal packing
To better understand the emission characteristics of these
highly efficient luminophores, suitable single crystals for X-ray
diffraction were obtained for 1-CN, 2-CN and 3-CN by
crystalline state. A high fluorescence half lifetime of τ = 10.8 ns
evaporation of a concentrated dichloromethane solution.
Unfortunately, 4-CN showed only microcrystallite formation,
which were not suitable for single crystal analysis.
Crystallographic data are summarized in Table 4 and S2–S19 and
depicted in Figure 9 and S108–S147.
and low radiationless decay pathways (kr = 0.69 × 108 s–1 and
knr = 0.24 × 108 s–1) verified the highly potent solid-state
emission nature of 3-CN. Together with its strong emission
properties in solution, 3-CN represents a dual state emission
(DSE) luminophore. In comparison, 4-CN showed relatively
weak blue-cyan fluorescence in the microcrystalline state with
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View Article Online
DOI: 10.1039/D0TC00776E
Fig. 9 Molecular structures of 1-CN (top), 2-CN (middle) and 3-CN (bottom) showing 50% probability ellipsoids, including top view and sideview of the single structure, molecular
packing as well as the π-π-interactions of adjacent molecules.
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Table 4 Hydrogen bond (∙∙∙) and crystal parameters of 1-CN, 2-CN and 3-CN.
View Article Online
DOI: 10.1039/D0TC00776E
Compd
O1–H1∙∙∙N2
Dihedral angle
C1–C2–C8–N2 [°]
0
–5.6(3); –3.8(3)
–0.05(17)
Slip angles [°]
Plane distance [Å] with corresponding
centroid-centroid distance ([Å])
3.329 (3.586)
3.379 (3.476); 3.400 (3.565)
3.298 (4.051); 3.311 (3.750)
Length [Å]
1.6704(3)
1.8592(19)
1.8289(11)
Angle [°]
1-CN
2-CN
3-CN
153.294(3)
147.790(12)
148.005(8)
68.2
72.5; 76.4
54.5; 62.0
The O1–H1∙∙∙N2 hydrogen bond distances in 1-CN, 2-CN and 3- explains the increased ФF values in the solid-state. For 1-CN,
CN were determined to be 1.670, 1.859 and 1.829 Å, several intermolecular interactions,33 like C–H∙∙∙O (2.679 Å) or
respectively, while the corresponding angles are assigned to be C–H∙∙∙N (2.658 Å) are identified (see the supporting
153.29°, 147.79°, and 148.01°. Dihedral angles between the information, Fig. S120), while antiparallel molecule cross-
phenol and oxazoline rings (i.e., the C1–C2–C8–N2 angle) were stacking takes place along the b-axis in a sheet structure.
found to be 0° for 1-CN, –5.6° and –3.8° (C21–C22–C28–N22) for Interplanar spacing is determined to 3.329 Å, whereas the
2-CN and –0.05° for 3-CN, respectively. These crystal data adjacent
π-skeleton distance is 3.586 Å with a slip angle of 68.2°
revealed that the O–H∙∙∙N hydrogen bond in 1-CN is the (Fig. 9). Thus, relatively strong
π-π-interactions lead to intense
strongest, which in turn facilitates intramolecular proton H-aggregation. Similarly, numerous intermolecular interactions,
transfer to the adjacent nitrogen atom and rationalizes the like C–H∙∙∙O (2.584 Å) or C–H∙∙∙N (2.627 Å) were detected for 2-
extremely high ФF. To gain a deeper understanding of the CN (see the supporting information, Fig. S131). Molecules are
increased ФF values in the solid-state, the crystal stacking cross-stacked in an antiparallel sheet structure along the a-axis
modes were examined. 1-CN (orthorhombic, Pnma), 2-CN with interplanar spacing of 3.379 and 3.400 Å to the adjacent
(triclinic, P–1) and 3-CN (monoclinic, P21/c) exhibit different molecules, respectively. π-Skeleton distances of 3.476 Å and
packing modes in their respective crystal lattice, including 3.565 Å with slip angles of 76.4° and 72.5° were determined,
various types of intermolecular interactions. This variety of indicating strong H-aggregation.
contacts leads to a restriction of intramolecular rotation and
Wavenumber (cm-1
20.000
)
Wavenumber (cm-1
20.000
)
28.000
24.000
16.000
28.000
24.000
16.000
a)
b)
1,0
0,8
0,6
0,4
0,2
0,0
1,0
0,8
0,6
0,4
0,2
0,0
Calculation
Experimental (80 K)
Calculation
Experimental (80 K)
350
400
450
500
550
600
650
700
350
400
450
500
550
600
650
700
Wavelength (nm)
Wavelength (nm)
Wavenumber (cm-1
20.000
)
Wavenumber (cm-1
20.000
)
28.000
24.000
16.000
28.000
24.000
16.000
c)
d)
1,0
0,8
0,6
0,4
0,2
0,0
1,0
0,8
0,6
0,4
0,2
0,0
Calculation
Experimental (80 K)
Calculation
Experimental 80 K
350
400
450
500
550
600
650
700
350
400
450
500
550
600
650
700
Wavelength (nm)
Wavelength (nm)
Fig. 10 Comparison of calculated fluorescence spectra (TD-B97/cc-pVDZ level of theory; red lines) with experimental emission spectra (black lines) measured in 2-MeTHF at 80 K of
1-CN (a), 2-CN (b), 3-CN (c) and 4-CN (d).
10 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
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ARTICLE
In 3-CN crystals, molecules are cross-stacked along the b-axis Calculated Emission Spectra
View Article Online
DOI: 10.1039/D0TC00776E
with an interplanar distance of 3.298 and 3.311 Å. Slip angles of
Fluorescence spectra of the keto tautomers of 1-CN, 2-CN, 3-CN
54.5° and 62.0° result in π-skeleton distances of 4.051 Å and
and 4-CN were calculated with Time-Dependent Density
Functional Theory (TDDFT)36 at the TD-B9737/cc-pVDZ38 level of
theory to rationalize the experimental emission spectra. This
level of theory was chosen because of the accurate
reproduction of the experimental emission data (Fig. 10).
Natural transition orbitals (NTOs)39 were applied to characterize
the first electronically excited singlet (S1) state at the S1
equilibrium geometry of the keto tautomer, out of which
fluorescence takes place. NTOs typically allow the
characterization of an electronically excited state via a single
excitation from the Highest Occupied Natural Transition Orbital
(HONTO) into the Lowest Unoccupied Natural Transition Orbital
(LUNTO) with a remarkably high eigenvalue. In all cases
3.750 Å. In contrast to the previously discussed solid-state
structures , sheet structure distortion of 12.4° is detected (see
the supporting information, Fig. S145). Again, intermolecular
interactions such as C–H∙∙∙O (2.413 Å) or C–H∙∙∙N (2.729 Å) lead
to a restriction of intramolecular rotation, pointing to weak H-
aggregation (see the supporting information, Fig. S146).
Interestingly, 1-CN exhibits a bent nitrile bond (N1–C7–Cipso
)
with an angle of 173.01°, whereas the respective bond in 2-CN
and 3-CN is almost linear (see the supporting information,
Tables S11 and S17). This observation can be explained through
adjacent orbital interactions of the nonbonding orbital of the
oxazoline-oxygen (O2) and a π*-orbital of the nitrile bond. The
interaction energy was calculated by means of natural bond
orbital (NBO) analysis and determined to 1.19 kcal/mol (see the
supporting information, chapter 6).
considered here, the S1 state can be characterized as an n-π*-
state with eigenvalues between >0.99 (Fig. 11). The excitations
are quite local, signifying only insignificant charge-transfer
character. Due to the accurate reproduction of the
experimental emission spectra by TDDFT, the calculated spectra
provide further evidence that emission takes place from the S1
state of the keto tautomer of each molecule presented here.
Commonly, H-aggregation results in diminished quantum yields,
whereas J-aggregation results in improved emission.34 1-CN
,
2-
CN and 3-CN exhibit high to extremely high quantum yields (ФF
)
in the solid-state, as already reported for other H-aggregated
structures.35 However, those literature known luminophores
exhibit larger π-conjugated systems and are not constructed on
a minimalistic single-benzene scaffold like the presented
emitters. Usually, highly potent solid-state emitters based on a
single-benzene
aggregation.24,21,23,22 1-CN demonstrates its powerful emission
ФF = 87.3%) with a well-arranged crystal lattice, including a
crystal system of relatively high symmetry (orthorhombic),
several short intermolecular contacts (C–H∙∙∙O/N) and strong
-interactions (centroid-centroid-distance = 3.586 Å). A short
core
are
commonly
showing
J-
(
π-
π
hydrogen bond distance (1.670 Å), a high hydrogen bond angle
(153.29°) as well as a zero dihedral angle confirms the great
solid-state emission potential of 1-CN. The origin of the high
photoluminescence quantum yield of 1-CN can be multifarious.
Intermolecular interactions in the crystal lattice such as C‒H‧‧
‧
N interactions or antiparallel coupling of local dipoles as
described by Park and Adachi for styrylbenzene derivatives can
cause such unusual optical behavior.28b,d
Finally, TGA/DSC measurements were conducted to investigate
the thermal stability of the discussed luminophores. All of these
novel low molecular weight phenols (MW = 216.1), exhibit
melting points TM between 121 and 170 °C and decomposition
temperatures Tdecomp above 170 °C as determined by
thermogravimetric analysis (TGA) and differential scanning
calorimetry (DSC) (Table 5). The corresponding spectra are
shown in the supporting information (Fig. S149-152).
Fig. 11 Excitations from the Highest Occupied Natural Transition Orbital (HONTO) into
the Lowest Unoccupied Natural Transition Orbital (LUNTO) that characterize the first
electronically excited singlet (S1) state at the S1 equilibrium geometry of the keto
tautomer, out of which the fluorescence of each of the four isomers takes place. E is
the electronic excitation energy of the S1 state in the keto form and  is the eigenvalue
of the HONTO→LUNTO transition.
Table 5 Thermal stability properties.
Conclusions
Compd
1-CN
2-CN
3-CN
4-CN
Onset/Endset T [°C]
199/233
TM [°C]
172.4
134.5
122.2
122.8
Tdecomp [°C]
229.5
216.5
219.9
224.8
In summary, minimalistic, nitrile-substituted single-benzene
ESIPT-based luminophores exhibiting powerful emission in the
solid-state have been successfully synthesized and investigated
towards their emission properties. Depending on the nitrile
188/220
190/224
198/229
position, AIEE
(1-CN and 2-CN), DSE (3-CN) or ACQ
characteristics (4-CN) with blue to green-cyan emission colors
and high Stokes shifts were identified for all investigated
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ARTICLE
Journal Name
Gompper and H.-U. Wagner, Angew. Chem. Int. Ed., 1988,
fluorophores. Quantum yields are remarkably high and with 1-
CN the lowest molecular weight ESIPT-based luminophore with
a quantum yield above 85% in the solid-state was discovered
(MW = 216.1; ΦF = 87.3%). Through careful single crystal
analysis, we identified the well-shaped molecular structure and
the eminent organized crystal lattice as the origin of these
outstanding photoluminescence properties. TDDFT calculated
emission spectra gave evidence for emission from the S1 state
of the keto tautomer. This exceptionally straightforward
concept for the creation of minimalistic, highly emissive
fluorophores opens up the opportunity for future design studies
yielding luminophores with superior properties. Applications of
the discussed fluorophores in optoelectronic materials are
underway in our laboratories.
View Article Online
DOI: 10.1039/D0TC00776E
27, 1437;
6
7
8
J. Mei, N. L. C. Leung, R. T. K. Kwok, J. W. Y. Lam and B. Z.
Tang, Chem. Rev., 2015, 115, 11718.
J. Mei, Y. Hong, J. W. Y. Lam, A. Qin, Y. Tang and B. Z. Tang,
Adv. Mater., 2014, 26, 5429.
B. Z. Tang, A. Qin, Aggregation-Induced Emission.
Fundamentals, Wiley; John Wiley & Sons Inc, Chichester,
2013.
a) B.-K. An, J. Gierschner and S. Y. Park, Acc. Chem. Res.,
2012, 45, 544; b) D. Oelkrug, A. Tompert, J. Gierschner, H.-
J. Egelhaaf, M. Hanack, M. Hohloch and E. Steinhuber, J.
Phys. Chem. B, 1998, 102, 1902;
9
10 Z. Zhao, J. W. Y. Lam and B. Z. Tang, J. Mater. Chem., 2012,
22, 23726.
11 S. Benson, A. Fernandez, N. D. Barth, F. de Moliner, M. H.
Horrocks, C. S. Herrington, J. L. Abad, A. Delgado, L. Kelly, Z.
Chang, Y. Feng, M. Nishiura, Y. Hori, K. Kikuchi and M.
Vendrell, Angew. Chem. Int. Ed., 2019, 58, 6911.
Conflicts of interest
There are no conflicts to declare.
12 M. Shimizu and T. Hiyama, Chem. Asian. J., 2010, 5, 1516.
Acknowledgements
13 a) A. C. Sedgwick, L. Wu, H.-H. Han, S. D. Bull, X.-P. He, T. D.
James, J. L. Sessler, B. Z. Tang, H. Tian and J. Yoon, Chem.
Soc. Rev., 2018, 47, 8842; b) V. S. Padalkar and S. Seki,
Chem. Soc. Rev., 2016, 45, 169; c) A. P. Demchenko, K.-C.
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E. Kwon and S. Y. Park, Adv. Mater., 2011, 23, 3615;
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Charaf-Eddin, A. D. Laurent, D. Jacquemin, G. Ulrich and R.
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We thank Prof. Dr. Nadja-C. Bigall, Dr. Dirk Dorfs, and Pascal
Rusch (all from Leibniz University Hannover) for supporting the
photophysical measurements.
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