Synthesis and spectrophotometric analysis of 1-azafluorenone derivatives

Article abstract of DOI:10.3390/molecules25153358

A new extension for the 'one pot' construction of diverse 1-azafluorene derivatives featuring a Diels-Alder/retro-Diels-Alder cycloaddition is reported. Conditions were also determined for oxidation to the derived azafluorenones. The spectrophotometric analysis of five different azafluorenones were performed. Moderate fluorescence was observed with azafluorenone derivatives that bear an imbedded pyridone motif; whereas those bearing substituted pyridines do not fluoresce.

Full text of DOI:10.3390/molecules25153358

molecules
Article
Synthesis and Spectrophotometric Analysis of
1-Azafluorenone Derivatives
Nicholas H. Angello, Robert E. Wiley, Christopher J. Abelt * and Jonathan R. Scheerer *
Department of Chemistry, The College of William & Mary, P.O. Box 8795, Williamsburg, VA 23187, USA;
nangello@email.wm.edu (N.H.A.); rewiley@email.wm.edu (R.E.W.)
* Correspondence: cjabel@wm.edu (C.J.A.); jrscheerer@wm.edu (J.R.S.)
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
ꢀꢁꢂꢃꢄꢅꢆ
Received: 10 June 2020; Accepted: 21 July 2020; Published: 24 July 2020
Abstract: A new extension for the ‘one pot’ construction of diverse 1-azafluorene derivatives
featuring a Diels–Alder/retro-Diels–Alder cycloaddition is reported. Conditions were also determined
for oxidation to the derived azafluorenones. The spectrophotometric analysis of five different
azafluorenones were performed. Moderate fluorescence was observed with azafluorenone derivatives
that bear an imbedded pyridone motif; whereas those bearing substituted pyridines do not fluoresce.
Keywords: azafluorenone; synthesis; fluorescence
1. Introduction
The spectrophotometric properties of fluorenone have been studied for over a century, and the
molecule remains of current interest to a diverse community of physical and organic chemists,
photochemists and material scientists [
9-fluorenone ( ) continue to be refined [
various applications such as incorporating fluorenone-based molecules for resistive memory devices [
Recent advances in modern organic photochemistry have returned to fluorenone as well defined
molecule that can in some cases provide superior results as compared to other photosensitizers [ ].
1
]. The fundamental photophysical properties for the parent
] and the resulting enhanced understanding has enabled
].
1
2
3
4,5
Several analogs of fluorenone have been prepared with heteroatom substitutions on or within the
core. The inclusion of more basic functionalities permits the tuning of the donor–acceptor properties.
In particular, 3-aminofluorenone derivatives (e.g.,
enhanced quantum yield and strong emissive properties stemming from the intramolecular charge
transfer excited state [ ]. The emission of these derivatives is strongly quenched by hydrogen-bonding
2) have been extensively studied and show both an
6
interactions with solvents. The mechanism (in plane/out of plane) and stoichiometry of the interactions
with the emissive excited state are currently under investigation [7].
Pyridine analogs of fluorenone have been known for some time as natural products [
Renewed isolation efforts have revealed nearly a dozen substituted 4-azafluorenone derivatives
related to onychine ( ), the simplest member of the family [ 10]. Due to potent biological activities,
a number of synthetic azafluorenone derivatives have been prepared and show promising therapeutic
potential against several diseases and cancer [11 12]. The analysis of the photophysical properties
of azafluorenone derivatives have been more limited; 3-azafluorenone has been spectroscopically
characterized [13 14] and 1,8-diazafluorenone has been advanced in the field of forensic analysis for
8].
3
9,
,
4
,
the fluorescent illumination of finger print residue [15]. Since the pyridine motif can impart favorable
aqueous solubility properties, azafluorenones containing this structural feature have recently emerged
as a new class of biocompatible imaging reagents. The 1-aza- and 2-azafluorenone derivatives
are illustrated as representative examples (Figure 1) [16,17].
5 and 6
Molecules 2020, 25, 3358; doi:10.3390/molecules25153358
www.mdpi.com/journal/molecules

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Figure 1. Fluorenone derivatives and the proposed synthesis and evaluation of 1-azafluorenones.
We recently developed a new synthetic strategy for the construction of 1-azafluorene derivatives
8
and
9
in one reaction vessel from 2-alkynylbenzaldehyde derivatives (e.g., ) [18 19]. This manuscript
7
,
both advances the related methods for the rapid construction of diverse 1-azafluorene structures
and determines the photophysical properties of a series of derived azafluoreneones represented by
structures 10 and 11. As such, the products of both the synthetic effort and spectroscopic characterization
are described. We were particularly interested in comparing the properties of 1-azafluorenone
derivatives bearing either the pyridine structure motif in 11 and the pyridone functionality present in
10. The pyridone 10 bears an in-plane N–H bond in close proximity to the carbonyl and we wondered
what effect such an orientation would have on the optical properties.
2. Results and Discussion
2.1. Synthesis
In our previous research, we discovered a domino reaction pathway starting from either the
diketopiperazine or oxazinone precursors 12 or 13 that enabled a rapid assembly of azafluorene
derivatives [18,19]. Although the reaction conditions can vary somewhat for different precursors,
both substrates undergo a cascade reaction sequence comprised of an aldol condensation,
alkene isomerization, Diels–Alder cycloaddition, and cycloreversion steps (Reactions (1) and (2)).
The foundational discovery highlighted in Reaction (1) affords pyridone-containing structures
8 after a
base-promoted extrusion of the lactimide bridge. Starting with the lactim-lactone precursor 13, the
derived intermediate [2.2.2]-bicycloadduct is not isolable under the reaction conditions (PhMe, 110 ◦C)
and undergoes the cycloreversion and extrusion of carbon dioxide to reveal the 2-methoxy-1-azafluorene
product 9a (Reaction (2)). The reactivity and scope for these reaction sequences have been disclosed in
initial communications.

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We now wish to report that a modified and analogous reaction sequence with phenyl
substituted dihydrooxazinone 14 was achieved using very mild reaction conditions (Reaction (3)).
We determined that the entire sequence to give azafluorene product 9b—the aldol, isomerization,
[4+2], and retro-[4+2]—could be accomplished with a non-hygroscopic weak fluoride base (TBAT,
tetrabutylammonium difluorotriphenylsilicate) at ambient temperatures and in reliable yields
(50–70%). To the best of our knowledge, this is the lowest temperature observed for any
cycloaddition/cycloreversion sequence with an oxazinone derivative. In practice, due to the propensity
for degradation or polymerization on storage, dihydrooxazinone 14 used in this sequence was prepared
in situ via Staudinger reductive cyclization from the derived azide precursor (see Supplementary
Materials). Azafluorene prepared in this manner gave more consistent results, on average a 70% yield
over the complete operation. Overall, the three related domino reaction sequences highlighted in
Reactions (1)–(3) enable the rapid preparation of diverse azafluorene derivatives.
We selected a series of azafluorenes to advance through oxidation to the derived fluorenone
products for spectroscopic analysis. In our hands, we found that the fluorenone product 11b could
be most conveniently accessed by treating a DMF solution of the tricyclic fluorene 9b with Cs2CO3
and air (Scheme 1). This oxidation proved general and another four products were prepared in
an analogous fashion. Thus, five 1-azafluorenone substrates (10a, 10b, 11a–11c) were advanced for
spectrophotometric analysis. These 1-azafluoreneone derivatives were selected in order to possess a
representative mix of both pyridine and pyridone structural motifs. Additionally, varied substitutions
at the C2 position, including carbon (Ph, 11b), oxygen (OMe, 11a; O, 10a and 10b) and halogen (Cl,
11c), were included for substrate diversity.

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Scheme 1. Preparation of the diverse 1-azafluorenones for spectroscopic characterization.
2.2. Optical Properties
The electronic absorption spectra of 10a
,
10b, and 11a
–
11c are shown in Figure 2. All have the
expected strong, short-wavelength UV bands below 300 nm. Methoxyazafluorenone 11a and the related
phenyl derivative 11b have moderate bands in the medium UV (300–330 nm) and very weak bands
around 400 nm, giving the solutions of these compounds a yellow color. The substituted azafluorenones,
especially the phenyl derivative, have larger molar absorptivities than the pyridones. The pyridone
derivatives 10a and 10b show three bands spaced between 300 and 480 nm of similar strength.
1000
800
600
400
200
0
60000
50000
40000
30000
20000
10000
0
ε
350
400
450
280
330
380
wavelength (nm)
430
480
Figure 2. Absorption and long wavelength expansion (inset) of pyridones 10a
azafluorenones 11a
— —; 10b (F), – – –.
,
b
in ethanol and
–c
in EtOAc. 11c (Cl), ——; 11b (Ph), — . . —; 11a (MeO), — . —; pyridone 10a (H),
The pyridone derivatives 10a and 10b showed moderate fluorescence in ethanol, while the
fluorenones 11a 11c were essentially non-fluorescent (Figure 3). Quantum yields are 0.04 0.01 for
10a; 0.07 ± 0.01 for 10b.
–
±

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10
8
6
4
2
0
Int
(arb)
420
470
520
570
620
670
wavelength (nm)
Figure 3. Normalized fluorescence of pyridinones 10a and 10b in ethanol (10a (H), — —; 10b (F),– – –).
2.3. Theoretical Calculations
The electronic structures of the ground (DFT, B3YLP, 6-311G+(2d,p)) and the first excited (TD-DFT,
B3LYP, 6-31G+(2d,p)) [20] states for 10a 10b, and 11a 11c were calculated with Gaussian 16 [21].
,
–
The molecular orbitals for the unsubstituted pyridone 10a and the phenylazafluorenone 11b are
representative of the two groups. The frontier molecular orbitals, calculated for the ground state
structures, are shown in Figure 4. For both molecule sets, the HOMO and LUMO orbitals are π-systems,
whereas the HOMO-1 orbital is an n-type orbital. Electron density in this orbital includes contributions
from the carbonyl oxygen and the pyridine nitrogen. In 11c and 11a, the chlorine and methoxy also
contribute. For the pyridone set, the HOMO-3 is another n-orbital that is centered on the pyridone
carbonyl. The calculations predict that strong, short-wavelength (<280 nm) absorption bands were
π→π*, whereas the longer wavelength bands have a partial to mostly n→π* character (Table 1).
Table 1. The calculated absorption bands (
and 11.
λ
max
(nm), oscillator strength, % n→π* character) for 10
Compound
λ
277
4489
0
286
4
100
301
4
100
306
123
58
max
f × 104
11c (Cl)
11b (Ph)
11a (MeO)
10a (H)
% n
λ
288
4995
0
306
21
0
308
59
0
316
423
45
322
3219
0
327
907
52
max
f × 104
% n
λ
max
279
4040
0
279
3
100
303
6
100
306
471
0
427
1
100
f × 104
% n
λ
max
295
3065
0
302
743
0
324
4
0
349
0
100
403
0
100
f × 104
% n
λ
291
3042
0
306
1006
44
322
194
41
347
0
64
399
0
36
max
f × 104
10b (F)
% n

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Figure 4. Frontier molecular orbitals (top, LUMO; middle, HOMO; bottom, HOMO-1) for pyridone
10a (left) and phenylazafluorenone 11b (right).
2.4. Discussion of Fluorescence
The azafluorenones 11a–11c show no appreciable fluorescence, whereas the pyridones 10a and
10b fluoresce abstemiously. Biczók and coworkers found that rapid intersystem crossing from the
lowest singlet excited state (S1) to lower energy triplet levels reduced the fluorescence quantum yields
of 3-azafluorenone derivatives to the order of 10−2
[13,14]. For 1-azafluorenones, 11a–11c, the lowest
energy singlet state (S1) has n→π* character, while the lowest energy triplet state (T1) has only π→π
*
character. These triplet states lie at 11.9, 14.8 and 14.9 kcal/mol lower in energy than the initial S1 states.
As a result, the typical spin-orbit coupling mechanism may be responsible for rapid intersystem
crossing and explain the relative lack of fluorescence.
In contrast to the azafluorenones, the pyridones 10a and 10b show moderate fluorescence. Here,
the π,π* triplet states lie at 37.1 and 36.6 kcal/mol lower in energy than the n,π* singlet states. The large
energy gap may impede the intersystem crossing. The nature of the fluorescent state for the pyridones is
not definite. The structures of the pyridones allow for the possibility of the excited-state intramolecular
proton transfer (ESIPT). Proton transfer to the amide carbonyl would give an iminol structure that
is similar to 11a (Figure 5, right). Since 11a does not show fluorescence, we conclude that this enol
structure is not likely to be the source of the pyridone emission. Proton transfer to the fluorenone
carbonyl gives the enol structure (Figure 5, left). This structure has an extended, cross-conjugated
π
-system. For the emission, the calculations predict bands at 504 nm for 10a and 452 nm for 10b
However, both of these transitions have some n, * character and zero oscillator strength. For the enol
structures, the longest wavelength emission bands are calculated to be at 483 and 489 nm, respectively.
These bands are pure * and have non-zero oscillator strengths (both 0.0004). The possibility of
ESIPT in the pyridone derivatives remains under investigation.
.
π
π,π

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Figure 5. Tautomers of 10a.
3. Materials and Methods
All reactions were carried out under an atmosphere of nitrogen in flame-dried or oven-dried
glassware with magnetic stirring unless otherwise indicated. Acetonitrile, THF, toluene, and Et2O
were degassed with argon and purified by passage through a column of molecular sieves and a
bed of activated alumina [22]. Dichloromethane was distilled from CaH2 prior to use. All reagents
were used as received unless otherwise noted. Flash column chromatography was performed using
SiliCycle siliaflash P60 silica gel (230–400 mesh) [23]. Analytical thin layer chromatography was
performed on SiliCycle 60Å glass plates. Visualization was accomplished with UV light, anisaldehyde,
ceric ammonium molybdate, potassium permanganate, or ninhydrin, followed by heating. Infrared
spectra were recorded using a Digilab FTS 7000 FTIR spectrophotometer. 1H-NMR spectra were
recorded on a Varian Mercury 400 (400 MHz) spectrometer and are reported in ppm using solvent as
an internal standard (CDCl3 at 7.26 ppm) or tetramethylsilane (0.00 ppm). Proton-decoupled 13C-NMR
spectra were recorded on a Mercury 400 (100 MHz) spectrometer and are reported in ppm using
solvent as an internal standard (CDCl3 at 77.00 ppm). All compounds were judged to be homogeneous
(>95% purity) by 1H and 13C NMR spectroscopy, unless otherwise noted. Mass spectra data analysis
was obtained through positive electrospray ionization (w/ NaCl) on a Bruker 12 Tesla APEX–Qe
FTICR-MS with an Apollo II ion source. Absorption and fluorescence data were obtained using a
fiber optic system with an Ocean Optics Maya CCD detector, a miniature deuterium/tungsten lamp
(uv/vis) and a 365 nm LED light source (fluorescence). Relative quantum yields were determined
using anthracene as the reference (
(1) the electronic noise was subtracted, (2) the wavelength values were converted to wavenumbers,
(3) the corresponding net intensity values were multiplied by
2/( max)2 to account for the effect of
Φ = 0.30). The emission intensity data was treated as follows:
λ
λ
the abscissa-scale transformation and (4) the resulting intensity values were divided by the spectral
response of the Hamamatsu S10420 CCD. Electronic structure calculations were conducted using
Gaussian 16. Ground state geometries were optimized using the DFT B3YLP method employing the
6-311G+(2d,p) basis set. Excited states were optimized using the TD-DFT B3LYP method employing
the 6-31G+(2d,p) basis set.
4. Conclusions
We successfully prepared several 1-azafluorenones and related pyridones using an aldol/
isomerization/Diels–Alder cycloaddition/cycloreversion cascade reaction sequence followed by fluorene
oxidation. The photophysical properties of these compounds were characterized and modeled through
computations. The pyridones fluoresce moderately, but the azafluorenones bearing the pyridine motif
do not.
Supplementary Materials: The following are available online: the experimental procedures, characterization
data, 1H and the 13C-NMR spectra for all new compounds.
Author Contributions: J.R.S. and C.J.A. conceived and designed the experiments, analyzed the data, and prepared
the manuscript. N.H.A. and R.E.W. performed experiments and analyzed the data. All authors have read and
agreed to the published version of the manuscript.
Funding: This research was funded in part by the National Institutes of Health (R15GM107702 to J.R.S.). Additional
support was also provided by the Camille and Henry Dreyfus Foundation (Henry Dreyfus Teacher
−
Scholar
Award to J.R.S.).

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Acknowledgments: The authors acknowledge William & Mary Research Computing for providing computational
resources and/or technical support that have contributed to the results reported within this paper. URL:
https://www.wm.edu/it/rc.
Conflicts of Interest: The authors declare no conflict of interest.
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Sample Availability: Samples of the compounds are available from the authors.
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2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access
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(CC BY) license (http://creativecommons.org/licenses/by/4.0/).