Synthesis and photophysical properties of a series of cyclopenta[b] naphthalene solvatochromic fluorophores

Article abstract of DOI:10.1021/ja3055029

The synthesis and photophysical properties of a series of naphthalene-containing solvatochromic fluorophores are described within. These novel fluorophores are prepared using a microwave-assisted dehydrogenative Diels-Alder reaction of styrene, followed by a palladium-catalyzed cross coupling reaction to install an electron donating amine group. The new fluorophores are structurally related to Prodan. Photophysical properties of the new fluorophores were studied and intriguing solvatochromic behavior was observed. For most of these fluorophores, high quantum yields (60-99%) were observed in methylene chloride in addition to large Stokes shifts (95-226 nm) in this same solvent. As the solvent polarity increased, so did the observed Stokes shift with one derivative displaying a Stokes shift of ～300 nm in ethanol. All fluorophore emission maxima, and nearly all absorption maxima were significantly red-shifted when compared to Prodan. Shifting the absorption and emission maxima of a fluorophore into the visible region increases its utility in biological applications. Moreover, the cyclopentane portion of the fluorophore structure provides an attachment point for biomolecules that will minimize disruptions of the photophysical properties.

Full text of DOI:10.1021/ja3055029

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Communication
Synthesis and Photophysical Properties of a Series of
Cyclopenta[b]naphthalene Solvatochromic Fluorophores
Erica Benedetti, Laura Susan Kocsis, and Kay M Brummond
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja3055029 • Publication Date (Web): 13 Jul 2012
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Journal of the American Chemical Society
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Synthesis and Photophysical Properties of a
Series of Cyclopenta[b]naphthalene Solvato-
chromic Fluorophores.
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Erica Benedetti, Laura S. Kocsis and Kay M. Brummond*
Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260
KEYWORDS (Word Style “BG_Keywords”). If you are submitting your paper to a journal that requires keywords, provide
significant keywords to aid the reader in literature retrieval.
Supporting Information Placeholder
ABSTRACT: The synthesis and photophysical properties of a
series of naphthalene-containing solvatochromic fluorophores
is described within. These novel fluorophores are prepared
using a microwave-assisted dehydrogenative Diels-Alder reac-
tion of styrene, followed by a palladium-catalyzed cross cou-
pling reaction to install an electron donating amine group. The
new fluorophores are structurally related to Prodan. Photo-
physical properties of the new fluorophores were studied and
intriguing solvatochromic behavior was observed. For most of
these fluorophores, high quantum yields (60-99%) were ob-
served in methylene chloride in addition to large Stokes shifts
(95-226 nm) in this same solvent. As the solvent polarity in-
creased so did the observed Stokes shift with one derivative
displaying a Stokes shift of ~300 nm in ethanol. All fluoro-
phore emission maxima, and nearly all absorption maxima
were significantly red-shifted when compared to Prodan.
Shifting the absorption and emission maxima of a fluorophore
into the visible region increases its utility in biological appli-
cations. Moreover, the cyclopentane portion of the fluorophore
structure provides an attachment point for biomolecules that
will minimize disruptions of the photophysical properties.
A common structural feature in many fluorophores is a con-
jugated pi-system functionalized with both an electron-
withdrawing and an electron-donating group. For many of
these fluorescent compounds, the pi-system is comprised of a
naphthalene moiety. The synthesis of these fluorescent chro-
mophores usually begins with commercially available naph-
thalene derivatives or alternatively, new naphthalene-based
dyes are developed by modifying an existing fluorophore. An
excellent example of the latter protocol involves Prodan (1a),
a compound whose fluorescent emission wavelength and
quantum yield are unusually dependent upon solvent polarity
(Figure 1). For example, in cyclohexane the fluorescent emis-
sion of Prodan is 410 nm and in water it is 534 nm, a batho-
chromic shift of 124 nm.5 Environmentally sensitive dyes are
of special interest as probes, and Prodan and its analogs are
considered to be state of the art for application in biological
systems.6 Structural variants of Prodan include the lipophilic
Laurdan (1b); the thiol reactive Acrylodan (1c) and Badan
(1d); and the amino acid-containing Aladan (1e).7 The latter
dye is important for monitoring protein interactions and dy-
namics.8 In addition, a spectrally red-shifted compound, An-
thradan (1f), has been prepared that incorporates an anthracene
ring between the electron donating and electron withdrawing
groups resulting in an emission spectra in hexanes of 483 nm
and of 604 nm in methanol.9 Recently, a fluorene analog of
Prodan was prepared that showed excellent brightness when
compared to Prodan.10 In addition, Prodan has been conju-
gated to four nucleosides to aid in the structure determination
of DNA (structures of the latter two examples are not
shown).11
Fluorescent-based tools are widely used to monitor envi-
ronments of biological events.1 Fluorescence is used for a va-
riety of reasons, including the nondestructive nature of these
measurements. Of the fluorescent probes available, small or-
ganic fluorophores are gaining in popularity due to rapid re-
sponse times for monitoring real-time events with excellent
spatial resolution.2 In addition, their relatively small size
minimizes disruption of the environment being studied. The
widespread use of these probes is enabled by the commercial
availability of hundreds of fluorescent dyes.3 However, there
are many elements to consider when choosing the optimal
fluorescent tool, including quantum yield, absorption and
emission wavelengths, photo- and chemical stability, and
Stokes shift to name just a few. Each of the commercially
available dyes comes with a list of advantages and compro-
mises that must be weighed for any application. Thus, new
small molecule-based fluorescent probes are continually being
developed to encompass all the desired elements while mini-
mizing compromises.4
Figure 1. Prodan and Its Derivatives
1a, R = CH2CH3, n = 1
1b, R = (CH2)10CH3, n = 1
1c, R = CHCH2, n = 1
1d, R = CH2Br, n = 1
1e, R = CH2C(CO2H)NHFmoc, n = 1
1f, R = CH2CH3, n = 2
O
y
x
R
N
n
For each of these Prodan derivatives, the donor-acceptor
substituents are located along the x-axis (longer axis) of the
naphthalene pi-system, and the amino group can be character-
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Page 2 of 5
ized as exonuclear and sterically unhindered. For the anthra- ing N,N-dimethylamine substituted cyclopenta[b]naphthalenes
4 and 6 in 70% and 49% yield, respectively.16 Different palla-
dium catalysts (Pd(OAc)2) and bases (K3PO4, Cs2CO3) were
also screened, but all resulted in lower yields of the coupling
products. Attempts were not made to further optimize the reac-
tion conditions. Each amine compound in Scheme 2 was ob-
tained from the corresponding chloronaphthalene using these
exact reaction conditions. Interestingly, a mixture of 3b and
3d gave two products, amine 5 in 52% yield along with the
dehalogenated cyclopenta[b]naphthalene. It is hypothesized
that this latter compound arises from a palladium-catalyzed
dehalogenation reaction of 3d facilitated by the close proxim-
ity of the methyl ketone. The photophysical properties of these
first generation fluorophores, including the solvatochromic
properties were examined (below). For our second generation
of fluorophores, a number of amines were coupled with 3a.
Secondary cyclic amines such as pyrrole, piperidine, and mor-
pholine gave the corresponding tertiary amines 7, 8, and 9 in
59%, 45% and 58% yield, respectively. Primary amines such
as benzylamine, aniline, and para-methoxyaniline were also
successfully coupled with 3a to afford 10, 11 and 12 in 89%,
78% and 71% yield, respectively. Compounds 13, 14 and 15
were prepared in a similar manner from aldehyde 3e and ester
3f. Finally, ketones 16 and 17 were prepared from the corre-
sponding cyclopentanones 3g and 3h. Desilylation of silyl
naphthalene 17 with tetra-n-butyl ammonium fluoride afforded
18.
1
2
3
4
5
6
7
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cene analog, even though the emission wavelength was sig-
nificantly red-shifted, the quantum efficiency was lower. The
synthesis of fluorophores possessing more desirable properties
could be significantly enhanced by more efficient and versatile
methods for the construction of naphthalene derivatives.12
In the previous paper, we described the synthesis of a series
of structurally novel naphthalene compounds using a micro-
wave-assisted dehydrogenative Diels-Alder reaction of styrene
(Scheme 1).13 One of these cycloadducts was functionalized with
acetyl and dimethylamino groups to give compound 4. This com-
pound showed very interesting optical properties when compared
to Prodan in that the absorption and emission maxima in methyl-
ene chloride (CH2Cl2) were significantly red-shifted by 22 and 70
nm, respectively. Moreover, this compound was highly fluores-
cent with a quantum yield of 99%! Thus, simply changing the
positions of the electron-donating and electron-withdrawing
groups on the naphthalene ring resulted in a photophysical behav-
ior previously only available by adding an additional ring to the
pi-system, as in both Anthradan (1e) and a fluorene analog of
Prodan.9 However, unlike these Prodan derivatives, compound 4
is relatively small, thereby limiting potential disruption of the
system under study, while displaying a high quantum yield. Fur-
thermore, the bathochromic shift of the absorption of compound 4
(22 nm) into the visible region allows for excitation with visible
light, compared to Prodan which absorbs in the UV range limiting
its application to biology. These interesting properties prompted
us to prepare a number of structurally related compounds in
search of fluorescent dyes with superior photophysical properties.
Moreover, the versatility of our synthetic approach using the de-
hydrogenative Diels-Alder reaction enables a systematic study
and rationally designed fluorophores with desirable properties for
biological applications.
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With a series of novel fluorophore compounds in hand, ab-
sorption and emission maxima along with quantum yields
were measured in CH2Cl2 (Scheme 2). Notable trends for this
series of compounds were observed. For example, compound
6 containing a 1,5-substituted cyclopenta[b]naphthalene moi-
ety absorbed light at a much shorter wavelength (334 nm) and
fluoresced at a much longer wavelength (562 nm) than either
the 1,7- or 1,6-disubstituted compounds 4 or 5 (absorption and
emission maxima for both are ~375 nm and ~510 nm). Inter-
estingly, the absorption and emission maxima along with the
quantum yield of compounds 5 and 6 are almost identical to
previously reported, and structurally related, analogs of Pro-
dan lacking the five-membered ring.17 Thus, it can be con-
cluded that the cyclopentane ring has little effect on the photo-
physical properties of these new fluorophores. The tertiary
cyclic amino ketone series of compounds 7, 8, and 9 showed a
range of absorption maxima (355-390 nm), while the emission
maxima remained relatively constant (508-515 nm). For ex-
ample, the absorption of the pyrrole-substituted naphthalene 7
was red-shifted by 13 nm when compared to the dimethy-
lamine derivative 4. This subtle change in structure affords a
compound that may be more amenable to biological studies
because the absorption maxima is in the visible region. The
emission spectra of the secondary amines 10, 11, and 12 were
significantly blue-shifted (482-495 nm) when compared to the
tertiary amines, but there were not significant differences in
absorption maxima between each of the secondary amines.
Scheme 1. Dehydrogenative Diels-Alder Reaction
X
R
R
X
µW
Cl
Cl
2
3
With an eye towards the preparation of a series of aminon-
aphthalene derivatives, ortho-, meta-, and para-chlorinated
styrenyl Diels-Alder precursors 2 were subjected to the dehy-
drogenative Diels-Alder reaction to afford the corresponding
chlorinated cyclopenta[b]naphthalene derivatives 3 (Scheme
1). Next, cross coupling reactions were examined for the in-
troduction of electron-donating amino groups via the chloro
group of the naphthalene derivatives 3. For this process, a
palladium-catalyzed amination of aryl chlorides emerged as a
valuable tool for producing the fluorophore structure; the ad-
vantages to carrying a chloro group through a synthetic se-
quence instead of the more reactive bromo or iodo groups
cannot be overstated.14,15 For the first generation of fluorescent
compounds, dimethylamine and chloronaphthalenes 3a, 3b, 3c
and 3d were synthesized and reacted with dimethylamine to
produce Prodan-like fluorophores so that photophysical prop-
erties of these first generation fluorophores could be directly
compared to that of Prodan (Scheme 2). The coupling reaction
of 3a and 3c using a commercially available RuPhos precata-
lyst (2.5 mol %) and lithium hexamethyldisilyamide
(LHMDS) in tetrahydrofuran (THF) afforded the correspond-
Substitution of the ketone group with an aldehyde or ester
significantly affects the photophysical properties. For exam-
ple, the absorption maxima of aldehydes 13 and 14 are red-
shifted by 50-60 nm when compared to ketones 4 and 7. How-
ever, the pyrrole group of aldehyde 14 had almost no effect on
the absorption maxima when compared to aldehyde 13. The
emission maxima for both aldehydes 13 and 14 were red-
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Journal of the American Chemical Society
shifted by 30 nm when compared to ketones 4 and 7. Substitu- being that for all three fluorophores, the emission maxima are
1
2
3
4
5
6
7
8
tion of the ketone group with an ester group results in a blue
shift in the emission maxima of 22 nm (see compounds 4 and
15 for a comparison).
significantly red-shifted as the solvent polarity increases. For
example, the emission maxima for compounds 4, 5, and 6 are
466, 445 and 480 nm in cyclohexane and 578, 605, and 634
nm in ethanol, respectively. The range of emission and absorp-
tion wavelengths for these fluorophores along with the large
Stokes shift of 300 nm for compound 6 are potentially valu-
able for multiplexing experiments.19 Moreover, the solvato-
chromic emission spectra are significantly red-shifted when
compared to Prodan, which emits at 389 nm in hexanes and
485 nm in ethanol. Red-shifted fluorescent emissions are im-
portant for biological applications where background fluores-
cence can limit the magnitude of the fluorescent change for
fluorophores emitting and absorbing at shorter wavelengths.15
A brief examination of the photobleaching properties was per-
formed on compounds 4, 5 and 6 in CH2Cl2. All three fluoro-
phores were found to have photodegradation curves similar to
that of Prodan (see supporting information).
Finally, a series of compounds with a carbonyl in the five
membered ring was synthesized and displayed a significant
red shift in the absorption and emission maxima comparable to
that observed for aldehydes 13 and 14. For example, fluoro-
phores 16, 17 and 18 have absorption maxima at 427 nm, 442
nm and 422 nm, respectively, with respective emission
maxima of 535 nm, 537 nm and 521 nm. The effect that the
silyl group has on the absorption is in agreement with the ba-
thochromic shift that was previously reported for other monos-
ilyl substituted naphthalene derivatives, only more pronounced
(20 nm vs 9 nm).18 Finally, the quantum yields for all fluoro-
phores in Scheme 2 were extremely high (82-99%), with four
exceptions, compound 6, 14, 16, and 17.
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Scheme 2. Pd-catalyzed Cross Coupling Reaction and Pho-
tophysical Properties of Functionalized Naphthalenes
In conclusion, ready access to a series of environmentally
sensitive fluorescent compounds has been accomplished by
using a dehydrogenative Diels-Alder cycloaddition reaction
followed by a palladium-catalyzed cross coupling reaction. It
is anticipated that these compounds or other readily prepared
fluorophores will prove useful as fluorescent markers in bio-
logical systems. Moreover, the ease in which the compounds
can be prepared, and our ability to incorporate a diversity of
electron-withdrawing and electron-donating groups into the
synthetic route, will aid in a deeper understanding of the pho-
tophysical properties of not only this class of fluorophores but
others, thus contributing to the arsenal of rationally designed
fluorescent probes. While some chemical modifications of the
Prodan scaffold has been studied, systematic construction of a
library of Prodan derivatives has not been reported, in part,
due to synthetic challenges. Our approach provides ready ac-
cess to a variety of Prodan-like fluorophores. Furthermore, it
is concluded that the fused five-membered ring does not affect
the photophysical properties of naphthalene-containing com-
pounds 4-15. The fused ring provides a point of attachment for
biomolecules or water solublizing groups with the ultimate
goal of developing new probes that can be used more effec-
tively for imaging in living cells and whole organisms. Fi-
nally, the red-shift of the absorption and emission maxima of
the cyclopentanone-containing compounds is intriguing and
supports the premise that this class of compounds emit from a
planar intramolecular charge transfer (PICT) excited state.20
Studies are ongoing to delineate the effect these subtle struc-
tural changes have on the photophysical properties and to de-
sign improved fluorophores for application to biological sys-
tems.
5
Precat.:
6
Cl
R2
H
N
R1
N
R2 R1
7
NH2
8
Pd
Cl
X
X
R
R
LHMDS 2 equiv
precat 2.5 mol %
THF, N2, 3 h, 85 °C
L
3a 7-chloro, X = H, R = Ac
3b 6-chloro, X = H, R = Ac
3c 5-chloro, X = H, R = Ac
3d 8-chloro, X = H, R = Ac
3e 7-chloro, X = H, R = CHO
L = RuPhOS
iPr
PCy2
3f 7-chloro, X = H, R = CO2Me
3g 7-chloro, X = O, R = Ph
3h 7-chloro, X = O, R = TMS
iPr
N
N
N
O
O
O
4: 70% yield
377 nma, 510 nmb, 99c
5: 52% yieldd
372 nma, 509 nmb, 82c
6: 49% yieldd
334 nma, 562 nmb, 60c
N
N
N
O
O
O
O
7: 59% yieldd
390 nma, 513 nmb, 96c
8: 45% yieldd
362 nma, 515 nmb, 99c
9: 58% yieldd
355 nma , 508 nmb, 99c
O
N
N
N
H
H
O
O
H
O
10: 89% yieldd
11: 78% yieldd
367 nma , 493 nmb, 95c
12: 71% yieldd
368 nma, 482 nm b
,
97c
372 nma, 495 nm b 89c
,
N
N
N
H
O
H
O
H3CO
O
13: 19% yieldd
437 nma, 540 nmb, 98c
14: 19% yieldd
440 nma, 543 nmb, 27c
15: 19% yieldd
378 nma, 488 nmb , 99c
Supporting Information. Detailed experimental pro-
cedures and characterization data for all new compounds
along with fluorescent emission spectra for compounds 4-
18 are provided. Photodegradation decay curves for com-
pounds 4, 5 and 6 are included in this section. This material
is available free of charge via the Internet at
N
N
N
O
O
O
Ph
TMS
16: 50% yieldd
427 nma, 535 nmb, 61c
17: 54% yieldd
442 nma, 537 nm b, 68c
18: 60-80d
422 nma, 521 nmb, 85c
aAbsorption maxima in CH2Cl2; bEmission maxima in CH2Cl2;c
Fluorescence quantum yield vs Prodan in DMSO (91%); Yield
optimizations were not performed on this substrate.
d
http://pubs.acs.org.
Finally, solvatochromic properties for the first generation
fluorophores 4, 5, and 6 were measured in a number of sol-
vents varying in polarity (Table 1 and Figure 2). Several im-
portant findings emerged from these measurements, the first
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1
2
3
4
5
6
Table 1. Spectroscopic Properties of Fluorophores 4, 5 and 6 in Comparison to Prodan
N
O
N
N
O
N
7
O
O
8
9
Prodan
5
4
λem
nm
/
466
490
495
497
510
516
529
536
578
6
λem
a
b
a
b
c
a
b
a
b
λabs
λem
φf
%
2.0
/
56
75
78
98
/
λabs
φf
λabs
λem
nm
/
φfc
%
/
λabs
φfc
%
/
Entry
Solvent
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
nm
340
/
346
346
348
355
/
nm
389
/
416
422
430
440
/
nm
/
%
/
nm
/
nm
/
nm
/
1
2
3
4
5
6
7
8
9
n-Hexane
Cyclohexane
Toluene
1,4-Dioxane
THF
CH2Cl2
CHCl3
Acetonitrile
DMSO
373
376
376
377
377
377
375
377
377
45
62
46
75
99
77
99
85
5
363
368
370
370
372
373
370
374
374
445
481
495
505
509
511
545
558
605
8
314
322
319
330
334
334
316
334
334
480
520
534
543
562
562
590
598
634
28
47
44
39
60
40
23
48
1
25
38
34
82
41
18
37
3
350
357
362
455
462
485
80
91
71
10
EtOH
aAbsorption maxima. bEmission maxima. cFluorescence quantum yield vs Prodan in DMSO (91%), excitation at 334 nm (10-5 M).
(9) Lu, Z.; Lord, S.J.; Wang, H.; Moerner, W.E.; Twieg, R.J. J.
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AUTHOR INFORMATION
Corresponding Author
*kbrummon@pitt.edu
Notes
The authors declare no competing financial interest.
Funding Sources
(13) Kocsis, L.S.; Benedetti, E.; Brummond, K.M. Manuscript
Submitted for Publication.
We thank the National Science Foundation for supporting this
work (CHE0910597).
(14) Surry, D.S.; Buchwald, S.L. Chem. Sci. 2011, 2, 27-50; Hart-
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ACKNOWLEDGMENT
We thank Professor Michael Trakselis for helpful discussions
regarding quantum yield measurements. We also thank Kristy
Gogick and Robin Sloan for their assistance in collecting fluores-
cence data for this manuscript.
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