7-azabicyclo[2.2.1]heptane as a unique and effective dialkylamino auxochrome moiety: Demonstration in a fluorescent rhodamine dye

Full text of DOI:10.1021/ja8075617

Published on Web 12/05/2008
7-Azabicyclo[2.2.1]heptane as a Unique and Effective Dialkylamino
Auxochrome Moiety: Demonstration in a Fluorescent Rhodamine Dye
Xiangzhi Song, Alexis Johnson,§ and James Foley*
Rowland Institute at HarVard UniVersity, 100 Edwin H. Land BouleVard, Cambridge, Massachusetts 02142
Received September 23, 2008; E-mail: foley@rowland.harvard.edu
Table 1. Photophysical Data for 221SR and TMSR in Methanol
Despite the spectacular successes that have been achieved using
organic fluorophores in contemporary pure and applied science, the
full potential of most luminescence-based technologies has yet to
be realized largely because the goal of developing monomolecular
chromophores that simultaneously have long-term photostabilities
and high fluorescent efficiencies remains an unmet challenge. The
limitations imposed by currently available fluorophores on cutting-
edge experiments and technologies has led to calls for the
development of improved, more versatile materials.1
a
dye
λabs (nm)
λfl (nm)
ε (M-1 cm-1
)
Φ20 °C
Φ60 °C
221SR
TMSR
548
550
567
568
112 000
108 000
0.95
0.65
0.95
0.38
a Calculated by Φ60 °C ) Φ20 °C(τ60 °C/τ20 °C)(n260 °C/n220 °C).
rhodamine than TMSR would be to find a replacement for the
dimethylamino donor having diminished steric requirements and/
or a higher ionization potential. These considerations led us to
consider using 7-azanorbornane for this purpose for the following
reasons: (1) With the use of 7-azanorbornyl benzene as a model to
qualitatively gauge pertinent steric interactions, geometry optimiza-
tion calculations [AM1] for both the neutral aniline and for its
corresponding cation radical predicted norbornyl C1-N-C4 bond
angles of 95.0° and 97.3°, respectively, and a “closest approach”
distance between the bridgehead hydrogen and ortho-benzene
hydrogen atoms of 2.100 and 2.094 Å, respectively. These values
correspond to considerably less steric interference than those
calculated for N,N-dimethylaniline and its cation radical, which were
predicted to have CH3-N-CH3 bond angles of 113.4° and 118.2°,
respectively, and methyl-benzene hydrogen atom separation dis-
tances of 1.609 and 1.594 Å, respectively. (2) Experimental
adiabatic ionization potentials for 7-azanorbornane and dimethy-
lamine are reported to be 8.40 and 8.24 eV, respectively.6
The targeted new dye 221SR was readily synthesized by heating
3,6-dichlorosulfofluoran with excess 7-azanorbornane. The dye was
purified by flash chromatography and characterized by HPLC, TLC,
and spectroscopic methods.
Spectroscopic measurements revealed that 221SR and TMSR
have nearly identical absorption maxima, fluorescence maxima,
extinction coefficients, and mirror image band shapes (Table 1).
Thus, in these regards 7-azanorbornane behaves like a typical
dialkylamino donor albeit with transition energies higher than would
have been expected for a similarly sized bis-n-dialkyl analogue.7
On the other hand, 221SR has an impressive fluorescence quantum
yield of 0.95 which is significantly higher than the 0.65 value
measured for its tetramethyl analogue. Moreover, despite being
untethered, 221SR has an emission efficiency that rivals those of
the very best rhodamines, regardless of the nature or flexibility of
the donor groups.5
Theoretical and experimental advances made in recent years in
two disparate fields of chemistry have allowed us to use their
combined databases and predictions to rationally design improved
fluorescent reporter dyes. One of these sources of information arose
from the continuing controversy associated with the TICT mech-
anism that has been proposed to explain dual fluorescence and
enhanced rates of nonradiative decay in a wide variety of
donor-acceptor (DA) dyes. This controversy has spawned a large
amount of research into elucidating pertinent structure-function
relationships that govern the photophysical behavior of these
materials.2,3 In addition, our effort was aided by the pioneering
studies of the Nelsen and Mann groups aimed at understanding the
physical chemistry of apex-N-substituted bicycloalkanes which have
shown some members of this class of amines to be extraordinarily
resistant to oxidative degradation.4 In this communication we
present the preliminary results of a study designed to test the idea
that some of the deficiencies that have been linked to the use of
ordinary dialkylamines as auxochrome moieties in DA fluorophores
could be ameliorated, or, at the least, minimized, by the use of
suitably configured, geometrically constrained amino groups.
Specifically, using sulforhodamine 221SR as a model fluorophore
and its tetramethyl analogue, TMSR, as a benchmark, we demon-
strate the benefits of using 7-azabicyclo[2.2.1]heptane (7-azanor-
bornane) as an electron donating moiety in place of a more
conventional dimethylamino group. These include (a) higher
fluorescence quantum yields, (b) fluorescence lifetimes and quantum
yields that are nearly independent of temperature, and (c) improved
resistance to photo-oxidative dealkylation.
It has long been known that DA dyes bearing untethered
dialkylamino donor moieties, such as the pendant dimethylamino
groups in rhodamine TMSR, have lower fluorescent quantum yields
than do analogues having lower degrees of substitution (NHR or
NH2) or where amino group rotation is prevented by cyclization.5
This increased propensity to undergo nonradiative decay has been
found to correlate with both (i) steric requirements at the R-position
of the alkylamino substituents that promote donor-acceptor non-
planarity and (ii) the ability of the donor group to stabilize a cationic
charge as measured by the ionization potential of the pertinent
amine.3 It follows that the key to designing a more efficient
Unlike the typical dialkyamino DA dye, TMSR, whose fluores-
cence lifetime (τ) and quantum yield (Φ) decrease significantly with
increasing temperature, the τ for 221SR was found to increase and
the Φ to remain constant when the temperature was ramped from
20 to 60 °C (Table 1, Figure 2). These findings validate the high
Φ values measured for 221SR in light of the relationships: Φ )
kr/(kr + knr) and τ ) 1/(kr + knr), where kr and knr are radiative and
nonradiative rate constants, respectively.8 In cases where kr . knr,
that is, where Eact for nonradiative decay is abnormally large, τ
should increase with temperature (kr R n2 of the solvent)8 whereas
§ Current address: Department of Chemistry, University of Wisconsins
Madison, 1101 University Ave. WI 53706.
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17652 J. AM. CHEM. SOC. 2008, 130, 17652–17653
10.1021/ja8075617 CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
Figure 1. Structures of pertinent dyes depicted to emphasize connectivity.
Figure 3. Photodegradation of dyes as a function of irradiation time.
achieving photostability in a rhodamine fluorophore and potentially
in other members of the DA family of dyes as well.
In summary, to the best of our knowledge this investigation
constitutes the first use of an apex-N-substituted azabicycloalkane
as an amino auxochrome group in a donor-acceptor chromophore.
We have demonstrated that the use of the 7-azanorbornyl moiety
as the electron donating component in a rhodamine chromophore
induces remarkable beneficial effects on emission efficiency and
photostability as compared to a tetramethylrhodamine analogue.
We expect that the findings of the present study will be broadly
applicable to the wide variety of fluorescent dyes, such as the
dansyls, coumarins, and oxazines, as well as the numerous
nonfluorescent classes of dyes that comprise the donor-acceptor
family of chromogens. Work along these lines, in addition to
extending the concepts that underscore the use of the 7-azanorbornyl
prototype to higher order apex-substituted azabicycloalkane ana-
logues, is currently in progress.
Figure 2. Fluorescent lifetimes of 221SR and TMSR vs temperature in
methanol (values reported in Supporting Information).
Φ should approach unity and be little affected by temperature, as
is the case for 221SR.
For a fluorophore to be of practical value, it must be reasonably
resistant to photodegradaton. Although the light-induced decom-
position of DA dyes can be complex, in this report we focus on
the products that arise from the degradation of donor groups. In
general, electron rich N-alkylated amino groups tend to undergo
stepwise photo-oxidative dealkylation reactions which cause a blue
shift in the absorption maximum of the dye.9 A key step in this
process is thought to be an R-elimination reaction in an initially
formed aminium radical (CH3NR2•+) to give a reactive immonium
(CH2dNR2+) salt.4 Because a parallel sequence of reactions for a
7-azanorbornyl donor would require the formation of a highly
energetic anti-Bredt immonium salt,10 in accord with the findings
of oxidation studies of several related azabicyclics,4 221SR should
be resistant to this type of oxidative photodegradation. To test this
hypothesis, we evaluated the photostabilities of 221SR and TMSR
using relatively innocuous trifluoroethanol as solvent. Because
preliminary studies in our laboratory and by others11,12 found
several tetramethylrhodamine derived dyes to be extremely resistant
to photodegradation under aerobic conditions, our stability studies
were conducted in a mildly alkaline, anaerobic environment,
conditions designed to accelerate the photo-oxidative decomposition
of the dyes;13,14 illumination was provided by a 1.7 W, 514 nm
CW laser. As is evident from inspecting Figure 3, 221SR is
considerably more stable than TMSR; the former dye loses only a
small amount of optical density at its absorption maximum without
changing color during 30 min of intense illumination while, under
identical conditions, the latter undergoes a dramatic decrease in its
absorption maximum and a concomitant significant blue shift which
is attributable to oxidation-induced demethylation. Thus, at least
where photo-oxidative degradation is concerned, the use of 7-aza-
norbornane as an electron donor represents a new paradigm for
Acknowledgment. This research was generously supported by
the Rowland Institute at Harvard.
Supporting Information Available: Preparation and characteriza-
tion details for 221SR and TMSR, Tables of “Fluorescent lifetimes as
a function of temperature” for 221SR and TMSR. This material is
available free of charge via the Internet at http://pubs.acs.org.
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