The Photophysics of Three Naphthylmethylene Malononitriles

Article abstract of DOI:10.1021/jp509882q

The solvent dependence of the photophysical properties of three naphthylmethylene malononitriles, 1-(1-naphthalenylmethylene)-propanedinitrile (1-MN), 2-(2-naphthalenylmethylene)-propanedinitrile (2-MN), and 2-(3,4-dihydro-1(2H)-phenanthrenylidene)-propanedinitile (r2-MN), was studied in order to determine their potential utility as fluidity probes and to make comparisons to the better studied benzylidene malononitriles. Density functional calculations were used to understand the possible conformational states related to rotation about the vinyl-aromatic bond ("τ"). Absorption and emission frequencies, extinction coefficients, fluorescence quantum yields, and fluorescence lifetimes were measured in 11 representative solvents. Both the computational and experimental results indicate that the S₀ → S₁ transitions of these molecules have substantial charge-transfer character and produce highly polar excited states. Emission appears to result from relaxed S₁ states which do not differ qualitatively from the Franck-Condon states reached by absorption. In 2-MN, time-resolved emission reveals the presence of two ground-state conformers ("a" and "b" differing by ～180 rotation about τ) coexisting in low-polarity solvents. In contrast, 1-MN appears to exist primarily as a single dominant ground-state conformer. Fluorescence lifetimes vary from ～1 ps in 1-MN to ～200 ps in 2-MN(a) at room temperature. With the exception of 2-MN(a), the lifetimes vary systematically with solvent in a manner similar to what is observed in the benzylidene malononitriles. Both solvent polarity and fluidity appear to be important determinants of lifetime. The primary mechanism of fluorescence decay in naphthylmethylene malononitriles is likely to be the same as that of the benzylidene malononitriles - twisting about the double bond in S₁, which leads to rapid internal conversion via a conical intersection with S₀. (Graph Presented).

Full text of DOI:10.1021/jp509882q

Article
pubs.acs.org/JPCB
The Photophysics of Three Naphthylmethylene Malononitriles
Jens Breffke,† Brian W. Williams,‡ and Mark Maroncelli*,†
†Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802, United States
‡Department of Chemistry, Bucknell University, Lewisburg, Pennsylvania 17837, United States
S
* Supporting Information
ABSTRACT: The solvent dependence of the photophysical properties of three
naphthylmethylene malononitriles, 1-(1-naphthalenylmethylene)-propanedini-
trile (1-MN), 2-(2-naphthalenylmethylene)-propanedinitrile (2-MN), and 2-
(3,4-dihydro-1(2H)-phenanthrenylidene)-propanedinitile (r2-MN), was studied
in order to determine their potential utility as fluidity probes and to make
comparisons to the better studied benzylidene malononitriles. Density
functional calculations were used to understand the possible conformational
states related to rotation about the vinyl−aromatic bond (“τ”). Absorption and
emission frequencies, extinction coefficients, fluorescence quantum yields, and
fluorescence lifetimes were measured in 11 representative solvents. Both the
computational and experimental results indicate that the S0 → S1 transitions of
these molecules have substantial charge-transfer character and produce highly polar excited states. Emission appears to result
from relaxed S1 states which do not differ qualitatively from the Franck−Condon states reached by absorption. In 2-MN, time-
resolved emission reveals the presence of two ground-state conformers (“a” and “b” differing by ∼180° rotation about τ)
coexisting in low-polarity solvents. In contrast, 1-MN appears to exist primarily as a single dominant ground-state conformer.
Fluorescence lifetimes vary from ∼1 ps in 1-MN to ∼200 ps in 2-MN(a) at room temperature. With the exception of 2-MN(a),
the lifetimes vary systematically with solvent in a manner similar to what is observed in the benzylidene malononitriles. Both
solvent polarity and fluidity appear to be important determinants of lifetime. The primary mechanism of fluorescence decay in
naphthylmethylene malononitriles is likely to be the same as that of the benzylidene malononitrilestwisting about the double
bond in S1, which leads to rapid internal conversion via a conical intersection with S0.
high viscosity solvents10 or polymers11 their fluorescence yields
and lifetimes increase markedly.
1. INTRODUCTION
Benzylidenemalononitriles are a class of molecules which have
long been studied for their biological activity1,2 and
Some disagreement still exists concerning whether the
fluorescence sensing properties.3−5 We have recently examined
three such molecules, shown in the top row of Scheme 1, in
mechanism responsible for this environmental sensitivity is a
twisted intramolecular charge transfer (TICT) process12
involving rotation about the benzene−vinyl single bond (τ in
Scheme 1) or a (null) “isomerization” about the double bond
(ω). Computational work by our group6 and others13,14 favors
the isomerization mechanism, which leads to a conical
intersection between S1 and S0 and rapid internal conversion.
In the asymmetric variant CCVJ, this mechanism can be
verified experimentally because the isomerization leads to long-
lived photoproducts.7 (We note that CCVJ and related
molecules have overshadowed DMN and JDMN as biological
“rotor probes”,15,16 but the potential presence of such
photoproducts means that they must be used with caution.)
Some support for the TICT mechanism was reported in a
recent study of JDMN by Gaffney and co-workers.17 They
found evidence that a long-lived TICT state may also be
present in these molecules, at least when excited with
Scheme 1
order to better understand the origins of their environmentally
sensitive fluorescence.5−7 DMN and JDMN have been widely
used as local fluidity sensors in a variety of contexts since the
pioneering work of Law and Loutfy in the early 1980s.8,9 In low
viscosity solvents, these molecules are weakly fluorescent, with
fluorescence lifetimes in the few picosecond range,5 whereas in
Special Issue: Branka M. Ladanyi Festschrift
Received: September 30, 2014
Revised: November 11, 2014
© XXXX American Chemical Society
A
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significant excess energy. Other work has made it clear that the
fluorescence of DMN, JDMN, and CCVJ respond very similarly
to environment,5,7,18 with both friction and polarity playing
important roles.5 However, even in simple solvents, it remains
unclear how one should interpret the often weak viscosity
scaling and apparent polarity dependence of these “fluidity”
sensors.
∼400-fold at 74 kbar relative to atmospheric pressure. Wang et
al. analyzed this pressure dependence in terms of pressure
affecting the relative populations of two ground-state con-
formers which independently give rise to the LE and ICT
emission, rather than to an LE → ICT reaction occurring in the
excited state. However, we believe that this analysis and
interpretation are incorrect. These authors neglected the fact
that the 325 nm excitation used in their experiments excites
into S2 not S1. On the basis of relative absorption and emission
frequencies, it is clear that, rather than LE and ICT emission,
what was being observed was emission from S2 and S1, with this
violation of Kasha’s rule being allowed by the very weak
character of the S1 emission even in a polymer host at low
pressures.
Two additional studies should also be mentioned relative to
the present work. Liu and co-workers22 measured the effect of
UV irradiation on the absorption spectra of 2-MN and 1-MN in
a 77 K glass in ordered to provide evidence for the “hula-twist”
mechanism of isomerization.23 In this mechanism, isomer-
ization about a double bond is accomplished by concerted
motion of the double bond and a neighboring bond in a
manner that minimizes the volume required. This mechanism
has a higher intrinsic activation energy than direct rotation
about the isomerizing bond, but it may be preferred in rigidified
media. In the case of 2-MN and 1-MN, the hula-twist
mechanism converts an extended conformer into a more
compact conformer (a → b in Scheme 2). Liu and co-workers
The present study seeks to provide additional perspective on
such questions by exploring the photophysics and solvent
sensitivity of several related molecules, depicted in the lower
portion of Scheme 1. In these naphthylmethylene malononi-
triles, the dimethylaniline donor of the former probes is
replaced by naphthalene. We anticipated that this change would
significantly alter the details of the S1 potential energy surface
on which the reaction is expected to sensitively depend6 while
maintaining their electronic spectra in an experimentally
convenient range. The differences in electronic and steric
interactions between the donor and acceptor groups in 1-MN
versus 2-MN might also be expected to produce significantly
different excited-state behavior in these two molecules. Finally,
the conformational restrictions imposed in the variant r2-MN
should eliminate the possibility of TICT state formation,
providing additional perspective on the deactivation mecha-
nism. In the present work, we provide the first systematic look
at the electronic structures and photophysical properties of
these molecules in a selected collection of solvents and relate
their behavior to that of DMN and JDMN.
Compared to the extensive work on benzylidene malononi-
triles, only limited and sporadic attention has been paid to vinyl
malononitriles with other aromatic donors. Three papers have
reported systematic studies of such systems.19−21 In a brief
note, Aihara et al.19 measured the absorption frequencies and
extinction coefficients of a collection of nine aromatic
malononitriles (ArCHC(CN)2) in chloroform. They
observed a clear correlation between the frequencies of the
lowest energy absorption bands and the ionization potentials of
the aromatic subsystem, indicating that the (Franck−Condon)
S1 states of these molecules are of intramolecular charge
transfer (ICT) character.
Scheme 2
The most extensive study to date was performed by Katritzky
et al.20 who measured absorption and emission spectra of five
ArCHC(CN)2 molecules with Ar = benzene, naphthalene
(1-MN), phenanthrene, anthracene, and pyrene in a collection
of eight solvents. They found that, whereas the absorption
bands did not vary much with solvent polarity, the emission
spectra did. They interpreted this observation as indicating that
the emission comes from an ICT state which is distinct from
the Franck−Condon state reached by absorption. They also
noted the fluorescence of these molecules is very weak
(quantum yields typically less than 10−3), which led them to
suggest that this ICT state was likely to be a TICT state,
achieved by a 90° rotation about the aryl−vinyl bond.
reported changes to the absorption spectra upon irradiation
consistent with the hula-twist mechanism occurring in 1-MN.
In 2-MN, either the change did not occur or the spectroscopic
changes produced were too small to observe.
Finally, we note that the present work was initiated partly as
a follow-up to a survey of the solvent dependence of the
absorption and emission spectra of 2-MN and r2-MN by
Donnovan24 under the guidance of one of us (B.W.W.). This
earlier study examined the solvatochromism of both of the
solutes, and some of these prior results are comparable to what
we report herein. However, on the basis of the present work,
we now believe that the r2-MN emission data reported by
Donnovan are incorrect due to the presence of low levels of a
fluorescent impurity which dominated its very weak fluo-
rescence. All of the results reported here have been collected
subsequent to this earlier work and with independently
synthesized solutes.
In the most recent study, Wang et al.21 measured the
emission spectra of 1-MN and the same anthracene and pyrene
variants as Katritzky et al. in poly(methyl methacrylate) at
pressures of up to nearly 80 kbar. They reported emission of all
three solutes to consist of two distinct bands, which they
interpreted as emission from a locally excited (LE) state and an
ICT state. With increasing pressure, the intensity of the overall
emission as well as the relative intensity of the ICT state
increased dramatically. In the anthracene case, the main focus
of their work, they observed the total emission intensity to
increase approximately 100-fold and the ICT intensity by
B
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where n is the solvent refractive index, I the integrated emission
intensity, and A the absorbance of the sample (S) and reference
(R).
2. EXPERIMENTAL AND COMPUTATIONAL METHODS
Solutes. 1-(1-Naphthalenylmethylene)-propanedinitrile (1-
MN) was synthesized by Knoevenagel condensation25,26 as
follows. 1-Naphthalenecarboxaldehyde (4.81 g; 30.8 mmol),
propanedinitrile (2.16 g; 32.6 mmol), and zinc chloride (4.51 g;
33.1 mmol) were mixed and ground together. The mixture was
placed in a glass test tube immersed in a boiling water bath,
allowed to melt, and stirred occasionally for 20 min. The crude
product was then dissolved in 5% aqueous ethanol, washed
twice in diethyl ether, and dried overnight under a vacuum,
resulting in 3.08 g of 1-MN (15.1 mmol; 49%). No further
Time-resolved data were acquired using two different
instruments. The first is a Kerr-gated emission (KGE)
instrument with subpicosecond time resolution, which has
been described in detail previously.32 The KGE system is based
on a 250 kHz amplified Ti:sapphire laser system (Coherent
Innova 400/Mira 900/RegA 9050) tuned to 775 nm with a
power output of 1.5 W. Frequency doubling provided 387 nm
light for excitation of the sample contained in a 0.5 mm flow
cell. The fundamental was used to gate the emission in a 0.7
mm thickness cell of benzene which served as the Kerr
medium. The instrumental response from this system was
∼350 fs (fwhm) based on the solvent Raman signal. Data sets
were time corrected for group velocity dispersion and intensity
corrected for CCD responsitivity and gating efficiency. A
deconvolution procedure was also applied in some cases to
increase confidence in the kinetics observed at early times.
The time correlated single photon counting (TCSPC)
technique employed here utilizes a cavity dumped Ti:sapphire
oscillator (Coherent Mira 900+PulseSwitch) pumped by a CW
laser (Coherent Verdi V-5) at 532 nm. Output pulses have a
width of <300 fs at a variable repetition rate of 5.43 MHz or
lower, and a tunable wavelength range of 760−900 nm. The
mode-locked output of the laser is frequency doubled in a 2
mm BBO crystal. Beam intensity is attenuated by an adjustable
λ/2 plate in front of a Glan-Thompson polarizer to provide
vertical polarization with restricted excitation energy to ensure
single photon collection. The system allows alignment of the
excitation beam for either right angle or front face detection.
Emission from the sample passes through an optical filter to
remove scattered excitation light and a Glan-Thompson
polarizer. Finally, the emission is spectrally resolved using a
100 mm single grating monochromator (Instruments SA, Inc.
H-10) with a 4 nm band-pass prior to detection with a 6 μm
microchannel plate photomultiplier (MCP-PMT; Hamamatsu,
R3809U). The MCP-PMT signal is amplified (Becker & Hickl
HFAC-26) and sent to a photon counting module (Becker &
Hickl SPC-130) on which the constant fraction discriminator,
time-to-amplitude converter (TAC), analog-to-digital convert-
er, data processing logic, and data memory are integrated.
Photon counting acquisition is operated in a reversed start−
stop mode in which an emission photon triggers the TAC and a
delayed reference pulse from a photodiode (Optoelectronics
PD-30) is used to synchronize a termination signal. The
instrument response function determined by a scattering
solution is typically 25 ps fwhm. The experiments performed
here utilized 380−390 nm excitation light, right angle collection
from a 1 cm square cuvette, a GG-400 optical filter to reject
scattered excitation light, and magic angle detection.
1
purification was required. H NMR (300 MHz, CDCl3): δ
7.711 (3H, m), 8.104 (1H, t), 8.296 (3H, m), 9.207 (1H, s).
2-(2-Naphthalenylmethylene)-propanedinitrile (2-MN) was syn-
thesized similarly using 2-naphthalenecarboxaldehyde (2.36 g;
15.11 mmol), propanedinitrile (1.06 g; 16.04 mmol), and zinc
chloride (2.15 g; 15.77 mmol). A 2.36 g portion of 2-MN
(12.87 mmol; 85%) was obtained. 1H NMR (300 MHz,
CDCl3): δ 7.701 (2H, m), 8.130 (4H, m), 8.484 (2H, m).
2-(3,4-Dihydro-1(2H)-phenanthrenylidene)-propanedinitile (r2-
MN) was synthesized according to Ettenger et al.27 using 3,4-
dihydro-1(2H)-phenanthreone (1.91 g; 9.73 mmol), propane-
dinitrile (2.10 g; 31.17 mmol), and sodium acetate (1.80 g;
21.94 mmol). Purification was done by column chromatog-
raphy using 5% ethyl acetate in hexanes three times. A 0.24 g
portion of r2-MN (1 mmol; 10%) was obtained. 1H NMR (300
MHz, CDCl3): δ 2.260 (2H, m), 3.114 (2H, t), 3.438 (2H, t),
7.693 (2H, m), 7.930 (1H, d), 8.003 (1H, d), 8.170 (1H, d),
8.257 (1H, d). Unfortunately, even after three purification
steps, a fluorescent impurity at the level of 1−2% remained in
this sample. The impurity has a nanosecond lifetime, and its
emission sufficiently dominated the steady-state emission of r2-
MN that it was not suitable for steady-state emission
experiments.
Solvents. Most solvents used were from Sigma-Aldrich and
were of spectroscopic or HPLC grade. They were used as
received except for some being dried over molecular sieves and
filtered through a 0.45 μm syringe filter prior use.
Spectroscopic Measurements. A Hitachi U-3010 UV/vis in
combination with a SPEX Fluorolog FL212 spectrometer were
used for steady-state absorption and emission measurements.
Emission spectra were corrected with respect to spectral
responsitivity using secondary emission standards.28 Samples
were prepared from a concentrated 1,4-dioxane stock solution
(∼0.01 M) and diluted at least 300-fold with the target solvent.
At this level of dilution, we could not observe any effect of the
dioxane on the spectral shapes, positions, or lifetimes.
Measurements were carried out in 1 cm quartz cuvettes at
concentrations providing ∼1 OD for absorption spectra and
<0.1 for emission spectra, quantum yields and single photon
Computational Methods. Several quantum chemical
methods were used to study the molecular structures and
photophysical properties relevant to the excited-state dynamics.
Most calculations involved time-dependent density functional
theory (TDDFT) with the B3LYP and CAM-B3LYP func-
tionals and a 6-31G(d,p) basis set. Møller−Plesset perturbation
theory (MP2) was also used to obtain an S0 molecular
geometry for comparison to X-ray structures and the B3LYP
results. Solvent effects were explored using implicit solvent
calculations with the polarizable continuum model.33 Most
calculations were performed using Gaussian 09.34 For exploring
the nature of the excited-state potential upon rotation about the
counting experiments. Temperature was set at 25 °C
0.1
using a water chiller circulating fluid through sample holders in
all experiments except for the Kerr-gated experiments, which
were at room temperature, 21 2 °C. Deoxygenation did not
have a noticeable effect, and therefore, most samples were not
deoxygenated. Fluorescence quantum yields were measured
relative to quinine sulfate in 0.05 M H2SO4 excited at 390 nm
(φem = 0.508)29,30 and calculated according to31
2
2
−0.5·AR
⎛
⎜
⎝
⎞
⎟
⎠
⎛
⎝
⎞
⎟
⎠
⎛
⎞
nS
IS AR ·10
⎜
⎟
φS = φR
⎜
⎝
A 10−0.5·A
·
S
I
n
⎠
R
(1)
R
S
C
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a
Table 1. Calculated Ground-State Properties
molecule
2-MN(a)
method
ΔE (kJ mol−1
)
ΔG (kJ mol−1
)
τ (deg)
θ (deg)
ω (deg)
μ (D)
B3LYP
MP2
(0)
(0)
(0)
(0)
0.8
5.7
0
6
131.6
130.6
131.7
128.4
124.4
122.8
129.9
125.8
130.6
126.4
0.0
0.0
0.0
0.0
2.9
1.9
3.1
3.2
2.3
2.0
7.7
6.6
7.2
6.0
7.7
6.5
7.1
5.8
6.5
5.3
2-MN(b)
r2-MN
B3LYP
MP2
1.4
0.5
180
156
−26
−34
27
B3LYP
MP2
1-MN(a)
1-MN(b)
B3LYP
MP2
(0)
(0)
12.9
5.8
(0)
(0)
13.3
5.5
43
B3LYP
MP2
138
131
a
ΔE is the difference in electronic energy and ΔG is the difference in the Gibbs free energy (298 K) relative to the lowest energy conformer. τ, θ, and
ω are the angles defined in Scheme 2, and μ is the dipole moment.
double bond, a few spin-flip TDDFT calculations35−37 were
also performed using the GAMESS38 electronic structure
program. For these calculations Minezawa’s method36 and the
BHHLYP hybrid functional with the Dunnin−Hay double-ζ
plus polarization DH(d,p) basis set were used.
3. RESULTS AND DISCUSSION
A. Electronic Structure Calculations. 1-MN and 2-MN
can exist as two conformers which differ by rotation around the
malononitrile single bond, defined by torsion angle τ. As shown
in Scheme 2, we label the more extended conformer “a” (τ ∼
0°) and the other conformer “b” (τ ∼ 180°). Results of ground-
state optimizations originating from initial τ = 0 and 180°
geometries are compiled in Table 1.
We first consider 2-MN. For this isomer, DFT (B3LYP/6-
31G(d,p)) calculations predict both conformers a and b to be
planar and to have virtually identical energies and dipole
moments. MP2/6-31G(d,p) calculations predict either a small
(2-MN(a), 6°) or moderate (2-MN(b), 25°) degree of
nonplanarity caused by repulsion between one CN group and
a ring H atom. This repulsion is partly alleviated by an opening
of the vinyl angle θ (Scheme 2). No crystal structure of 2-MN
is available for comparison, but experimental data are available
for the related molecules benzylidene malononitrile (BzMN)
and 2-vinylnapthalene (2-VN). Spectroscopic studies of 2-VN
suggest that the a conformer is planar but are indecisive
concerning the b conformer.39 Similarly to 2-MN, B3LYP
calculations predict both conformers of BzCN to be planar,
whereas MP2 calculations predict nonplanar forms, τ(a) = 15°
and τ(b) = 150°. In the crystal, BzMN is nonplanar, τ =
11°,40,41 and the B3LYP (τ = 0°) and MP2 (τ = 23°)
predictions bracket this value. Other comparisons to BzMN X-
ray data are provided in Table S1 of the Supporting
Information. On the basis of these comparisons, we conclude
that 2-MN(a) is likely to be planar or nearly so in the ground
state, whereas 2-MN(b) is probably twisted by a moderate
angle, comparable to the 11° twist measured for BzMN. Both
model chemistries predict only a slightly lower energy for the a
conformer and nearly equal dipole moments for the two
conformers. On this basis, we expect both the a and b
conformers of 2-MN to be significantly populated in the room-
temperature experiments conducted here.42
Figure 1. Relative potential energies as functions of the single bond
torsion angle τ calculated at the levels B3LYP/6-31G(d,p) (black
circles) and CAM-B3LYP/6-31G(d,p) (red triangles). (a) 2-MN in S0,
(b) 1-MN in S0, (c) 2-MN in S1, and (d) 1-MN in S1. These profiles
were made by fixing the dihedral angle shown in bold in Scheme 2 and
optimizing all other coordinates. S1 profiles are TDDFT calculations.
Energies were calculated over the entire range 0−360° and symmetry
related values at τ and 360 − τ averaged. Error bars indicate twice the
difference between these two energies.
triangles). In the ground state, similar results are obtained using
these two functionals. Both show barriers to interconversion of
a and b conformers to be ∼30 kJ/mol in 2-MN, which is
considerably larger than the barrier in 2-VN (10−20 kJ/mol).39
r2-MN was synthesized as a torsionally restricted analogue of
2-MN(a), and we will show that it shares many spectroscopic
features in common with 2-MN (a or b). Nevertheless, joining
the naphthalene ring and malononitrile groups in this way
renders r2-MN significantly nonplanar, τ ∼ 30° (Table 1). This
nonplanarity relieves some of the strain on the vinyl angle,
allowing θ to decrease, but it also causes some twisting of the
CC bond, denoted by ω (see Scheme 2).
The ground-state potential energy profile of 2-MN as a
function of the torsion angle τ is shown in Figure 1a. Because of
the possibility of TICT state formation in S1, we performed
torsional scans using both the B3LYP functional (black circles)
and its Coulomb attenuated version,43 CAM-B3LYP (red
In the case of 1-MN, steric repulsion leads to both a and b
conformers being significantly nonplanar (Table 1; see also
Figure 3). The a conformer is calculated to be preferred in the
gas phase, and it is the form found in the crystalline state,44
D
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a
Table 2. Excited-State and Transition Properties from TD Calculations at the S0 Geometries
μ0
μ1
|μ − μ |
ν1←0 (103
cm−1
M1←0
μ2
|μ − μ |
ν2←0 (103
cm−1
M2←0
⃗
⃗
⃗
⃗
0
0
molecule
2-MN(a)
functional
(D)
(D)
1(D)
)
(D)
f1←0
(D)
2(D)
)
(D)
f 2←0
B3LYP
7.7
7.4
7.2
6.9
7.7
7.5
7.1
6.8
6.5
6.1
15.0
13.5
14.6
13.0
14.5
12.8
13.2
12.2
11.8
11.6
7.5
6.2
7.5
6.3
7.4
5.9
6.2
5.5
6.1
6.1
26.5
31.3
25.3
30.1
26.1
30.9
26.0
30.3
25.8
30.4
3.3
4.4
2.9
4.0
3.1
4.1
4.8
5.3
4.1
4.4
0.13
0.29
0.10
0.23
0.12
0.25
0.29
0.40
0.21
0.28
10.8
10.5
9.8
3.2
3.2
2.7
3.0
3.0
3.0
6.3
4.1
6.2
3.7
30.9
34.1
30.3
33.5
30.6
34.2
30.7
35.2
30.8
35.4
6.5
5.8
5.8
5.0
5.6
4.8
1.4
1.3
1.2
1.0
0.62
0.55
0.48
0.39
0.44
0.36
0.03
0.03
0.02
0.02
CAM-B3LYP
B3LYP
2-MN(b)
r2-MN
CAM-B3LYP
B3LYP
9.9
10.3
9.9
CAM-B3LYP
B3LYP
1-MN(a)
1-MN(b)
13.4
10.8
12.1
9.4
CAM-B3LYP
B3LYP
CAM-B3LYP
a
μ is the permanent electric dipole moment of state S , i = 0, 1, 2, and |μ − μ |, νi←0, Mi←0, and fi←0 are the change in dipole moment, the transition
⃗
⃗
i
i
i
0
energy, the transition dipole moment, and the oscillator strength of the i ← 0 transition.
where τ = 38°. In light of the calculated energy differences and
its larger dipole moment (see below), it seems likely that 1-
MN(a) is the dominant ground-state conformer in solution. In
this conformer, there is also a modest twist about the double
bond, predicted to be ω ∼ 3° by DFT and MP2 calculations
and measured to be ω ∼ 5° in the crystal.44 The torsional
potential of 1-MN (Figure 1b) shows a slightly smaller barrier
separating the a and b conformers than in 2-MN, but the
overall barrier to 180° rotation is comparable. Comparisons to
available X-ray data on 1-MN44 and r2-MN27 (Supporting
Information, Tables S2 and S3) indicate slightly greater
accuracy for the DFT predictions of S0 properties, and we
will primarily focus on these calculations in the remainder of
the discussion.
Characteristics of the lowest energy electronic transitions and
electric dipole moments based on gas-phase TDDFT
calculations are summarized in Table 2. Additional calculations
including implicit solvent are provided in Table S4 of the
Supporting Information. In all molecules/conformers, both
Figure 2. Experimental absorption spectra in n-hexane (smooth
functionals predict S1 to be separated from S2 by 2900−5000
cm−1. Vertical transition energies (ν1←0, ν2←0) calculated with
the CAM-B3LYP functional are all 3200−4800 cm−1 larger
than the B3LYP calculations. As far as these transition energies
are concerned, the B3LYP calculations are the more realistic.
Figure 2 compares experimental absorption spectra in n-hexane
to gas-phase B3LYP calculations. As illustrated here, the B3LYP
calculations are in reasonable agreement with experiment as
regards the frequencies and relative oscillator strengths of the
low-energy transitions. The same is true in the case of r2-MN,
whose transition properties are close to those of 2-MN(a). Self-
consistent reaction field calculations in cyclohexane (Table S4,
Supporting Information) are similar to those shown in Figure 2
but with the S1 and S2 transitions red-shifted by about 800
cm−1. The comparison between experiment and CAM-B3LYP
calculations (Figure S1, Supporting Information, and Table 2)
is much poorer; all transition frequencies are calculated higher
than observed. As will be seen later, the transition strengths
calculated using CAM-B3LYP are also in poorer agreement
with experiment. Thus, the Coulomb attenuation correction is
not helpful for describing these transitions, despite their partial
charge-transfer character. We will therefore focus on the B3LYP
calculations henceforth. A final observation to be made on the
basis of Figure 2 (and Table 2) is that these calculations predict
that it would be difficult to distinguish between the a and b
conformers of either 2-MN or 1-MN in the experimental
spectra.45
curves) compared to gas-phase TD-B3LYP-calculated vertical
excitation energies and oscillator strengths (bars) of 2-MN and 1-
MN. The first six S0 → Sn transitions are shown. a and b conformer
calculations are shown in blue and red, respectively. The vertical scales
of the experimental spectra are arbitrary.
The dipole moments listed in Table 2 indicate all molecules/
conformers to be significantly more polar in S1 than in S0. The
increase in polarity results from transfer of electron density
from the naphthalene ring to the vinyl malononitrile group.
Ground-state dipole moments are all 7−8 D, and as illustrated
in Figure 3, they are in all cases oriented from the malononitrile
group to the naphthalene ring. These dipole moments suggest
significant charge transfer is present in the ground state.
Mulliken and ESP-fit atomic charges46 both indicate the
naphthalene ring to be positive by 0.17−0.21e in all cases. The
S − S differences, |μ − μ |, entail loss of an additional ∼0.3e
⃗
⃗
1
0
1
0
from the naphthalene ring, so that there is approximately a net
charge separation of 0.5e in the S1 state. In the case of 2-MN,
the S2 state is of intermediate polarity compared to S0 and S1,
whereas in 1-MN the S1 and S2 states are of similar polarity.
Figure 3 also indicates the directions of the electric transition
dipole moments between S0, S1, and S2 (B3LYP calculations at
the S0-optimized geometries). In 2-MN, the S0 → S1 and S0 →
⃗
⃗
S2 transition moment directions (M1←0 and M2←0) are within
25° of one another, with the stronger S2 transition lying closer
to the ring−malononitrile direction. In 1-MN, the S0 → S1
E
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Figure 3. Directions of the S0 state permanent electric dipole (μ0,
green) and transition dipole moments of the transitions S0 → S1
⃗
⃗
(M1←0, red) and S0 → S2 (M2←0, blue). Geometries and μ0 are from
B3LYP/6-31G(d,p) calculations of S0, and transition moments are
from TD-B3LYP calculations at the S0 geometry.
Figure 4. Absorption and emission spectra of 1-MN and 2-MN in five
solvents of varying polarity. Solvents are n-hexane, di-n-butylether,
methyl acetate, methanol, and acetonitrile.
transition is also aligned with the ring−malononitrile direction
(close to μ ), but in this case, the S → S transition is closer to
⃗
0
0
1
orthogonal to this direction, lying along the long naphthalene
axis.
of 11 solvents. Representative data are shown in Figure 4. The
spectra of r2-MN closely resemble those of 2-MN (Figure S3,
Supporting Information) and are therefore not included here.
As already seen in Figure 2, the absorption spectra of 2-MN
above 300 nm (34 000 cm−1) appear to consist of two
overlapping bands, whereas those of 1-MN appear as a single
peak. On the basis of the calculations of the previous section,
we interpret the absorption of 2-MN in terms of a slightly
structured S1 absorption centered near 27 000 cm−1 and a
stronger, less structured S2 absorption near 30 000 cm−1. Both
of these absorption bands are likely to contain contributions
from closely spaced 2-MN(a) and 2-MN(b) conformers. The
calculations of the previous section suggest that the properties
of the two conformers are similar, and in the present section,
we treat them as an effective average species. The lowest energy
absorption band of 1-MN is attributed to a single electronic
transition from the single conformer 1-MN(a). As illustrated in
Figure 4, the absorption bands of both compounds shift to the
red with increasing solvent polarity. The emission spectra of 1-
MN and 2-MN are similar in shape. In weakly polar solvents,
vibronic structure is observed, but this structure is largely
obscured in solvents of even moderate polarity. The emission
bands shift considerably more with solvent polarity than do the
absorption bands, and the shift is larger in 2-MN compared to
1-MN.
The molecular orbitals and main configurations comprising
the S1 and S2 states are summarized in Figure S2 and Table S5
(Supporting Information). As noted by Pfanstiel and Pratt
concerning 2-VN, the π orbitals of 1-MN (a or b) resemble
those of naphthalene much more than do those of 2-MN (a or
b) or r2-MN, which are more like those of a linear polyene
including the malononitrile group. In all cases, the S1 state is
comprised mainly of the HOMO → LUMO excitation, which is
not purely charge-transfer in character.
We finally consider the relaxed S1 torsional potentials
displayed in the bottom panels of Figure 1. In contrast to the
S0 potentials, the predictions of the B3LYP (black) and CAM-
B3LYP (red) calculations differ significantly. In 2-MN, the
primary minima in S1 still lie near τ = 0 and 180°, but the
barrier separating these minima is much larger in the CAM
calculations, >35 kJ/mol vs ∼5 kJ/mol. Whereas the B3LYP
calculations suggest that conformer interconversion might be
possible during the lifetime of S1, the CAM-B3LYP predictions
preclude such interconversion. In the case of 1-MN, the B3LYP
calculations predict a global minimum near 90°, and a state
significantly different in character from the Franck−Condon
(FC) states of 1-MN(a) or 1-MN(b) (see Table S4, Supporting
Information). Whereas all of the other S1-optimized geometries
have dipole moments and transition properties close to those at
their respective FC geometries, this ∼90° state is predicted to
have a dipole moment ∼4 D larger, and much weaker and
largely red-shifted emission, hallmarks of a twisted intra-
molecular charge transfer (TICT) state. The B3LYP torsional
profile in S1 is such that Franck−Condon excitation of 1-MN(a
or b) would lead to a rapid conformational change to this TICT
state. In contrast, no TICT state or important torsional
relaxation is predicted by the CAM calculations. As discussed
later, we believe the CAM calculations are more consistent with
experiment in regard to the shape of the S1 potential surface.
B. Steady-State Spectra and Solvatochromism. Steady-
state absorption and emission spectra were recorded in a series
In order to quantitatively analyze solvent-induced shifts and
other spectral parameters of the absorption of 2-MN and r2-
MN, it is necessary to effect an approximate separation of their
overlapping S1 and S2 absorption bands. As illustrated in Figure
5 for the case of 2-MN, this separation is performed by
assuming that the overlapping bands can be fit by a sum of two
log-normal line shape functions:47
2
⎧
⎨
⎩
h exp{−ln(2)[ln(1 + α)/γ] } for α > −1
L(ν) =
0
for α < −1
(2)
F
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The Journal of Physical Chemistry B
Article
Summaries of solvent-dependent frequencies of 2-MN and 1-
MN are provided in Tables 3 and 4, and analogous results for
r2-MN are presented in Table S6 (Supporting Information).
abs
pk
Peak frequencies ν and extinction coefficients εmx are those
directly measured from the spectra, i.e., without separating S1
and S2. In 2-MN and r2-MN, first moment frequencies in
(1)
abs
(2)
abs
absorption ⟨ν ⟩ and ⟨ν ⟩ are from the log-normal fits (eq 3),
whereas those for 1-MN and those reported for the emission of
all solutes are numerically integrated values.
To quantify and partially interpret the solvent dependence of
the solvatochromic shifts, we employ a simple polarizable point
dipole solute + dielectric continuum solvent model described
previously in refs 5 and 48. Within this model, absorption and
emission frequencies are related to the refractive indices nD and
relative permittivities εr of solvents according to
2
Figure 5. Decomposition of the lowest-frequency band of 2-MN
(black crosses) into approximate S1 (green) and S2 (red)
contributions. The heavier black curves at the bottom of the lower
panels are the residuals, differences between the observed and fit
relative absorbance. These residuals are shown on expanded scales in
the top panels ( 2%).
ν
= ν0
+ Aabs,emdc(nD ) + Cabs,em
abs,em
abs,em
2
× [dc(εr) − dc(nD )]
(4)
with
dc(x) = (x − 1)/{2(1 − c)x + (1 + 2c)}
(5)
2
2
The factors dc(nD ) and [dc(εr) − dc(nD )] characterize the
electronic and nuclear (total minus electronic) polarizabilities
of the solvent, and ν0, A, and C are related to solute properties,
as is the constant c, which is proportional to its electronic
polarizability.48,5 Here we choose c = 0.25 and use the factors dc
to correlate observed shifts. (Solvent properties, including these
where α = 2γ(ν − ν0)/Δ. The parameter γ defines the
asymmetry of the band, ν0 is the peak frequency, and Δ is a
width parameter related to the full width at half-maximum Γ by
Γ = Δ sinh(γ)/γ. In highly polar solvents like acetonitrile, all
eight parameters {hi, ν0,iΓi, γi for i = 1, 2} are reasonably
determined. In less polar solvents and especially in nonpolar
solvents like n-hexane, the structure present in the S1
absorption cannot be captured by a log-normal function, and
as a result, the parameters cannot all be determined with
confidence. To achieve consistent and sensible representations
for all solvents studied, we fixed the following parameters to the
average values observed in polar solvents: Γ1 = 4000 cm−1, γ1 =
0.38, and γ2 = 0.26. We believe that such fits afford relative
absorbances accurate to 10% and first-moment frequencies
dc, are provided in Table S7, Supporting Information.)
(I)
abs
Fits of ⟨ν ⟩ to eq 4 are shown in Figure 6, and all regression
equations are summarized in Table S8 (Supporting Informa-
tion). Note that toluene has been omitted from these fits
because the permittivity of this primarily quadrupolar solvent
poorly represents its total polarizability in relation to molecular
solvation.49,50 As illustrated by the spread of points in the top
panel of Figure 6, the solvent sensitivity of the S1 absorption
bands to solvent increases in the order 1-MN < 2-MN < r2-
MN. The difference between 1-MN and 2-MN is anticipated on
the basis of the larger S0 and S1 dipole moments calculated for
2-MN (Table 2), but the even larger solvent sensitivity of r2-
MN is contrary to the similar dipole moments of 2-MN and r2-
MN and the larger size of r2-MN. The much smaller solvent
2
⎡
⎤
⎛
⎞
3γ
Δ
2γ
⎢
⎥
⟨ν⟩ = ν
+
exp
− 1
⎜
⎟
⎠
pk
⎢
⎣
⎥
⎦
4 ln 2
⎝
(3)
accurate to 300 cm−1 (S1) or 50 cm−1 (S2).
a
Table 3. Solvent-Dependent Photophysical Properties of 2-MN (25 °C)
abs
(1)
(2)
#
solvent
n-hexane
ν
pk
εmx
⟨ν
⟩
M1←0
⟨ν ⟩
abs
M2←0
⟨νem
⟩
φem (10−3
)
⟨τem⟩
krad
M1→0
τa
τb
abs
1
30.53
30.47
30.36
30.37
30.46
30.49
30.22
30.30
30.48
30.43
29.88
27.0
27.3
27.2
26.1
27.1
26.1
27.1
24.5
25.7
26.7
22.5
28.33
28.29
28.20
28.06
28.07
28.02
27.78
27.76
27.93
27.88
27.72
3.2
3.2
3.3
3.3
3.3
3.3
3.5
3.4
3.3
3.4
3.3
31.26
31.22
31.12
31.08
31.16
31.20
30.95
31.02
31.17
31.13
30.76
5.2
5.2
5.2
5.2
5.3
5.3
5.4
5.2
5.3
5.4
5.0
24.05
24.04
23.95
23.25
22.86
21.99
21.87
20.97
21.09
20.90
22.36
4.8
5.3
5.3
7.0
8.3
7.3
7.0
9.4
7.7
7.8
5.4
128
135
98
3.75
3.90
5.42
6.08
6.97
5.33
4.24
5.67
5.00
5.05
4.49
2.1
2.2
2.5
2.8
3.2
3.0
2.6
3.2
3.1
3.2
2.4
257
248
184
146
147
136
164
166
154
154
154
21
22
18
39
39
2
n-heptane
3
cyclohexane
di-n-butyl ether
diethyl ether
methyl acetate
1-pentanol
4
115
119
136
164
166
154
154
121
5
6
7
8
propylene carbonate
acetonitrile
9
10
11
methanol
toluene
a
νpabks, ⟨ν(a1b)s⟩, ⟨νa(b2)s⟩, and ⟨νem⟩ are the frequencies of the peak of the S1+S2 absorption band and the first moment frequencies of the S1 and S2 bands
and the emission, respectively, all in units of 103 cm−1. εmx is the decadic molar absorption coefficient of the S1+S2 band at the maximum in units of
103 M−1 cm−1, the Mi↔j are the transition dipole moments of the i ↔ j transition in D, and φem is the emission quantum yield. ⟨τem⟩ is the average
decay time and τa and τb the component decay times observed in biexponential fits in units of ps, and krad is the (average) radiative rate in units of
107 s−1.
G
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Table 4. Solvent-Dependent Photophysical Properties of 1-MN (25 °C )
Article
a b
abs
(1)
#
solvent
n-hexane
ν
pk
εmx
⟨ν
⟩
abs
M1←0
⟨νem
⟩
φem (10−4
)
⟨τem⟩
krad
M1→0
⟨τν⟩
1
27.40
27.35
27.25
27.18
27.28
27.46
26.99
27.23
27.44
27.32
26.73
13.2
13.3
13.2
12.4
12.6
11.6
12.1
11.1
11.6
10.8
28.63
28.56
28.49
28.38
28.49
28.71
28.22
28.51
28.70
28.59
27.93
4.4
4.5
4.5
4.4
4.5
4.5
4.6
4.5
4.5
4.4
4.3
23.10
22.98
22.93
22.52
22.28
21.55
21.70
20.75
20.79
20.97
21.82
1.9
1.9
2.1
1.1
2.3
2.2
1.9
3.3
2.2
2.7
2.2
0.52
0.64
0.84
0.87
0.91
1.7
3.67
2.96
2.52
1.31
2.57
1.24
1.11
1.22
1.15
1.90
2.62
7.1
6.4
5.8
4.4
6.4
4.7
4.2
4.7
4.8
6.2
6.1
2
n-heptane
3
cyclohexane
di-n-butyl ether
diethyl ether
methyl acetate
1-pentanol
4
5
6
0.41
3.0
7
1.7
8
propylene carbonate
acetonitrile
2.7
1.7
9
1.9
0.13
6.1
10
methanol
1.41
0.84
11
toluene
11.4
a
b
Lifetime data are at 21 2 °C rather than 25 °C. νapbks and ⟨νa(b1)s⟩ are the peak and first moment frequencies of the S1 absorption band, and ⟨νem⟩ is
the first moment frequency of the emission band in units of 103 cm−1. εmx is the maximum decadic molar absorption coefficient of the S1 band in
units of 103 M−1 cm−1, the Mi↔j are the transition dipole moments of the i ↔ j transition in D, and φem is the emission quantum yield. ⟨τem⟩ is the
average time associated with the emission intensity decay and ⟨τν⟩ the time associated with the frequency shift of the emission spectrum in units of
ps. krad is the radiative rate in units of 108 s−1.
Figure 7. Stokes shifts of 2-MN (red) and 1-MN (blue) versus the
solvent nuclear polarizability factor dc(εr) − dc(nD2). Circles denote
aprotic and squares alcohol solvents. The asterisk marks 1-MN in 1-
pentanol.
are found between (νabs − νem) and the solvent nuclear
2
polarizability dc(εr) − dc(nD ); values of r2 are 0.99 (2-MN) and
Figure 6. Correlations of first-moment absorption band frequencies
0.90 (1-MN). The poorer fit in the case of 1-MN is due to the
according to eqs 4 and 5. Filled red symbols denote 2-MN, open green
1-pentanol datum (asterisk in Figure 7). As will be seen later, 1-
symbols r2-MN, and filled blue symbols 1-MN. Circles denote aprotic
MN lifetimes are short (∼1 ps), and because 1-pentanol
and squares alcohol solvents. Toluene data are omitted. These
solvates much more slowly (∼100 ps51) than the remaining
regressions are the best linear or bilinear regressions shown in boldface
solvents, the steady-state emission of 1-MN is not equilibrated,
in Table S8 (Supporting Information).
an assumption implicit in eq 6. The slopes of the fits in Figure 7
are 5900 cm−1 (2-MN) and 4900 cm−1 (1-MN). The value of
sensitivity of the S2 absorption bands of 2-MN and r2-MN
compared to S1 is as expected based on the smaller calculated
S2 dipole moments (Table 2), but the greater sensitivity of r2-
MN compared to 2-MN is again unexpected.
Using the solvatochromic model above, rough estimates of
the change in the (gas-phase) dipole moments between S1 and
Δμ derived from these slopes is highly dependent upon the
choice of cavity radius a. Plausible values of a range between 3.6
Å, the radius of a sphere equal to the van der Waals volume of
the solute (188.8 Å3 based on van der Waals increments52), and
this value augmented by the radius of a typical solvent molecule
(∼2.5 Å). These values, a = 3.6−5.1 Å, yield values of Δμ =
11.4−5.1 D for 2-MN and Δμ = 10.4−4.7 D for 1-MN. The
TD-B3LYP/6-31G(d,p) calculated values are Δμ = 6.7 D for 2-
MN and 6.0 D for 1-MN. Thus, the calculated values are
consistent with the observed solvatochromism of these
molecules.
S , Δμ = |μ − μ |, can be determined from the observed Stokes
⃗
⃗
0
1
0
shifts via48,5
− ν = (ν0 − ν0 ) +
2(Δμ)2
2
ν
[dc(εr) − dc(nD )]
abs
em
abs
em
a3
Katritzky et al.20 previously measured the solvatochromism
of 1-MN but in a smaller collection of solvents (only six
omitting the quadrupolar solvents toluene and dioxane). Using
a = 3.7 Å and neglecting any dependence of νabs on solvent,
these authors estimated Δμ = 3.2 D. A slightly different analysis
of these same data by Ravi et al.53 using the same value of a
(6)
where a is the radius of the effective spherical cavity
representing the solute. In Figure 7, plots of the Stokes shifts
of 2-MN and 1-MN are provided. (Corresponding data on r2-
MN could not be obtained due to interference from impurity
emission.) As illustrated in Figure 7, good linear correlations
H
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Article
later produced a slightly larger value Δμ = 3.9 D. Both values
fall significantly below the already large range of values Δμ =
10.4−4.7 D derived here for 1-MN. The difference is partly due
to assumptions made in the analysis, but it is also due to
differences in the spectroscopic data. Obtaining reliable
emission data for 1-MN is difficult because of its low emission
quantum yield, and the differences may reflect the effect of
impurities in the emission data.54
polarity. The change is in all cases less than 10% over the full
range of dc(εr). In the case of 1-MN, the change is negligible.
We have also determined emission transition moments
(M1→0) of 2-MN and 1-MN using58,55
3
⎡
⎢
⎤
krad
1
3hc
|M1→0|2 =
⎥
⎦
3
4
3
⎣
nD f(nD) 64π
ν
̃
(9)
em
C. Transition Dipole Moments. Quantities related to the
strength of the S0 ↔ S1 and S0 → S2 transitions of these
molecules are provided in Tables 3 and 4 and Table S6
(Supporting Information). Maximal absorption coefficients εmx
for the S1/S2 composite bands of 2-MN and r2-MN average 2.6
× 104 and 2.2 × 104 M−1 cm−1, respectively, whereas that of the
S1 band of 1-MN is roughly half of these values, 1.2 × 104 M−1
cm−1. Previously reported values of εmx are 2.81 × 104 M−1
cm−1 (2-MN) and 1.42 × 104 M−1 cm−1 (1-MN) in
chloroform19 and 1.4× 104 M−1 cm−1 for 1-MN in
tetrahydrofuran.20
In this expression, krad is the radiative rate, calculated from the
emission quantum yield φem and average emission decay time
⟨τem⟩ via krad = φem/⟨τem⟩, and νem
̃
3 = {∫ F(ν) dν/∫ F(ν)ν−3 dν}
with F(ν) being the emission spectrum (ν in s−1). Values of
M1→0 for 2-MN and 1-MN are shown in Figure 8 (open
symbols). The large error bars here reflect the difficulty
associated with accurately measuring quantum yields in the
range 10−4−10−3 as well as uncertainties in the lifetime
measurements. In the case of 1-MN, the emission transition
moments average 5.5 1.0 D, compared to 4.5 0.1 D in
absorption. Typically, emission transition moments are slightly
smaller than absorption moments for well-isolated transitions.
We suspect that the apparently larger average value of M1→0
here as well as the scatter may reflect the presence of long-lived
impurities. In the case of 2-MN, the average values are more in
For quantitative comparison to calculated transition
strengths, we compute transition dipole moments via55−57
⎡
⎤
ε(ν)
ν
1
2303 3hc
|MI←0|2 =
dν
⎢
⎥
⎦
∫
3
n f(n )
N
A
bandI
⎣ 8π
(7)
(8)
D
D
keeping with expectations: M1→0 = 2.8 0.4 D versus M1←0
=
3.3 0.1 D. There is, however, an unexpectedly large variation
of M1→0 with solvent, 40% over the range of solvents
considered. The time-resolved data discussed in the next
section indicate that the two conformers of 2-MN have
markedly different lifetimes and their relative populations vary
substantially with solvent polarity. It may be that this variation
in conformer populations is somehow responsible for the
variations in M1→0 observed here.
D. Time-Resolved Emission. As indicated by the ∼30-fold
difference in quantum yields of 2-MN and 1-MN (Tables 3 and
4), the lifetimes of these molecules differ substantially. For this
reason, different techniques were used to measure time-
resolved emission.
with
f(x) =
9x2
(2x2 + 1)2
where h, c, and NA are Planck’s constant, the speed of light, and
Avogadro’s number. For separating the S1 and S2 absorption
bands, we used the log-normal fits described in the previous
section. Solvent data for 2-MN and r2-MN are plotted as
functions of the total solvent polarity measure dc(εr) in Figure
8. Experimental values of M1←0 average 3.3, 3.3, and 4.5 D in 2-
In the case of 2-MN, lifetimes are on the order of 100 ps,
sufficiently slow that time-correlated single photon counting
(TCSPC) is able to faithfully record the emission dynamics. In
most solvents studied, the decay kinetics observed using
TCSPC were independent of emission wavelength. The main
exception was 2-MN in 1-pentanol, where multiexponential
wavelength-dependent kinetics were observed. We attribute this
behavior to dynamic Stokes shifting of the emission due to
solvation dynamics slow enough to be registered by the 25 ps
time resolution of TCSPC. (The average solvation time in 1-
pentanol is ∼100 ps.51) A ∼5 ps lifetime component was
detected on the blue side of the emission spectrum in
methanol, which we also ascribe to solvation dynamics. In
other solvents, we observed either single-exponential emission
decays with time constants in the 100−300 ps range or
biexponential decays with one component in this same range
and another component between 20 and 50 ps. The latter
behavior is observed only in the least polar solvents, the alkanes
and ethers (#1−5). The amplitude of the short component
decreases with increasing solvent polarity, from ∼50% in n-
hexane to ∼25% in diethyl ether.
Figure 8. Absorption (filled symbols) and emission (open symbols)
transition dipole moments plotted as functions of the total solvent
polarizability factor dc(εr). Circles denote aprotic solvents, squares
alcohols, and the hexagon toluene.
MN, r2-MN, and 1-MN, respectively. These values are in
excellent agreement with gas-phase TD-B3LYP/6-31G(d,p)
calculations, which predict 3.1, 3.1, and 4.8 D (Table 2).
Average values of the S0 → S2 transition moments are 5.2 D in
2-MN and 4.7 D in r2-MN. These quantities are overestimated
by the TDDFT calculations, which predict 6.2 and 5.6 D,
respectively. The absorption transition moments of 2-MN and
r2-MN appear to increase slightly with increasing solvent
We attribute the biexponential decay kinetics of 2-MN in
low-polarity solvents to the coexistence of two conformers (a
and b) with distinct lifetimes in these solvents. Evidence for this
hypothesis is provided in Figure 9, where we show emission of
2-MN in n-hexane monitored at 420 nm as a function of
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Figure 9. Excitation-wavelength-dependent emission decays of 2-MN
in n-hexane. The left panel shows the absorption spectrum with
excitation wavelengths indicated. The right panel shows emission
decays collected at 420 nm. Decays were collected to 104 counts in the
peak channel except for the 400 nm decay, which was collected to
5000 counts and rescaled for comparison. “IRF” labels the instrument
response function.
Figure 10. KGE spectra of 1-MN in acetonitrile showing (a) “raw”
data (prior to deconvolution) and (b) spectra after thinning,
conversion to frequency, and multiexponential fitting to remove
instrumental broadening (points). The solid curves in part b are log-
normal fits to the data. The dashed curve is the 10 ps spectrum
renormalized to highlight the spectral shift.
excitation wavelength. The left panel shows the red edge of the
absorption spectrum with the excitation wavelengths indicated.
Between 390 and 400 nm, a subtle change in the slope of the
absorption edge suggests the possible presence of a weaker
transition underlying the primary absorption. Excitation at 390
nm (used for most TCSPC experiments) or redder wavelengths
greatly enhances the amplitude of the short (22 ps) component
relative to the long (280 ps) component. The latter dominates
the emission (70−80%) when excitation is at wavelengths
shorter than 385 nm. On the basis of the TDDFT calculations
(Figure 2), we tentatively assign the short lifetime component
to emission from the b conformer and the long component to
emission from the a conformer. Assuming approximately equal
oscillator strengths, as suggested by calculation, these data
indicate ∼25% of the 2-MN adopts the b conformer in n-
hexane at 25 °C. This relative population is consistent with
TDDFT energy calculations in cyclohexane (Table S4,
Supporting Information). In solvents more polar than the
alkanes, the absorption band shifts to the red such that 390 nm
excitation is no longer on the edge of the absorption where it
emphasizes b emission. Nevertheless, we would expect to be
able to observe an ∼25% amplitude fast component in the
TCSPC decays if one were present. The fact that we fail to
observe a shorter component in solvents more polar than
diethyl ether suggests that the a conformer becomes
increasingly favored with increasing solvent polarity, such that
in methyl acetate and more polar solvents it contributes
negligibly to the emission of 2-MN. Such a change with solvent
is also supported by the DFT calculations in Table S4
(Supporting Information).
The emission lifetimes of 1-MN are all considerably shorter
than those of 2-MN, and Kerr-gated emission (KGE) was
therefore used to monitor emission dynamics. Representative
spectra of 1-MN in acetonitrile are shown in Figure 10 to
illustrate the analyses employed. Figure 10a shows “raw”
spectra after correction for temporal dispersion and detection
sensitivity. The sharp features at times comparable to the
instrument resolution (350−400 fs fwhm) are solvent Raman
bands. The raw spectra exhibit both a rapid intensity decay and
a frequency shift over the course of a few picoseconds. To
partially remove the effects of instrumental broadening from
the results, these raw spectra were thinned and fit to a 4-
exponential model, resulting in the deconvoluted spectra shown
in panel b. The latter spectra (points) were fit to log-normal
functions (time-dependent versions of eq 5) excluding the
Raman region. These fits are shown as the smooth curves in
Figure 10b; they are used to provide simple metrics of the
spectral evolution. Two spectral characteristics obtained in this
manner, the integral intensities and peak frequencies of 1-MN
in three solvents, are shown in Figure 11. In all three solvents,
Figure 11. Characteristics of the spectral evolution of 1-MN emission
in acetonitrile, propylene carbonate (PC), and methanol. The left
panel shows the decay of integral emission intensity and the right
panel the peak frequency.
the intensity decays are significantly nonexponential. In highly
polar solvents such as those illustrated, peak frequencies shift by
1500−2000 cm−1. This shift is also nonexponential in time and
occurs on a time scale comparable to the intensity decay. The
widths of the spectra (not shown) decrease from ∼5000 to
∼4000 cm−1 over this same time range. We fit both the
intensity and frequency decays to biexponential functions of
time to extract the integral times listed in Table 4. No obvious
correlation exists between the times associated with the loss of
emission intensity ⟨τem⟩ and the peak shift ⟨τν⟩. Rather, in four
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of the five solvents where comparisons are possible, the peak
shift times are close to the Stokes shift times measured with the
coumarin 153 (C153) probe.51 These observations suggest that
the spectral shift is due to polar solvation and is not closely tied
to the process leading to loss of emission. The exception is 1-
pentanol, where the value of ⟨τν⟩ obtained here (∼3 ps)
appears unrelated to the time associated with C153 solvation
(∼100 ps). This discrepancy is likely the result of the short
lifetime of 1-MN (2 ps) obscuring the dominant slow solvation
time constants (τ > 20 ps51) present in 1-pentanol.
The lifetimes of r2-MN fall in the range 5−20 ps,
intermediate between those of 2-MN and 1-MN. Due to the
very limited amount of r2-MN available, we only measured its
dynamics using TCSPC, which requires much less sample than
the KGE experiment. Given the fact that the lifetimes of r2-MN
are smaller than the 25 ps instrument response function, only
an overall time constant associated with the emission decay
could be determined. These lifetimes, which are averages over
decays measured at four to five emission wavelengths, are
collected in Table S6 (Supporting Information). It should be
noted that the impurities which preclude accurate determi-
nation of the steady-state emission of r2-MN do not influence
these lifetime data. In the TCSPC data, the impurities are
manifest as a 1−2% component with a lifetime of ∼3 ns.
Although the difference in the r2-MN and impurity lifetimes
means that impurities dominate the steady-state spectra, the
opposite is true of the TCPC data, where the r2-MN
component constitutes 98% or more of the decay amplitude.
We finally take an empirical look at the solvent dependence
of the lifetimes observed for all three solutes. We consider three
quantities potentially relevant to the emission deactivation
process: the total solvent polarizability function, dc(εr), the
solvent viscosity η, and the average emission lifetime of the
benzylidene malononitriles DMN and JDMN, τBzMN. (Because
prior work showed a high degree of correlation between the
lifetimes of DMN and JDMN,5 we use the average value here to
represent this class of molecule.) The function dc(εr) is
expected to be relevant to the decay kinetics if some polarity-
dependent barrier controls the deactivation. The viscosity, a
coarse measure of solvent friction, is expected to be relevant if
deactivation is controlled by some large amplitude motion
through solvent. Finally, if the same mechanism of relaxation
controls the fluorescence of the naphthalene and benzylidene
malononitriles, one might expect τBzMN to correlate the
observed solvent dependence.
Figure 12 displays the lifetimes of 2-MN and 1-MN as
functions of these three quantities. Consider first 2-MN. The
decay time τa, attributed to the dominant a conformer of 2-MN,
is approximately constant for most solvents except the alkanes,
where it is significantly larger. As indicated by the small values
of the coefficients of determination (“r2”) in Figure 12 (top
values), none of the three quantities examined here explains
most of the variance in τa with solvent. In the case of τb,
attributed to 2-MN(b), decay times are most strongly
correlated to dc(εr), they exhibit a slightly weaker correlation
to τBzMN, and they appear unrelated to viscosity. The same
pattern is observed in the case of the integral decay times of 1-
MN. The strongest correlation is found with dc(εr), and the
weakest correlation with η. As shown by the dashed line of
equality (“1:1”) in Figure 12, in 7 out of the 11 solvents
studied, the intensity decay times of 1-MN are nearly equal to
τBzMN, but in the 4 other solvents, the 1-MN lifetimes are
significantly shorter than those of the benzylidene malononi-
Figure 12. Decay times plotted as functions of the total polarizability
factor dc(εr), the solvent viscosity η, and the average decay time
estimated for benzylidene malononitriles τBzCN (see text). Circles
denote aprotic solvents, squares alcohols, and the hexagon toluene.
Inset numbers are the coefficients of determination of linear fits to the
data. In the case of 2-MN, the top values are for τa (filled points) and
bottom values τb (open symbols) regressions. The two small points in
the top panel are 2-MN (τa) data for the additional solvents CH2Cl2
(τa ∼100 ps) and ethyl acetate. These points are not included in the
correlations.
triles. Finally, in the case of r2-MN, the decay times correlate
moderately well with all three of these quantities (r2 = 0.5−
0.6).
4. SUMMARY AND CONCLUSIONS
We conclude by discussing the nature of the emitting states and
origins of the short lifetimes of these naphthalene malononi-
triles in light of the results collected here as well as those
available in the literature. The two studies to previously
investigate emission in these molecules described the emission
of 1-MN (and several other ArCHC(CN)2 systems) as
reflecting the presence two distinct excited states, a locally
excited (LE) state and an ICT state having much greater
intramolecular charge transfer character.20,21 The time-resolved
spectra of 1-MN and the other solutes recorded here indicate
that excitation into S1 leads to emission from only a single
excited state. Comparisons between absorption and emission
transition dipole moments (Figure 8) indicate that the
absorbing and emitting states do not differ markedly. Although
the solvent dependence of the emission moments of 2-MN is
puzzling (and those of 1-MN scattered), these data are
inconsistent with the idea that emission arises from a TICT
state, as was originally suggested.20
Calculations predict the ground-state dipole moments of all
three solutes to be large, in the range 6−8 D. The solvent
dependence of the Stokes shifts observed here (Figure 7)
suggests that their S1 state dipole moments are substantially
higher, Δμ > 5 D. These estimates are consistent with values of
Δμ from TDDFT calculations, which all fall into the range 5−8
D (Table 2). Thus, the S1 state can be reasonably described as
an ICT state, as previously suggested. However, calculations
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indicate that the charge transfer is only partialthe
naphthalene ring is already positive by ∼0.2e in S0, and this
charge only increases to ∼0.5e in S1.
order to the lifetimes: 2-MN(a) (6°) < 2-MN(b) (25°) < r2-
MN (34°) < 1-MN(a) (43°). In some cases, relief of the steric
interference reflected in these τ angles also results in some
nonplanarity of the double bond (ω ≠ 0) in the ground-state.
MP2 calculations indicate ω = 0 for both conformers of 2-MN,
whereas in r2-MN and 1-MN this angle has been measured to
be ∼1 and ∼6°, respectively.27,44 Thus, there is a correlation
between twisting of the τ angle, which is perhaps accompanied
by some slight nonplanarity of the double bond in the ground
state, and the rate of the excited-state reaction. A similar
correlation between the ground-state τ angle and barriers to
excited-state isomerization in styrene derivatives was previously
noted by Lewis and Zuo.59
With the exception of 2-MN(a), these naphthalene
malononitrile reactions display a solvent dependence not
unlike those previously found for the benzylidene malononi-
triles DMN and JDMN.5 For the collection of 11 solvents
considered here, we find that the lifetimes of 2-MN(b), r2-MN,
and 1-MN all show significant correlations (r2 > 0.5) with the
average lifetimes of DMN and JDMN (Figure 12), which are
themselves strongly correlated. It therefore seems reasonable to
conclude that the origins of the solvent dependence are similar.
Despite the poor correlations with viscosity in Figure 12, we
expect solvent friction to be one important factor controlling
the lifetimes and emission yields of these molecules, as it is in
the benzylidene malononitriles. Evidence comes from the
ability to greatly enhance the emission yields upon environ-
mental rigidification by either temperature60 or pressure21
variation. The weak correlation with solvent viscosity observed
here is likely a manifestation of the fact that solvent viscosity is
a poor indicator of the friction operative on these reactions, at
least when one considers an assortment of different solvents as
was done here. In addition, solvent polarity, which provides
some of the best correlations observed here (Figure 12), also
appears to play a role in determining the solvent dependence of
both classes of fluorophores, making them imperfect as simple
reporters of fluidity. Disentangling frictional and energetic
effects on these reactions is a challenging task, one which must
await further investigation.
2-MN and 1-MN can exist as two ground-state conformers
which differ in the torsional angle τ between the naphthalene
ring and the vinylmalononitrile units. In both cases, the more
extended a form (Scheme 2) is preferred. Calculations suggest
that in 2-MN the conformers are sufficiently close in energy
that both forms would be expected to be present in room-
temperature samples, whereas in 1-MN the energy difference is
sufficiently great that the a conformer is expected to dominate
(Table 1). In both 2-MN and 1-MN, the electronic properties
of the two forms are calculated to be sufficiently similar (Figure
2, Table 2) that it would be difficult to distinguish them on the
basis of their absorption or emission spectra. Time-resolved
emission of 2-MN does, however, reveal the presence of two
emitting species with distinct lifetimes in low polarity solvents.
We interpret this behavior in terms of a ground-state
equilibrium in which the slightly more polar a form begins to
dominate as solvent polarity increases.
Like the benzylidene malononitriles, the lifetimes of all three
of these naphthalene malononitriles are short and indicative of
some fast excited-state process leading to a nonfluorescent
state. In 2-MN and 1-MN, one cannot rule out the possibility
that this process involves a TICT mechanism, i.e., twisting
about τ to a virtually dark state at τ ∼ 90°. B3LYP calculations
suggest that such a state might represent a minimum on the S1
potential energy surface (PES) of 1-MN. In contrast, CAM-
B3LYP calculations (Figure 1) do not predict a TICT state,
even in the presence of polar solvent (Table S4, Supporting
Information) As discussed previously in regard to calculations
on DMN,6 it is likely that the presence of this minimum is an
artifact. The fact that r2-MN, which cannot undergo a TICT
process, behaves similarly to 2-MN and 1-MN also argues
against a TICT mechanism. Instead, it seems more likely that
the fluorescence deactivation mechanism operative here is the
same as that in DMN and JDMN“isomerization” about the
double bond which leads to an intersection between S1 and S0
near ω = 90°. We did not pursue multiconfigurational
calculations of the sort needed to adequately explore the S1
potential energy surfaces of these molecules with respect to this
coordinate. However, preliminary spin-flip TDDFT calcula-
tions35−37 did demonstrate the presence of such a conical
intersection at large ω.
Assuming this common mechanism, the question remains
why significant differences exist among the lifetimes of these
molecules: 2-MN(a) (∼100 ps) > 2-MN(b) (∼30 ps) > r2-MN
(∼10 ps) > 1-MN (∼1 ps). No definitive conclusions are
possible at this point, but a few observations can be made. First,
if one views these differences as resulting from changes to
energy barriers along the reaction coordinate, factors of 10
equate to barrier height variations of ∼6 kJ/mol (∼2 kBT at
room temperature). Thus, one might reasonably ascribe these
variations in lifetime to rather subtle differences in their S1 PESs
(differences which would be hard to capture with current
electronic structure methods). Second, there are structural
differences among the three solutes with respect to the torsion
angle τ, which might give rise to such differences in the S1 PES.
Although we believe that the excited-state reaction is directly
related to twisting about ω rather than τ, it is reasonable to
assume that these two angles are coupled. If the MP2
predictions are taken as the best estimators of ground-state
geometries, one finds the torsion angle τ to vary in inverse
ASSOCIATED CONTENT
■
S
* Supporting Information
Comparisons of ground-state geometries predicted by B3LYP
and MP2 calculations to X-ray data, tabulations of additional
DFT calculations, descriptions of the frontier orbitals of all
solutes, solvent-dependent photophysical properties of r2-MN,
solvent properties, results of solvatochromic regressions, and
comparisons of the absorption spectra of 2-MN and r2-MN.
This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank Johannes Belmar for help with synthesizing
the solutes, Andrew Sirjoosingh and Lasse Jensen for assistance
in performing the spin flip calculations, and Dylan Donnovan
for helpful discussions during the early stages of this work. This
research was supported by the Division of Chemical Sciences,
Geosciences, and Biosciences, Office of Basic Energy Sciences
L
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of the U.S. Department of Energy through grant DE-FG02-
12ER16363.
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N
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