Dynamics of Isolated 1,8-Naphthalimide and N-Methyl-1,8-naphthalimide: An Experimental and Computational Study

Article abstract of DOI:10.1021/acs.jpca.6b01226

In this work we investigate the excited-state structure and dynamics of the two molecules 1,8-naphthalimide (NI) and N-methyl-1,8-naphthalimide (Me-NI) in the gas phase by picosecond time- and frequency-resolved multiphoton ionization spectroscopy. The energies of several electronically excited singlet and triplet states and the S₁ vibrational wavenumbers were calculated. Nonadiabatic dynamics simulations support the analysis of the radiationless deactivation processes. The origin of the S₁ ← S₀ (ππ?) transition was found at 30 082 cm^-1 for NI and at 29 920 cm^-1 for Me-NI. Furthermore, a couple of low-lying vibrational bands were resolved in the spectra of both molecules. In the time-resolved scans a biexponential decay was apparent for both Me-NI and NI. The fast time constant is in the range of 10-20 ps, whereas the second one is in the nanosecond range. In accordance with the dynamics simulations, intersystem crossing to the fourth triplet state S₁ (ππ?) → T₄ (nπ?) is the main deactivation process for Me-NI due to a large spin-orbit coupling between these states. Only for significant vibrational excitation internal conversion via a conical intersection becomes a relevant deactivation pathway. (Figure Presented).

Full text of DOI:10.1021/acs.jpca.6b01226

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Article
The Dynamics of Isolated 1,8-Naphthalimide and N-Methyl-1,8-
Naphthalimide: An Experimental and Computational Study
Thiemo Gerbich, Hans-Christian Schmitt, Ingo Fischer, Roland Mitric, and Jens Petersen
J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b01226 • Publication Date (Web): 13 Mar 2016
Downloaded from http://pubs.acs.org on March 15, 2016
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Revised version, submitted J. Phys. Chem. A, March 10 , 2016
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The Dynamics of Isolated 1,8ꢀNaphthalimide and Nꢀ
Methylꢀ1,8ꢀNaphthalimide: An Experimental and
Computational Study
Thiemo Gerbich, HansꢀChristian Schmitt, Ingo Fischer,* Roland Mitrić and Jens Petersen*
Institute of Physical and Theoretical Chemistry, University of Würzburg, Am Hubland, Dꢀ97074
Würzburg, Germany
AUTHOR INFORMATION
Corresponding Authors
ingo.fischer@uniꢀwuerzburg.de, phone: +49ꢀ931/31ꢀ86362
jens.petersen@uniꢀwuerzburg.de, phone: +49ꢀ931/31ꢀ88832
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ABSTRACT
In this work we investigate the excited state structure and dynamics of the two molecules 1,8ꢀ
naphthalimide (NI) and Nꢀmethylꢀ1,8ꢀnaphthalimide (MeꢀNI) in the gas phase by picosecond
timeꢀ and frequencyꢀresolved multiphoton ionization spectroscopy. The energies of several
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electronically excited singlet and triplet states and the S vibrational wavenumbers were
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calculated. Nonadiabatic dynamics simulations support the analysis of the radiationless
*
ꢀ1
deactivation processes. The origin of the S ← S (ππ ) transition was found at 30 082 cm for NI
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ꢀ1
and at 29 920 cm for MeꢀNI. Furthermore, a couple of lowꢀlying vibrational bands were
resolved in the spectra of both molecules. In the timeꢀresolved scans a biꢀexponential decay was
apparent for both MeꢀNI and NI. The fast time constant is in the range of 10 – 20 ps, while the
second one is in the nanosecond range. In accordance with the dynamics simulations, intersystem
*
*
crossing to the fourth triplet state S (ππ ) → T (nπ ) is the main deactivation process for MeꢀNI
1
4
due to a large spinꢀorbit coupling between these states. Only for significant excitation internal
conversion via a conical intersection becomes a relevant deactivation pathway.
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INTRODUCTION
The excitedꢀstate dynamics of several photoexcited heterocycles seems to be characterized by a
competition between intersystem crossing (ISC) and internal conversion (IC) occurring on a
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1,2
picosecond time scale, as shown in previous studies on 9ꢀfluorenone and 1,8ꢀnaphthalic
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anhydride (NDCA). Furthermore, in isolated NDCA the deactivation rate shows a strong
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dependence of the vibrational excess energy , which can be explained by a barrier separating the
energy minimum of the lowest excited singlet state from a conical intersection (CI) to the ground
state. Since the details of the electronic structure seem to influence the competition between IC
and ISC, we extended our investigation to isolated 1,8ꢀnaphthalimide (NI) and Nꢀmethylꢀ1,8ꢀ
naphthalimide (MeꢀNI), depicted in Scheme 1. In both molecules the electronic structure is
altered by replacing one oxygen atom of NDCA by a nitrogen atom.
Scheme 1. Structure of a) NI (C Symmetry) and b) MeꢀNI (C Symmetry). The coordinate axes
2
v
s
used for determining the symmetry are shown in grey. The primarily planar molecules are
orientated in the yꢀz plane.
A second motivation for understanding the dynamics of radiationless deactivation after
photoexcitation in these molecules is their relevance to energy and electron transfer in organic
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materials. Heterocycles like 1,8ꢀnaphthalimide (NI) and its derivatives show interesting
optoelectronic properties and are used in a variety of applications, such as dyes, liquid crystal
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displays and in organic light emitting diodes.
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Until now no gas phase experiments of isolated naphthalimides have been reported. Nevertheless,
these molecules have been of interest in a couple of previous studies. UV/Vis absorption spectra
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of several substituted NIs in solution have been reported. The photophysical properties of NI and
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MeꢀNI in solution were examined by Wintgens et al. For both molecules high quantum yields
for intersystem crossing and short fluorescence lifetimes of τ < 130 ps were found in nonpolar
solvents. However, the ISC quantum yield decreases for more polar solvents, and fluorescence
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lifetimes in the nanosecond range were observed. Computational studies of NI investigated the
influence of solvent on absorption and fluorescence spectra and elucidated charge transfer
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processes of NI in different solvents.
The photophysical properties and charge transfer
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processes in more complex derivatives of NI have been addressed in further studies as well.
To gain insight into the photophysical changes that occur upon replacing O by N, we investigated
the excited state dynamics of isolated NI and MeꢀNI in a free jet by timeꢀresolved
photoionization and compare it to previous studies on NDCA. MeꢀNI was studied in addition to
NI, because (a) due to its higher vapour pressure signals are larger and (b) neither tautomers nor
hydrogenꢀbonded clusters that potentially perturb the spectra are expected to be present in the jet.
The experiments are supported by timeꢀdependent density functional theory (TDDFT)
calculations to provide energies of lowꢀlying electronically excited singlet and triplet states.
Additionally, to study the coupled electronꢀnuclear dynamics and to interpret the deactivation
processes, surface hopping simulations were carried out.
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EXPERIMENTAL SECTION
MeꢀNI (m = 211 amu) was synthesized in one step from 1,8ꢀnaphthalic anhydride (NDCA) and
methylamine (MeNH , 25 % aqueous solution) with a yield of nearly 100 %. The synthesis
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followed the route of Nicholson et al. In contrast, NI and NDCA were obtained commercially
from SigmaꢀAldrich and used without further purification. While NI has a high melting point of
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approximately 300 °C, for MeꢀNI a lower value of about 205 °C has been reported. The gas
phase experiments were carried out by heating the samples in a homeꢀbuild oven attached to a
pulsed solenoid valve. The molecules were heated up to 200 ꢀ 220 °C in an argon atmosphere
(
p0(Ar) = 1.4 – 2.0 atm) and were expanded through a 1.0 mm diameter nozzle into a
differentially pumped vacuum chamber. The detailed experimental setup has been described in a
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previous publication.
For excitation and ionization of the supersonically cooled molecules picosecond pulses provided
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from a 10 Hz laser system introduced in ref. were used. The system operates at a fundamental
wavelength of 1053 nm delivered by a solid state Nd:YLF (neodymiumꢀdoped yttrium lithium
fluoride) laser. In an optical parametric generator (OPG) 85 % of the third harmonic (7ꢀ8 mJ) was
employed to generate tunable pump pulses with wavelengths in the range of 280 to 350 nm and
pulse energies of 30 – 100 µJ. The residual part of the third harmonic with 200 – 300 µJ was sent
over a motorized linear translation stage and subsequently used as the probe pulse. The
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bandwidth of the pulses was on the order of 20 cm and the instrument response function (IRF)
was found to be around 5 ps. The pump and probe pulses were overlapped on a dichroic mirror
and slightly focused into the molecular jet. After acceleration of the molecular ions in a TOF
mass spectrometer the ion signal was recorded on a multichannel plate detector. The data was
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averaged for 75 – 100 shots per point for every scan and up to 12 scans were averaged to
optimize the SNR.
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COMPUTATIONAL SECTION
The structures of MeꢀNI and NI have been optimized in the framework of density functional
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theory (DFT) employing the CAMꢀB3LYP functional
combined with the def2ꢀTZVP basis
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set, and the energy minima have been confirmed by calculating the vibrational frequencies.
Subsequently, the vertical electronic absorption spectra have been calculated using timeꢀ
dependent density functional theory (TDDFT) employing the same functional and basis set. In
order to determine the origin of the vibronic spectra between the electronic ground state and the
lowest electronically excited state, the structures have been also optimized in the excited state,
and the vibrational frequencies have been determined. All DFT calculations have been performed
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using the Gaussian 09 program package.
The coupling between singlet and triplet states has been investigated by calculating spinꢀorbit
matrix elements for selected DFTꢀoptimized geometries of MeꢀNI. The spinꢀorbit coupling has
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been calculated using the Molpro program package,
employing the multiꢀreference
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configuration interaction method (MRꢀCI)
with reference configurations taken from a
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complete active space selfꢀconsistent field (CASSCF) calculation. For this purpose, the 6ꢀ31G
basis set has been employed, and the active space consisted of 12 electrons in 7 orbitals, while for
the MRCI all single and double excitations from the active orbitals to higher virtual orbitals were
taken into account.
The nonadiabatic relaxation from the first excited singlet state to the electronic ground state of
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MeꢀNI was investigated by employing Tully’s surface hopping method . To this end, a total
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number of 100 initial nuclear coordinates and momenta were sampled from 30 ps long
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trajectories employing the semiempirical OM3 method in the electronic ground state at 30 K or
300 K, respectively. Subsequently the nonadiabatic dynamics was performed using the OM3
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method combined with MRꢀCI
in the framework of the MNDO program, . The active
space consisted of 14 electrons in 11 orbitals, and all single and double excitations out of the
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ground state SCF and the HOMOꢀLUMO excited configuration were taken into account.
The
nuclear motion was simulated by numerically solving Newton’s equations of motion with a time
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step of 0.2 fs using the velocity Verlet algorithm. The electronic state coefficients needed for
determining the hopping probabilities between electronic states were calculated along each
trajectory by numerical integration of the timeꢀdependent Schrödinger equation including the
nonadiabatic coupling.
Similarly, also nonadiabatic dynamics simulations within the four lowest triplet states were
performed employing the same set of 100 initial conditions as for the singlet state dynamics. As
the initial state, the T state was chosen, which lies energetically close to the first excited singlet
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state. These simulations served to determine the order of magnitude of the nonadiabatic couplings
within the triplet state manifold. For this purpose, a propagation time of 100 fs, during which
population transfer from the T to all of the lowerꢀlying states occurred, was sufficient to obtain
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average values for the nonadiabatic coupling elements (see below).
In addition to the nonadiabatic dynamics in the singlet states, the electronic state population
dynamics in the full manifold of two singlet and four triplet states has been simulated on a model
level. For this purpose, the timeꢀdependent Schrödinger equation
ꢉꢊ
ꢀ
ℏꢁ
ꢂ
= ꢄ ꢁ − ∑ ꢅꢀℏꢆ − ꢈ ꢋꢁ
ꢇ
(1)
ꢃ
ꢃ ꢃ
ꢇ
ꢃꢇ
ꢃꢇ
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including the electronic energies ꢄ , the nonadiabatic couplings ꢆ and the spinꢀorbit couplings
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has been integrated numerically over a time duration of 50 ps. The energies and spinꢀorbit
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couplings have been initially assigned the values obtained for the S state minimum structure.
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The nonadiabatic couplings have been averaged from surfaceꢀhopping simulations in the singlet
and triplet manifolds as described above. Additionally, in order to mimic the fluctuations of
energies and couplings due to nuclear motion, the initial values have been modified in each time
step by a random term, which for the energies could assume a value up to 0.05 eV, while for the
couplings a variation up to the size of the initial coupling was made possible. In this way, timeꢀ
dependent populations of all involved singlet and triplet states have been obtained.
RESULTS
The vertical excitation energies of the lowest singlet and triplet states of NI and MeꢀNI calculated
by TDDFT are summarized in Table 1. For comparison the excitation energies of NDCA from
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ref. are listed as well. While the C symmetric NI is planar (like NDCA), MeꢀNI shows only C
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v
s
symmetry due to the nonꢀplanarity of the methyl group. Nevertheless the excitation energies to
the first excited states are very similar for NI and MeꢀNI. For both molecules the lowest excited
singlet state is accessible upon nearꢀUV excitation and bears a high oscillator strength. The
second excited singlet state is located 0.23 eV above the S for MeꢀNI and 0.24 eV above for NI.
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The calculated adiabatic energy differences between the vibronic origins of S and S even
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amount to 0.42 eV in both cases. Therefore, electronic coupling between these states can most
likely be neglected.
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*
*
Close to or below the (ππ ) S state several triplet states are located, at least one of them with nπ
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character. According to the ElꢀSayed rules significant spin orbit coupling and thus considerable
*
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intersystem crossing rates can be expected between states of nπ and ππ character.
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Table 1: Vertical excitation energies of the lowꢀlying electronic states of isolated MeꢀNI, NI and
NDCA at the S minimum geometry, determined by TDDFT calculations.
0
Vertical excitation energies /eV
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MeꢀNI
NI
NDCA (Ref. )
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1
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1
1
*
T1 A‘ (ππ ) 2.32 T1 A (ππ ) 2.32 T1
A (ππ )
2.32
3.55
3.83
4.17
4.21
4.40
4.63
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T2
A‘ (ππ ) 3.57 T2 B (ππ ) 3.56
2
T2
B (ππ )
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*
T3 A‘ (ππ ) 3.87 T3 B (ππ ) 3.86 T3
B (ππ )
2
2
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*
3
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*
T4 A‘‘ (nπ ) 4.00 T4 B (nπ ) 4.01
S1
A
1
(ππ )
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1
*
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S1 A‘ (ππ ) 4.13 S1 A (ππ ) 4.14 T4
B (nπ )
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*
1
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*
T5
A‘‘ (nπ ) 4.23 S2 B (ππ ) 4.38
2
S2
B (ππ )
2
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S2 A‘ (ππ ) 4.36 S3 B (nπ ) 4.39
S3
B (nπ )
1
1
NꢀMethylꢀ1,8ꢀNaphthalimide
The oneꢀcolor photoionization mass spectrum of MeꢀNI recorded at 314 nm excitation is depicted
in the upper trace of Figure 1. The molecular signal at m/z = 211 shows the highest intensity in
the spectrum and is accompanied by a couple of fragments. The signals at m/z = 183 and m/z =
1
67 originate from a loss of CO or CO , respectively. The smaller fragments at m/z = 154 and
2
m/z = 126 were generated by dissociation of the (ꢀNꢀMe) group and are equivalent to the
3
fragments found in mass spectra of NI, recorded at 332 nm (lower trace), or NDCA. All
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fragments originate from dissociative photoionization because the recorded REMPI spectra are
similar in all mass channels.
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Figure 1. Mass spectrum of MeꢀNI (top graph) and NI (bottom graph) recorded at 314 nm and
332.4 nm respectively. The peak at m/z=211 is due to MeꢀNI ion, while the peak at m/z = 197
corresponds to NI.
The vertical ionization energy of MeꢀNI was determined by photoelectron spectroscopy to be
36
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.57 eV. Therefore two 351 nm (3.53 eV) photons are necessary for ionization from the S
1
state. By scanning the excitation laser from 320 nm to 340 nm a [1+2’] REMPI spectrum was
ꢀ1
recorded. The first band in the spectrum shown in Figure 2 was found at 29 920 cm and is
ꢀ1
assigned to the origin of the S state. Furthermore three distinct peaks are observed at +208 cm ,
1
ꢀ1
ꢀ1
+308 cm and +418 cm and represent S vibrationally excited states. With higher excitation
1
energies the background signal increases, probably due to the large density of vibronic states, and
prohibits an assignment of further bands in the MeꢀNI spectrum. The small shoulder at the
ꢀ1
beginning of the spectrum around 29 700 cm is due to hot band or sequence band transitions.
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54
55
56
57
58
59
60
Figure 2. The twoꢀcolor REMPI spectrum of isolated MeꢀNI recorded at t . The peak at 29 920
0
ꢀ1
cm is assigned to the band origin of the S ← S transition.
1
0
Employing TDDFT, the origin of the vibronic excitation spectrum was computed at an energy of
ꢀ1
3
.735 eV (30 120 cm ), which agrees within 25 meV with the experimentally determined value
ꢀ
1
of 3.710 eV (29 920 cm ). The difference of 0.4 eV compared to the vertical excitation energy
given in Table 1 indicates a slight geometry change upon excitation.
In Table 2 the experimentally observed vibrational bands are compared to the values computed
by TDDFT calculations. The majority of the bands is assigned to totally symmetric bending
modes of the molecule. Even in case of a free torsion of the NꢀMeꢀgroup, which would lead to a
C symmetry instead of the originally assumed C symmetry, these bands represent allowed
2
v
S
transitions into A bands. Due to the similar minimum energy structure of S and S , Franckꢀ
1
1
0
Condon factors for nonꢀtotally symmetric transitions will be small.
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1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Table 2. Comparison of experimental and calculated wavenumbers for the vibrational bands of
the S state of MeꢀNI.
1
rel. wavenumber
λ /nm
exc
calc. vibrational
assignment
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
ꢀ1
ꢀ1
/
cm
transition /cm
ꢌ
2
9 920
334.2
30 120
+271
0
ꢌ
+
208
46 A’
45 A’
+
308
+363
42 A’ / 41 A’/
+
418
+419 / +454
329.6
326.5
2
(
46 A’)
39 A’
38 A’
+
547
650
+567
+635
+
2
+706
848
+695 / +731
+821
37 A’ / (12 A’’)
36 A‘
+
In the next step timeꢀresolved pumpꢀprobe spectra were measured for most of the vibronic bands
summarized in Table 2. In Figure 3 three exemplary delay scans at different excitation energies
are depicted. In all scans the temporal decay is fitted by a biexponential function. The second
time constant τ exceeds the timeꢀscale of the experiment and was considered in the fit function
2
with a fixed time constant of 1 ns. For the S origin a fast decay of τ = (14.1±1.4) ps was
1
1
observed (top graph of Figure 3). Other vibronic bands of the S state with higher excitation
1
energies yielded similar lifetimes in the range between 12 and 16 ps and show a similar longꢀ
lived offset.
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43
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45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 3. Selected delay scans of MeꢀNI with different excitation energies. In every scan a twoꢀ
step deactivation is observed with a fast time constant of 12ꢀ16 ps and a second, much slower
process.
1,8ꢀNaphthalimide
In addition to MeꢀNI we investigated the chemically similar molecule NI (see Scheme 1). Its
3
6
vertical ionization energy was determined to be 8.68 eV, slightly higher compared to MeꢀNI.
Again two probe photons are required for ionization from the S state. The mass spectrum of NI
1
(m/z = 197), given in the lower trace of Figure 1 and recorded at 332.4 nm, also shows fragments
at m/z = 154 and m/z = 126 resulting from CO/NHꢀ and 2× CO/NHꢀloss, respectively, due to
dissociative photoionization. Unlike the cases of MeꢀNI and NDCA, there are no signals resulting
from COꢀ or CO ꢀloss. Note that no dimer signal is present. Cluster formation has been
2
suppressed in the experiments by recording spectra early in the jet. Using the same delay between
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1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
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4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
laser and pulsed valve as in the experiments on MeꢀNI, a substantial amount of dimers and higher
clusters was observed.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
ꢀ1
Figure 4. The twoꢀcolor REMPI spectrum of isolated NI at t . The peak at 30082 cm is assigned
0
to the origin of the S ← S transition.
1
0
The first excited state was also investigated by a [1+2’] REMPI. The pumpꢀpulses were tuned
between 320 and 336 nm and the third harmonic of the Nd:YLF laser was again used for
ionization, the resulting spectrum is depicted in Figure 4. Like in MeꢀNI, the background signal
increases with higher excitation energies due to the increasing density of states. Two peaks 126
ꢀ1
ꢀ1
ꢀ1
cm apart appear around 30000 cm . It is tempting to assign the first peak at 29 956 cm to the
ꢀ1
S origin, because a shift of +36 cm for methylꢀsubstitution appears reasonable. Two factors
1
ꢀ1
conflict with this assignment: First, a band at +126 cm does not match the computed value of
any totally symmetric vibration. Second, there is a pattern of three closeꢀlying vibronic bands that
ꢀ1
ꢀ1
ꢀ1
also appears in the spectra of MeꢀNI and NDCA. Values of +354 cm , +437 cm and +511 cm
would correspond to significantly higher vibrational energies than in the related molecules.
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9
ꢀ1
ꢀ1
ꢀ1
Choosing the band at 30082 cm as the origin leads to vibrational bands at +228 cm , +311 cm
ꢀ1
ꢀ1
ꢀ1
and +385 cm . Further bands then appear +535 cm and 742 cm . The vibrational spacings are
then very similar for NI and MeꢀNI, as we would expect for such similar molecules. We therefore
prefer the latter assignment. Based on TDDFT calculations nearly all appearing bands can be
10
11
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46
47
48
49
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attributed to inꢀplane bending modes of A symmetry. A summary of all observed vibronic bands
1
ꢀ1
and their assignment is given in Table 3. Only the lowest energy band at +228 cm is assigned to
the first overtone of a B vibration. Such a band was also visible in NDCA and has also been
1
3
assigned to a B overtone .The rightꢀhand column in Table 3 lists observed bands in NDCA for
1
comparison. As expected the observed spacings are similar to those in MeꢀNI summarised in
Table 2.
ꢀ1
Consequently, we have to discuss the assignment of the band at 29 956 cm , redꢀshifted by ꢀ126
ꢀ1
cm . There are three possible reasons for this band, none of them fully convincing. First it might
ꢀ1
be assigned to the lactim tautomer of NI. In fact, a transition energy of 29097 cm was computed
for the origin of its S ← S vibronic excitation spectrum. Since the lactim tautomer lies roughly
2
0
ꢀ1
8
0 kJ⋅mol above the lactam, significant population in a free jet is rather improbable. Second, the
band might originate from a cluster fragment. In the mass spectrum (cf. lower trace Figure 1) no
cluster signals are visible, so we would have to assume a complete fragmentation of the cluster.
However, the wavenumber shift is in the range that could be expected for the cluster with one
water molecule. Due to the significant geometry change upon ionization this cluster might
ohotoionize dissociatively. Finally the band might be assigned to a hotꢀ or sequence band. In
order to avoid cluster formation we chose comparably warm conditions in the jet that might lead
to insufficient cooling of lowꢀfrequency modes. This is evidenced in the spectrum by the broader
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1
1
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1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
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5
5
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5
5
6
bands in comparison to MeꢀNI. Nevertheless the band would be unusually intense for a hot or
sequence band.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Table 3. Experimental and computed wavenumbers for the vibrational bands of the S State of
1
NI.
rel. wavenumber
λ /nm
exc
calc. vibrational
tentative
observed band
ꢀ
1
ꢀ1
ꢀ1
/
cm
126
0 082
228
transition /cm
assignment
NDCA /cm
ꢀ
ꢌ
3
332.4
329.9
329
30 252
+2⋅137
0
ꢌ
2
+
(11 B1)
+249
+366
+432
+555
+713
+311
+385
+356
21 A1
+440 / +465
+465 / +583
+698
20 A / 19 A
1
1
1
+
535
742
19 A / 18 A
1
+
17 A1
In order to study the dynamics of the first excited state we also performed psꢀtimeꢀresolved
pumpꢀprobe experiments. Several bands visible in Figure 4 were excited and their lifetimes were
probed. Figure 5 shows some selected delayꢀtraces. Like in MeꢀNI all bands show a fast time
constant τ between 10 to 17 ps and a longꢀlived signal, which is still present after 250 ps and
1
indicates a second much slower decay. In isolated NI no systematic dependence of the lifetime on
the excitation energy of the vibronic states can be found.
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60
Figure 5. Selected timeꢀdelay traces of isolated NI. The pump wavelength is indicated in the
figure. A twoꢀstep decay is assumed with short time constants of roughly 10 – 20 ps and a
second, slower decay.
Surface Hopping Dynamics Simulations
To gain a mechanistic insight into the excited state deactivation pathways, molecular dynamics
simulations were carried out for MeꢀNI. In a first step, the nonadiabatic relaxation within the
singlet manifold was investigated by Tully’s surface hopping method.
All molecules were initialized in the first excited singlet state. Two different temperatures (30 K
and 300 K) were simulated by choosing different initial conditions of spatial coordinates and
momenta. The resulting population dynamics of the S and S state is depicted in Figure 6.
1
0
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0
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0
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0
1
2
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9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 6. Evolution of the S and S population of MeꢀNI for 30 K and 300 K obtained from
1
0
surface hopping dynamics.
In the case of cold molecules with a small amount of vibrational energy, corresponding to the
conditions assumed in the free jet experiments, no population transfer into the ground state was
found within 20 ps. Simulation with a temperature of 300 K yielded a partial deactivation of the
S1 state through a conical intersection (CI). The latter is characterized by the methyl group
moving out of the ring plane as well as by a bond elongation between nitrogen and one of the
neighboring carbonyl carbon atoms. However, 70 to 80 % of the population stays in S . This can
1
be explained by an energy barrier on the S surface that must be overcome by the molecules in
1
order to reach the CI. The effective height of this barrier can be estimated from the dynamics
simulations to be 0.5ꢀ0.6 eV. Far away from the CI the nonadiabatic coupling between the S and
0
ꢀ1
the S state is weak (approx. 6 cm ) and would only result in a slow deactivation via internal
1
conversion. This is apparent from the simulation with cold molecules at 30 K shown in Figure 6.
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In the second step the depopulation of the S state of MeꢀNI via an intersystem crossing (ISC) to
1
the triplet manifold is considered. The matrix elements for the spin orbit coupling between S , S1
0
and the four lowest triplet states were calculated at the DFTꢀoptimized S geometry, as described
1
10
11
12
13
14
15
16
17
18
19
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21
22
23
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43
44
45
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47
48
49
50
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56
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above in the computational section. The resulting spinꢀorbit couplings are mostly negligible (<
ꢀ2
ꢀ1
*
5
·10 cm ) except for the transition from the S and S states, which bear ππ character, to the
0 1
3
*
ꢀ1
fourth triplet state ( A’’ nπ ), with matrix elements of 46 and 15 cm respectively (for the values
,
of all spinꢀorbit couplings employed in the simulation cf. Supporting Information (SI), Table S2).
This is in accordance to the ElꢀSayed rules that predict an allowed electronic transition between
1
*
3
*
35
(
ππ ) and (nπ ) states. Within the triplet manifold, nonadiabatic couplings with averaged
ꢀ1
values of 30ꢀ590 cm were determined from surfaceꢀhopping calculations (cf. SI, Table S3), thus
efficient IC processes populating the lowꢀlying triplet states will take place after ISC from the S1
to the T state.
4
To investigate the coupled population dynamics including the two lowest singlet and four triplet
states, an additional simulation was performed on a model level. For this purpose, the electronic
energies obtained at the optimized S geometry were employed (for the values cf. SI, Table S1),
1
as well as the nonadiabatic and spinꢀorbit couplings alluded to above (cf. SI, Tables S2 and S3),
and the timeꢀdependent Schrödinger equation was integrated in this manifold of states as
described in detail in the Computational section. Note that in the simulation a deactivation via a
conical intersection is not included. The resulting population dynamics of MeꢀNI is shown in
Figure 7, indicating a depopulation of the initially excited S state on the psꢀtime scale.
1
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2
2
2
2
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0
1
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0
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0
1
2
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0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 7. Calculated electronic relaxation dynamics of MeꢀNI involving singlet and triplet states.
Population dynamics of the states S , S and the sum of all four triplet states T obtained by a
0
1
1ꢀ4
model simulation employing the DFT energies for the optimized S state geometry as well as
1
nonadiabatic and spinꢀorbit couplings.
After 50 ps the population of the S state has decreased to 55 % while the population of the triplet
1
states T1ꢀ4 has grown accordingly to 45 %. A significant population transfer to electronic ground
state is not observed. The model confirms an effective deactivation to the triplet manifold due to
ISC from the S to the T state. The corresponding time constant is in the order of 10ꢀ100 ps.
1
4
DISCUSSION
The results of the nonadiabatic dynamics simulations of MeꢀNI immediately lead to an
interpretation of the two time constants observed in the experiment: The short time constant τ1,
*
which is on the order of 10 – 20 ps is assigned to ISC from the photoexcited S (ππ ) state to the
1
*
*
T (nπ ), while τ , which is assumed to be > 500 ps corresponds to the lifetime of the T (ππ )
4
2
1
state. Although the computed time constant is larger than the experimental one, the order of
magnitude agrees. The intermediate internal conversion within the triplet manifold is very fast
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and cannot be resolved within the time resolution of our experiment. The fast ISC can be
explained by a large spinꢀorbit coupling in accordance with the El Sayed rules and the small
ꢀ1
energy gap. The T state was computed to lie 0.13 eV (~ 1000 cm ) above the S state. Given the
4
1
10
11
12
13
14
15
16
17
18
19
20
21
22
23
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25
26
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41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
accuracy of the computational methods the two states can be considered to be almost degenerate.
3
7
According to Fermis golden rule a high deactivation rate can be expected under these
conditions. This interpretation is in accordance with transient absorption measurements of MeꢀNI
in nonpolar solutions where the quantum yield ΦISC for a S → T transition was determined to be
1
x
9
close to 100 %. Such fast ISC is not uncommon in carbonyl compounds. In benzophenone, for
38
example, a time constant of 5 ps was found for the T (ππ*)→S (nπ*) nonꢀradiative transition.
2
1
On the other hand the probability for IC within the singlet manifold is small at the temperature of
a free jet (30 K). The surface hopping simulations show that a deactivation of the excited S to
1
the ground state via a conical intersection is only possible at high internal energies, because of a
barrier on the excited state surface.
Although the dynamics was only simulated for MeꢀNI, a similar excitedꢀstate deactivation is
assumed for NI, since the additional methyl group has little influence on the electronic structure
and because similar results have been obtained in the experiments. The S and T states (see
1
4
Table 1) are again almost degenerate, and ISC should be efficient in accordance with the Elꢀ
Sayed rules. The deactivation of the S state of NI is thus also attributed to ISC.
1
It is interesting to compare the dynamics of NI and MeꢀNI to the one of NDCA, which has an
3
oxygen atom instead of the NꢀH or NꢀMe group and was investigated previously. NDCA
deactivates around the S origin by ISC within roughly 1 ns, but IC increasingly dominates at
1
higher vibronic excitation. In contrast, IC is not important for isolated MeꢀNI and NI at low
temperatures. ISC is much faster and no significant dependence on the excess energy was found.
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2
2
2
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2
2
2
2
2
2
3
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4
4
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4
4
4
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5
5
5
5
5
5
5
6
While the S /T energy gap is similar in all three molecules, calculations yield a SOC matrix
1
4
ꢀ1
ꢀ1
element of 15 cm for MeꢀNI, somewhat larger than the value 10 – 11 cm computed for NDCA.
The average coupling element for S → S internal conversion is on the other hand only slightly
1
0
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
ꢀ1
ꢀ1
smaller in MeꢀNI compared to NDCA (6 cm vs. 9 cm ). While for NDCA the simulations
showed internal conversion to be efficient at 300 K, it remains incomplete in MeꢀNI.
Interestingly the barrier to the S /S conical intersection on the excited state surface is slightly
1
0
smaller in MeꢀNI (0.6 eV compared to 0.7ꢀ0.9 eV in NDCA). It seems that access to the CI
requires a larger degree of structural rearrangement in NI and MeꢀNI and thus ISC remains the
dominant deactivation pathway even at significant internal excitation.
CONCLUSION
The dynamics of the S state of isolated MeꢀNI and NI has been investigated by both picosecond
1
laser spectroscopy and molecular dynamics simulations. The S state origins were found at
1
ꢀ1
ꢀ1
29 920 cm (3.710 eV) for MeꢀNI and 30 082 cm (3.729 eV) in excellent agreement with
ꢀ1
TDDFT computations. In NI a further band appears at 29 956 cm (3.714 eV), which might be
assigned to the lactim tautomer, a cluster fragment or a hot or sequence band. The similar
adiabatic excitation energies indicate a small influence of the methyl group on the electronic
structure. The REMPI spectra show a few vibronic bands which appear at slightly different
relative energies in the two molecules due to the presence of the Nꢀmethyl group. They can
mostly be assigned to inꢀplane bending modes. A twoꢀstep deactivation was observed for both
molecules. The first time constant, which is in the range of 10ꢀ20 ps for both molecules is
assigned to a fast ISC into the T (nπ*) state. ISC is followed by rapid internal conversion to the
4
lowest triplet state. The second much slower time constant on the nanosecond range corresponds
to photoionization from the lowest triplet state. In contrast to the related molecule NDCA
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intersystem crossing remains the dominant deactivation pathway over the investigated energy
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ASSOCIATED CONTENT
Supporting Information. Additional computational information (Electronic energies obtained
at the optimized S geometry, nonadiabatic and spinꢀorbit couplings) and full references 4 and 23.
1
This material is available free of charge via the Internet at http://pubs.acs.org
ACKNOWLEDGMENT
The work was founded by the Deutsche Forschungsgemeinschaft within project FI 575/9ꢀx. H.ꢀC.
S. acknowledges financial support by the “Fonds der Chemischen Industrie”. We would like to
thank Eileen Welz for her help in some of the experiments.
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