Solution-state photophysics of: N -carbazolyl benzoate esters: Dual emission and order of states in twisted push-pull chromophores

Article abstract of DOI:10.1039/c6cp04619c

The stepwise photoinduced charge transfer in a series of N-carbazolyl benzoate ester push-pull chromophores has been studied in solution. Dual emission from the locally excited (LE, the lowest-energy singlet excited state of ¹L_b natu

Full text of DOI:10.1039/c6cp04619c

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DOI: 10.1039/C6CP04619C
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Solution-State Photophysics of N-Carbazolyl Benzoate Esters: Dual
Emission and Order of States in Twisted Push-Pull Chromophores.
Received 00th January 20xx,
Accepted 00th January 20xx
Liubov M. Lifshits, Darya S. Budkina, Varun Singh, Sergey M. Matveev, Alexander N. Tarnovsky,
and Jeremy K. Klosterman*
DOI: 10.1039/x0xx00000x
The stepwise photoinduced charge transfer in a series of N-carbazolyl benzoate ester push-pull chromophores has been
studied in solution. Dual emission from the locally excited (LE, the lowest-energy singlet excited state of 1Lb nature
localized on the carbazole donor) and the highly polarized, intramolecular charge-transfer states of (pre)-twisted type
(TICT states) is observed in non-polar and polar solvents. Ultrafast transient spectroscopy reveals that the excitation into
the 1Lb LE is followed by rapid (~ps) charge separation into an emissive TICT state. Excitation into the second singlet excited
state localized on the carbazole (S2) with 1La nature, results in sub-100 fs population of both 1Lb and TICT states.
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Introduction
N
H
N
Organic chromophores are key components in molecular
optoelectronic application organic light emitting diodes and
photovoltaics.1–3 Among the many chromophores available,
carbazole and its many derivatives tower as essential
functional building blocks due to their ideal photophysical
properties, e.g. efficient emission, large band gaps and high
triplet energies and synthetic accessibility.4 Indeed, carbazole
biphenyls (CBPs) are ubiquitous host materials.5–9 As with most
organic chromophores, the resulting bulk optoelectronic
behaviors strongly depend on the local environment. Non-
emissive excimers can form upon aggregation in the solid
materials and serve as trap states that result in non-optimal
device performance.10–13
Cbz
CzP
N
N
N
MeO
OMe
OEt
O
O
O
O
OEt
CzI
1
CMB
2
CPB
3
Fig. 1. Chemical structures of carbazole, Cbz, N-phenyl-carbazole, CzP, and push-pull
carbazoles with dimethyl isophthalate, CzI 1, and ethyl benzoate acceptors CMB 2 and
CPB 3.
Recently we reported enhanced solid-state emission from
5-(9H-carbazol-9-yl)-isophthlates incorporated within metal-
organic frameworks (MOF) constructed of diamagnetic zinc
ions.14,15 Within the confines of the MOF, the carbazole ligands
were rigidly held such to prevent cofacial overlap of adjacent
carbazole π-systems and preclude quenching via non-emissive
excimers. Furthermore, restricted rotation around the central
C–N bond was hypothesized to impede formation of twisted
intramolecular charge transfer (TICT) states.
highly solvatochromic emission indicative of a TICT state
wherein the donor and acceptor moieties adopt a 90˚
conformation in the excited state. The Zachariasse25,26 and
Kapturkiewicz groups,27–30 however, examined a series of N-
aryl donor-acceptor carbazoles with cyano and keto electron
withdrawing substituents and concluded that, although
twisted, an orthogonal donor-acceptor TICT state was unlikely.
We now report the photophysical behavior of dimethyl 5-
(9H-carbazol-9-yl) isophthalate 1 and two new analogues,
ethyl 3-(9H-carbazol-9-yl) benzoate 2, and ethyl 4-(9H-
carbazol-9-yl) benzoate 3, each with a single electron
withdrawing ester groups (Fig. 1). X-ray crystallography and
DFT calculations confirm a pre-twisted (θ ∼ 50˚) ground state
structures retaining electronic communication between the
two halves. Fluorescence spectroscopy reveals emission from
localized carbazole LE states in the nonpolar cyclohexane and
solvatochromic emission from twisted ICT states in increasing
solvent polarity. Ultrafast transient absorption spectroscopy
Rettig16–20 and others21–24 showed that N-(4-cyanophenyl)
carbazole in particular, along with many N-aryl carbazole
derivatives but not N-phenyl carbazole, displayed broadened,
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indicates that photoexcitation selectively accesses Frank
Condon states, localized on the carbazole chromophore, which
rapidly react, via electron transfer, to form the emissive
Twisted ICT states.
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DOI: 10.1039/C6CP04619C
Results and discussion
Absorption
The room temperature absorption spectra of 1-3 above
250 nm resemble that of N-phenyl carbazole with prominent
1
1
bands at 290 nm and ~340 nm arising from the La ← A and
1
1Lb ← A transitions of carbazole, respectively (Fig 2). These
π→π* transitions are polarized in molecular plane along the
respective long and short axis of carbazole.31,32 The π→π*
transitions of the isophthalate and benzoate esters contribute
to the intensity of the band at 290 nm.33,34 Broad charge
transfer (CT) bands appear as shoulders at 306 nm in CzI 1 and
at 314 nm for CPB 3, which confirms the push-pull nature of
these new carbazole derivatives with increasing acceptor
strength. CzI 1, due its greater electron withdrawing ability,
also exhibits lower energy transitions at 352 nm. Although the
p-benzoic ethyl ester of CPB 3 possesses a single electron
withdrawing ester, as compared to the two esters in
isophthalate acceptor moiety of CzI 1, the charge transfer
band at 314 nm is much more intense due to resonance.16 The
charge transfer band of the m-benzoic ester CMB 2, on the
other hand, is effectively negligible as resonance is prohibited
due to lack of orbital overlap. The intensity of the CT bands at
~310 nm shift to the blue and weaken with increasing solvent
polarity. Here, increasing solvent polarity stabilizes the
zwitterionic nature of the ground state and increases the
contribution of CT transitions which lowers the overall band
intensity as CT transitions typically display small oscillator
strengths, due to poor orbital overlap.35 A similar phenomenon
has been previously observed in the CT absorption spectra of
push-pull chromophores.28
Fig. 2 Absorption spectra of 9H-carbazolyl benzoate esters 1-3 in cyclohexane (purple),
diethyl ether (blue), tetrahydrofuran (cyan), dichloromethane (green), acetonitrile
(orange) and ethanol (red) at room temperature. The absorption spectrum of CzP in
acetonitrile is provided for comparison (grey).
Emission
Similar to previous 9-aryl carbazoles,16,27–30 emission of
carbazol-9-yl benzoic esters exhibit highly solvatochromic
emission behavior; increasing the solvent polarity from
cyclohexane to ethanol results in large red shifts and spectral
broadening indicative of highly polarized TICT (twisted
intramolecular charge-transfer) excited states (Fig. 3). As steric
repulsion of hydrogens enforces a pre-twisted ground state
conformation (θ ~ 50˚) and prevents coplanarity in the excited
state, we refer to this CT state as a twisted excited state (TICT),
in accordance with earlier work.16
Fig. 3 Emission spectra of 9H-carbazolyl benzoates 1-3 in cyclohexane (purple), diethyl
ether (blue), tetrahydrofuran (cyan), dichloromethane (green), acetonitrile (orange)
and ethanol (red) at room temperature, λex = 290 nm. Emission spectrum of CzP in
acetonitrile is provided for comparison (grey). The second harmonic appears at 580 nm.
In nonpolar cyclohexane, the roughly structured emission
of CzI 1 in cyclohexane with broadened peaks at 377 and 390
nm closely resembles the emission of N-phenyl carbazole CzP
but is red-shifted ~30 nm to lower energies due to the
stabilization by the greater electron affinity and extended
conjugation of the isophthalate acceptor. Fluorescence from
N-phenyl carbazole is structured in both polar and non-polar
solvents and non-solvatochromic. Thus CzP emission has been
attributed to the initially populated, locally excited (LE) state
as excited state charge transfer does not occur.16 We therefore
refer to the structured emission band of CzI 1 in cyclohexane
as LE emission, in accordance with literature precedent.36 The
LE emission from CMB 2 in cyclohexane is also broadened, but
only slightly red-shifted, ~3 nm relative to CzP as the reduction
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Table 1. Solvatochromic spectroscopic data of 1 – 3: Absorption and emission maxima
ꢀ!and ꢀ!, Stokes shift Δꢀ and emission band full-width at half-maxima (fwhmf ) in
wavenumbers (103 cm-1).
View Article Online
DOI: 10.1039/C6CP04619C
solvent
∆fa
ꢀ!
ꢀ!
Δꢀ
fwhmfb
103 cm-1
103 cm-1 103 cm-1
103 cm-1
1
2
CH
Et2O
THF
DCM
ACN
EtOH
0.100
0.256
0.308
0.319
0.393
0.379
29.76
29.85
29.76
29.76
29.85
29.85
25.51
23.92
22.78
22.27
21.05
20.88
4.25
5.93
6.98
7.49
8.80
8.97
3.22
3.60
3.72
3.84
5.64
4.35
Fig. 4 Lippert-Mataga plots of TICT fluorescence maxima for 1 – 3 vs. solvent polarity
CH
Et2O
THF
DCM
ACN
EtOH
0.100
0.256
0.308
0.319
0.393
0.379
29.76
29.76
29.85
29.50
29.59
29.67
27.93
25.84
24.94
23.47
22.68
21.60
1.83
3.92
4.91
6.03
6.91
8.08
2.96
3.70
3.92
4.04
4.22
4.54
ꢀ ꢀ
−
!! ꢀ ꢀ! for aprotic solvents CH, Et2O, THF, DCM, and ACN.
Table 2. Onsager Radii, calculated ground state dipole moments, ꢀ!, and excited state
dipole moments ꢀ! estimated from solvatochromic plots of 1 – 3:
polar aprotic solvents
all solvents
!
!
!
!!
!
! !!
! !
b
!
!
!
a
c
aa ꢀ!
ꢀ!
ꢀ!
D
ꢀ!
D
R2
R2
!
!
!
!
!
!
3
CH
Et2O
THF
DCM
ACN
EtOH
!!!
0.100
0.256
0.308
0.319
0.393
0.379
29.76
29.85
29.67
29.67
29.85
29.76
29.33
26.32
25.32
23.98
22.88
22.03
0.44
3.54
4.36
5.69
6.97
7.74
3.10
4.74
3.59
5.31
4.72
4.53
Å
D
eV
D
eV
1
2
3
4.8 2.48d
4.8 1.65d
5.2 1.68
1.34
1.45
1.55
0.98 16.7
0.91 16.9
0.88 19.5
0.99
1.23
1.50
0.93 14.5 15.4d
0.92 15.6 15.6d
0.94 19.1 17.1
a
b
where the Onsager radius a = 1/2 the length of the molecular axis calculated
using DFT (B3LYP/6-31G*).c calculated with TD DFT (B3LYP/6-31G*). daverage
dipole moment. (See ESI†)
!
!
!!
aΔꢀ =
−
as defined in the text; full width at half-maximum of
b
!
!!!!
!! !!
emission
different orbital origins, i.e. LE and CT states, it is advisable to
simply use the fluorescence shift:36
potential of the benzoate acceptor is less than that of
isophthalate. Emission from CPB 3 in cyclohexane, however,
retains vibrational structure and is blue-shifted ~6 nm to
higher energies. In polar acetonitrile, all compounds emit
strongly from broad TICT bands centered at 475 nm, 441 nm,
and 436 nm for 1 – 3, respectively. Importantly, excitation
spectra (Fig. S12, ESI†) indicate that the emissive LE and CT
states are efficiently populated upon excitation into both the
!!!
! !!!!
! !!!!!
ℎꢀꢀ! = ℎꢀꢀ!! + !!! !!!!!
−
(2)
!
!
!!!!
!
Lippert-Mataga plots of the CT emission maxima vs. ∆f for 1 –
3 show a linear relationship (average R2 ≥ 0.93) (Fig. 4).
Deviations from linearity, “data scatter,” often arise from
specific solvent-solute interactions, i.e. hydrogen bonding in
EtOH.40 For CzI 1 and CMB 2, removal of the ethanol and
cyclohexane data points from fitting significantly improved the
correlation factors due to the difficulty in unambiguously
separating the LE and CT contributions to emission in
cyclohexane and hydrogen bonding in EtOH.28 For CPB 3,
excluding the EtOH and CH data points from fitting significantly
lowered the correlation factors but did not affect the final
calculated values (See ESI† for more details). Using the McRae
solvent polarity function, which corrects for contributions of
solvent-induced dipole moments, or Reichardt's normalized
microscopic solvent polarity parameters also improved the fits
(average R2 ≥ 0.97 and 0.98, respectively) (See ESI†).40
However, ꢀ! ꢀ! − ꢀ! /ꢀ!! values from the slope of the Lippert-
Mataga plots are more commonly employed and are used here
to aid comparison of the excited state dipole moments within
similar model frameworks.
1
1La (λex ~290 nm) and Lb (λex ~340 nm) states of carbazole in
all the solvents irrespective of their polarity.
Excited State Dipole Moments
The emission wavelength maxima and Stokes shifts
increase with increasing solvent polarity over the range of
solvents examined. The solvent polarity dependence of the
Stokes shift is related to the difference in the ground and
excited state dipole moments by: 37,38
!
! !!!!!
!!!
!!!!
!!!!!
ℎꢀ ꢀ! − ꢀ! = ℎꢀ ꢀ!! − ꢀ!!
+
−
(1)
!
!
!!!!
!
where h is Planck’s constant (erg·s), c is the speed of light (in
cm-1), ꢀ! and ꢀ! are the respective band positions of
absorption and emission in wavenumbers (cm-1), ꢀ!! and ꢀ!! are
the respective extrapolated gas phase absorption and emission
band positions (cm-1), ꢀ!and ꢀ!are the respective dipole
moments of the ground and excited states (in esu), a0 is the
radius of the Onsager cavity (in cm-1),39 ε is the solvent static
dielectric constant,40 and n is the solvent index of refraction. In
cases where the absorption and emission transitions have
From the solvatochromic slopes (Table 2), it is readily
apparent that the emissive excited states are a) highly
polarized (i.e. ꢀ! ≫ ꢀ!); b) similarly stabilized by polar aprotic
solvents; and c) comparable with related N-benzonitrilyl
carbazoles (solvatochromic slopes of 1.35 – 1.51 eV).25,28
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Table 3. Quantum Yields, Decay time, Radiative and non-radiative constants and
transition dipole moments for 1 – 3.
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DOI: 10.1039/C6CP04619C
b
τf
ns
ꢀ!!
107 s-1
1.26
0.79
0.73
0.55
0.44
0.33
knr
Mc
D
Solvent
Φf
107 s-1
23.90
6.91
3.99
3.65
3.51
7.84
1
2
3
CH
Et2O
THF
DCM
ACN
EtOH
0.05
0.10
0.15
0.13
0.11
0.04
3.98
13.00
21.16
23.79
25.33
12.25
0.91
0.85
0.83
0.74
0.78
0.67
CH
Et2O
THF
DCM
ACN
EtOH
0.11
0.15
0.28
0.31
0.32
0.04
4.03
8.69
16.52
27.82
36.23
8.71
2.76
1.71
1.71
1.10
0.88
0.49
22.00
9.80
4.34
2.49
1.88
11.02
1.17
1.14
1.12
0.98
1.00
0.76
Fig. 5. Representative ORTEP plots of a) CzI 1, b) CMB 2, and c) CPB 3 at 50% probability
with numbering schemes and side view showing the twist angle between planes of the
donor carbazole and acceptor isophthalate/benzoates. Hydrogen atoms have been
omitted for clarity.
CH
Et2O
THF
DCM
ACN
EtOH
0.55
0.58
0.80
0.72
0.60
0.23
4.60
5.48
7.59
9.98
12.48
5.21
12.0
10.6
10.6
7.18
4.79
4.41
9.73
7.68
2.59
2.84
3.23
14.78
2.28
2.74
2.73
2.40
2.29
2.29
respectively, whereas the lifetime of 3 only reaches 12.5 ns in
ACN. As expected from the quantum yields, hydrogen-bonding
interactions result in shortened lifetimes for 1 – 3 in ethanol.
The radiative rate constants of CzI 1 and CMB 2, calculated
from ꢀ!! = ꢀ!/ꢀ! with the assumption that irreversible
formation of the emissive species occurs with 100% efficiency,
are lesser than the radiative rates of CPB 3. The small ꢀ!! rate
constants, particularly in the case of 1 and 2, strongly indicate
efficient charge separation in the emissive CT state, as
compared to the nonpolar LE and ground states. In all cases
the radiative rates decrease with increasing solvent polarity
a Excited at 340 nm and recorded at λmax b kf = Φf/ τ, and knr = (1 – Φf)/ τ.
.
Calculation of estimated excited state dipole moments
using eq. (4) is highly dependent25 on the selection of a, the
Onsager radius, and multiple methods for deriving spherical
approximation of non-spherical molecules have been
proposed (See ESI†).22,26,41,42 Using half the length of the long
molecular axis (as measured from single crystal X-ray
structures) and ground state dipole moments, ꢀ!, determined
using DFT calculations, excited state dipole moments of 17 –
20 D are obtained (Table 2). These large values suggest that
full electron transfer from the carbazole nitrogen atom occurs
to roughly the center of the aromatic benzoate acceptors upon
excitation of 1 – 3.36 Excited dipole moments calculated from
the Stokes shifts are quite similar (See ESI†). Excited state
dipole moments from TD DFT calculations (see computational
section) are similar in magnitude and trend.
1
(~3 fold in acetonitrile), characteristic of a forbidden So ← CT
radiative transition between
a largely decoupled, i.e.
significantly (pre-) twisted, donor-acceptor pair.16,46,28,47–49 The
electronic transition dipole moments, M, as calculated
according eq (3) decrease slightly with increasing solvent
polarity. The small magnitude of the M values are typical of
twisted intramolecular CT states.36,50
!
ꢀ! = !"! ꢀ!ꢀ!!
ꢀ
(3)
!
!!
X-ray Crystal Structures of CzI 1, CMB 2 and CPB 3
Single crystals of CMB 2 and CPB 3 were obtained by slow
evaporation of saturated solutions and analysed by X-ray
diffraction.‡ The structural solution for CPB 3 was non-trivial
and will be reported shortly elsewhere.51 The crystal structure
of CzI 1 was previously reported.14 All of the push-pull
chromophores are non-planar, due to steric repulsions, and
adopt a twisted conformation with angles (θ) of 49.5˚, 47.5˚,
and 44 – 55˚, between the best-fit planes of the donor-
carbazole and acceptor isophthalate/benzoate ring atoms (Fig.
5). For CMB 2 the ethyl ester is effectively coplanar whereas
ester groups in CzI 1 and CPB 3 display slightly larger
deviations from co-planarity (α = 6.70/7.21˚, 1.31˚, and ∼ 9.52˚
for 1 – 3 respectively). Bond distances and angles are similar to
other N-aryl substituted carbazoles and no strong indications
of ground state CT interactions are apparent (Table S14,
ESI†).25,11,52–55
Quantum Yields
Quantum yields of 9H-carbazolyl benzoate esters 1 – 3
gradually increase with greater solvent polarity but level off at
THF (∆f = 0.308) potentially indicating dielectric saturation or,
more likely, the concurrent stabilization of competitive non-
radiative pathways (Table 3).43–45 CPB 3 is highly emissive, Φf
=
55 to 80% whereas CzI 1 and CMB 2 are weakly and
moderately emissive, 5 to 10% and 10 to 30% respectively. For
all three compounds, emission decreases in EtOH, relative to
the less polar ACN and THF. Specific solute-solvent interactions
in protic solvents, i.e. ethanol, are well-established,36 and
hydrogen bonding interactions often augment non-radiative
decay processes resulting in quenched emission.
Radiative Rates
Lifetimes of CzI 1 and CMB 2 range from 4 to 25 and 36 ns,
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Fig. 6. HOMO, LUMO and relevant frontier molecular orbital diagrams (DFT B3LYP/6-31G*) for a) carbazole Cbz), b) N-phenyl carbazole (CzP), c) CzI 1,-syn away - conformer and
the methoxy esters of d) CMB OMe 2’ - away – conformer and e) CPB OMe 3’. Hydrogen atoms have been omitted for clarity.
Fig. 8. Side view showing the twist angle between planes of the donor carbazole and
acceptor isophthalate/benzoates in the optimized first excited state (TD DFT B3LYP/6-
31G*) of CzI 1, CMB OMe 2’, and CPB OMe 3’. Hydrogen atoms omitted for clarity.
Fig. 7. HOMO, LUMO and relevant LUMO+n gas-phase energies (DFT B3LYP/6-31G*) of
carbazole (Cbz), N-phenyl carbazole (CzP), and lowest energy conformers of 9H-
on the electron rich carbazole moiety but can also be found at
the ortho and para positions on the isophthalate/benzoate
rings indicating ground state electronic coupling between the
two halves (Fig. 6). In the case of 3, electron density is also
found on carbonyl oxygen of the para- ester groups.
Population densities of the LUMOs are almost completely
localized on the isophthalate/benzoate halves for 1 and 2. The
LUMO of CPB 3, however, extends from the central pyrrole
ring to the acceptor benzoate indicating CT interactions. These
results are consistent with the presence of CT bands strongly
contributing to the absorption spectra of 3, but only weakly
present for 1 and 2 (Fig. 2). Comparison to the frontier orbitals
carbazolyl methyl benzoates 1 – 3. MOs corresponding to the Cbz 1La and 1Lb states are
highlighted in blue and green, respectively.
DFT Calculations
DFT calculations (B3LYP/6-31G*) generated non-planar,
twisted ground state conformations for 9H-carbazolyl
benzoate methyl esters of 1-3, twist angles (θ) of 54.9˚, 54.7˚
and 52.0˚, respectively, which correspond well to the observed
X-ray crystal structures (Fig. 6). The ester groups are effectively
coplanar (α = ∼0.9˚ for 1 – 3, respectively). The calculated
ground state dipole moments56 align towards the vector of the
non-carbonyl ester oxygen atom and are, therefore, strongly
dependent on the orientation of the ester substituents; the
dipole moment of the crystallographically observed anti-
conformation of 1 is 1.75 D whereas the dipole moment of the
favored solution state syn- conformers57–59 are 2.14 and 3.55 D
(Fig. S23 and Table S15, ESI†). The calculated dipole moments
of the meta- CMB 2 conformers are 0.40 and 2.90 D. The
dipole moment of CPB 3 is 1.68 D.
1
of Cbz and CzP enables the identification of the carbazole Lb
state (LUMO for Cbz and CzP) as LUMO+2 for 1 and LUMO+1
for 2 and 3. The carbazole 1La state (LUMO+1 for Cbz, LUMO+3
for CzP) is now the LUMO+3 for 1, 2 and 3. The energies of
these states remain fairly constant across the series (Fig. 7)
1
and correspond to Franck-Condon excitation into the Cbz Lb
and 1La states at 340 nm and 290 nm, respectively.
Optimization of the first excited states using single-point
time-dependent DFT (TD DFT) calculations (B3LYP/6-31G*)60–62
Orbital density in the HOMOs of 1 – 3 is located primarily
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Fig. 9 ΔA spectra of CzI 1, CMB 2 and CPB 3 in a 0.1-mm flowing jet of acetonitrile at a – c) 0–1 ps, d – e) 1–20 ps and f– h) 20–1000 ps. Excitation at 340 nm (pulse energy, 5.3 μJ).
Delay times between the excitation and probe pulses (in picoseconds) are shown in the legends.
Table 4 Parameters obtained from global fit analyses of transient absorption spectra of CzI 1, CMB 2 and CPB 3 in acetonitrile upon excitation at 340 and 288 nm.
CzI 1
CMB 2
CPB 3
λex 340 nm
τg / ps
ε1 /nm
λex 288 nm
ε1 / nm
0.096 412, 537
0.18 600, 697
λex 340 nm
τg / ps
ε1 / nm
λex = 288 nm
ε1 / nm
0.072 384, 613, 744
λex 340 nm
τg / ps
ε1 / nm
λexc 288 nm
τg / ps
ε1 / nm
0.076 399, 717
τg /ps
τg / ps
τ0g
τ1g 0.07
τ2g 1.40
τ3g 27
640
594, 703
398, 562, 714 19
396, 558, 711
0.10
619
624
0.17
1.69
611
0.11
637, 723
616
0.15
2.1
620
595, 717
2.24 401, 567–586, 717 1.45
394, 477, 616, 725 1.65
400, 461, 707
400, 458, 701
404, 566, 704
399, 557, 712
36
399, 457, 712 31
399, 456, 706
42
404, 466, 711 17
403, 463, 713
405, 468, 720
403, 461, 707
final2
1 ε band maxima of evolution-associated difference spectra from the global fit, 2 permanent spectrum.
revealed relaxation into fully perpendicular geometries (θ = delay time, the reported ΔA signals are due to excitation of CzI
88.2˚, 90.0˚, and 90.0˚, for the methyl esters of 1 – 3, 1, CMB 2, and CPB 3.
respectively) (Fig. 9 and Table S16, ESI†). The esters are nearly Excitation at 340 nm In the first measurement set, solutions of
perfectly coplanar (α
= 0.0˚ – 0.2˚). The orthogonal CzI 1, CMB 2, and CPB 3 in acetonitrile were excited at 340 nm,
conformation greatly stabilizes the LUMOs (-0.54 to -0.62 eV corresponding to the carbazole S0→1Lb transition. This
relative to the GS) localized on the isophthalate/benzoate excitation wavelength was selected at the red edge of the UV-
acceptor, lending credence to a twisted intramolecular CT vis absorption spectra of these molecules to minimize excess
state. The calculated excited state dipole moments of 12.8– vibrational energy in the S1 (1Lb) state. The spectral and
18.0 for the conformers of CzI 1, 14.6 and 16.6 for CMB 2, and temporal evolution of ΔA signals at short – (0.1 – 0.5 ps) and
17.1 D for CPB 3, are consistent with the experimental dipole intermediate (0.5 – 20 ps) delay times between excitation and
moments calculated from the solvatochromic emission plots probe pulses is similar for all three compounds (Fig. 9). At 100
(Fig. 4).63 The HOMO→LUMO transitions, directly leading to fs, intense transient absorption bands are centered at 600 –
1
ICT, are forbidden (f ∼ 10-4), whereas the So → Lb transitions 620 nm (Band I). These bands undergo spectral narrowing
are partially allowed (f ∼ 0.04) (Table S17, ESI†). These results within 200 fs, and then decay on a time scale of 2 ps with the
are consistent with excitation primarily localized on the formation of three transient absorption bands. Band II is
carbazole moiety, which then relaxes into a twisted ICT state.
Transient Absorption Measurements
centered at ~ 400 nm (sharp) and Band IV at ~ 700 nm (broad)
for all three molecules. Band III is located at ~ 550 nm for CzI 1
and ~ 470 nm for both CMB 2 and CPB 3. For CzI 1 and CMB 2,
Bands II – IV remain without changes, within experimental
accuracy, to the end of the measurement (1.2 ns). For CPB 3,
Band IV at 700 nm decays slightly from 20 to 200 ps while
absorptions below 590 nm, including Bands II & III, slightly
Acetonitrile Solutions Two sets of measurements (at different
excitation wavelengths) on CzI 1, CMB 2, and CPB 3 were
performed in acetonitrile solutions. Any noticeable transient
absorption (ΔA) signals from neat acetonitrile occurred at
delay times between −50 and 50 fs. Starting from a 100 fs
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Scheme 1. Kinetic models obtained from the evolution-associated difference global
analysis of the transient absorption data measured for CzI 1, CMB 2, and CPB 3 in
acetonitrile solutions upon a) 340 nm excitation into the S1 (1Lb) state and b) 288-nm
excitation into the S2 (1La) state. Intramolecular vibrational energy redistribution (IVR)
leads to vibrationally equilibrated (equilib.) solute species in which the excess of
vibrational energy is distributed statistically. Vibrational cooling of solutes via energy
transfer to solvent leads to relaxed species.
Fig. 10. Evolution-associated difference spectra (εi) extracted from the global fit of the
ΔA spectra of 1 – 3 in CH3CN upon excitation at 340 nm.
grow. The resulting ΔA spectrum decay somewhat towards 1.2
ns, indicating that the final product states exist on a
nanosecond time scale similar to CzI 1and CMB 2.
nm. Bands I, centered at, 602 nm (CzI 1), 624 nm (CMB 2) and
614 nm (CPB 3), (corresponding to the εB spectra in Fig. 10)
appear as early as 100 fs after excitation and are thus assigned
to absorption from the initially populated, locally excited (LE)
states. The preceding spectra, εA, are due to vibrationally hot
species formed upon the S0→1Lb excitation. The spectral
narrowing and changes observed are typical manifestations of
the vibrational relaxation of polyatomic molecules.
Intramolecular vibrational energy redistribution (IVR) of large
organic molecules takes place within a few hundred fs,64 which
are close to the τ1g values observed. Vibrational cooling of
polyatomic molecules in polar solvents, such as acetonitrile,
The results of global fitting analyses of ΔA spectra and
multiexponential fits of ΔA kinetic traces are found to be in
mutual agreement, and confirm that all major changes in the
360-720 nm ΔA spectra (λex = 340 nm) of 1 – 3 occur before
200 ps. The product states present at 200 ps are too long-lived
for the ns decay time constants to be determined from the
transient measurements (see Table 3). A satisfactory global fit
of the ΔA spectra from 50 fs to 200 ps required a sum of three
major evolution-associated difference spectra (εi) obeying
exponential time dependences (time constants τ1g, τ2g, and τ3g)
and a permanent spectrum (Table 4 and Fig. 10). The τ1g
component is associated with the blue shift and spectral
narrowing of the initial ΔA signals leading to the formation of
Bands I at 640, 619 and 614 nm with τ1g = 0.07, 0.10 and 0.11
ps for 1 CzI 1, CMB 2, and CPB 3, respectively. The τ2g
component describes the decay of Bands I at ~ 620 nm and the
rise of Bands II (398, 399 and 404 nm), Bands III (562, 456 and
463 nm) and Bands IV (711, 706 and 713 nm) where τ2g = 1.40,
1.46 and 1.65 ps for CzI 1, CMB 2, and CPB 3, respectively. A
third time component is required to account for the rise in the
higher energy portion of the ΔA spectrum (λ < 590 nm) for CPB
3 with τ3g = 42 ps. Upon close inspection this third time
component is also found for CzI 1 and CMB 2 with τ3g = 27 and
36 ps respectively, albeit of much smaller amplitude.
takes place on
a longer time scale, typically several
picoseconds.65 The observed τ1g values are also similar to the
characteristic solvation time constants in acetonitrile.66,67
Solvation phenomena are known to cause a blue shift in
excited state absorption.68,69 Therefore, the τ1g time
component (εA → εB) can be assigned to IVR from a non-
equilibrated LE state to an equilibrated LE state possibly
accompanied by solvation.
The τ2g time component is related to the decay of the LE
state species and the formation of a new very long-lived state
that is assigned to a TICT state where an electron transfers
from carbazole moiety to the isophthalate/benzoate ring
moiety (Scheme 2). Bands II (~ 400 nm) and IV (~ 700 nm) are
characteristic of the excited-state absorption (ESA) of
carbazole cation species (Fig. S26a, ESI†).25,70,71 The ensuing
isophthalate and benzoate radical anion species are expected
to absorb in the similar spectral range for CMB 2 and CPB 3 but
at longer wavelengths in CzI 1 due to the lower reduction
potential. This is in agreement with our observations of Bands
III, at 454 and 460 nm bands in CMB 2 and CPB 3, and at lower
energies (550 nm) in CzI 1. The similar τ2g values observed for
CzI 1, CMB 2, and CPB 3 indicates that the electronic and
With kinetic data in hand, the following photoinduced
dynamics involving the LE and TICT states of carbazol-9-yl
benzoic esters 1 – 3 is proposed (Scheme 1a). Excitation at 340
nm corresponds to the carbazole S0→1Lb transition wherein
the excited state is localized on the carbazole ring (LE). As this
chromophore is common to all three molecules, CzI 1, CMB 2
and CPB
3 molecules should exhibit similar transient
absorptions in the same spectral range after excitation at 340
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Fig. 11. Jablonski energy level diagrams showing the sequence of states in acetonitrile (ACN) upon excitation at 340 nm for a) CzI 1 and CMB 2 and b) CPB 3; excitation at 280 nm
for c) CzI 1 and CMB 2 and d) CPB 3; and excitation at 288 nm for 1 – 3 in cyclohexane.
*
*
D*–A
N
D*–A
N
*
*
N
N
MeO
OMe
MeO
OMe
EtO
O
EtO
O
O
O
O
O
CPB 3
Scheme 2. Photochemical formation of the carbazole radical-cation and isophthalate
radical-anion for CzI 1.
Scheme 3. Hindered rotation of the ester in the excited state of CPB 3.
resonance. Kapturkiewicz et al. proposed the existence of
similar conformers, based on discrepancies in reorganization
energies due to differences in the resultant dipole moments,
for carbazoles with aromatic ketone acceptors but with meta-
regiochemistry.27 Unambiguous assignment of the τ3g
component to a transition from TICT1 to a second TICT2
requires further synthetic and computational work and is
currently in progress.
Excitation at 288 nm In the second measurement set,
solutions of CzI 1, CMB 2 and CPB 3 in acetonitrile were
excited at 288 nm, corresponding to the carbazole S0→1La
transitions. The initial (0 and 100 fs delay times) ΔA spectra are
more congested in comparison to the ΔA spectra measured
upon excitation at 340 nm (Fig. 12). Transient absorption
bands corresponding to the LE state reached upon excitation
at 340 nm are present at 600-620 nm (Bands I) but are much
broader. The initial spectra of all three molecules show the
broad ~ 400 and ~ 700 nm ΔA bands (Bands II and IV)
characteristic of the excited-state absorption for the carbazole
structural factors controlling the electron transfer processes in
these related molecules are not that different.
The τ3g component is very pronounced in CPB 3 but not in
CzI 1 or CMB 2. We hypothesize that this component is related
to the para- ester carboxylate acceptor. The radical anion of
the TICT state, initially formed on the benzoate ring based on
excited dipole moments, can be further stabilized by
delocalization of charge, via resonance, to a coplanar carbonyl
group (Scheme 3). Resonance gives a greater double-bond
character to the connecting C–C single bond and torsional
motion about this hindered C–C bond should cause the charge
to become less dispersed and more localized. This, in turn,
should increase the dipole moment and stabilize the energy of
the excited state, forming a second ICT state at lower energy
(TICT2) (Fig. 11a vs. 11b). The τ3g time constant (42 ps) of CPB 3
is consistent with hindered rotation with a small potential
barrier. The magnitude of the effect is much less pronounced
in CzI 1 and CMB 2 where the substituent(s) are in meta-
position(s), and cannot effectively hinder rotation via
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Fig. 12. . ΔA spectra of CzI 1, CMB 2, and CPB 3 in a 0.1-mm flowing jet of acetonitrile at a – c) 0–1 ps, d – e) 1–20 ps and f– h) 20–1000 ps. Excitation at 288 nm (pulse energy, 2.2
μJ). Delay times between the excitation and probe pulses (in picoseconds) are shown in the legends.
cation. The broad visible transient absorptions assigned to rule, S2 is expected to undergo radiationless relaxation into S1,
1
1
isophthalate/benzoate radical anion species (Bands III) are i.e. La → Lb. The observation of the excited-state absorption
present for CzI 1 and CMB 2. Bands I (600-620 nm) again from the LE state as early as at the 0-fs delay time between
decay on a 2 ps time scale concomitant with the development excitation and probe pulses suggests that the radiationless
1
of Bands II (~400 nm), Bands III (550, 454 and 460 nm for CzI 1, relaxation of the La state occurs much faster than the ~ 40 fs
CMB 2 and CPB 3, respectively) and Bands IV (~700 nm) in all pulse duration. At the same delay time, the spectroscopic
three molecules. Bands II – IV undergo some spectral signatures of the carbazole radical cation (Bands II and IV),
sharpening between 20 and 200 ps and afterwards only minor characteristic to the formation of the TICT state, are
changes occur up to 1 ns.
immediately present in all three molecules. Therefore, the
1
Global fit analyses of the ΔA spectra for 288 excitation decay of the La state directly populates the TICT state, in
show four major processes (Fig. S27 in ESI†and Table 4). The addition to population of the LE. This conclusion is consistent
first two (τ0g and τ1g) describe ultrafast spectral reshaping of with the steady state excitation spectra (Fig. S12, ESI†). The
the initial ΔA spectra and have ~ 70 and ~ 170 fs time molecules in the nascent LE and TICT states initially occupy
constants. We note that the τ0g, εA and τ1g, εB components high-energy vibrational levels and the τ0g, εA and τ1g, εB
should be viewed as a guide to the eye rather than the model components likely reflect intramolecular vibrational relaxation
results for two reasons: (i) these product species are strongly of these species. The relaxation of the equilibrated LE state
out-of-equilibrium and their absorption line shapes are species (τ2g, εC) leads to the formation of the hot TICT species,
expected to change with time, in contrast to the which undergo vibrational cooling (τ3g, εD) to produce the
presupposition of the constant band shapes by the global fit, relaxed, equilibrated TICT species (εE). It is interesting that the
and (ii) vibrational relaxation of such highly internally excited immediate observation of the TICT spectroscopic signatures
species is not necessarily single-exponential but depends on (sub-50 fs) rules out the involvement of any significant twisting
the position of the species within the potential well. The third motion during the TICT formation. Therefore, any rotation
component (τ2g ~ 1.5 - 2 ps) describes the decay of Bands I about the C–N bond to achieve the prototypical 90˚
(600-620 nm) and the development of Bands II – IV. This conformation (from the pre-twisted ~50˚) occurs after TICT
component is quite similar to τ2g obtained from 340 nm formation.
excitation. At 288 nm, this process is most pronounced in CPB Excitation at 288 nm in Cyclohexane Solutions Cyclohexane
3 and least pronounced in CzI 1. The fourth and final solutions CzI 1, CMB 2, and CPB 3 were excited at 288 nm and
component, τ4g, corresponds to spectral sharpening of the transient absorption spectra recorded. These experiments
product bands over several tens of picoseconds.
proved to be difficult due to limited solubility of these
Taken together, these data paint out the following molecules in cyclohexane solvent and gave decreased signal-
photoreaction mechanism for the case of 288 nm excitation to-noise ratio in experimental data. As observed above (Fig. 2),
(Scheme 1b and Fig. 11c,d). For all three molecules, excitation the steady-state emission spectra CzI 1, CMB 2, and CPB 3 in
at 288 nm accesses the 1La state of carbazole. Based on Kasha's
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cyclohexane are red-shifted and broadened relative to LE in the ∆A spectrum. The LE → TICT photoreaction occurs
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emission observed from CzP, which does not form an ICT state. quickly (kICT = 1/ τ2 = 50 – 70 × 1010 s ) for 1 – 3 and the
-1
DOI: 10.1039/C6CP04619C
Transient absorption spectra in all three CzI 1, CMB 2, and resultant TICT state slowly decays on the nanosecond time
CPB 3 solutions at short times after 288-nm excitation in scale. A second TICT state is clearly observed for CPB 3 and is
cyclohexane (Fig. S28, ESI†) are similar to spectra obtained ascribed to torsional motion between the ester carbonyl and
upon 288 nm excitation of these molecules in acetonitrile (Fig. phenyl ring. Higher carbazole-localized excited states, e.g. the
S26, ESI†). It follows that the interpretation of the spectra is 1La state, are reached upon excitation at 288 nm, which rapidly
also similar (Scheme 2b and Fig. 11e); 288 nm excitation into (≤ 40 fs) relax into both LE 1Lb and TICT states simultaneously.
the 1La state is followed by ultrarapid (< 50 fs) population of LE
In summary, these results indicate that rigidification of
and TICT states. The ΔA spectra remain stable until about 200 push-pull chromophores (i.e. in MOFs or solid matrices) should
ps. From 200 ps to 1000 ps, all three molecules behave not preclude the rapid LE → TICT photoreaction, which can
similarly; ESA bands from the LE (Bands I: ~600 nm) decay, occur even when the donor-acceptor pairs are weakly coupled,
while TICT absorptions of the carbazole cation moiety (Bands II e.g. a pre-twisted θ ~ 45˚. Rather, any enhancement of solid-
and IV: ~400 and ~700 nm) and the radical anion species state emission should arise from suppression of non-radiative
(Bands III: ~550 and 450 nm for CzI 1 and CMB 2/CPB 3) either decay pathways, which likely involve torsional motion.73
decay significantly slower and even rise. This can be
interpreted as radiationless relaxation between the LE and
TICT states. This relaxation occurs on time scale longer than
Experimental Section
several hundreds of picoseconds, and possibly occurs in
competition with fluorescence. Band integral analysis72 yields
time constants of ca. 2, 4.1 and 3.2 ns for CzI 1, CMB 2, and
CPB 3 for the decay process, (Fig. S29, ESI†). Finally, slow decay
of the TICT states should follow on a time scale longer than LE.
Decay of the TICT state contributes to the observed steady-
state emission in CH, apparent as shoulders at longer
wavelengths (Fig. S30) but the overall contribution is quite
small. Further emission decay profiles were recorded at 390
nm, 425 and 450 nm for 1 and 355, 375 and 410 nm for 3. The
major component was ~4ns for 1 and 3 (consistent with Table
3). Biexponential fitting of emission decays at longer
wavelengths (450 and 410 nm, respectively) confirmed the
presence of a second component (Fig S31). The contributions
of the second component increased with increasing
wavelength (from 390 to 450 nm and 355 to 410 nm,
respectively) and but the numerical values of these
components (4 – 5 ns) were too close to the LE decay (3 – 4 ns,
respectively) to be confidently resolved.
Materials
The synthesis of dimethyl 5-(9H-carbazol-9-yl)isophthalate
1 has been described previously.14 The new ethyl 3-(9H-
carbazol-9-yl)benzoate and ethyl 4-(9H-carbazol-9-
2
yl)benzoate 3 were prepared from carbazole and ethyl 3-
iodobenzoate and ethyl 4-iodobenzoate, respectively, using
standard Copper cataly8zed Ullman procedures74 in good
yields, 64% for 2 and 70% for 3. N-phenyl-carbazole CzP was
prepared from carbazole and bromobenzene. All compounds
were purified with flash column chromatography on silica and
recrystallized from methanol (1 and 3), hexanes (2) or ethanol
(CzP). Compounds were fully characteårized by 1H and 13C
NMR, HIRES mass spectrometry and elemental analysis.
Synthesis details are included in the supporting information.
Spectroscopic quality cyclohexane (CH), diethyl ether (Et2O)
tetrahydrofuran (THF), dichloromethane (DCM), acetonitrile
(ACN), and ethanol (EtOH) was purchased from EMD millipore
and used without further purification.
Absorption and Emission measurements
UV-Vis absorption spectra were recorded with Shimadzu
UV-2401 PC spectrophotometer at concentrations of 10-5 M.
Steady state fluorescence measurements were performed on
Horiba Fluorolog FL3-11 spectrofluorometer using a 450 W
xenon lamp with computer controlled excitation shutter.
Emission spectra were recorded upon excitation at the
absorption global maximum (290 nm) for each compound.
Excitation spectra were recorded at the emission maximum.
Lifetime decays were measured using time-correlated single
photon counting (TCSPC) with a 340 nm NanoLED excitation
source. Absolute quantum yields were measured at
Hamamatsu absolute Pl quantum yield spectrometer C11347.
Measurements were repeated on multiple (6+) freshly
prepared solutions and observed standard deviations are
reported. All solutions were deaerated by bubbling nitrogen
gas for 15 minutes.
Conclusions
This study provides a comprehensive examination of the
photophysical processes in the twisted, push-pull carbazol-9-yl
benzoic esters 1 – 3. Dual LE and CT emission is observed in
polar and non-polar solvents for meta- benzoic ester CzI 1 and
CMB 2 whereas only CT emission is observed for the para-
benzoic ester CPB 3. X-ray crystal structures and DFT
calculations support a partially twisted (θ ~ 50˚) ground state
that, after photoirradiation, can relax into a fully orthogonal CT
excited state. Emission from a decoupled donor-acceptor pair
is supported by the dependence of the radiative rate on
solvent polarity and small magnitude of the transition dipole
moments.
Of particular relevance is the sequence of states in the
photochemical ICT reaction deduced from the transient
absorption spectroscopy (Fig. 11). Excitation at 340 nm
effectively populates the locally excited (LE) 1Lb states localized
on the carbazole chromophore, based on Bands I at ~620 nm
Transient Absorption Measurements
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The set-up used in this work is based on a 35-fs Center. ANT acknowledges NSF support (CVAieRwEAErtRicle OCnHlinEe-
DOI: 10.1039/C6CP04619C
regeneratively amplified Ti:sapphire laser system (Spitfire Pro, 0847707, CHE-0923360 and DMR-1006761). The authors also
Spectra-Physics, 1 kHz), the output of which is split to a pump wish to thank Matthias Zeller for his assistance with the X-ray
TOPAS-C optical parametric amplifier to produce 288 and 340 crystallography of CMB 2 and CPB 3;51 Hoi Ling Luk for
nm excitation pulses and to a pump CaF2 window to produce a assistance with DFT calculations and Mayokun Ayodele for
white-light continuum probe spanning from 345 to 760 nm assistance with GC-MS measurements.
(See ESI for a full description). The solutions were circulated
through a flowing jet with a 0.1-mm path thickness. The zero
delay time is obtained from the non-resonant electronic
Notes and references
response from neat solvents measured at the same
experimental conditions.75 The typical excitation energy was
5.3 µJ pulse-1 and the linearity of solute ∆A signals with
excitation energy confirmed that single-photon excitation is
responsible for the measured data. The polarization of the
excitation beam was set at the magic angle (54.7o) with
respect to that of the probe beam. All experiments were
performed at 21oC.
Multiexponential fits of ∆A kinetic traces and evolution-
associated global fits76 of ΔA spectra were employed to
characterize the temporal evolution of the reactive species in
acetonitrile and cyclohexane. Solvent transient absorption
signals75,77 persisted only for 50 fs after excitation and were
much smaller than signals from solutes: nevertheless, the 0-50
fs region was excluded from fitting. The ΔA kinetic traces were
fitted to a sum of exponential functions:
‡ Crystal data for CMB 2: C21H17NO2, Mr = 315.36, colourless
prism, 0.13 × 0.14 × 0.17 mm, Monoclinic, P 21/c, a, b, c =
20.7631(5), 3.95300(10), 18.9724(4) Å, α, β, γ = 90°, 99.9960(10),
90°, V = 1533.55(6) Å3, Z = 4, ρ = 1.366 gcm-3, Cu Kα, F(000) =
664.0, μ = 0.699 mm-1, t = 100K, 2θmax = 79.282, 3249 unique
reflections used, 2697 with Io > 2σ(Io), Rint = 0.052, 218
parameters, 0 restraints, GoF = 1.062, R = 0.042 [Io >2σ(Io)], wR2
= 0.095 (all reflections), 0.18 < Δρ < –0.32 e Å3
1
C. W. Schlenker and M. E. Thompson, in Unimolecular and
Supramolecular Electronics I, ed. R. M. Metzger, Springer Berlin
Heidelberg, Berlin, Heidelberg, 2011, vol. 312, pp. 175–212.
2
P. J. Low, M. A. J. Paterson, D. S. Yufit, J. A. K. Howard, J. C.
Cherryman, D. R. Tackley, R. Brook and B. Brown, J. Mater.
Chem., 2005, 15, 2304.
3
L. . Hung and C. . Chen, Mater. Sci. Eng. R Rep., 2002, 39,
143–222.
4
J. V. Grazulevicius, P. Strohriegl, J. Pielichowski and K.
Pielichowski, Prog. Polym. Sci., 2003, 28, 1297–1353.
5
C.-L. Ho, L.-C. Chi, W.-Y. Hung, W.-J. Chen, Y.-C. Lin, H. Wu, E.
∆ꢀ = ∆ꢀ! + ! ꢀ!ꢀ!!
(4)
!
!
Mondal, G.-J. Zhou, K.-T. Wong and W.-Y. Wong, J Mater Chem,
2012, 22, 215–224.
6
P. Strohriegl, D. Wagner, P. Schrögel, S. T. Hoffmann, A.
where τi are the time constants, Ai are the amplitudes, ΔA0 is
the permanent spectrum at infinitely long times (>> 1 ns). Also,
the ΔA spectra were globally fitted to a sum of exponents
Köhler, U. Heinemeyer and I. Münster, eds. F. So and C. Adachi,
2013, p. 882906.
7
B. W. D’Andrade and S. R. Forrest, Adv. Mater., 2004, 16,
!!
1585–1595.
!
!,!
! ꢀ! ꢀ
(5)
8
K. Brunner, A. van Dijken, H. Börner, J. J. A. M. Bastiaansen,
N. M. M. Kiggen and B. M. W. Langeveld, J. Am. Chem. Soc.,
2004, 126, 6035–6042.
where τj,g are the time constants, and ꢀ (ꢀ) are the decay-
associated spectra characteristic of the τj,g time components.
From these data, the evolution-associated difference spectra
!
9
A. van Dijken, J. J. A. M. Bastiaansen, N. M. M. Kiggen, B. M.
W. Langeveld, C. Rothe, A. Monkman, I. Bach, P. Stössel and K.
Brunner, J. Am. Chem. Soc., 2004, 126, 7718–7727.
10 S. A. Bagnich, S. Athanasopoulos, A. Rudnick, P. Schroegel, I.
Bauer, N. C. Greenham, P. Strohriegl and A. Köhler, J. Phys.
Chem. C, 2015, 119, 2380–2387.
(ꢀ ꢀ or EADS, see p 97 in ref [76]) were reconstructed from
!
based on the assumption of
a consecutive reaction
mechanism. The j-EADS grows with a j-lifetime. The global fit
assumes that the product absorption line-shapes do not 11 C. J. Gleason, J. M. Cox, I. M. Walton and J. B. Benedict,
CrystEngComm, 2014, 16, 7621–7625.
12 V. Jankus, C. Winscom and A. P. Monkman, J. Chem. Phys.,
2009, 130, 074501.
13 S. A. Jenekhe and J. A. Osaheni, Science, 1994, 265, 765–768.
14 L. M. Lifshits, B. C. Noll and J. K. Klosterman, Chem.
Commun., 2015, 11603–11606.
15 L. M. Lifshits, C. Campana and J. K. Klosterman, Acta
Crystallogr. Sect. E Crystallogr. Commun., 2015, 71, m152–m153.
16 W. Rettig and M. Zander, Chem. Phys. Lett., 1982, 87, 229–
change with time, which can affect the resulting time
constants on a scale of several picoseconds typical of
vibrational relaxation in solution.
DFT Calculations
Ground state geometry optimizations were performed using
the density functional method B3LYP78 in vacuo with the 6-
31G* basis set.79 The absence of imaginary frequencies for
fully optimized ground state geometries of 1-3 was confirmed 234.
17 W. Rettig, J. Mol. Struct., 1982, 84, 303–327.
by second derivative (Hessian) analysis. Molecular orbital
contour plots were visualized with GaussView05.
18 W. Rettig and M. Zander, Z Naturforsch, 1984, 39a, 41–48.
19 N. Mataga, H. Yao, T. Okada and W. Rettig, J. Phys. Chem.,
1989, 93, 3383–3386.
20 J. Dobkowski, Z. R. Grabowski, J. Waluk, W. Kühnle, W.
Rettig, C. Rullière, W. Yang, J. Adamus and J. Gebicki, .
21 D. Nie, Z. Bian, A. Yu, Z. Chen, Z. Liu and C. Huang, Chem.
Phys., 2008, 348, 181–186.
22 C. Zhang, L. Feng and Z. Chen, Chem. Phys. Lett., 2006, 420,
330–335.
Acknowledgements
This work was supported by Bowling Green State University,
the BGSU Building Strength program and in part by an
allocation of computing time from the Ohio Supercomputer
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PhysicPalelaCsehedmo insotrtyaCdjhuestmmicaarlgPinhsysics
Page 12 of 12
ARTICLE
Journal Name
23 J. Catalán, C. Díaz, V. López, P. Pérez and R. M. Claramunt, J. 56 A. L. Hickey and C. N. Rowley, J. Phys. Chem. VAie,w2A0rt1ic4le,O1n1lin8e,
Phys. Chem., 1996, 100, 18392–18398. 3678–3687.
24 Y. Mo, F. Bai and Z. Wang, J. Photochem. Photobiol. Chem., 57 A. G. Pinkus and E. Y. Lin, J. Mol. Struct., 1975, 24, 9–26.
DOI: 10.1039/C6CP04619C
1995, 92, 25–27.
58 A. R. Katritzky, M. V. Sinnott, T. T. Tidwell and R. D. Topsom,
25 V. A. Galievsky, S. I. Druzhinin, A. Demeter, P. Mayer, S. A. J. Am. Chem. Soc., 1969, 91, 628–636.
Kovalenko, T. A. Senyushkina and K. A. Zachariasse, J. Phys. 59 E. N. Gur’yanova and N. . Grishko, Zhurnal Strukt. Khimii,
Chem. A, 2010, 114, 12622–12638.
1963, 4, 368–71.
26 T. Yoshihara, V. A. Galievsky, S. I. Druzhinin, S. Saha and K. A. 60 C. J. Jödicke and H. P. Lüthi, J. Am. Chem. Soc., 2003, 125,
Zachariasse, Photochem. Photobiol. Sci., 2003, 2, 342. 252–264.
27 A. Kapturkiewicz and J. Nowacki, J. Phys. Chem. A, 1999, 103, 61 E. K. U. Gross and W. Kohn, in Advances in Quantum
8145–8155. Chemistry, Elsevier, 1990, vol. 21, pp. 255–291.
28 A. Kapturkiewicz, J. Herbich, J. Karpiuk and J. Nowacki, J. 62 M. E. Casida, J. Mol. Struct. THEOCHEM, 2009, 914, 3–18.
Phys. Chem. A, 1997, 101, 2332–2344. 63 R. D. Amos, Chem. Phys. Lett., 2002, 364, 612–615.
29 A. Kapturkiewicz, J. Herbich and J. Nowacki, Chem. Phys. 64 T. Elsaesser and W. Kaiser, Annu. Rev. Phys. Chem., 1991, 42,
Lett., 1997, 275, 355–362. 83–107.
30 J. Herbich, A. Kapturkiewicz and J. Nowacki, Chem. Phys. 65 I. Martini and G. V. Hartland, J. Phys. Chem., 1996, 100,
Lett., 1996, 262, 633–642.
19764–19770.
31 G. E. Johnson, J. Phys. Chem., 1974, 78, 1512–1521.
66 M. Maroncelli, J. Mol. Liq., 1993, 57, 1–37.
32 R. W. Bigelow and G. E. Johnson, J. Chem. Phys., 1977, 66, 67 M. L. Horng, J. A. Gardecki, A. Papazyan and M. Maroncelli, J.
4861.
Phys. Chem., 1995, 99, 17311–17337.
33 N. Berman, C. Ruof and H. Howard, Anal. Chem., 1951, 23, 68 P. F. Barbara and W. Jarzeba, in Advances in Photochemistry,
1882–1883.
eds. D. H. Volman, G. S. Hammond and K. Gollnick, John Wiley &
34 Sadtler Research Laboratories and W. W. Simons, Eds., The Sons, Inc., Hoboken, NJ, USA, 1990, pp. 1–68.
Sadtler handbook of ultraviolet spectra, Sadtler Research 69 R. Jimenez, G. R. Fleming, P. V. Kumar and M. Maroncelli,
Laboratories, Philadelphia, Pa, 1979.
Nature, 1994, 369, 471–473.
35 J. Hicks, M. Vandersall, Z. Babarogic and K. B. Eisenthal, 70 T. Ueda, R. Fujisawa, H. Fukumura, A. Itaya and H. Masuhara,
Chem. Phys. Lett., 1985, 116, 18–24. J. Phys. Chem., 1995, 99, 3629–3635.
36 Z. R. Grabowski, K. Rotkiewicz and W. Rettig, Chem. Rev., 71 H. Miyasaka, T. Moriyama, S. Kotani, R. Muneyasu and A.
2003, 103, 3899–4032.
Itaya, Chem. Phys. Lett., 1994, 225, 315–321.
37 E. Lippert, Z. Für Naturforschung A, 1955, 10.
72 T. Bultmann and N. P. Ernsting, J. Phys. Chem., 1996, 100,
38 N. Mataga, Y. Kaifu and M. Koizumi, Bull. Chem. Soc. Jpn., 19417–19424.
1956, 29, 465–470.
39 L. Onsager, J. Am. Chem. Soc., 1936, 58, 1486–1493.
73 D. A. Shultz and M. A. Fox, J. Am. Chem. Soc., 1989, 111,
6311–6320.
40 C. Reichardt, Solvents and solvent effects in organic 74 C. Sambiagio, S. P. Marsden, A. J. Blacker and P. C.
chemistry, Wiley-VCH, Weinheim, 3rd, updated and enl. ed., McGowan, Chem. Soc. Rev., 2014, 43, 3525.
2003.
75 S. A. Kovalenko, A. L. Dobryakov, J. Ruthmann and N. P.
41 J. Karpiuk, Y. N. Svartsov and J. Nowacki, Phys. Chem. Chem. Ernsting, Phys. Rev. A, 1999, 59, 2369–2384.
Phys., 2005, 7, 4070.
76 I. H. M. van Stokkum, D. S. Larsen and R. van Grondelle,
42 R. S. Butler, P. Cohn, P. Tenzel, K. A. Abboud and R. K. Biochim. Biophys. Acta BBA - Bioenerg., 2004, 1657, 82–104.
Castellano, J. Am. Chem. Soc., 2009, 131, 623–633.
77 M. Rasmusson, A. N. Tarnovsky, E. Åkesson and V.
43 J. R. Lakowicz, Principles of fluorescence spectroscopy, Sundström, Chem. Phys. Lett., 2001, 335, 201–208.
Springer, New York, 3rd ed., 2006. 78 C. Lee, W. Yang and R. G. Parr, Phys. Rev. B, 1988, 37, 785–
44 Y. Niko, S. Kawauchi and G. Konishi, Chem. - Eur. J., 2013, 19, 789.
9760–9765.
79 R. Ditchfield, J. Chem. Phys., 1971, 54, 724.
45 N. J. Turro, Modern molecular photochemistry, Univ. Science
Books, Sausalito, Calif, 1991.
46 M. Maus, W. Rettig, D. Bonafoux and R. Lapouyade, J. Phys.
Chem. A, 1999, 103, 3388–3401.
47 S. Sasaki, K. Hattori, K. Igawa and G. Konishi, J. Phys. Chem.
A, 2015, 119, 4898–4906.
48 J. Herbich and A. Kapturkiewicz, Chem. Phys., 1991, 158,
143–153.
49 J. Herbich and A. Kapturkiewicz, Chem. Phys., 1993, 170,
221–233.
50 S. Murali, V. Kharlanov, W. Rettig, A. I. Tolmachev and A. V.
Kropachev, J. Phys. Chem. A, 2005, 109, 6420–6429.
51 L. M. Lifshits, M. Zeller and J. K. Klosterman, Manuscript in
Preparation.
52 L.-Q. Chen, C.-L. Yang, X.-G. Meng and J.-G. Qin, Acta
Crystallogr. Sect. E Struct. Rep. Online, 2005, 61, o3073–o3075.
53 S. Saha and A. Samanta, Acta Crystallogr. C, 1999, 55, 1299–
1300.
54 C. Avendaño, M. Espada, B. Ocaña, S. García-Granda, M. del
R. Díaz, B. Tejerina, F. Gómez-Beltrán, A. Martínez and J. Elguero,
J Chem Soc Perkin Trans 2, 1993, 1547–1555.
55 Y.-Z. Xie, J.-Y. Jin and X.-C. Qu, Acta Crystallogr. Sect. E Struct.
Rep. Online, 2012, 68, o1199–o1199.
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