Dicyano and pyridine derivatives of β-carotene: Synthesis and vibronic, electronic, and photophysical properties

Article abstract of DOI:10.1021/jp106293s

Density functional theory and time-dependent density functional theory calculations provide pictures of the molecular orbitals involved in the ground and excited states of two cyano derivatives of 8′-apo-β-caroten- 8′-al synthesized via an acid-base-catal

Full text of DOI:10.1021/jp106293s

ARTICLE
pubs.acs.org/JPCA
Dicyano and Pyridine Derivatives of β-Carotene: Synthesis and
Vibronic, Electronic, and Photophysical Properties
A. J. Cruz,† K. Siam,‡ and D. P. Rillema*,†
†Department of Chemistry, Wichita State University, Wichita, Kansas 67260, United States
‡Department of Chemistry, Pittsburgh State University, Pittsburgh, Kansas 66762, United States
ABSTRACT: Density functional theory and time-dependent
density functional theory calculations provide pictures of the
molecular orbitals involved in the ground and excited states of
two cyano derivatives of 80-apo-β-caroten-80-al synthesized
via an acid-base-catalyzed Knoevenagel condensation reac-
tion. Population analysis shows that the symmetry-allowed
transition, S0 (1Ag) f S2 (1Bu) based on the C2h symmetry is a
HOMO (highest occupied molecular orbital) to LUMO
(lowest unoccupied molecular orbital) π f π* transition
with electron densities located mostly on the polyene chain.
Calculated and actual steady-state absorption spectra show similar features with low-energy peak maxima between 550 and 600 nm.
’ INTRODUCTION
Carotenoids play an important role in photosynthesis, parti-
cularly in their capacity to harvest light and initiate photosensi-
tized energy transfer and electron transport in photosystems
bound to cell membranes.1-3 β-Carotenes, for instance, serve as
antennas for absorbing light in spectral regions where chloro-
phylls do not absorb, and thus initiate photosynthesis.4-10 In the
past, group theoretical treatments based on the C2h point
group4,5 for carotenes revealed that the electronic ground state
is 1Ag (S0) in character, whereas the symmetry-allowed transition
found in the visible region, responsible for pigment coloration in
plant leaf tissues and light-harvesting properties in photosystems,
is due to the population of a second excited state, 1Bu (S2),2,4-22
shown in Figure 1. The S0 f S2 transition has been well-
characterized by steady-state absorption measurements con-
joined with molecular orbital treatments.4-22 The first 1Ag
(S1) excited state has also been identified1,11-13,23 and is known
to lie between the ground state and the 1Bu excited state. Previous
studies involving carotenoids involved in the xanthophyll cycle
Figure 1. Energy diagram and photochemical processes in carotenoids
(C2h point group).
1
1
showed that after the Ag (S0) f Bu (S2) transition, internal
conversion occurs populating the S1 excited state.23,24 This
process is the primary route in energy transfer and energy
dissipation in carotenoid-chlorophyll systems.23
also reported and were localized on the CdC atoms in the
polyene chain. The eigenvalues of the energy states were not
reported.33-35
To date, this excited state has been characterized using low-
temperature (77 K) emission studies25 and two-photon exci-
tation26,27 and time-resolved infrared spectroscopy.28-30 Triplet
states have also been investigated previously by flash photolysis
and pulse radiolysis and are known to be responsible for the
photoprotection of plants by quenching triplet chlorophyll.31,32
Previously, the 70,70-dicyano-70-apo-β-carotene was synthe-
sized and AM1 (Austin-model 1) calculations were carried out to
In this study, we reinvestigate the chemistry of 70,70-dicyano-
70-apo-β-carotene (3) synthesized via acid-catalyzed Knoevena-
gel condensation and a new cyanine derivative, 70-cyano-70-
pyridyl-70-apo-β-carotene (2), that can serve as antennas for
attachment to transition metal complexes. Their photophysical
Received: July 7, 2010
1
1
optimize the ground (1 Ag), first excited state (2 Ag), and
Revised:
December 20, 2010
second excited state (1 1Bu). Their relative charge densities were
Published: January 18, 2011
r
2011 American Chemical Society
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and vibronic properties have been examined. In a previous paper,
we used semiempirical calculations to establish the properties of
these states in short-chained systems.33 Here, density functional
theory (DFT) and time-dependent density functional theory
(TDDFT) calculations have been employed to ascertain their
spectral properties with respect to those determined experimen-
tally. Population analysis was also performed and correlated with
experimental results. Scheme 1 shows the structures of the
β-carotene analogues. Relative energies of the S1 state in carot-
enoids involved in the xanthophyll cycle have been identified
indirectly by fluorescence studies and observed in the near-IR
region.1,23,28-30
’ EXPERIMENTAL SECTION
Materials. The compound all-trans-80-apo-β-caroten-80-al
(1) was obtained from Sigma. Malononitrile was obtained
from Fluka; the compound 4-pyridyl acetonitrile monohy-
drochloride was purchased from Acros Scientific. Propionic
acid and ammonium carbonate were purchased from Aldrich.
Dried benzene, ethyl ether, HPLC grade methanol, chloro-
form, methylene chloride, and Optima grade acetonitrile for
UV-vis spectral determinations were purchased from Fisher
Scientific. IR grade potassium bromide salt used as a pellet for
infrared spectral analysis was obtained from Aldrich. Sulfur
powder used as standard to calibrate the Fourier transform
(FT) Raman spectrometer was provided by Thermo Nicolet. A
4:1 ethanol/methanol solution was prepared and used as
solvent for near-IR emission study.
Scheme 1. Chemical Structure of Naturally Occurring 80-
Apo-β-caroten-80-al (1) and Its Derivatives
Preparation of All-trans-70-cyano-70-pyridyl-70-apo-β-carot-
ene (2). The following preparation was performed under an
argon atmosphere. All-trans-80-apo-β-caroten-80-al (103.3 mg,
2.23 ꢀ 10-4 mol) was placed in a 10 mL round-bottom flask. The
compound was then dissolved in benzene and was allowed to stir
for 5 min. After the solid dissolved, 4-pyridylacetonitrile mono-
hydrochloride (58.3 mg, 5.30 ꢀ 10-4 mol) and ammonium
carbonate (98.6 mg, 1.03 ꢀ 10-3 mol) were added to the solu-
tion and the reaction was allowed to stir for about 5 min. Then,
1.00 mL of propionic acid was added dropwise into the solution.
After all the reagents were added, the reaction was refluxed
overnight with its temperature regulated at approximately 70 °C.
Figure 2. Knoevenagel condensation of 80-apo-β-caroten-80-al with 4-pyridylacetonitrile monohydrate and malononitrile.
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Figure 3. Mechanism for the acid- and base-catalyzed Knoevenagel condensation of 80-apo-β-caroten-80-al with 4-pyridylacetonitrile monohydrate.
Then, the reaction was allowed to cool in an ice bath, and the
solvent was removed by lyophilization yielding a red-orange solid
which was redissolved in chloroform and subjected to column
chromatography through an oven-dried neutral alumina (∼10.0 g)
stationary phase. The mobile phase used was 7:1 chloroform/
methylene chloride. The first and final fractions were discarded,
whereas the middle fractions were examined on thin-layer chro-
matography (TLC) plates for purity. The pure fractions were
combined, and the solid was recovered after freeze-drying the
solution to remove the solvent. The purple compound was dried
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Table 1. Vibrational Stretching Frequencies (cm-1) of CN and Polyene Groups
compound
80-apo-β-caroten-80-al, 1
functional group
experimental: (infrared)
experimental: (Raman)
CdO stretch
1669
1524
1567
1610
na
na
-CdC- skeletal
1521
dC-H in-plane bending
CN stretch
1148
2266
na
all-trans-70-cyano-70-pyridyl-70-apo-β-carotene, 2
2201
1406
1510
1546
na
-CdC- skeletal
dC-H in-plane bending
polyene -CH3 stretch
CN stretch
1232
na
2924
2205
1400
1504
1546
na
all-trans-70,70-dicyano-70-apo-β-carotene, 3
na
-CdC- skeletal
1506
dC-H in-plane bending
polyene -CH3 stretch
1164
na
2962
in a vacuum oven and was stored in a desiccator. Color: purple,
82%. Anal. Calcd for C37H44N2: C, 85.98; H, 8.50; N, 5.40.
Found: C, 86.01; H, 8.19; N, 4.25. IR (KBr pellet, cm-1): (CN)
2201 m, (-CdC-) 1503 sh, 2924 w. Raman (solid, cm-1):
(CN) 2266, (-CdC-H) 1232.
Preparation of All-trans-70,70-dicyano-70-apo-β-carotene
(3). All-trans-80-apo-β-caroten-80-al (101.2 mg, 2.18 ꢀ 10-4
mol) was placed in a 10 mL round-bottom flask. The compound
was then dissolved in benzene and allowed to stir for 5 min. After
dissolution, malononitrile (58.3 mg, 5.30 ꢀ 10-4 mol) and
ammonium carbonate (98.6 mg, 1.03 ꢀ 10-3 mol) were added
to the solution and the reaction was allowed to stir further for
about 5 min. Then, 1.00 mL of propionic acid was added drop-
wise into the solution. After all the reagents were added, the
reaction was refluxed overnight with its temperature regulated at
approximately 70 °C. Then, the reaction was allowed to cool in
an ice bath and the solvent was removed by lyophilization. The
purple-black solid was redissolved in chloroform and was sub-
jected to column chromatography through an oven-dried neutral
alumina (∼10.0 g) stationary phase. The mobile phase used was
7:1 chloroform/methylene chloride. The first and final fractions
were discarded, whereas the middle fractions were examined for
purity on TLC plates. After combining the fractions, the solid was
recovered after freeze-drying the solution to remove the solvent.
The dark compound was placed in a vacuum oven to dry. Color:
black, 85%. Anal. Calcd for C33H40N2: C, 85.28; H, 8.68; N, 6.03.
Found: C, 85.09; H, 8.41; N, 5.86. IR (KBr pellet, cm-1): (CN)
2205 m, (-CdC-) 1501 sh, 2962 w. Raman (solid, cm-1):
(-CdC-) 1506, (-CdC-H) 1164
Measurements. Infrared spectra were obtained using Perkin-
Elmer model 1600 FT-IR and Nicolet Avatar model FT-IR
spectrophotometers. All samples were prepared as potassium
bromide pellets. Raman spectra were obtained using a Thermo
Nicolet FT Raman module spectrophotometer. The Nicolet
instruments were accompanied by Omni software programs.
Proton (1H) and carbon (13C) NMR spectra were obtained
using Varian Mercury 300 MHz and Varian Inova 400 MHz FT
NMR spectrometers (internal standard, TMS). Ultraviolet spec-
tra were obtained using a Hewlett-Packard model 8452A diode
Figure 4. Raman spectrum of 80-apo-β-caroten-80-al (solid).
array spectrophotometer interfaced with an OLIS software
program. Elemental (C, H, and N) analysis was performed by
MHW Laboratories. Near-IR emission was carried out using a
SPEX Nanolog spectrometer. Samples for emission studies were
degassed via the freeze-pump-thaw method36 with the absor-
bance set at 0.1 λex = 519 nm.
Calculations. Calculations were effected using Gaussian ’03
(rev. B.03)37 for UNIX. The molecules were optimized using
Becke’s three-parameter hybrid functional B3LYP38 with the
local terms39 of Lee, Yang, Parr and the nonlocal term40 of
Vosko, Wilk, and Nassiar. The basis set 6-311G** was chosen
for all atoms, and the geometry optimizations were all ran in the
gas phase. TDDFT41-47 calculations were employed to pro-
duce a number of singlet excited states48 in the gas phase based
on the optimized geometry. Simulated absorption spectra were
generated using the Gauss sum 2.2 software package for the
TDDFT data.
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Figure 5. Infrared spectra: 2a and 3a (solid). Raman Spectra: 2b and 3b (solid).
’ RESULTS AND DISCUSSION
acid catalyst was added to protonate the carbonyl oxygen gene-
rating the carbonyl carbocation necessary to initiate the con-
densation reaction.
Synthesis. The preparation of 2 and 3 followed the Kno-
evenagel49,50 reaction which involved the condensation of the
aldehyde, 80-apo-β-caroten-80-al, with substrates lacking R-hy-
drogen atoms adjacent to the coupling site. This criterion is
required to enhance the acidity of the methylene (-CH2-) pro-
tons. Traditionally, this condensation reaction is catalyzed using
only a weak base such as piperidine or NaNH2.49 In our case, we
used simultaneous base- and acid-catalyzed condensation reac-
tions to enhance the yield. The base catalyst used was NH4CO3,
whereas the acid catalyst chosen was propionic acid (CH3CH2-
COOH). The reaction scheme is shown in Figure 2.
Vibrational Properties. The infrared and Raman spectral
data of carotene reactant and products are listed in Table 1. The
Raman spectrum of 80-apo-β-caroten-80-al is shown in Figure 4.
The IR spectrum of 80-apo-β-caroten-80-al for skeletal vibrations
was consistent with previously published results.28 Both IR and
Raman spectra of the products are shown in Figure 5. As found in
Table 1, the carbonyl (CdO) stretch of 80-apo-β-caroten-80-al is
IR-active but not Raman-active. This stretching frequency occurs
at 1669 cm-1. Results show that the Raman behavior of the long-
chained derivatives follows the same trend as their short-chained
counterpart.34 The Raman scattering in 2 is a little weaker than
that of 3. In the case of the pyridine adduct, the CN stretching
frequency is both IR- and Raman-active with the latter having a
higher energy than its cyano-pyridine counterpart. Their energies
are found at 2201 and 2266 cm-1, respectively. As expected,
The mechanism of formation of the pyridyl adduct is pre-
sented in Figure 3. This can be extended to the dicyano deriv-
ative. In this reaction, the base catalyst was added first to the
reaction mixture to neutralize the hydrochloride and to depro-
tonate one of the methylene (-CH2-) protons. Then, the weak
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Table 2. Percentage of Electronic Distribution in Carotene
Derivatives
all-trans-70-cyano-70-
pyridyl-70-apo-β-
carotene, 2
all-trans-70,70-dicyano-
70-apo-β-
states:
carotene, 3
LUMO þ 3
5 (chain)
0 (β-ring)
90 (CN)
78 (chain)
18 (β-ring)
4 (CN)
5 (pyridyl)
77 (chain)
7 (β-ring)
1 (CN)
LUMO þ 2
LUMO þ 1
LUMO
87 (chain)
8 (β-ring)
5 (CN)
15 (pyridyl)
83 (chain)
3 (β-ring)
10 (CN)
89 (chain)
3 (β-ring)
7 (CN)
3 (pyridyl)
88 (chain)
1 (β-ring)
7 (CN)
91 (chain)
1 (β-ring)
8 (CN)
4 (pyridyl)
90 (chain)
6 (β-ring)
2 (CN)
HOMO
90 (chain)
7 (β-ring)
3 (CN)
2 (pyridyl)
76 (chain)
16 (β-ring)
5 (CN)
Figure 6. Orbital pictures of selected states involved in optical transi-
tion in 2 and 3.
HOMO - 1
HOMO - 2
HOMO - 3
74 (chain)
20 (β-ring)
6 (pyridyl)
the -CdC- skeletal stretching frequencies of the cyano pro-
ducts are lowered compared to those of 80-apo-β-caroten-80-al.
The -CdC- IR stretch in the cyano-pyridyl adduct can be
found at 1503 cm-1. In case of the dicyano derivative, this
stretching frequency is both IR- and Raman-active and located at
1501 and 1506 cm-1, respectively. The polyene -CH3 stretch
of the pyridine and dicyano adduct are found at 2924 and 2962
cm-1, respectively.
3 (pyridyl)
57 (chain)
35 (β-ring)
6 (CN)
56 (chain)
38 (β-ring)
6 (pyridyl)
2 (pyridyl)
62 (chain)
29 (β-ring)
8 (CN)
70 (chain)
24 (β-ring)
7 (pyridyl)
2 (pyridyl)
’ CALCULATIONS (DFT/TDDFT)
Molecular Orbitals. The highest occupied molecular orbitals
(HOMOs) and lowest unoccupied molecular orbitals (LUMOs)
of the cyano-pyridyl adduct determined by DFT calculations are
shown in Figure 6. The electron density is somewhat shifted from
the HOMO to the LUMO toward the pyridyl ring even though
the majority of the electron density is localized on the polyene
chain. This trend is similar to the short-chained counterpart, as
reported previously.36
The same electron density shift can also be observed in the
nature of the HOMO/LUMO orbitals of the dicyano-carotenoid
derivative as shown in Figure 6. Here the shift in the electron
density is even greater from the β-ring and moves toward the
nitrile groups than for cyano-pyridine derivative.
Population Analysis. Table 2 lists the population analysis
data and the percentage electronic distribution of some selected
states in 2 and 3 as shown in Figure 6. All calculations were
effected using TDDFT. Results show similar trends as their
short-chained counterparts.36 The HOMO is predominantly
polyene in character with 90% occupancy in both 2 and 3. The
most remarkable result is that both have equal electronic densities
Table 3. UV-Visible Data of the S0 1Ag f S2 1Bu Transition
in 1, 2, and 3
compound:
E (λmax) (exptl)a
ε (M-1 cm-1
)
E (λmax) (calcd)
1
2
3
448
517
519
7.11 ꢀ 105
8.00 ꢀ 104
6.33 ꢀ 104
541
609
625
a Dried acetonitrile.
in the conjugated chain, where in dicyano’s case, the β-ring and
CN groups do not play an important contribution. The same
trend was observed in the case of the LUMO of both 2 and 3.
Although the LUMO character lies on the polyene chain, the
electron density of the LUMO in 2 is distributed throughout the
molecule and is shifted to the newly condensed group, 4% pyridyl
group contribution.
This pyridyl group plays an important role in hosting the
excited electron. In some cases the methyl groups of the polyene
chain have minor contributions in one of the states such as that
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Figure 7. Experimental and calculated UV-vis data for 2 and 3.
Table 4. Calculated UV-Visible Data for 2 and 3
2
3
λ (nm)
F
major contributors
λ (nm)
F
major contributors
612
486
3.2580
0.7496
HOMO f LUMO
622
483
2.9117
1.0436
HOMO f LUMO
HOMO - 1 f LUMO
HOMO f LUMO þ 1
HOMO - 2 f LUMO
HOMO - 1 f LUMO þ 1
HOMO f LUMO þ 1
HOMO - 3 f LUMO
HOMO - 2 f LUMO þ 1
HOMO f LUMO þ 2
HOMO f LUMO þ 4
HOMO - 2 f LUMO þ 1
HOMO f LUMO þ 3
HOMO f LUMO þ 3
HOMO - 1 f LUMO þ 2
HOMO - 1 f LUMO
HOMO f L þ 1
372
309
0.1123
0.0851
369
352
0.0422
0.0132
HOMO - 2 f LUMO
HOMO - 1 f LUMO þ 1
HOMO f LUMO þ 1
HOMO - 3 f LUMO
HOMO - 2 f LUMO
HOMO - 1 f LUMO þ 1
296
0.1509
306
0.0653
HOMO - 2 f LUMO þ 1
HOMO f LUMO þ 2
HOMO - 4 f LUMO
HOMO - 2 f LUMO þ 1
HOMO - 1 f LUMO þ 2
HOMO f LUMO þ 2
HOMO - 1 f LUMO þ 2
HOMO f LUMO þ 3
294
287
0.1115
0.0399
294
287
0.0152
0.2894
HOMO f LUMO þ 4
HOMO - 7 f LUMO
HOMO - 3 f LUMO þ 1
264
0.0266
277
0.0262
observed in HOMO - 2. In the latter case, the π-bonding
interaction observed in retinoids arises every third atom of the
polyene chain. All low-lying LUMO orbitals in 2 have major
contributions from the pyridyl ring. The electronic distributions
of other low-lying HOMO and LUMO orbitals are pictured in
Figure 6. The electronic density distribution in the higher
LUMOs of 3 is localized mostly on the polyene chain where
the cyano groups have less effect on the π-conjugation. However,
as the electron density shifts toward the dicyano group, the β-ring
electronic density depletes as one goes from the LUMO to the
higher LUMO orbitals. For instance, the β-ring has a 1% con-
tribution in the electron density of its LUMO and an 18%
contribution in the LUMO þ 3.
Electronic Spectra. The UV-vis spectral data of the com-
pounds are given in Table 3. Both experimental and calculated
data showed bathochromic shifts in the visible region of the
absorption spectrum, as expected, with respect to the starting 80-
apo-β-caroten-80-al derivative. For the cyano-pyridine derivative,
λmax was shifted by 69 nm from 448 to 517 nm. Unlike the short-
chained version,34 λmax of the dicyano derivative did not shift
much relative to the pyridyl adduct, which is observed at 519 nm.
However, a similar bathochromic trend is observed with respect
to the energy positions of the λmax in the visible spectrum. This is
due to the lengthening of the polyene chain (1-D box). After
condensation, a new double bond was generated between
the carbonyl carbon of the retinal and the methylene groups of
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As also shown in Figure 7, the simulated UV-vis spectra
obtained using TDDFT calculations gave rise to four distinct
theoretical bands in 2, whereas three bands were calculated for 3.
Table 4 lists selected transitions, their corresponding oscillator
strengths, and the major molecular orbitals that contribute in
each transition. As expected, the HOMO to LUMO transition of
2 (S0 1Ag f S2 1Bu) has the highest oscillator strength (f =
3.2580), in agreement with its high molar extinction coefficient,
and is the predominant peak calculated in the UV-vis spectrum
with its maximum centered at 609 nm. A theoretical peak located
at 486 nm with oscillator strength of 0.75 involves the following
transitions: HOMO - 1 f LUMO and HOMO f LUMO þ 1.
The other calculated peaks merge pairwise into the peaks shown
in Figure 7. For example, the peaks at 372 and 296 nm appear as
one peak. The calculated peak at 296 and 294 nm gave an average
oscillator strength of 0.13. Under this band lie the following
transitions: HOMO - 2 f LUMO þ 1 and HOMO f LUMO
1
þ 3. This band can be attributed to the cis peak (S0 1Ag f S3 Ag)
as the transition reported earlier in this energy region for
initiating cis/trans isomerization.34 The molecular orbital pic-
tures showing these transitions are shown in Figure 6. The
calculated UV-vis data of 3 are also listed in Table 4. Once again,
the HOMO-LUMO transition (S0 1Ag f S2 1Bu) dominates
with high oscillator strength 2.5 times greater than the others.
The theoretical peak observed at 483 nm (f = 1.04) involves a
similar type of electronic transitions as the pyridyl adduct
assigned as HOMO - 1 f LUMO and HOMO f LUMO þ 1.
Both of these transitions may lie under the lowest energy
absorption band of 2 and 3. A theoretical peak in the UV region
was observed, this time at 287 nm. It has oscillator strength
of 0.29 involving the following electronic transitions: HOMO
- 2 f LUMO þ 1, HOMO - 1 f LUMO þ 2, and HOMO f
LUMO þ 2. This band can be attributed to the cis peak (S0 1Ag
f S3 1Ag).
Figure 8. Near-IR emission spectrum of 3 in 4:1 EtOH/MeOH.
CN-C-py (for 2) and CN-C-CN (for 3). The experimental
molar extinction coefficients are also tabulated in Table 3. Results
gave a value of 8.00 ꢀ 104 M-1 cm-1 for 2 and 6.33 ꢀ 104 M-1
cm-1 in 3. The UV-vis spectra of 2 and 3 are shown in Figure 7.
The theoretical UV-vis absorption data of 1, 2, and 3 were
determined using TDDFT calculations and are overlaid with the
actual electronic spectra (Figure 7). The calculated λmax are also
listed in Table 3. Similar to the short-chained derivatives,34
a
hypsochromic shift was observed in the actual electronic spectra
of the carotenoid derivatives compared to the simulated ones.
However, in the latter case, the absorption spectra in gas phase
appear approximately 100 nm more red-shifted compared to the
ones in liquid solution. The lowest energy π f π* transitions in 2
underwent an actualblue-shiftfrom 609nmin gas phaseto 517 nm
in acetonitrile solution. For 3, the shift was from 625 to 519 nm in
solution. Calculations revealed that the lowest energy transition
occurred from the HOMO to LUMO in both 2 and 3, similar to
the ones observed in their retinoid counterparts.31 Experimen-
tally, a weak absorption band was also observed for both cases
appearing in the UV region (Figure 7). Overall, the features in
both experimental and theoretical spectra are similar despite the
energy shift observed. The shift is often observed due to the gas-
phase calculations compared to the experimental results in
solution.
The assignment of these transitions can be related to those
determined from group theoretical treatment4-22 given in
Figure 1 with the Ag and Bu symmetry orbital labels applied to
both cyano carotenoid derivatives to describe the nature of the
π f π* band (S0 1Ag f S2 1Bu). As expected, the π f π* located
on the polyene chain is the major contributor to the lowest
energy transition. The HOMO of 2 (Figure 6) shows the
electron density as located on the polyene chain with a small
contribution from the β-ring, whereas the LUMO shows some
electron density of the polyene chain extending toward the pyri-
dyl group.
Emission Spectrum. Excitation of all-trans-70,70-dicyano-70-
apo-β-carotene, 3 at 519 nm leads to weak emission in the near-
IR region which peaked at 1055 nm and is consistent with that
determined by two-photon excitation.28 The emission spectrum
of 3 is shown in Figure 8. This emission is attributed to indirect
population of the low-lying S1 (2 1Ag) state, which results after
direct population of the S2 (1 1Bu) followed by internal conver-
sion. Previous studies involving transient IR and visible spectros-
copy of 80-apo-β-caroten-80-al and its derivatives29,30 also lead to
a possibility that the near-IR emission observed in these systems
may be dependent on excitation wavelength and will be explored
further.
’ SUMMARY
Two cyanine derivatives of β-apo-80-carotenal were synthe-
sized using the acid-base-catalyzed Knoevenagel condensation
reaction. A CN stretch at 2201 cm-1 was observed in the infrared
and at 2266 cm-1 in the Raman for 2 and at 2205 cm-1 in
the Raman for 3. Absorption maxima were red-shifted com-
pared to the parent β-apo-80-carotenal. The Ag and Bu symmetry
orbital labels based on group theoretical calculations reported
earlier4-22 were applied to both cyano carotenoid derivatives to
describe the nature of the colorful π f π* band. DFT and
TDDFT calculations established the nature and predict the
location of the transitions involved in the optical spectra.
Calculations revealed that the symmetry-allowed S0 (1Ag) f
S2 (1Bu) transition can be assigned to the HOMO f LUMO in
The HOMO of 3 is spread out over the polyene chain, the
dicyano group, and the β-ring. Some p-orbital contribution in
the case of the methylene carbon is also observed. Its LUMO on
the other hand is less distorted compared to its retinoid derivative
and is very distinctive of polyene chain π*-orbital with a small
contribution from the dicyano groups.
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The Journal of Physical Chemistry A
ARTICLE
both cases, which are mostly polyene π f π* in character. Weak
S1 fS0 fluorescence in the case of the dicyano derivative was
found for the first time for carotenoids in the near-IR region at
1055 nm.
(34) O’Neil, M. P.; Wasielewski, M. R.; Khaled, M. M.; Kispert, L. D.
J. Chem. Phys. 1991, 95, 7212.
(35) Wasielewski, M. R.; Johnson, D. G.; Bradford, E. G.; Kispert,
L. D. J. Chem. Phys. 1989, 91, 6691.
(36) Cruz, A. J.; Siam, K.; Moore, C.; Islam, M.; Rillema, D. P. J. Phys.
Chem. A 2010, 114, 6766.
’ ACKNOWLEDGMENT
(37) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.;
Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J. J.;
Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson,
G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.;
Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.;
Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.;
Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.;
Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.;
Voth, A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.;
Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.;
Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.;
Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.;
Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham,
M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.;
Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian
03, revision B.04; Gaussian Inc.: Wallingford, CT, 2004.
(38) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.
We thank the Wichita State University High Performance
Computing Center, the Wichita State University Office of
Research Administration, and the Department of Energy for
support.
’ REFERENCES
(1) Polivka, T.; Herek, J. L.; Zigmantas, D.; Akerlund, H. E.;
Sundstrom, V. Proc. Natl. Acad. Sci. U.S.A. 1999, 96, 4914.
(2) Frank, H. A.; Cogdell, R. Photochem. Photobiol. 1996, 63, 257.
(3) Demmig-Adams, B.; Adams, W. W. Trends Plant Sci. 1996, 1, 21.
(4) Cogdell, R. J.; Frank, H. A. Biochim. Biophys. Acta 1987, 895, 63.
(5) Young, A. J.; Britton, G. Carotenoids in Photosynthesis, 1st ed.;
Chapman and Hall: London, 1993.
(6) Frank, H. A.; Young, A. J.; Britton, G.; Cogdell, R. J. The
Photochemistry of Carotenoids; Klewer Academic: Dordrecht, The
Netherlands, 1999.
(39) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785.
(40) Vosko, S. H.; Wilk, L.; Nusair, M. Can. J. Phys. 1998, 109, 8218.
(41) Andrae, D.; Hauessermann, U.; Dolg, M.; Stoll, H.; Preuss, H.
Theor. Chim. Acta 1990, 77, 123.
(7) Borland, C. F.; McGarvey, D. J.; Truscott, T. G.; Cogdell, R. J.;
Land, E. J. J. Photochem. Photobiol., B 1987, 93.
(8) Griffiths, M.; Sistrom, W. R.; Cohen-Bazire, G.; Stanier, R. Y.
Nature 1955, 176, 1211.
(42) Stratmann, R. E.; Scuseria, G. E.; Frisch, M. J. J. Chem. Phys.
1998, 109, 8218.
(9) Foote, C. S.; Chang, Y. C.; Denny, R. W. J. Am. Chem. Soc. 1970a,
92, 5218.
(43) Bauernschmitt, R.; Ahlrichs, R. Chem. Phys. Lett. 1996, 256,
454.
(44) Casida, M. E.; Jamorski, C.; Casida, K. C.; Salahub, D. R.
(10) Foote, C. S.; Chang, Y. C.; Denny, R. W. J. Am. Chem. Soc.
1970b, 92, 5216.
(11) Koyama, Y.; Kuki, M.; Andersson, P. O.; Gillbro, T. Photochem.
J. Chem. Phys. 1998, 108, 4439.
Photobiol. 1996, 63, 243.
(45) Cossi, M.; Barone, V. J. Chem. Phys. 2001, 115, 4708.
(46) Barone, B.; Cossi, M. J. Phys. Chem. A 1998, 102, 1995.
(47) Cossi, M.; Rega, N.; Scalmani, G.; Barone, V. J. Comput. Chem.
2003, 24, 669.
(48) Stoyanov, S. R.; Villegas, J. M.; Rillema, D. P. Inorg. Chem. 2002,
41, 2941.
(12) Orlandi, G.; Zerbetto, F.; Zgierski, M. Z. Chem. Rev. 1991, 91, 867.
(13) Kohler, B. E.; Samuel, I. D. J. Chem. Phys. 1995, 103, 6253.
(14) Zoos, Z. G.; Ramasesha, S. Phys. Rev. B 1984, 29, 5410.
(15) Christensen, R. L.;Kohler, B. E. Photochem. Photobiol. 1973, 18, 293.
(16) Labhart, H. J. Chem. Phys. 1957, 27, 957.
(17) Tavan, P.; Schulten, K. J. Chem. Phys. 1979, 70, 5407.
(18) Tavan, P.; Schulten, K. J. Chem. Phys. 1986, 85, 6602.
(19) Petek., H.; Bell, A. J.; Choi, Y. S.; Yoshihara, K.; Tounge, B. A.;
Christensen, R. L. J. Chem. Phys. 1995, 102, 4726.
(20) Snyder, R.; Arvidson, E.; Foote, C.; Harrigan, L.; Christensen,
R. L. J. Am. Chem. Soc. 1985, 107, 4117.
(49) Knoevenagel, E. Chem. Ber. 1894, 27, 2345.
(50) Ren, Z.; Cao, W.; Tong, W. Synth. Commun. 2002, 32, 3475.
(21) Granville, M. F.; Holtom, G. R.; Kohler, B. E. J. Chem. Phys.
1980, 72, 4671.
(22) Gillbro, T.; Cogdell, R. J. Chem. Phys. Lett. 1989, 158, 312.
(23) Frank, H. A.; Cua, A.; Chynwat, V.; Young, A. J.; Gosztola, D.;
Wasiewelski, M. R. Photosynth. Res. 1994, 41, 389.
(24) Shreve, A. P.; Trautman, J. K.; Frank, H. A.; Owens, T. G.;
Albrecht, A. C. Biochim. Biophys. Acta 1991, 1058, 280.
(25) Frank, H. A.; Bautista, J. A.; Josue, J. S.; Young, A. J. Biochemistry
2000, 39, 2831.
(26) Walla, P. J.; Linden, P. A.; Hsu, C. P.; Scholes, G. D.; Fleming,
G. R. Proc. Natl. Acad. Sci.U.S.A. 2000, 97, 10808.
(27) Zimmerman, J.; Linden, P. A.; Vaswani, H. M.; Hiller, R. G.;
Fleming, G. R. J. Phys. Chem. B 2002, 106, 9418.
(28) Pang, Y.; Prantil, M. A.; Van Tassle, A. J.; Jones, G. A.; Fleming,
G. R. J. Phys. Chem. B 2009, 113, 13086.
(29) Pang, Y.; Jones, G. A.; Prantil, M. A.; Fleming, G. R. J. Am.
Chem. Soc. 2010, 132, 2264.
(30) Pang, Y.; Fleming, G. R. Phys. Chem. Chem. Phys. 2010, 12,
6782.
(31) Evans, D. F. J. Chem. Soc. 1960, 1735.
(32) Raubach, R. A.; Guzzo, A. V. J. Phys. Chem. 1973, 75, 983.
(33) Hand, E. S.; Belmore, K. A.; Kispert., L. D. J. Chem. Soc., Perkin
Trans. 2 1993, 659.
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dx.doi.org/10.1021/jp106293s |J. Phys. Chem. A 2011, 115, 1108–1116