Photophysical properties and electronic structure of retinylidene-chlorin- chalcones and analogues

Article abstract of DOI:10.1039/c3pp50421b

Synthetic chlorins can accommodate diverse substituents about the macrocycle perimeter. Simple auxochromes (e.g., vinyl, acetyl, phenyl) allow systematic tuning of spectral and photophysical features. More extensive spectral tailoring may be achieved by using more potent, highly conjugated substituents that themselves bring new absorption into a target spectral region, if deleterious excited-state quenching processes can be avoided. To explore such an expanded substituent space, herein the spectral and photophysical properties of four chlorin-chalcones are reported. The molecules are free base and zinc chlorins with substituents at the 13-position that include a chalcone and an extended chalcone derived by reaction of the 13-acetylchlorin with benzaldehyde and all-trans-retinal, respectively. Measurements of the spectral and photophysical properties (Φ_f, τ_s, k _f, k_ic, k_isc) are accompanied by density functional calculations that examine the characteristics of the frontier molecular orbitals. The chlorin-chalcones in nonpolar (toluene) and polar (dimethylsulfoxide) media exhibit bathochromically shifted (and intense) Q _y absorption bands. The presence of the retinylidene group adds new absorption in the blue-green region where the chlorins are typically transparent; excitation in this region leads to quantitative formation of the chlorin Q_y excited state. The spectral properties generally correlate with substituent effects on the frontier MOs. The four chlorin-chalcones in the solvent toluene have high fluorescence yields (0.24-0.30) and multi-nanosecond singlet excited-state lifetimes (3.7-8.4 ns), in addition to the added absorption imparted by the chalcone moiety. Collectively, the studies reported herein provide insight into the fundamental properties of chlorins and illustrate the utility of chalcones as a means of both tuning and augmenting the spectral properties of these chromophores.

Full text of DOI:10.1039/c3pp50421b

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PAPER
Photophysical properties and electronic structure
of retinylidene–chlorin–chalcones and analogues†
Joseph W. Springer,a Masahiko Taniguchi,b Michael Krayer,b Christian Ruzié,b
James R. Diers,c Dariusz M. Niedzwiedzki,a,d David F. Bocian,*c
Jonathan S. Lindsey*b and Dewey Holten*a
Cite this: Photochem. Photobiol. Sci.,
2014, 13, 634
Synthetic chlorins can accommodate diverse substituents about the macrocycle perimeter. Simple auxo-
chromes (e.g., vinyl, acetyl, phenyl) allow systematic tuning of spectral and photophysical features. More
extensive spectral tailoring may be achieved by using more potent, highly conjugated substituents that
themselves bring new absorption into a target spectral region, if deleterious excited-state quenching pro-
cesses can be avoided. To explore such an expanded substituent space, herein the spectral and photo-
physical properties of four chlorin–chalcones are reported. The molecules are free base and zinc chlorins
with substituents at the 13-position that include a chalcone and an extended chalcone derived by reaction
of the 13-acetylchlorin with benzaldehyde and all-trans-retinal, respectively. Measurements of the spec-
tral and photophysical properties (Φf, τs, kf, kic, kisc) are accompanied by density functional calculations
that examine the characteristics of the frontier molecular orbitals. The chlorin–chalcones in nonpolar
(toluene) and polar (dimethylsulfoxide) media exhibit bathochromically shifted (and intense) Qy absorption
bands. The presence of the retinylidene group adds new absorption in the blue-green region where the
chlorins are typically transparent; excitation in this region leads to quantitative formation of the chlorin Qy
excited state. The spectral properties generally correlate with substituent effects on the frontier MOs.
The four chlorin–chalcones in the solvent toluene have high fluorescence yields (0.24–0.30) and multi-
nanosecond singlet excited-state lifetimes (3.7–8.4 ns), in addition to the added absorption imparted by the
chalcone moiety. Collectively, the studies reported herein provide insight into the fundamental properties
of chlorins and illustrate the utility of chalcones as a means of both tuning and augmenting the spectral
properties of these chromophores.
Received 7th December 2013,
Accepted 21st January 2014
DOI: 10.1039/c3pp50421b
www.rsc.org/pps
near-UV (NUV) region, known as the Soret (or B) bands, corre-
sponding to absorption from the ground electronic state (S0) to
Introduction
The utilization of tetrapyrrole macrocycles in solar light- upper (S3, etc.) excited states. The long-wavelength absorption
harvesting and photomedical applications requires the ability band, corresponding to absorption to the lowest (S1) excited
to tailor optical and photophysical properties as well as create state, is weak and in the visible region for porphyrins, stronger
integrated architectures that incorporate additional constitu- and in the red region for chlorins, and stronger still and in the
ents such as accessory pigments. The main tetrapyrrole near-infrared (NIR) region for bacteriochlorins.1 We have pre-
families are porphyrins, chlorins, and bacteriochlorins, which viously found that the long-wavelength band of chlorins (the
contain zero, one or two reduced pyrrole rings, respectively. All Qy band) could be shifted bathochromically in ∼10 nm incre-
three families of tetrapyrroles have strong absorption in the ments from ∼605 nm to ∼670 nm region by placement of
simple auxochromes (phenyl, vinyl, ethynyl, acetyl, formyl) at
the 3- or 13-position or both.2–4 Analogous spectral shifts from
aDepartment of Chemistry, Washington University, St. Louis, Missouri 63130-4889,
∼710 to ∼770 nm were attained upon similar attachment of
USA
such auxochromes to bacteriochlorins.5–7 For both families, a
bDepartment of Chemistry, North Carolina State University, Raleigh, North Carolina
bathochromic shift is accompanied by intensification of the
absorption band. Such spectral tuning is largely achieved by
effects of the simple auxochromes on the relative energies of
key frontier molecular orbitals (MOs). The MO energy shifts
alter the extent of mixing of the associated excited-state elec-
tronic configurations, and thereby shift absorption intensity
27695-8204, USA
cDepartment of Chemistry, University of California, Riverside, California 92521-
0403, USA. E-mail: holten@wustl.edu, jlindsey@ncsu.edu, david.bocian@ucr.edu
dPhotosynthetic Antenna Research Center, Washington University, St. Louis,
MO 63130-4889, USA
†Electronic supplementary information (ESI) available: Synthesis procedures
and characterization data. See DOI: 10.1039/c3pp50421b
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between the upper excited states (NUV B bands) and the lowest
excited state (red or NIR Qy band).
A potential means to elicit more profound spectral effects is
to employ ‘substituents’ that inherently provide absorption in
the NUV to NIR spectral region. Compared to simple auxo-
chromes (e.g., acetyl, phenyl), such entities are necessarily
characterized by (1) a more extensive degree of electron delocal-
ization (e.g., extended polyene or arene), and (2) frontier MOs
that are closer in energy to, and thus have the potential of
mixing more substantially with, those of the tetrapyrrole
(porphyrin, chlorin, bacteriochlorin) to which they are
appended. As such, more potent, highly conjugated substitu-
ents may not only cause larger spectral shifts and intensity
redistribution than simple auxochromes, but also introduce
additional oscillator strength. The latter may include absorp-
tion in spectral regions in which a tetrapyrrole is normally
transparent, thereby also serving as a tightly coupled accessory
absorber. Of course, such highly conjugated species are
necessarily more easily oxidized/reduced than simple auxo-
chromes. Thus, a parallel question is the extent to which
the target effects on light-harvesting capacity can be attained
without compromising desirable photophysical properties
such as long excited-state lifetimes.
To examine such possibilities, we recently explored the use
of “chalcone” (i.e., enone) substituents at the 3- and 13-posi-
tions of bacteriochlorins and extended the absorption even
further into the NIR region, again with an increase in
intensity.8 One significant attraction of chalcones is the ease
of formation. Of equal importance is the absorption provided
by the chalcone moiety itself, which can be tailored by the length
of the polyene unit and the nature of the terminal groups.
Indeed, the term chalcone was first used at the end of the 19th
century to denote the characteristic copper-like color of the
enone condensation product of benzaldehyde and aceto-
phenone.9 The literature on chalcones (i.e., α,β-enones) is now
vast8 owing to preparation of diverse synthetic products10,11
and recognition that the chalcone substructure appears in a
wide variety of natural products.12–15
Chart 1 Chlorin–chalcones (top) and benchmark compounds
(bottom).
Herein, we use static and time-resolved optical spectroscopy
to examine the absorption spectra and other photophysical
properties of synthetic chlorins that contain chalcone substitu-
ents. The chlorin–chalcones explored are shown in Chart 1
and include the free base (Fb) form and zinc (Zn) chelate of
the chlorin (C). Each chlorin–chalcone contains a 10-mesityl
group, which is present in 13-acetyl benchmarks ZnC-A and
FbC-A but not in the unsubstituted parent chlorins ZnC and
FbC. The retinylidene–chlorin–chalcones (FbC-6, ZnC-6) are
examples of two commonplace chromophores joined via a
readily constructed linker. Two additional chlorin–chalcones
(FbC-1, ZnC-1) contain the shortest chalcone possible. Bench-
mark chalcone I that contains six carbon–carbon double
bonds was also prepared by reaction of all-trans-retinal and
acetophenone (Chart 1).
tightly coupled to a chlorin introduces new absorption in the
blue-green region (to upper excited states of the combined con-
struct), (2) the presence of the chalcone does not substantially
alter the photophysical properties of the chlorin singlet excited
state except in a few cases (depending on chalcone and solvent
polarity), and (3) the effects of chalcone versus simple auxo-
chromic substituents can be generally rationalized by correla-
ting with the energies and electron-density distributions of the
frontier MOs. Collectively, the study has produced chlorins with
enhanced absorption and favorable excited-state properties for
utility in light-harvesting systems.
Results
Synthesis
Analysis of the photophysical properties is aided by molecular-
orbital (MO) characteristics obtained from density-functional The chlorin–chalcones described herein were prepared pre-
theory (DFT) calculations. Key findings are (1) a chalcone viously16 with the exception of ZnC-6. The zinc chlorin ZnC-6
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Scheme 1 Synthesis of benchmark retinylidene–chalcone I.
Fig. 1 Absorption spectra of the retinylidene–chalcone I in toluene
(solid) and DMSO (dashed) normalized at the local maxima (423,
434 nm).
Table 1 1H NMR chemical shifts (δ, ppm) of retinylidene–chalcones
Proton positions
All-trans-retinala
Ib
IIc
IIId
1-gem-CH3
2-CH2-
3-CH2-
4-CH2-
5-CH3
9-CH3
13-CH3
7-H
8-H
10-H
11-H
12-H
1.03
1.47
1.62
2.03
1.72
2.03
2.32
6.33
6.16
6.18
7.14
6.37
5.97
10.10
—
1.04
1.47
1.61
2.02
1.73
2.01
2.13
6.27
6.16
6.18
6.89
6.41
6.38
7.91
7.01
1.03
1.45
1.58
2.14
1.70
2.01
2.09
6.30
6.19
6.25
7.04
6.58
6.51
7.90
7.48
1.13
1.48
1.59
1.97
1.79
1.77
1.67
6.39
6.31
6.04
6.82
6.31
8.32
8.05
—
Chart 2 Retinylidene–chalcones.
was obtained by zincation17 of the free base chlorin FbC-6.
The benchmark chlorins ZnC,18 FbC,19 ZnC-A18 were prepared
as described previously. The known benchmark chlorin
FbC-A16 was prepared here by demetalation of the zinc chelate
ZnC-A. The benchmark chalcone (I) was obtained as a red oil
upon reaction of acetophenone and all-trans-retinal
(Scheme 1). The reaction with microwave irradiation afforded a
substantially higher yield versus traditional heating.
14-H
15-H
16-H
a The chemical shifts are from ref. 24; assignments are from ref. 20;
all in CDCl3. b This work, in CDCl3. c Ref. 20, in DMSO-d6. d Ref. 21,
in benzene-d6.
The structure of I can be compared with those of other
known retinylidene–chalcones II and III (Chart 2). The absorp-
tion spectrum of I in toluene and in dimethylsulfoxide
(DMSO) at room temperature is shown in Fig. 1. The main
absorption band of compound I (ε423 nm = 37 400 M−1 cm−1 in
toluene) can be compared with values for II (ε442 nm = 44 600
M−1 cm−1 in methanol)20 or III (ε484 nm = 65 000 M−1 cm−1 in
n-hexane;21 ε490 nm = 49 800 M−1 cm−1 in ethanol22). The 1H
NMR spectra of retinals have been thoroughly examined and
complete assignments are available.21,23 The 1H NMR spec-
trum of I has been examined by H–H COSY and NOESY proto-
cols. The assignments are listed in Table 1 together with those
for two other retinylidene–chalcones (II, III) and for all-trans-
retinal.
13-acetylchlorin benchmark (ZnC-A), and the 13-chalcone–
chlorins (ZnC-1 and ZnC-6) in toluene. Spectra for the
same four compounds in DMSO are given in Fig. 2B. Spectra
for the free base analogues are presented in Fig. 2C and 2D.
Each spectrum contains the basic elements expected for
a chlorin, plus additional blue-region absorption for ZnC-6
and FbC-6. Progressing from longer to shorter wavelengths,
the main tetrapyrrole features (and rough spectral ranges)
are Qy (590–690 nm), Qx (470–560 nm), Bx (390–430 nm), and
By (390–430 nm). Each origin band is typically accompanied
by
a
weaker vibronic overtone feature 1000–1500 cm−1
at shorter wavelength. The weak Qx bands (origin and over-
tone) are more apparent for the free base chlorins than
the zinc chelates. The nominal Bx and By (Soret) features
are coalesced for ZnC and ZnC-A and become separated by
Photophysical characterization of chlorins
Absorption spectra. Fig. 2A shows the absorption spectrum varying degrees for ZnC-1 and ZnC-6 and the free base
of the zinc chelates of the parent chlorin (ZnC), the analogues.
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Fig. 2 Absorption spectra of the zinc chlorins (A, toluene; B, DMSO) and free base chlorins (C, toluene; D, DMSO) normalized at the maximum in
the Soret region.
The spectra in Fig. 2 are normalized at the Soret (B) ΣQ /ΣB (Table 2), the latter taking into account the Qy band-
y
maximum, which is a common manner of presenting spectra width, which is not included in the normalization used in
for photosynthetic chromophores and analogues to help visualize Fig. 3. A 10-mesityl (or phenyl or p-tolyl) group (not present in
variations in the peak intensity of the Qy band. Such normaliza- ZnC or FbC) causes a bathochromic shift of only 3–5 nm and
tion does not always provide a quantitative assessment, because instead a small diminution of Qy intensity.3,4,17 Replacement
substituents may alter the overlap of the Bx and By components of the 13-acetyl group with the short chalcone group of ZnC-1
as well as the relative bandwidths of these features and the Qy and FbC-1 gives rise to an additional bathochromic shift
band. [Plots on the basis of molar absorption (extinction) co- and intensification of the Qy band, albeit generally not as
efficients are also often misleading due to significant measure- substantial as the addition of the acetyl group to a parent
ment errors.] To roughly capture variations in the Soret chlorin. There is a somewhat larger (relative to the short
manifold, Fig. 3 presents the spectra normalized to the inte- chalcone) bathochromic shift upon replacement with the
grated absorption in the 350–450 nm region. This range encom- longer chalcone group of ZnC-6 and FbC-6. Consideration of
passes the main Soret absorption while rejecting most of the the ΣQ /ΣB values in Table 2 (and associated errors) indicates
y
additional absorption (>450 nm) associated with the presence that potential intensification of the Qy band for the chalcone
of the 13-retinylidene group of ZnC-6 and FbC-6. This normali- groups containing six versus one carbon–carbon double bond(s)
zation method also has shortcomings (such as not accounting may depend on the metalation state (zinc versus free base)
for changes in Qy bandwidth), but provides a more faithful first- and/or solvent (toluene versus DMSO).
order visualization of the variation in Qy peak intensity among
The Soret (B) maximum also shifts bathochromically along
compounds. The 450 nm cutoff used to construct Fig. 3 is the series ZnC < ZnC-A < ZnC-1 < ZnC-6 and the series FbC <
arbitrary, and this presentation is not meant to imply that the FbC-A < FbC-1 < FbC-6 (Fig. 2). The shift upon incorporation
13-retinylidene group and chlorin macrocycle can be considered of a 13-acetyl group and then chalcone moieties bearing one
independent entities, which they are almost certainly not or six carbon–carbon double bonds likely reflects an increase
(vide infra). Values for the peak position, full-width-at-half- in splitting of the By and Bx components as well as an increase
maximum (FWHM), Qy/B peak-intensity ratio (IQ /IB) and Qy/B in breadth of these features. The most substantial effects on
y
integrated-intensity ratio (ΣQ /ΣB) are collected in Table 2.
the Soret contour are observed for the chlorin–chalcones that
y
Previous studies have shown that the 15–20 nm batho- contain the retinylidene unit, and are greater for ZnC-6 than
chromic shift and intensification of the Qy band for the 10,13- FbC-6. The broadening of the Soret contour for the latter two
substituted ZnC-A and FbC-A versus parent chlorins ZnC and FbC compounds (in both toluene and DMSO) likely derives from
derive primarily from the 13-acetyl group.2,3 The intensification the above-noted effects plus additional absorption in the
of Qy is relative to the Soret and is indicated by both IQ /IB and 450–500 nm region due to the presence of the long chalcone
y
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Fig. 3 Absorption spectra of the zinc chlorins (A, toluene; B, DMSO) and free base chlorins (C, toluene; D, DMSO) normalized by the integrated
absorption in the 350–450 nm (Soret) region.
Table 2 Spectral properties of chlorin–chalcones and benchmarksa
Compound
Solvent
Bmax abs (nm)
Qy abs (nm)
Qy abs FWHM (nm)
IQ /IB
ΣQ /ΣB
Qy flu (nm)
Qy flu FWHM (nm)
y
max
y
ZnC
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
398
418
422
423
389
414
426
421
602
631
640
644
633
658
665
666
11
14
17
20
10
12
15
17
0.28
0.45
0.69
0.89
0.42
0.52
0.62
0.62
0.23
0.32
0.37
0.36
0.17
0.21
0.26
0.25
604
635
646
648
635
661
669
670
12
15
18
23
12
14
16
18
ZnC-A
ZnC-1
ZnC-6
FbC
FbC-A
FbC-1
FbC-6
ZnC
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
403
423
604
636
645
650
631
655
661
663
14
19
28
31
12
15
20
24
0.16
0.32
0.59
0.72
0.31
0.36
0.46
0.49
0.17
0.34
0.45
0.45
0.12
0.21
0.25
0.31
608
642
650
658
633
660
668
665
15
20
31
33
14
17
21
25
ZnC-A
ZnC-1
ZnC-6
FbC
FbC-A
FbC-1
FbC-6
429
402/427b
395
414
424
418
a All absorption (abs) and fluorescence (flu) data were obtained at room temperature. b The two Soret features have approximately equal intensity.
substituent. Broad absorption in this blue-green region is versus toluene is paralleled by a decrease in relative Qy/B peak-
exhibited by the long-chalcone benchmark I (Fig. 1).
intensity ratio (IQ /IB) and is taken into account in the inte-
y
Subtle changes in spectral properties of the chlorins (chal- grated-intensity ratio (ΣQ /ΣB).
y
cone-substituted and benchmarks) are observed as the solvent
Fluorescence spectra. The fluorescence spectra for the
is changed from toluene to DMSO (Fig. 2 and 3 and Table 2). chlorin–chalcones and corresponding benchmarks are domi-
The switch to the more polar solvent gives a bathochromic nated by the Qy origin band (Fig. 4). The spectra are typical
shift in the Soret (B) maximum (4–7 nm), a bathochromic shift of previously studied chlorins.2,4,17 The Qy fluorescence
in the Qy maximum for the zinc chelates (2–6 nm), a hypso- maximum for each zinc chlorin is bathochromically shifted
chromic shift in the Qy band for the free base chlorins 2–8 nm in DMSO versus toluene, paralleling the solvent effect
(2–4 nm), and a broadening of the Qy band for all compounds on the Qy absorption maximum (Table 2). The opposite effect
(20–65%). The greater Qy FWHM for the compounds in DMSO is seen for the free base chlorins, with a 1–5 nm hypsochromic
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Fig. 4 Normalized fluorescence spectra of zinc chlorins (A, toluene; B, DMSO) and free base chlorins (C, toluene; D, DMSO).
at the Soret maximum. The excellent match of the two spectra,
including in the 450–500 nm region, indicates that the new
absorption due to the retinylidene group in ZnC-6 and FbC-6
leads to the quantitative formation of the Qy excited state.
Similar results are obtained for FbC-6 in toluene and both
compounds in DMSO, albeit with differences in inherent fluo-
rescence yield due to differences in the decay properties of the
Qy excited state (vide infra).
Excited-state properties. The lowest-energy singlet excited
state (S1) of each chlorin–chalcone and chlorin benchmark
decays by S1→S0 spontaneous fluorescence, S1→S0 internal
conversion, and S1→T1 intersystem crossing with respective rate
constant kf, kic or kisc; yield Φf, Φic or Φisc; and lifetime τS = (kf +
kic + kisc)
−1. Table 3 lists the measured values of τS, Φf and Φisc
and the value of Φic calculated from the formula Φic = 1 − Φf −
Φisc. The table also gives the three rate constants, obtained
using the expression ki = Φi/τS (with i = f, ic, isc).
Fig. 5 Normalized absorptance (1 − T) spectrum and fluorescence-
excitation spectrum (λem = 650 nm) for ZnC-6 in toluene.
The chlorin–chalcones (ZnC-1, ZnC-6, FbC-1, FbC-6) in
shift in DMSO compared to toluene, also consistent with the toluene exhibit fluorescence yields (0.24–0.30) that are gener-
Qy absorption shift. Both the zinc and free base chlorins ally greater than those for the benchmarks (ZnC-A, FbC-A) and
exhibit broader emission peaks in DMSO versus toluene, with all the synthetic chlorins that we have studied,2–4 approaching
the largest change observed for the zinc chlorin–chalcones that of the native chlorophyll a (0.325).25,26 The large Φf values
(10–13 nm FWHM increase; Table 2).
reflect the increased kf values (because Φf = τSkf), which in
A pertinent question is whether the new absorption in the turn are related via the Einstein coefficients27 to the above-
450–500 nm region for ZnC-6 and FbC-6, which is not observed noted Qy (S0→S1) absorption. The kf (and Φf) values and Qy
for typical chlorins such as ZnC-A and FbC-A (Fig. 2 and 3), absorption strength (as indicated by ΣQ /ΣB) increase in the
y
leads to chlorin Qy fluorescence. This issue is addressed in order ZnC < ZnC-A < ZnC-1 and FbC < FbC-A < FbC-1 for the
Fig. 5, which compares the absorptance [= 1 − transmittance compounds in toluene and DMSO (Tables 2 and 3). The kf
(T)] spectrum of ZnC-6 with the excitation spectrum of the value for FbC-6 is comparable to that for FbC-1, while that for
fluorescence (λdet = 650 nm) from this compound. Both ZnC-6 cannot be obtained because the τS value is not due to
spectra are for the compounds in toluene and are normalized the (pure) S1 state (vide infra). Overall, the good correlation of
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Table 3 Excited-state properties of chlorin–chalcones and benchmarksa
Compound
Solvent
τS (ns)
Φf
Φisc
0.88
0.64
0.64
0.47
0.61
0.52
0.41
0.39
Φic
kf−1 (ns)
kisc−1 (ns)
kic−1 (ns)
ZnC
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
1.7
4.7
4.6
3.7
8.8
9.9
8.4
7.2
0.06
0.24
0.26
0.28
0.20
0.26
0.30
0.24
0.06
0.12
0.10
0.25
0.19
0.22
0.29
0.37
27
20
18
13
44
38
28
30
2
7
7
29
39
45
15
46
45
29
19
ZnC-A
ZnC-1
ZnC-6
FbC
FbC-A
FbC-1
FbC-6
8
14
19
20
18
ZnC
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
1.8
5.6
0.070
0.25
0.15
0.006
0.17
0.24
0.24
0.13
0.80
0.06
0.45
27
22
21
2
8
29
7
b
ZnC-A
ZnC-1
ZnC-6
FbC
FbC-A
FbC-1
FbC-6
3.2
0.40
≤0.01
0.67
0.61
0.51
0.015c
9.3
0.19
0.22
0.29
0.39
44
38
28
32
14
19
20
11
46
45
29
9
9.3
7.3
1.0/7.4d
0.48
a All measurements were made at room temperature. The errors in the measured quantities are as follows: τS 5%, Φf 10%, Φisc 10%. b The
apparent Φisc (0.79) is too high to allow reliable estimation of rate constants. c The decay measured transient absorption has a dominant 15 ps
component and a minor (∼5%) 0.4 ns phase. Rate constants are not calculated because the chlorin τS may be shorter than 15 ps, which instead
would reflect the decay of a lower energy state fed by the chlorin S1 (Qy) state. d Two decay components with equal amplitudes. The rate constants
are averages based on the two lifetime components.
internal conversion (kic, Table 3) in one of two (or more)
excited-state populations (vide infra).
The most significant exception to the trends in photo-
physical properties is found for ZnC-6 in DMSO, where
internal conversion is so enhanced that, despite an expected
increased kf (based on ΣQ /ΣB), Φf is reduced to 0.006 and τS to
y
∼15 ps or less (Table 3). Fig. 7 shows representative transient
absorption data for ZnC-6 in DMSO, revealing the ultrafast
excited-state decay and (complete) ground-state recovery com-
pared to the multi-nanosecond excited-state decay (and high
yield of the long-lived T1 excited state) for the compound in
toluene. Furthermore, the broad absorption between 500 and
630 nm (Fig. 7B) versus the relatively featureless absorption in
toluene (Fig. 7A) suggests some difference in character of
Fig. 6 Intensity ratio of the B and Qy absorption manifolds versus the
the excited state observed in the two media. Such a difference
in spectra is suggestive of charge-transfer (CT) character.
However, the amount of CT character is not clear because ana-
logous spectral differences are observed for ZnC-1 and ZnC-A
in DMSO versus toluene, but not for ZnC.
radiative rate constant determined from the fluorescence yield and
single excited-state lifetime. Data for the four zinc (Zn) chelates and free
base (Fb) analogues in toluene and DMSO are color coded as indicated.
The symbols further identify the compounds as follows: ZnC and FbC
(circles), ZnC-A and FbC-A (squares), ZnC-1 and FbC-1 (down triangles),
and ZnC-6 and FbC-6 (up triangles).
Photophysical properties of retinylidene benchmark I
the kf and ΣQ /ΣB values shown in Fig. 6 reflects consistency of
the various measurements and analysis.
The benchmark for the chalcone substituent with six carbon–
carbon double bounds (I) is essentially non-fluorescent in
both toluene and DMSO. Thus, the excited-state decay pro-
perties were studied using ultrafast transient absorption
spectroscopy. The transient absorption spectra are generally
similar for I in the two media (Fig. 8). The spectra display
broad (positive) absorption from 500 to 700 nm, with the
expected (negative) ground-state bleaching feature dominating
at shorter wavelengths. Global fits of the time profiles across
the spectral region probed return lifetimes of 6.5 ps and 60 ps
(1 : 1 ratio) in toluene, and 15 ps in DMSO (Fig. 8C).
y
One of two exceptions to the above-noted trends is FbC-6 in
DMSO, for which Φf (0.13) is about half that for FbC-1 in
DMSO (0.24), and the excited-state decay has τS components of
∼1 ns and ∼7.4 ns with roughly equal amplitudes. The longer
τS component is similar to that for FbC-1 in DMSO. Nonethe-
less, the kf of (32 ns)−1, calculated from Φf and the amplitude-
weighted τS, is comparable to the value of (28 ns)−1 for FbC-1
and correlates with the relative Qy intensity (Fig. 6). The
reduced Φf and short τS component reflect enhanced S1→S0
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Fig. 7 Transient absorption spectra for ZnC-6 in toluene (A) and DMSO
(B), and representative kinetic traces at 633 nm (C), obtained using exci-
tation with ∼100 fs flashes at ∼420 nm.
Fig. 8 Transient absorption spectra for I in toluene (A) and DMSO (B),
and time profiles (C), obtained using ∼100 fs excitation flashes at
420 nm.
Molecular-orbital characteristics
the shorter 13-chalcone group of ZnC-1 and FbC-1 (benzylidene-
acetophenone, denoted IV). MO energies and the
LUMO–HOMO energy gap for all the molecules are listed in
Table 4.
The MO characteristics were obtained from DFT calculations.
Fig. 9 shows the electron-density distributions and energies
for the highest occupied MO (HOMO), the lowest unoccupied
MO (LUMO) as well as the HOMO−1 and LUMO+1 for the zinc
chlorin–chalcones and chlorin benchmarks. Fig. 10 shows
results for the free base analogues. The lettered arrows in
Fig. 9 and 10 indicate correlation of an orbital or boxed pair of
orbitals with one of the four “normal” chlorin frontier MOs
(HOMO−1, HOMO, LUMO, LUMO+1 of ZnC or FbC) as the
13-substituent is changed from none to acetyl to short chal-
cone (one carbon–carbon double bond) to long chalcone
(retinylidene; six carbon–carbon double bonds). Fig. 11 shows
MO characteristics for I, which also serves as a benchmark for
the long 13-chalcone substituent of ZnC-6 and FbC-6. Fig. 11
also shows the MO properties for a benchmark for the
13-acetyl group of ZnC-A and FbC-A (acetophenone) and for
Discussion
The ability to tailor the spectral and photophysical properties
of hydroporphyrins is of fundamental interest for capturing
red and NIR light. The ability to integrate such hydroporphy-
rins with other constituents and in larger architectures is essen-
tial to meet the needs for diverse applications. The use of the
chalcone substituent may satisfy both objectives in affording a
bathochromic shift of the Qy absorption band and, most
importantly, imparting absorption in a region where the
chlorin absorbs little if at all. Indeed, the chlorin–chalcones
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Fig. 9 Characteristics of the three lowest unoccupied and three highest occupied MOs of zinc chlorins. The lettered arrows indicate correlation of
an orbital or boxed pair of orbitals with one of the four “normal” chlorin frontier MOs (HOMO−1, HOMO, LUMO, LUMO+1 of ZnC) as the 13-substitu-
ent is changed from none to acetyl to short chalcone to long chalcone.
(ZnC-1, FbC-1, ZnC-6, FbC-6) in nonpolar (toluene) and polar excited-state lifetimes (3.7–8.4 ns) that can support efficient
(DMSO) media exhibit bathochromically shifted (and intense) excited-state energy or charge transfer depending on the appli-
Qy absorption bands versus that of most other synthetic chlor- cation. Even in DMSO the Φf and τS values of ZnC-1 (0.15, 3.2
ins.2,3 Furthermore, the presence of the retinylidene group in ns), FbC-1 (0.24, 7.3 ns) and FbC-6 (0.13, 1.0/7.4 ns) are sub-
ZnC-6 and FbC-6 adds new absorption in the 450–500 nm stantial. Only for ZnC-6 (and the short τS component for
region where chlorins are typically transparent. This new FbC-1) in DMSO does the presence of the (long) chalcone
absorption increases the integrated (350–750 nm) absorption group result in significant quenching of the chlorin Qy excited
capacity by 10–15% (compared to benchmarks that lack a chal- state (0.006, ≤15 ps). We now turn to exploring how these pro-
cone substituent) and leads to quantitative formation (via perties derive from the composition and electronic structure of
internal conversion) to the chlorin Qy excited state of ZnC-6 the architectures.
and FbC-6. Moreover, the four chlorin–chalcones in toluene
In the following subsections, we begin with an overview of
have high fluorescence yields and multi-nanosecond singlet molecular design, which is followed by a brief comparison of
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Fig. 10 Characteristics of the three lowest unoccupied and three highest occupied MOs of free base chlorins. The lettered arrows are as in Fig. 9.
the excited-state properties of the retinylidene–chalcone Molecular design
benchmark (I) with related compounds studied previously. We
then turn to the chlorin systems. We first discuss the effects of One of the challenges in artificial photosynthesis is to identify
the 13-substituent (acetyl, chalcone with one or six carbon– rapid routes for the preparation of molecular architectures for
carbon double bond(s)) on the energies and electron-density solar light-harvesting and energy conversion. The retinylidene–
distributions of the frontier MOs. A feature of this discussion chlorin–chalcone is an example of two commonplace chromo-
is how interactions between substituent and chlorin macro- phores joined via a readily constructed linker. The joining
cycle are manifested in the nature and number of orbitals in reaction that creates the chalcone linker makes use of all-trans-
the normal HOMO−1 to LUMO+1 energy regime. We then retinal and a synthetic acetylchlorin. Advances in chlorin syn-
examine the relationship of the MO properties to the absorp- thetic chemistry have now made available a wide variety of
tion spectra, followed by a description of the relationship to chlorins either by de novo synthesis or by semisynthesis from
the photophysical properties and an outlook for the utility of porphyrins or chlorophylls.28–30 The acetylchlorin employed
the new architectures.
herein contains a geminal dimethyl group in the reduced
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Fig. 11 Characteristics of the three lowest unoccupied and three highest occupied MOs of benchmarks acetophenone (for ZnC-A and FbC-A), IV
(for ZnC-1 and FbC-1), and I (for ZnC-6 and FbC-6).
(pyrroline) ring, a feature that precludes adventitious dehydro-
a handful of meso-tetraarylporphyrins bearing appended
genation leading to the porphyrin. The acetyl group is located chalcones.32 The retinylidene–chalcone moiety has some
at the 13-position, which is the location of the keto group of structural resemblance with carotenoids. In this regard,
the isocyclic ring of chlorophylls and also lies along the axis numerous carotenoid–tetrapyrrole constructs have been pre-
coincident with the transition that gives rise to the long-wave- pared in pioneering and comprehensive work by the team
length (Qy) absorption band. All-trans-retinal, of course, is of Gust, Moore, and Moore (for reviews, see ref. 33–35;
derived by oxidative homolysis of β-carotene. A wide variety of for selected reports, see ref. 36–39).33–39 Such constructs
retinals are readily available via synthesis given the extensive have been examined for light-harvesting and photo-
studies of rhodopsins and bacteriorhodopsins.31 Nonetheless, protection properties that resemble those of the natural
surprisingly few retinylidene–chalcones have been pre- systems.40 The carotenoids in such dyads and larger
pared21,22 despite the facile methods for synthesis.10,11 architectures generally contain 8–11 double bonds in
Examples include II and III in Chart 2.
the polyene chain, which is substantially longer than the 6
To our knowledge, the only other examples of tetrapyrrole– double bonds of the retinylidene–chalcone moiety examined
chalcones include several bacteriochlorin–chalcones8 and herein.
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Molecular-orbital characteristics
The energies of all four standard chlorin frontier MOs (HOMO−1,
HOMO, LUMO, LUMO+1) are altered (typically stabilized)
by auxochromes such as the 13-acetyl group of ZnC-A and
FbC-A relative to analogues that lack such groups such as ZnC
and FbC; the 10-mesityl group has small effects by compari-
son.3,4 The LUMO is preferentially stabilized relative to the
HOMO, LUMO+1 and LUMO−1 (Fig. 9 and 10). Significantly
more (yet modest) electron density is observed on the acetyl
oxygen in the LUMO and HOMO than in the LUMO+1 and
HOMO−1. Such effects are designated by the arrows lettered
‘a’–‘d’ in Fig. 9 and 10.
Switching from the 13-acetyl group of ZnC-A and FbC-A to
the short (one carbon–carbon double bond) 13-chalcone group
of ZnC-1 and FbC-1 results in only small (≤0.05 eV) differences
in the HOMO−1, HOMO, and LUMO energies, and the elec-
tron distribution on the macrocycle is essentially unchanged
(arrows lettered ‘f’–‘h’ in Fig. 9 and 10). The electron density
on the substituent oxygen is comparable for chalcone
versus acetyl (along with density on the chalcone phenyl
ring) for the LUMO, less on chalcone versus acetyl for the
HOMO, and is minor on the chalcone enone moiety for the
HOMO−1.
Chart 3 Homologues of all-trans-retinal (Ret-5).
Excited-state dynamics of retinylidene benchmark I
Native carotenoids such as peridinin and fucoxanthin and syn-
thetic analogues bearing one or more carbonyl groups have
been studied extensively using time-resolved absorption
spectroscopy.41–48 A typical (but not universal) characteristic of
these systems is the role of an intramolecular charge transfer
(ICT) state involving the carbonyl moiety. Multi-exponential
decays are ascribed to internal conversions involving the S2, S1,
and ICT excited states and the S0 ground state. Solvent depen-
dence of the dynamics typically decreases as the length of the
π-system increases. The solvent effects are ascribed to stabiliz-
ation of the ICT state and thus, its energy relative to the S1 and
S0 states.
One such example is the compound denoted Ret-7 in
Chart 3 for which decay of the ICT state (or mixed ICT/S1 state)
to S0 decreases from ∼200 ps (in n-hexane or toluene) to
∼20 ps (in acetone or ethanol) to ∼7 ps (in methanol).45
Increasing the chain length by two double bonds in analogue
Ret-9 reduces the solvent effect such that the time constant
changes from ∼25 ps (in isooctane or toluene) to ∼15 ps (in
acetone or ethanol) to ∼8 ps (in methanol).45 Another example
is the retinylidene analogue III (Chart 2), which is similar to I
except for the presence of the second carbonyl group.48
Excited-state decay to S0 takes ∼6 ps in n-hexane, ∼11 ps
in toluene or acetone, and ∼14 ps in methanol. The results
and theoretical studies of III suggested that the dynamics
do not involve an ICT state but may involve an (n,π*) excited
state, at least in one excited-state population.
The lifetimes observed herein for return of I to the ground
state of 6.5 ps and 60 ps (1 : 1 ratio) in toluene and 15 ps in
DMSO are in the range found for the above-noted related com-
pounds and others.39,41–46,48 A change from 60 ps (at least in
one excited-state population) in toluene to 15 ps in DMSO and
companion changes in the shape of the transient absorption
spectrum (Fig. 8) could suggest an enhanced contribution of
an ICT state. Nevertheless, what is most germane herein is that
the retinyl-like excited states responsible for the rapid (tens of
picoseconds) decay of I to the ground state are not involved in
the decay of the chlorin Qy excited state of ZnC-6 or FbC-6 in
toluene, where multi-nanosecond S1 lifetimes are observed
(Table 3). It seems clear that some type of CT underlies the
short (∼15 ps) excited-state decay for ZnC-6 in DMSO and
the ∼1 ns component for FbC-6 in this solvent. We will return
to the nature of such states once a MO framework has been
established in the following.
A major difference upon switching the 13-substituent from
acetyl to the short chalcone is splitting of the normal chlorin
LUMO+1 into the LUMO+1/LUMO+2 pair of orbitals, as indi-
cated in the box preceded by arrow ‘e’ in Fig. 9 for ZnC-1 and
Fig. 10 for FbC-1. As can be seen from the figures and Table 4,
the two orbitals of the LUMO+1/LUMO+2 pair are split by only
0.04 eV for ZnC-1 and by 0.10 eV for FbC-1, with small (0–0.1
eV) individual shifts from the LUMO+1 of ZnC-A and FbC-A.
Both orbitals of the LUMO+1/LUMO+2 pair have substantial
electron density on both the short chalcone substituent and
the macrocycle, but with different relative magnitudes for
ZnC-1 and FbC-1.
These combined considerations suggest that electron pro-
motion from HOMO−1 or HOMO to a member of the
LUMO+1/LUMO+2 pair for ZnC-1 or FbC-1 produces an
excited-state configuration in the same energy regime as for
electron promotion from HOMO−1 or HOMO to LUMO+1 for
the benchmarks (ZnC-A, FbC-A, ZnC, FbC) but having substan-
tially more chlorin → chalcone CT character. This character
may differ for ZnC-1 and FbC-1 depending on the relative con-
tributions of the orbitals in the LUMO+1/LUMO+2 pair. Poten-
tial impacts on the absorption spectra and excited-state
properties are discussed below.
Related observations, but with significant differences in
detail, can be made for a switch of the 13-substituent from
acetyl to the long chalcone group (six carbon–carbon double
bonds) of ZnC-6 and FbC-6. Tracing arrows ‘c’, ‘g’ and ‘k’ in
Fig. 9 and 10 shows that for both ZnC-6 and FbC-6 the
chlorin HOMO is split into a HOMO−1/HOMO pair, spaced by
∼0.1 eV. Similarly, tracing arrows ‘b’, ‘f’ and ‘j’ in Fig. 9 and 10
shows that for both ZnC-6 and FbC-6 the chlorin LUMO is
split into a LUMO/LUMO+1 pair, spaced by 0.36–0.38 eV. Both
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Table 4 Molecular-orbital energies of chlorinsa
HOMO−1
HOMO
LUMO
LUMO+1
LUMO–HOMO
ZnC
−5.10
−5.23
−5.19
−5.12
−5.07
−5.14
−5.10
−5.03
−7.01
−6.66
−6.10
−4.79
−2.12
−1.55
2.67
2.53
2.48
2.61b
2.69
2.56
2.50
2.63b
5.25
4.21
2.71
ZnC-A
ZnC-1
ZnC-6
FbC
FbC-A
FbC-1
FbC-6
Acetophenone
IV
−4.95
−2.42
−1.68
−4.91
−2.43
[−1.68,−1.64]
−1.57
[−4.88,−4.79]
−4.89
[−2.41,−2.05]
−2.20
−1.67
−1.80
−5.04
−4.99
−2.48
−2.49
[−1.80,−1.70]
−1.68
[−4.98,−4.84]
−6.72
[−2.47, −2.09]
−1.47
−0.23
−6.31
−5.07
−2.10
−2.36
−0.65
I
−1.16
a From DFT calculations. Entries in columns 2–4 for the chlorin–chalcones (ZnC-1, FbC-1, ZnC-6, FbC-6) are the individual orbitals or pairs of
orbitals (in square brackets) that have electron-density distributions on the macrocycle that are the closest to the “normal” frontier MOs (e.g.,
those for ZnC or FbC), as can be seen from Fig. 9 and 10. For example, because the listed ‘HOMO’ of ZnC-6 is a pair of orbitals (the actual HOMO
and HOMO−1) the listed ‘HOMO−1’ is the HOMO−2 for this molecule (Fig. 9). b For ZnC-6 and FbC-6, the energies of the two orbitals listed as
the ‘HOMO’ are averaged and the energies of the two orbitals listed as the ‘LUMO’ are averaged in calculating the LUMO–HOMO gap.
members of the HOMO−1/HOMO pair and both members of Potential relationships of such differences in MO character-
the LUMO/LUMO+1 pair have substantial electron density istics with photophysical properties of the chlorins are given
spread along the length of the long 13-chalcone substituent of below.
ZnC-6 and FbC-6.
Optical properties
As a result of the splitting of the HOMO into the HOMO−1/
HOMO pair and the LUMO into the LUMO/LUMO+1 pair, the In a simple one-electron model, a trend in the LUMO–HOMO
benchmark chlorin HOMO−1 becomes the HOMO−2 (arrows
energy gap often correlates with the relative energies (and
‘d’, ‘h’ and ‘l’), and the chlorin LUMO+1 becomes the LUMO+2 absorption wavelengths) of the lowest singlet excited state (S1).
(arrows ‘a’, ‘e’ and ‘i’) for ZnC-6 and FbC-6. For these two The data in Fig. 11 and Table 4 indicate that the LUMO–
chlorins, the energy shifts of the HOMO−2 and LUMO+2 from
HOMO gap decreases as the conjugation length of the bench-
the respective HOMO−1 and LUMO+1 of ZnC-A and FbC-A is mark (and corresponding 13-substituent of the chlorin)
roughly 0.1 eV. The LUMO+2 of ZnC-6 and FbC-6 has essen- increases along the series I (2.71 eV; long chalcone) < IV
tially no electron density on the retinylidene unit; hence,
(4.21 eV; short chalcone) < acetophenone (5.25 eV; acetyl). The
electron promotion to this orbital from a member of the substantially lower LUMO–HOMO gap for I no doubt underlies
HOMO−1/HOMO pair gives an excited-state configuration with the shift of the long-wavelength transition into the visible
substantial CT character. Additionally, differences in the
region (Fig. 1).
electron-density on the long chalcone (retinylidene) group in
The LUMO–HOMO gap for I (2.71 eV) is comparable to the
the HOMO−2 for ZnC-6 versus FbC-6 and with the unoccupied typical LUMO–HOMO gap of chlorins (∼2.5 eV; Table 4). The
MOs suggests that electron promotions involving these orbitals
will produce configurations with varying degrees of CT by the LUMO–HOMO gap because the simple one-electron
Qy energy for each chlorin is considerably lower than indicated
character.
The splitting of the benchmark chlorin HOMO into a
(particle-in-a-circular-box) model is not sufficient. The com-
plexity is revealed in Gouterman’s four orbital model,49–51 in
HOMO/HOMO−1 for ZnC-6 must arise in part because the which the Qy and By excited states (and absorption bands)
HOMO energy of I (−5.07 eV; Fig. 11) is comparable to that of the
simple chlorins (−4.95 eV, ZnC-A), thereby facilitating chlorin–
derive from linear combinations of the HOMO → LUMO and
HOMO−1 → LUMO+1 configurations. The Qy is roughly a
chalcone MO mixing. The same phenomenon is true for 70/30 admixture of the two configurations and vice versa for
By.52–55 Similarly, Bx and Qx derive from the configurations
FbC-6. On the other hand, the HOMO energy for the shorter
(one carbon–carbon double bond) chalcone benchmark IV
(−6.31 eV) or the acetyl benchmark acetophenone (−6.72 eV) LUMO+1 and HOMO−1 to LUMO.
resulting from one-electron promotions from HOMO to
are substantially lower than the typical chlorin HOMO (for
Inspection of Fig. 9 and 10 indicates that within the set of
ZnC-A, FbC-A, ZnC, FbC) giving rise to less orbital mixing.
four zinc chlorins there is far less variation in the LUMO+1–
Similar arguments apply for the generation of a LUMO/ HOMO−1 energy gap than in the LUMO–HOMO energy gap.
LUMO+1 pair for ZnC-6 and FbC-6. In particular, the LUMO
The same is true of the four free base chlorins. Thus, although
energy of I (−2.36 eV) is comparable to that of the chlorins
consideration of such mixing is essential to understand
(−2.42 eV, ZnC-A). Additionally, comparison of these values the actual energies and relative intensities of the bands, the
and the similar LUMO energy of benchmark IV (−2.10 eV) may
underlie why the LUMO of ZnC-1 and FbC-1 has appreciable
trends in Qy position are reasonably reflected in the trend in
LUMO–HOMO energy gap. Such comparisons are relatively
electron density on the short chalcone group (Fig. 9 and 10). straightforward for understanding the bathochromic shift in
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the Qy upon the addition to ZnC (or FbC) of the 13-acetyl the simple trifurcation model for S1 excited-state decay used to
group to give ZnC-A (or FbC-A) and then switching to the short derive the rate constants (kf, kisc, kic) listed in Table 3.
chalcone group to give ZnC-1 (or FbC-1). The situation is more
The result for FbC-6 in DMSO is that Φf is about half that
complex for ZnC-6 and FbC-6 because pairs of orbitals are for FbC-1 in DMSO (0.13 versus 0.24) and τS has a short com-
derived from mixing of the chlorin HOMO and LUMO with ponent (∼1 ns) that is about 7-fold reduced from the longer
orbitals of the long (retinylidene) chalcone group (vide supra). component (∼7.4 ns) for this compound (FbC-6) and that for
Nonetheless, the nominal Qy and By bands (like the orbitals FbC-1 (7.3 ns) in this solvent. The quenching apparently
involved) are in the proper energy regime and retain substan- occurs in one of two (or more) excited-state populations that
tial characteristics of the chlorin macrocycle.
likely result from different conformers associated with the retin-
Given the MO characteristics, it is not certain whether the ylidene moiety and/or its disposition with respect to the
new absorption in the 450–500 nm region of ZnC-6 and FbC-6 chlorin macrocycle. The likely contribution of a state with
(relative to the benchmarks) is best described as having mixed chlorin ↔ chalcone CT character for ZnC-6 in DMSO results in
chlorin–retinylidene parentage or reflecting transition(s) more complete excited-state decay to the ground state with a time
localized on the long conjugated 13-substituent. Although the constant of ∼15 ps. This lifetime could reflect the decay of the
MO characteristics suggest the possibility that absorption in CT-like state and not the nominal S1 state from which it is
this region could involve excited states with some net macro- borne, which would have a shorter lifetime still.
cycle ↔ chalcone CT character, the dependence on solvent
Outlook
polarity is not substantially different than that for the other
absorption features (Fig. 2 and 3). Regardless, the absorption
in the 450–500 nm region of these chlorin–chalcones leads to
quantitative formation of the Qy excited state (vide supra). It is
the decay properties of that state to which we now turn.
The ability to design molecular architectures with desired
spectral properties is of fundamental importance in artificial
photosynthesis. Systematic tuning of spectral properties
(wavelength, intensity) of a given tetrapyrrole macrocycle
(porphyrin, chlorin, bacteriochlorin) in modest increments
across rather large spectral regions can be attained through the
use of simple auxochromes (e.g., phenyl, vinyl, ethynyl, acetyl,
formyl). However, such tuning is generally achieved by the
effects of the substituents on the energies of the tetrapyrrole
MOs and excited-state electronic configurations and a redistri-
bution of the oscillator strength within the tetrapyrrole
excited-state manifold. An approach to achieve more extensive
spectral coverage is to tightly couple to the tetrapyrrole more
potent, highly conjugated substituents that bring new absorp-
tion into a target spectral region. Herein, the chlorin–chal-
cones have been found to exhibit strongly bathochromically
shifted (and intense) Qy absorption bands and long excited-
state lifetimes. The long lifetimes and bright fluorescence are
accompanied by enhanced absorption in the 400–500 nm
region for retinylidene-containing FbC-6 and ZnC-6 where
chlorins alone – or chlorins bearing simple auxochromes –
have little or no oscillator strength. The enhanced absorption
in the 400–500 nm region gives rise to quantitative formation
of the chlorin Qy excited state, thereby affording enhanced
solar coverage.
Excited-state properties
The description of the frontier MOs given above affords a foun-
dation for understanding how, depending on compound and
solvent polarity, excited-states with chlorin ↔ chalcone CT
character could participate in photophysical properties such as
the fluorescence quantum yields and excited-state lifetimes.
For the unsubstituted chlorins (ZnC and FbC) and the acetyl
chlorins (ZnC-A and FbC-A), there is essentially no electron
density on the 13-substituent. Thus, all electron promotions
from HOMO−1 or HOMO to LUMO or LUMO+1 will be entirely
chlorin based. On the other hand, for the chlorin–chalcones,
and most notably ZnC-6 and FbC-6, a number of electron pro-
motions involving the frontier MOs will produce an excited
state with chlorin ↔ chalcone CT character. One example is
electron promotion from a member of the HOMO−1/HOMO
pair (derived from the chlorin HOMO) to LUMO+2 (basically
the benchmark chlorin LUMO+1). Additionally, the greater
electron density on the HOMO−2 for FbC-6 will produce less
CT character upon electron promotion to the LUMO or
LUMO+1 relative to ZnC-6. The latter difference may contribute
to the far less substantial reduction in Φf and τS for FbC-6
compared to ZnC-6 when the solvent is changed from toluene
to DMSO. Also contributing to that difference in behavior are
the relative redox properties of the zinc and free base chlo-
rins,56 as reflected in the relative HOMO (chlorin oxidation)
and LUMO (chlorin reduction) energies (Fig. 9 and 10).
Experimental methods
Photophysical studies
Fluorescence yields, singlet excited-state lifetimes, and inter-
The rationale for ascribing reduced Φf and τS values for system-crossing (triplet) yields were measured as described
FbC-6 and more so for ZnC-6 in DMSO versus toluene (Table 3) previously.4,6 In short, fluorescence lifetimes are typically the
to a state with chlorin ↔ chalcone CT character is that such average from measurements (Soret excitation) using a phase-
states would be stabilized in the more polar solvent. Such a modulation technique and a time-resolved instrument with a
state could mix with the normal chlorin S1 (Qy) or drop to ∼1 ns response function. Transient absorption studies typically
lower energy and act as a quencher. The consequence is an utilized ∼100 fs excitation pulses (in the Soret, Qx or Qy bands)
enhanced effective rate constant for internal conversion with with an energy of 0.5 μJ and a diameter of 1 mm. The standard
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Paper
Photochemical & Photobiological Sciences
for fluorescence quantum yields was chlorophyll a in deoxy-
genated toluene (Φf = 0.325),25 which is the value in deoxy-
genated benzene.26 Samples used for static absorption and
fluorescence studies typically had a chlorin concentration of
0.2–0.7 μM in a 1 cm path cuvette (A ≤ 0.1 at the excitation
wavelength) and those for transient absorption studies were
about 3-fold more concentrated in a 2 mm path cell.
4 J. W. Springer, K. M. Faries, H. L. Kee, J. R. Diers,
C. Muthiah, O. Mass, C. Kirmaier, J. S. Lindsey,
D. F. Bocian and D. Holten, Effects of substituents on syn-
thetic analogs of chlorophylls. Part 3: The distinctive
impact of auxochromes at the 7- versus 3-positions, Photo-
chem. Photobiol., 2012, 88, 651–674.
5 M. Taniguchi, D. L. Cramer, A. D. Bhise, H. L. Kee,
D. F. Bocian, D. Holten and J. S. Lindsey, Accessing the
near-infrared spectral region with stable, synthetic,
wavelength-tunable bacteriochlorins, New J. Chem., 2008,
32, 947–958.
6 E. Yang, C. Kirmaier, M. Krayer, M. Taniguchi, H.-J. Kim,
J. R. Diers, D. F. Bocian, J. S. Lindsey and D. Holten, Photo-
physical properties and electronic structure of stable,
tunable synthetic bacteriochlorins: Extending the features
of native photosynthetic pigments, J. Phys. Chem. B, 2011,
115, 10801–10816.
Molecular-orbital calculations
DFT calculations were performed with Spartan ’10 for
Windows version 1.2.0 using the hybrid B3LYP functional,
6-31G* basis set, and equilibrium geometries optimized using
the default parameters.57 Molecular-orbital (MO) images were
plotted from Spartan using an isovalue of 0.016.
7 C.-Y. Chen, E. Sun, D. Fan, M. Taniguchi, B. E. McDowell,
E. Yang, J. R. Diers, D. F. Bocian, D. Holten and
J. S. Lindsey, Synthesis and physicochemical properties of
metallobacteriochlorins, Inorg. Chem., 2012, 51, 9443–9464.
8 E. Yang, C. Ruzié, M. Krayer, J. R. Diers,
D. M. Niedzwiedzki, C. Kirmaier, J. S. Lindsey, D. F. Bocian
and D. Holten, Photophysical properties and electronic
structure of bacteriochlorin–chalcones with extended near-
infrared absorption, Photochem. Photobiol., 2013, 89, 586–
604.
9 S. V. Kostanecki and J. Tambor, Ueber die sechs isomeren
monooxybenzalacetophenone (monooxychalkone), Chem.
Ber., 1899, 32, 1921–1926.
10 D. N. Dhar, The Chemistry of Chalcones and Related Com-
pounds, John Wiley & Sons, Inc., New York, 1981.
11 S. N. A. Bukhari, M. Jasamai and I. Jantan, Synthesis and
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Acknowledgements
This work was supported by grants from the Division of
Chemical Sciences, Geosciences, and Biosciences, Office
of Basic Energy Sciences of the U.S. Department of Energy to
D.F.B. (DE-FG02-05ER15660), D.H. (DE-FG02-05ER15661) and
J.S.L. (DE-FG02-96ER14632). Mass spectra were obtained at
the Mass Spectrometry Laboratory for Biotechnology at North
Carolina State University. Partial funding for the facility was
obtained from the North Carolina Biotechnology Center and
the National Science Foundation. Transient absorption studies
were performed in the Ultrafast Laser Facility of the Photo-
synthetic Antenna Research Center (PARC), an Energy Frontier
Research Center funded by the U.S. Department of Energy,
Office of Science, Office of Basic Energy Sciences, under Award
No. DE-SC0001035.
12 R. E. Koes, F. Quattrocchio and J. N. M. Mol, The flavonoid
biosynthetic pathway in plants: function and evolution,
Bioessays, 1994, 16, 123–132.
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