Synthesis and photophysical characteristics of 2,3,12,13-tetraalkylbacteriochlorins

Article abstract of DOI:10.1039/c6nj00517a

Bacteriochlorins absorb strongly in the near-infrared spectral region and hence are of great interest across the field of photochemistry. The established de novo self-condensation of a dihydrodipyrrin-acetal has afforded stable bacteriochlorins bearing a

Full text of DOI:10.1039/c6nj00517a

NJC
View Article Online
View Journal
PAPER
Synthesis and photophysical characteristics of
2,3,12,13-tetraalkylbacteriochlorins†
Cite this:DOI: 10.1039/c6nj00517a
Shaofei Zhang,a Han-Je Kim,ab Qun Tang,c Eunkyung Yang,d David F. Bocian,*c
Dewey Holten*d and Jonathan S. Lindsey*a
Bacteriochlorins absorb strongly in the near-infrared spectral region and hence are of great interest
across the field of photochemistry. The established de novo self-condensation of a dihydrodipyrrin-
acetal has afforded stable bacteriochlorins bearing a variety of b-pyrrolic substituents, but the route was
incompatible with the presence of two alkyl groups. The lacuna stemmed from the instability of the
dialkyl-substituted dihydrodipyrrin-acetal. Here, two dihydrodipyrrin-carboxaldehydes, each bearing a
tert-butoxycarbonyl group at the pyrrole a-position, were prepared and found to be stable to routine
handling. Each ester-substituted dihydrodipyrrin-carboxaldehyde in acid underwent in situ ester
cleavage and ensuing self-condensation to provide the corresponding 2,3,12,13-tetraalkylbacteriochlorin.
The alkyl groups examined include methyl and methyl acetate. Such synthetic bacteriochlorins, while
previously unknown, provide valuable models of the natural chromophores. The absorption and
fluorescence characteristics of the tetraalkylbacteriochlorins are generally typical for this genre of
macrocycle. The X-ray structure of the tetramethylbacteriochlorin reveals that the pyrrolinic (reduced)
rings are modestly more twisted out-of-plane (torsional angle B111) than those of the nearly planar
(torsional angle B11) parent bacteriochlorin that lacks pyrrolic tetraalkyl groups. The resonance Raman
spectrum of the tetramethylbacteriochlorin is also far richer in the low- to mid-frequency region than
that of the unsubstituted analogue, but like the unsubstituted counterpart, is far sparser in the high-
frequency region than that of native bacteriochlorophyll. Access to tetraalkylbacteriochlorins constitutes
one step on the path from the most simple, fully unsubstituted bacteriochlorin to the fully decorated,
natural bacteriochlorophylls.
Received (in Montpellier, France)
16th February 2016,
Accepted 27th May 2016
DOI: 10.1039/c6nj00517a
www.rsc.org/njc
Introduction
Bacteriochlorophylls are Nature’s chosen chromophores for
light-harvesting in the near-infrared (NIR) spectral region.1,2
The bacteriochlorin chromophore of bacteriochlorophylls is a
tetrahydroporphyrin wherein two reduced pyrrole rings are located
at the opposite sides of the macrocycle. The structure of bacterio-
chlorophylls a, b and g are shown in Chart 1. Bacteriochlorins
a Department of Chemistry, North Carolina State University, Raleigh,
North Carolina 27695-8204, USA. E-mail: jlindsey@ncsu.edu
b Department of Science Education, Gongju National University of Education,
Gongju, Korea
c Department of Chemistry, University of California, Riverside,
California 92521-0403, USA. E-mail: david.bocian@ucr.edu
d Department of Chemistry, Washington University, St. Louis, Missouri 63130-4889,
USA. E-mail: holten@wustl.edu
† Electronic supplementary information (ESI) available: Acid catalysis survey of
the self-condensation of a dihydrodipyrrin-carboxaldehyde; X-ray structural data
table for BC-MM; absorption spectra for 10-Me and 10-CHO; and characterization
data for all new compounds. CCDC 1453311. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c6nj00517a
Chart 1 Natural bacteriochlorophylls.
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2016
New J. Chem.

View Article Online
Paper
NJC
group secures the pyrroline ring from adventitious oxidation that
would lead to unsaturated products (i.e., chlorins and porphyrins).
By modification of the substituents attached to the pyrrolic
units and/or the meso-positions, the bacteriochlorins can be
tailored with regards to the position of the long-wavelength
absorption band (ranging from 690 nm to 900 nm),14–17 the
polarity (hydrophobic, amphiphilic, or hydrophilic18) and the
presence of derivatizable groups for bioconjugation or building-
block applications.8,9,15,17,19–21
A key aspect of the self-condensation of a dihydrodipyrrin-
acetal is that different acidic conditions (acid composition and
concentration) and dihydrodipyrrin-acetal concentrations can
lead to distinct outcomes with regards to the macrocycles
formed: a free base 5-unsubstituted bacteriochlorin (HBC-type
macrocycle), a free base 5-methoxybacteriochlorin (MeOBC-
type macrocycle), a free base B,D-tetradehydrocorrin (TDC-type
macrocycle).10,11,22 As a general rule, the use of BF3ÁOEt2 at
modest concentrations in CH3CN typically provides a mixture of
two if not all three macrocycles, but with 140 mM BF3ÁOEt2 and
18 mM dihydrodipyrrin-acetal, the HBC-type macrocycle is the
dominant macrocyclic product (20–30% yield).10 On the other
hand, TMSOTf and 2,6-di-tert-butylpyridine (DTBP) in CH2Cl2
exclusively give the MeOBC-type macrocycle (40% yield),11
whereas Yb(OTf)3 in CH2Cl2 exclusively gives the corresponding
TDC-type macrocycle in high yield (70%).22
Scheme 1 Self-condensation of a dihydrodipyrrin-acetal.
One of our research objectives has been to install architectural
characteristically exhibit a strong (e B 105 MÀ1 cmÀ1) long- features about the bacteriochlorin for wavelength tuning, water-
wavelength absorption band (Qy band) located around 700– solubilization, and bioconjugation. An opposite thrust has been
900 nm.3 The strong NIR absorption makes bacteriochlorins to create sparsely substituted macrocycles, in particular to
attractive for fundamental studies as well as applications in systematically ‘‘build up’’ the substituent pattern of bacterio-
solar-energy conversion and in photomedicine.4–9
chlorophylls beginning with the fully unsubstituted bacteriochlorin.
Over some years now, a de novo synthesis route has been A progression of such synthetic, free base bacteriochlorins is shown
developed to gain access to diverse substituted bacterio- in Fig. 1. The progression begins with the fully unsubstituted HBC-
chlorins.10–13 The synthetic method relies on the self-condensation type bacteriochlorin BC0 and would add in (i) b-pyrrole substitution
of a dihydrodipyrrin-acetal (1-acetal), which contains a dimethyl without potent auxochromes (four methyl groups, BC-MM), (ii) a
group integral to the reduced, pyrroline ring and an acetal unit single auxochrome (acetyl, BC-A3), (iii) the isocyclic ring, which
located at the pyrroline a-position (Scheme 1). The gem-dimethyl contains a single auxochrome in the bacteriooxophorbine (BOP),
Fig. 1 Progression of structural complexity along the path to a free base derivative of BChl a.
New J. Chem.
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2016

View Article Online
NJC
Paper
(iv) perhaps other structures of intermediate complexity, and
finally (v) a structure (BOP-MA3) that contains two methyl
groups, one acetyl group, and the isocyclic ring as in methyl
bacteriopheophorbide a, the free base analogue of BChl a where
the phytyl group has been transesterified. While access to BC0
was achieved via the de novo route, we were unsuccessful in
preparing the tetraethyl analogue of the next compound in the
progression, the seemingly simple tetramethylbacteriochlorin.11
We felt the origin of the failure stemmed from the instability
of the diethyldihydrodipyrrin-acetal under the acidic reaction
conditions. Thus, the synthesis of the seemingly simple tetra-
alkylbacteriochlorins, paradoxically, has proved more difficult
than constructing architecturally elaborate bacteriochlorins.
In this paper, we report the synthesis of new b-dialkyldihydro-
dipyrrins and their conversion to tetraalkylbacteriochlorins. The
b-dialkyldihydrodipyrrins differ from the dihydrodipyrrin-acetals
used previously in two regards: (1) the acetal is replaced with a
carboxaldehyde, and (2) the pyrrole is stabilized at the a-position
with an acid-cleavable ester unit. The new b-dialkyldihydrodipyrrins
have provided access to two new 2,3,12,13-tetraalkylbacteriochlorins
(as well as one copper chelate), including BC-MM as desired in a
step along the progression of structural complexity. The electronic
spectral, vibrational, and excited-state properties of the bacterio-
chlorins have been investigated. In all, the present work should
provide a convenient route to a variety of 2,3,12,13-tetraalkyl-
bacteriochlorins that heretofore have been inaccessible.
Scheme 2 Failed access to tetraalkylbacteriochlorins.
Results and discussion
Western half precursor,25 and Jacobi similarly incorporated an
(I) Synthesis
(A) Reconnaisance. In our prior attempt to prepare tetra- a-ester with the b-alkyl-substituted dihydrodipyrrin II for use as
alkylbacteriochlorins,11 pyrrole-nitrohexanone 2, which bears a Southern half precursor26 (Chart 2). Note that II differs subtly
two ethyl groups at the b-pyrrole positions, was subjected to the from the dihydrodipyrrins employed in Schemes 1 and 2 in the
standard conditions for cyclization (via TiCl3) to form the position of the gem-dimethyl group in the pyrroline ring.
dihydrodipyrrin-acetal 3 (Scheme 2). The yield of the latter
The general approach shown in Scheme 2 has provided
was 7.6%, to be compared with typical yields of 20–50% for diverse access to diverse bacteriochlorins from the corresponding
substrates; but even worse, the product began decomposing dihydrodipyrrin-acetals, none of which contained pyrrole a-ester
within minutes. Dihydrodipyrrin-acetal 3 was quickly purified stabilization. Two strategies for preparing such dihydrodipyrrin-
(with limited characterization) and submitted to the bacteriochlorin- acetals (III) have been developed (Scheme 3). The original method
forming process under a handful of acid-catalysis conditions. In the employs Michael addition of a 2-(2-nitroethyl)pyrrole and an
best condition, only a slim signature (absorption bands, molecular a,b-unsaturated ketone-acetal (4-acetal) following by reductive
ion peak) of the HBC- and MeOBC-type bacteriochlorins was cyclization. A more recent method relies on Michael addition of
observed, and the yield of each on the basis of absorption spectro- a 2-(2-nitroethyl)pyrrole (IV) and mesityl oxide (4-Me) to form
scopy was o1%. From this result we surmised that a viable synthesis
of tetraalkylbacteriochlorins would require a strategy for stabilizing
the dialkyl-substituted precursors.
The synthesis of porphyrins bearing an alkyl group at each
b-pyrrole position has been known since the first half of the
20th century.23 One general synthetic approach has employed
pyrroles, dipyrrins or dipyrromethanes that are stabilized by the
presence of ester substituents at the a-position(s). The electron-
withdrawing effect of the esters counterbalances the electron-
releasing effect of the b-alkyl groups. The same approach has been
employed in rational routes to chlorins.24 Battersby incorporated
an a-ester to stabilize the b-alkyl-substituted dihydrodipyrrin I Chart 2 Dihydrodipyrrins used in chlorin syntheses.
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2016
New J. Chem.

View Article Online
Paper
NJC
Scheme 3 Retrosynthesis of dihydrodipyrrin-acetals.
the dihydrodipyrrin-Me (V). Subsequent oxidation (SeO2) gives
the dihydrodipyrrin-carboxaldehyde (VI), which is finally converted
to the dihydrodipyrrin-acetal (III).13 In prior examples of the
second route (for b-pyrrole substituents = ethyl and carboethoxy),
the intermediate dihydrodipyrrin-carboxaldehyde was not isolated
or purified but instead converted to the desired target, the
dihydrodipyrrin-acetal.13 Here, we adopted the latter route
to prepare and utilize two a-ester stabilized dihydrodipyrrin-
carboxaldehydes.
Scheme 4 Preparation of dihydrodipyrrin-carboxaldehydes.
(B) Preparation of dihydrodipyrrin-carboxaldehydes. We
began the methodology studies with a substrate for which data
were in hand concerning synthesis of the dihydrodipyrrin and in chlorinated solvents and during silica column chromatography,
conversion to a bacteriochlorin. Thus, the known p-tolyl-substituted but was stable for several days upon storage in solid form at À20 1C.
nitroethylpyrrole 511 was used in the synthesis of a dihydrodipyrrin- Compound 1-CHO was characterized by 1H NMR spectroscopy and
carboxaldehyde and the corresponding bacteriochlorin. Michael ESI-MS; however, a satisfactory 13C NMR spectrum was not obtained.
addition27 of nitroethylpyrrole 5 with mesityl oxide (4-Me) in the The 1H NMR spectrum of the purified product indicated the
presence of DBU gave g-nitrohexanone 6 in 74% yield. Subsequent presence of a minor impurity.
treatment of 6 with NaOMe followed by a buffered TiCl3 solution to
For the synthesis of the analogous dimethyldihydrodipyrrin-
achieve reductive cyclization10 afforded dihydrodipyrrin 1-Me as carboxaldehyde, a tert-butyl ester was introduced to the a-position
a yellow solid in 25% yield. Oxidation of dihydrodipyrrin 1-Me in the dihydrodipyrrin. Prior synthetic work in this area had
with SeO2 in 1,4-dioxane at room temperature, a procedure first resulted in transformations along the series of known compounds
reported by Jacobi and coworkers,13,28 gave the corresponding 7–9 and 10-Me for use in the formation of chlorins, where the
dihydrodipyrrin-carboxaldehyde 1-CHO in 47% yield (Scheme 4). dihydrodipyrrin-methyl compound 10-Me suffices, but not for
The conversion of 5 to 1-CHO was reported earlier.29 The bacteriochlorins. Thus, 830 was prepared beginning with acyclic
formation of the carboxaldehyde is readily observed by absorption precursors to 731 following the literature, whereas 932 and
spectroscopy given the bathochromic shift into the visible 10-Me32 were prepared via modified literature methods.11,27
region of the dihydrodipyrrin chromophore,13 as well as by Treatment of the ester-substituted dimethyldihydrodipyrrin
visual inspection (1-Me is light yellow whereas 1-CHO is orange-red). 10-Me with SeO2 in p-dioxane at room temperature afforded
Dihydrodipyrrin-carboxaldehyde 1-CHO was prone to decomposition the corresponding dihydrodipyrrin-carboxaldehyde 10-CHO in
New J. Chem.
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2016

View Article Online
NJC
Paper
63% yield (Scheme 4). The absorption spectra maxima in
(D) Preparation of a 2,3,12,13-tetramethylbacteriochlorin.
dichloromethane changed from 350 nm to 468 nm upon conversion The self-condensation of dihydrodipyrrin-carboxaldehyde 10-CHO
of 10-Me to 10-CHO. Unlike dihydrodipyrrin-carboxaldehyde 1-CHO, at B10-fold increased scale (0.21 mmol) afforded BC-MM in
10-CHO was relatively stable in a variety of solvents (including 29% yield (Scheme 5). Again, no other macrocycles were observed
CH2Cl2 and CHCl3) as well as during silica column chromatography. or isolated. The copper chelate was of particular interest for
(C) Self-condensation study of dihydrodipyrrin-carboxal- examination by resonance Raman spectroscopy, given the absence
dehydes. The typical conditions for the self-condensation of of fluorescence from copper tetrapyrroles. Following a standard
dihydrodipyrrin-acetals were used for the reactions with the procedure for metalation of bacteriochlorins,33 treatment of
dihydrodipyrrin-carboxaldehydes 1-CHO and 10-CHO. The two BC-MM with a large excess of sodium hydride and Cu(OAc)2
acidic conditions chosen for the self-condensation study with in THF at 60 1C for 16 h under argon afforded CuBC-MM.
18 mM dihydrodipyrrin-carboxaldehyde were as follows: (1) 140 mM Prolonging the reaction caused extensive byproducts. CuBC-MM
BF3ÁOEt2, or (2) 72 mM TMSOTf/144 mM DTBP (Table 1, entries 1 was unstable on silica column chromatography (due to conversion
and 2). A survey of other acidic conditions for the self-condensation to Cu(II)oxobacteriochlorins as found upon LD-MS analysis, or
is provided in the ESI.†
other decomposition) but could be purified on a short column,
The self-condensation of 1-CHO in CH3CN containing BF3Á albeit in low yield. Given that NMR spectroscopy is not readily
OEt2 gave BC-T in 12% yield, but little or no product was employed with copper tetrapyrroles, characterization relied on
obtained with the other two acids (entries 1–3). No other absorption spectroscopy, mass spectrometry, and absence of
macrocycles, including the HOBC-type or TDC-type species, fluorescence, all of which are diagnostic for conversion of a free
were reliably observed by TLC analysis or laser-desorption mass base tetrapyrrole to the copper chelate. The small quantity
spectrometry (LD-MS) of the crude sample from entry 1. The of CuBC-MM obtained was more than ample for resonance
self-condensation of 10-CHO gave nearly identical results in Raman spectroscopic characterization (vide infra).
1
BF3ÁOEt2 in CH3CN, with 11% yield for BC-MM (entry 4).
The bacteriochlorin BC-MM was characterized by H NMR
TMSOTf/DTBP gave no product but resulted in recovery of spectroscopy, 13C NMR spectroscopy, high-resolution mass
starting material, indicating the stability of the dihydrodipyrrin- spectrometry (ESI-MS) and absorption spectroscopy. The structure
carboxaldehyde (entry 5). On the other hand, the use of neat TFA of BC-MM also was determined by single-crystal X-ray analysis
gave BC-MM in 24% yield (entry 6). No other macrocycles were (Fig. 2). Comparison of the structure of BC-MM with that of BC033
detected on the basis of TLC analysis or attempted isolation. A leads to the following findings. (1) The average nitrogen–centroid
convenient feature of this latter result is that reaction occurred in distance of BC-MM is 2.099 Å, which is slightly longer than that of
1 h rather than 19 h for BF3ÁOEt2. The disparity in yield of BC (2.096 Å), indicating that the core size is slightly larger. (2) The
bacteriochlorin (0.2% versus 24%) in neat TFA despite identical torsion angle of the four carbons in the pyrroline ring (C6–C7–
reacting motifs for 1-CHO and 10-CHO is striking (entries 3 and 6). C8–C9; C16–C17–C18–C19) in BC-MM is 11.601, which is much
Regardless of mechanistic origin, the acid survey confirmed the larger than that of BC0 (1.111). The out-of-plane twists of the two
viability of the ester-substituted dihydrodipyrrin-carboxaldehyde as pyrroline rings in BC-MM are in opposite directions with respect
a means to access a tetraalkylbacteriochlorin.
to the macrocycle plane, imparting overall Ci symmetry.
Table 1 Acid survey for the self-condensation of dihydrodipyrrin-carboxaldehydesa
Entry
Reactant
R1
R2
R3
Acidic condition
Time (h)
Bacteriochlorin
Yield (%)
12%
1
2
3
4
5
6
1-CHO
1-CHO
1-CHO
10-CHO
10-CHO
10-CHO
p-tolyl
p-tolyl
p-tolyl
Me
Me
Me
H
H
H
Me
Me
Me
H
H
H
BF3ÁOEt2 in CH3CN
TMSOTf/DTBP in CH2Cl2
Neat TFA
BF3ÁOEt2 in CH3CN
TMSOTf/DTBP in CH2Cl2
Neat TFAd
19
19
1
19
19
1
BC-T
BC-T
BC-T
BC-MM
BC-MM
BC-MM
b
—
0.2%
11%
t
CO2tBu
CO2tBu
CO2 Bu
c
—
24%
a
All reactions were carried out (1.0 mL scale) with 18 mM dihydrodipyrrin-carboxaldehyde at room temperature. Concentrations of acid were
140 mM BF3ÁOEt2 or 72 mM TMSOTf/144 mM DTBP. Yields were determined by absorption spectroscopy of samples isolated by chromatography.
b
c
d
Not detected. Unreacted dihydrodipyrrin-carboxaldehyde remained. The same reaction in TFA-d (0.068 mmol 10-CHO) also gave BC-MM in
comparable yield with no isotopic incorporation.
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2016
New J. Chem.

View Article Online
Paper
NJC
Scheme 5 Preparation of BC-MM and CuBC-MM.
(E) A bacteriochlorin with acetate side-chains. The successful
synthesis of BC-MM prompted exploration of the synthesis of
more elaborate alkyl substituents. Thus, the bacteriochlorin
BC-AmAm was synthesized following the strategy reported
above (Scheme 6). Compounds 11,34 1235 and 1336 are known
but 12 and 13 were prepared here via alternative routes. Thus, a
rapid transesterification37 of pyrrole 11 at 220 1C gave the tri-benzyl
ester 12 in 68% yield, following by a selective base-catalyzed
transesterification37 at room temperature to give the dimethyl
monobenzyl ester 13. Next, hydrogenolysis followed by esterifi-
cation38 of the resulting carboxylic acid with tert-butyl alcohol
and DCC along with 4-(N,N-dimethylamino)pyridine (DMAP)
gave the new pyrrole 14. Treatment of the latter with thionyl
chloride followed by hydrolysis32 caused oxidation of the a-methyl
group in the presence of the three alkyl esters to give the
corresponding pyrrole-carboxaldehyde 15. Subsequent nitro-
aldol condensation11 and reduction by NaBH4 afforded nitroethyl-
pyrrole 16 in 39% yield. The Michael addition of 16 and mesityl
oxide (4-Me) in the presence of TBAF at room temperature39
Scheme 6 Synthesis of BC-AmAm.
afforded 17 in 54% yield. Reductive cyclization of 17 gave the
desired dihydrodipyrrin 18-Me in 18% yield. Finally, oxidation13,28
of dihydrodipyrrin 18-Me gave the resulting dihydrodipyrrin-
carboxaldehyde 18-CHO (not shown), which was sufficiently pure
for direct use in the next step. To our surprise, treatment with neat
TFA only afforded a trace amount of BC-AmAm. On the other
hand, BF3ÁOEt2-mediated self-condensation gave BC-AmAm in
6.6% yield for the two steps from 18-Me. The bacteriochlorin
1
was characterized by absorption spectroscopy, H NMR spectro-
scopy, and mass spectrometry (ESI-MS). We note that established
chemistry exists for the preparation of related dialkylpyrroles with
substituents such as 3-Me/4-Et,40 3-Et/4-Me,41 3-Me/4-CH2CH2-
CO2Me,42 and 3-CH2CH2CO2Me/4-Me,43 all of which may be
potential starting materials to make tetraalkylbacteriochlorins.
(II) Photophysical characterization
The photophysical data obtained upon characterization of the
new bacteriochlorins were compared with three benchmark
Fig. 2 ORTEP drawing of BC-MM. Ellipsoids are at the 50% probability
level and hydrogen atoms (except N–H) are omitted for clarity.
New J. Chem.
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2016

View Article Online
NJC
Paper
bacteriochlorins (Chart 3). One benchmark is the fully
unsubstituted bacteriochlorin BC0. Two others (BC-MEs and
BC-EEs) have alkyl (methyl or ethyl) groups at the 2,12-positions
and carboethoxy groups attached directly to the 3,13-positions
of the macrocycle. Note that the four carbomethoxy groups of
BC-AmAm are each removed from the macrocycle by a methylene
group and thus do not directly interact with the bacteriochlorin
p-system.
(A) Absorption and fluorescence spectra. Electronic ground-
state absorption spectra of parent BC0, new tetraalkyl derivatives
BC-MM and BC-AmAm, and copper chelate CuBC-MM are shown
in Fig. 3 (solid lines). The figure also shows fluorescence spectra
of the free base bacteriochlorins (dashed lines). Table 2 lists the
spectral characteristics of the free base compounds along with
those of additional benchmarks BC-MEs and BC-EEs studied
previously.14 The new tetraalkylbacteriochlorins BC-MM and
BC-AmAm are quite soluble in toluene and remain stable
even upon prolonged illumination without noticeable photo-
aggregation, an adverse phenomenon that has occurred with
other bacteriochlorins.44,45
The absorption spectrum of each bacteriochlorin consists of
four major bands: By, Bx in the near ultraviolet, Qx in the green-
yellow and Qy in the NIR region. Each origin, or (0,0), feature is
accompanied by at least one weaker vibronic satellite band to
higher energy. The Qy band of BC-AmAm (732 nm) and BC-MM
(721 nm) is bathochromically shifted from parent BC0 (714 nm)
but is hypsochromically shifted from benchmarks BC-MEs
(760 nm) and BC-EEs (761 nm), which contain 2,3-dimethyl
or 2,3-diethyl plus 3,13-dicarboethoxy groups (Table 2). Similar
trends are observed for the wavelength (nm) positions of the Qx
origin bands: BC0 (489) o BC-MM (490) o BC-AmAm (495) o
Fig. 3 Absorption (solid) and fluorescence (dashed) spectra of bacterio-
chlorins in toluene.
BC-MEs (520) o BC-EEs (521). Like the Qy feature, the Soret (B) maximum for the new tetra-substituted bacteriochlorins are
(By, Bx) bands (in nm) of BC-AmAm (347, 373) and BC-MM comparable to the benchmarks (0.85–1.1). Compared to the
(346, 374) are bathochromically shifted from parent BC0 (340, 365) parent and benchmarks, the Qx(1,0) vibronic satellite band of
and hypsochromically shifted from benchmarks BC-MEs (354, BC-MM and BC-AmAm has gained considerable intensity relative
384) and BC-EEs (354, 383). These trends thus reflect a general to the Qx(0,0) origin band. The Qx(0,0)/Qx(1,0) peak-intensity ratio
shift in the center of gravity of the spectrum among the follows the trend BC-MM (1.9) o BC-AmAm (4.8) o BC0 (7.9).
compounds.
Thus, the 2,3,12,13-tetraalkyl substituent pattern tends to turn
Several additional spectral characteristics are noteworthy: on vibronic activity, whether Franck–Condon or Herzberg–
(1) the full-width-at-half-maximum (FWHM) of the Qy absorption Teller in origin.
band of BC-AmAm and BC-MM is small and comparable to
The fluorescence spectra of BC-MM and BC-AmAm and
benchmark BC0, being in the range 12–13 nm or 229–243 cmÀ1 parent BC0 are shown in Fig. 3 (dashed lines). The fluorescence
(Table 2). These features are about 30% narrower than for spectrum of each bacteriochlorin is dominated by the Qy(0,0)
benchmarks BC-MEs and BC-EEs (19–20 nm, 337–343 cmÀ1
)
feature, which is positioned only 1–2 nm (19–29 cmÀ1) to longer
and bacteriochlorins with substituents that impart an even wavelength than the corresponding Q(0,0) absorption maximum.
longer wavelength Qy band.14 (2) The ratio of the Qy and Soret Such ‘Stokes shift’ is smaller than for BC-MEs or BC-EEs (Table 2)
or other bacteriochlorins bearing simple substituents (e.g.,
methyl, ethyl, phenyl, ester, acetyl, ethynyl) at the 2,3,12,13-
positions, which give an average shift of B4 nm (B90 cmÀ1).14
The FWHM (in cmÀ1) of the Qy emission feature of BC-MM and
BC-AmAm (296–308) is small, even slightly reduced from parent
BC0 (312) and smaller still versus benchmarks BC-MEs and
BC-EEs (350 and 368) (Table 2) and other bacteriochlorins
bearing simple b-pyrrole substituents.14
(B) Resonance Raman studies of CuBC-MM. Resonance
Chart 3 New and benchmark bacteriochlorins.
Raman (RR) measurements round out the static studies of
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2016
New J. Chem.

View Article Online
Paper
NJC
Table 2 Spectral characteristics of bacteriochlorins in toluene at room temperature
By(0,0) Bx(0,0) Qx(1,0) Qx(0,0)
Qy(1,0) Qy(0,0) Qy(0,0)
Qy(0,0)
Qy(0,0)
em FWHM em FWHM
em (nm) (nm)
Qy(0,0)
abs
abs
(nm)
abs
(nm)
abs
(nm)
IQ (0,0)/ abs
abs
(nm)
abs FWHM abs FWHM Qy(0,0)
(nm)
x
Compound (nm)
IQ (1,0) (nm)
(cmÀ1
)
(cmÀ1
)
IQ /IB
x
y
max
BC-MM
BC-AmAm 347
346
374
373
365
384
383
462
466
462
488
489
490
495
489
520
521
1.9
4.8
7.9
17.0
8.3
685
694
678
719
721
721
732
714
760
761
11.9
13.0
12.0
19.0
20.0
229
243
236
337
343
723
733
716
764
775
15.5
16.7
16.0
20.0
21.0
296
308
312
350
368
0.97
1.08
0.85
0.98
0.94
BC0
340
354
354
BC-MEsa
BC-EEsa
a
Data are from ref. 14.
the ground-state properties of the new tetraalkylbacteriochlorin 219, 652, 692, and 768 cmÀ1. The 736 cmÀ1 band of CuBC-MM is
constructs. The low- and mid-frequency regions (150–1700 cmÀ1 likely the analogue of the 727 cmÀ1 band of CuBC0. However,
of the Qy-excitation RR spectrum of CuBC-MM (lex = 737 nm) is the 736 cmÀ1 band of CuBC-MM is not the strongest feature
compared with that previously reported46 for CuBC0 (lex
in the spectrum, being less intense than either the 482 or
730 nm) in Fig. 4. For CuBC-MM, more noise is evident in the 612 cmÀ1 bands.
)
=
low-frequency than mid-frequency region of the spectrum owing
The rationale for examining the Qy-excitation RR spectrum
to the fact that a weak fluorescence background underlies the of CuBC-MM was to determine whether addition of the four
low-frequency region (see Experimental section). The striking methyl substituents at the b-pyrrolic positions of the macro-
aspect of the Qy-excitation RR spectrum of CuBC0 is that it is cycle would substantially alter the general characteristics of the
extremely sparse in both the low- and mid-frequency regions spectrum from that observed for CuBC0. As was previously
and is dominated by a single strong band at 727 cmÀ1. The noted, the Qy-excitation RR spectrum is quite unusual, exhibiting
mid-frequency region (800–1700 cmÀ1) of the RR spectrum of only a single strong band at 727 cmÀ1 plus a few other weak
CuBC-MM is similar to that of CuBC0 in the exhibition of features in both the low- and mid-frequency regions. These
relatively few weak bands. In contrast, the low-frequency region characteristics of the Qy-excitation RR spectrum of CuBC0
(150–800 cmÀ1) of the RR spectrum of CuBC-MM is much richer are strikingly different from those of the Qy-excitation RR
than that of CuBC0, exhibiting relatively strong bands at 359, spectra of natural bacteriochlorophylls, both in solution and in
482, 602, 612, and 736 cmÀ1 and somewhat weaker bands at proteins,47–52 and a Cu–bacteriochlorophyll model complex,53 all
of which exhibit dozens of bands that span the low- and mid-
frequency regions of the spectra. The BChl a macrocycle differs
from that of CuBC0 in that the former contains four b-pyrrolic
substituents, including two methyl groups, an acetyl group, and a
keto group embedded in the five-membered, isocyclic ring. The
isocyclic ring spans the 13- and 15-positions and also contains a
carbomethoxy substituent (Chart 1). Accordingly, addition of four
b-pyrrolic substituents to CuBC0, yielding CuBC-MM, was viewed
as one step in increasing the complexity of the macrocycle
towards that found in BChl a.
The results reported herein reveal that addition of the four
b-pyrrolic substituents does indeed increase the richness of the
RR spectra, but only in the low-frequency region. Thus, other
structural characteristic of the natural pigment must also come
into play in ultimately determining the overall RR spectral
pattern. The detailed analysis of the RR spectrum of CuBC-MM
is beyond the scope of this paper; however, the data acquired for
this complex serve as a starting point for our program aimed at
examining the RR spectra of a series of model complexes wherein
the substituents present in BChl a are systematically added to the
parent bacteriochlorin macrocycle.
(C) Excited-state properties. The S1 lifetime (tS) and yields
of S1 - S0 fluorescence (Ff) and S1 - T1 intersystem crossing
(Ff), the triplet yield, were measured to fully characterize the
decay properties of the S1 (Qy) excited state. The yield of S1 - S0
internal conversion (Fic) is obtained by the simple calculation
Fig. 4 Low- and mid-frequency regions of the Qy-excitation RR spectra
Fic = 1 À Ff À Fisc. The rate constants for the three processes
of CuBC0 (lex = 730 nm; CH2Cl2 solution) and CuBC-MM (lex = 737 nm;
benzene solution).
are then obtained using the expression ki = Fi/tS, where i = f, isc,
New J. Chem.
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2016

View Article Online
NJC
Paper
Table 3 Photophysical properties of bacteriochlorinsa
b
À1
À1
Qy
tS
kfÀ1 kisc
kic
SQy/
c
Compound (eV) (ns) Ff
Fisc Fic
(ns) (ns)
(ns) Stot
BC-MM 1.72 2.9 0.10 0.58 0.32 26
BC-AmAm 1.69 3.2 0.16 0.52 0.32 22
5
7
7
6
6
9
11
17
9
0.11
0.13
0.09
0.15
0.13
BC0
BC-MEs
BC-EEs
1.74 4.0 0.14 0.62 0.24 29
1.63 3.0 0.13 0.52 0.35 23
1.61 3.3 0.14 0.55 0.31 24
11
a
All measurements were made in toluene at room temperature. Data
for BC-MEs and BC-EEs are from ref. 14. The typical errors (percent of
value) of the photophysical properties are as follows: tS (Æ7%), Ff
(Æ5%), Fisc (Æ15%), Fic (Æ20%), kf (Æ10%), kisc (Æ20%), kic (Æ25%). The
error bars for tS, Ff, and Fisc were determined from select repeat
measurements, and those for the Fic, kf, kisc and kic were obtained
b
Fig. 5 Integrated intensity of the Qy absorption manifold [Qy(0,0) and
Qy(1,0)] versus that of the total spectrum (300–900 nm) when plotted in
wavenumbers (vertical axis) versus the radiative rate constant kf obtained
from the tS and Ff values (x-axis). The overall linear relationship reflects the
consistency of the measurements and analysis.
from propagation of errors. Average energy of the Qy absorption and
c
fluorescence bands. Integrated intensity of the Qy absorption manifold
[Qy(0,0) and Qy(1,0)] versus that of the total spectrum (300–900 nm) when
plotted in wavenumbers.
and ic. The values of the photophysical properties are collected
in Table 3.
Conclusions
The Ff values for BC-MM (0.10) and BC-AmAm (0.16) are
comparable to those of parent BC0 (0.14) and benchmarks
BC-MEs (0.13) and BC-EEs (0.14). The tS values (in ns) for
BC-MM and BC-AmAm (2.9 and 3.2 ns) are modestly shorter
than those of parent BC0 (4.0 ns) and more similar to bench-
marks BC-MEs and BC-EEs (3.0 and 3.3 ns). The intersystem-
crossing (triplet) yield values of BC-MM and BC-AmAm (0.58,
0.52) are comparable to those for the parent and benchmarks
(0.62, 0.52, 0.55). The internal-conversion yields for BC-MM and
BC-AmAm (both 0.32) are slightly higher than that for parent
BC0 (0.24) but comparable to benchmarks BC-MEs and BC-EEs
(0.35, 0.31).
Although the internal-conversion yields carry large (cumulative)
error because they derive from a series of measurements, the
differences appear to be significant and contribute to the
above-noted tS values. In general, as the Qy band of bacterio-
chlorins bearing simple substituents (mostly at the 2,3,12,13-
positions) moves to lower energy (longer wavelength), the tS
values are progressively reduced.3,16 The S1 lifetime and energy
are BC-MM (2.9 ns, 1.72 eV), BC-AmAm (3.2 ns, 1.69 eV),
BC-MEs (3.0 ns, 1.63 eV) and BC-EEs (3.3 ns, 1.61 eV). The tS
values for BC-MM and BC-AmAm are shorter than expected and
should be intermediate between those of the two benchmarks
and that of the parent BC0.
The use of a deactivating group at the a-position of the pyrrole
unit of a dihydrodipyrrin-acetal has enabled (1) the incorporation
of two alkyl groups at the pyrrole b-positions, and (2) the smooth
oxidation of the 9-methyl group to the corresponding aldehyde.
This synthetic approach blends features of longstanding routes to
natural porphyrins, where dialkylpyrrole-a-esters are common-
place, with gem-dimethyl-substituted dihydrodipyrrins to prepare
tetraalkylbacteriochlorins. The tetraalkylbacteriochlorins are
accessible, albeit in small quantities, via a relatively concise
synthesis. The tetraalkylbacteriochlorins constitute macro-
cycles of intermediate complexity along the progression from
the fully unsubstituted parent bacteriochlorin BC0 to the native
bacteriochlorophylls.
Experimental section
General methods
1H NMR (300 MHz) and 13C NMR (100 MHz) spectra were
collected at room temperature in CDCl3 unless noted other-
wise. Absorption spectra were obtained in toluene at room
temperature unless noted otherwise. Electrospray ionization
mass spectrometry (ESI-MS) data are reported for the molecular
ion or protonated molecular ion. THF used in all reactions was
freshly distilled from Na/benzophenone ketyl. Compounds 5,11
7,31 830 and 1134 were prepared as described in the literature.
Although this result could reflect in part differences (or
errors) in radiative rate constant (kf), those values (derived from
tS and Ff) are consistent with the integrated intensity of the
Qy band relative to the total spectrum (Fig. 5), as expected from
Self-condensation study of dihydrodipyrrin-carboxaldehydes
the relationship of the Einstein coefficients.54 The differences Each reaction (1.0 mL) was carried out in a microreaction vial
do not appear to reflect changes in intersystem-crossing rate equipped with a stir bar and cap; the latter was sealed with
constants (Table 3). The apparent origin is that BC-MM and Teflon tape to limit solvent evaporation. Reactions were done
BC-AmAm have more facile internal conversion than expected on a 0.018 mmol scale of dihydrodipyrrin-carboxaldehyde.
on the basis of the S1 (Qy) energy. It is interesting to speculate Anhydrous solvents (CH2Cl2, CH3CN) were reagent grade and
whether the more facile internal conversion of the two new were used as received.
2,3,12,13-tetraalkylbacteriochlorins is connected with the increased The acids used were (1) BF3ÁOEt2 in CH3CN, (2) TMSOTf/
vibronic activity manifest in the absorption spectrum and RR DTBP in CH2Cl2, and (3) neat TFA. The procedures for each
spectrum of CuBC-MM versus CuBC0 (vide supra). condition are as following: (i) the dihydrodipyrrin-carboxaldehyde
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2016
New J. Chem.

View Article Online
Paper
NJC
(0.018 mmol) in CH3CN (1.0 mL) was treated with BF3ÁOEt2 (17 mL, 3.90 mmol, 5.0 mol equiv.] and H2O (31 mL) were combined.
140 mmol) under argon using a microsyringe. The reaction mixture The solution was bubbled with argon for 10 min. NH4OAc
was stirred at room temperature for 19 h. TEA (60 mL, 0.043 mmol) (22.9 g, 297 mmol) was slowly added to buffer the solution to
was added to the reaction mixture. Then the reaction mixture was pH 6.0; and then THF (2.2 mL) was added under argon
concentrated. (ii) The dihydrodipyrrin-carboxaldehyde (0.018 mmol) bubbling (B20 min). The solution in the first flask containing
in CH2Cl2 (1.0 mL) was treated with DTBP (33 mL, 144 mmol, 8 molar the nitronate anion of 6 was transferred via a cannula to the
equiv.) followed by TMSOTf (13 mL, 72 mmol, 4 molar equiv.) under buffered TiCl3 solution in the second flask. The resulting
argon. The reaction mixture was stirred at room temperature mixture was stirred at room temperature for 6 h under argon.
for 19 h. The reaction mixture was concentrated. (iii) The Then the mixture was slowly poured into a stirred mixture of
dihydrodipyrrin-carboxaldehyde (0.018 mmol) was dissolved saturated aqueous NaHCO3 solution (120 mL) and ethyl acetate
in TFA (1.0 mL) under argon. The resulting solution was stirred (40 mL). After 10 min, the mixture was extracted with ethyl
at room temperature for 1 h. Then the reaction mixture was acetate. The combined organic extract was washed with water
poured into saturated aqueous NaHCO3 solution and extracted and then dried (Na2SO4). The solvent was removed under
with CH2Cl2. The extract was washed (brine), dried (Na2SO4) reduced pressure at room temperature. The crude product
and concentrated.
was passed through a short column [alumina, hexanes/ethyl
The crude samples were checked by TLC, UV-vis spectro- acetate (2 : 1)] to afford a yellow solid (55 mg, 25%): mp 112–
1
scopy and LD-MS. For TLC analysis, a tiny amount of the crude 113 1C; H NMR d 1.18 (s, 6H), 2.21 (s, 3H), 2.38 (s, 3H), 2.51
sample in CH2Cl2 was spotted directly onto the TLC plate (s, 2H), 5.97 (s, 1H), 6.27–6.28 (m, 1H), 6.84–6.86 (m, 1H), 7.21
[silica, hexanes/CH2Cl2 (1 : 1)]. TLC showed the consumption (d, J = 8.0 Hz, 2H), 7.36 (d, J = 8.0 Hz, 2H), 10.99–11.10 (br, 1H);
of starting material, and the formation of HBC-type and/or 13C NMR (75 MHz) d 20.7, 21.2, 29.1, 41.2, 53.7, 102.9, 108.9,
other macrocycles (if any). The diagnostic feature in the absorption 118.3, 123.3, 127.2, 128.5, 129.2, 134.4, 134.9, 161.2, 176.6; ESI-MS
spectrum was the characteristic bacteriochlorin Qy absorption obsd 279.1853, calcd 279.1856 [(M + H)+, M = C19H22N2].
band. If present, the bacteriochlorin was isolated by column
1-Formyl-3,3-dimethyl-7-(4-methylphenyl)-2,3-dihydrodipyrrin
(1-CHO)
chromatography [silica, hexanes/CH2Cl2 (1 : 1)]. The fractions
containing bacteriochlorins were collected, concentrated and
the yield was determined spectroscopically. The assumed value Following a general procedure,13,28 a solution of 1-Me (45 mg,
of eQ = 120 000 MÀ1 cmÀ1 for HBC-type bacteriochlorins10 was 0.16 mmol) in 1,4-dioxane (3.2 mL) was treated with SeO2 (27 mg,
y
identical to that for BC-MM upon experimental determination. 0.24 mmol) under argon. The mixture was stirred for 1.5 h at
room temperature. The reaction mixture was treated with
saturated aqueous NaHCO3 solution (4.0 mL) and extracted
with ethyl acetate. The organic extract was washed with water,
6-[3-(4-Methylphenyl)pyrrol-2-yl]-4,4-dimethyl-5-nitro-2-
hexanone (6)
Following a standard procedure,27 a mixture of 5 (460 mg, dried (Na2SO4), and chromatographed [silica, hexanes/ethyl
2.00 mmol) and mesityl oxide (458 mL, 4.00 mmol) in CH3CN acetate (3 : 1)] to give a dark orange solid (22 mg, 47%):
(4.0 mL) was treated with DBU (897 mL, 6.00 mmol). The 1H NMR d 1.22 (s, 6H), 2.41 (s, 3H), 2.72 (s, 2H), 6.34–6.35
reaction mixture was stirred for 24 h at room temperature, (m, 1H), 6.38 (s, 1H), 7.00–7.02 (m, 1H), 7.25 (d, J = 8.0 Hz, 2H),
diluted with ethyl acetate (15.0 mL) and washed with water and 7.35 (d, J = 8.0 Hz, 2H), 10.0 (s, 1H), 10.71–10.82 (br, 1H); FAB-MS
brine. The organic layer was dried (Na2SO4) and concentrated. obsd 293.1654, calcd 293.1654 [(M + H)+, M = C19H20N2O].
Excess mesityl oxide was removed under high vacuum. The
4,4-Dimethyl-6-(5-tert-butoxycarbonyl-3,4-dimethylpyrrol-2-yl)-
5-nitrohexan-2-one (9)
resulting oil was chromatographed [silica, hexanes/ethyl acetate
(2 : 1)] to afford a light brown oil (486 mg, 74%): 1H NMR
(400 MHz) d 1.08 (s, 3H), 1.19 (s, 3H), 2.10 (s, 3H), 2.37 (s, Following a standard procedure27 with modification, a mixture
3H), 2.55 (AB, 2J = 17.6 Hz, 2H), 3.21 (ABX, 3J = 2.6 Hz, 2J = of 8 (4.02 g, 15.0 mmol) and mesityl oxide (4-Me, 4.40 g,
15.6 Hz, 1H), 3.38 (ABX, 3J = 11.6 Hz, 2J = 15.6 Hz, 1H), 5.18 (ABX, 45.0 mmol) was treated with DBU (6.7 mL, 45.0 mmol) at room
3J = 2.6 Hz, J = 11.6 Hz, 1H), 6.22–6.24 (m, 1H), 6.67–6.69 (m, temperature. After 16 h, the reaction mixture was diluted with
3
1H), 7.20 (d, J = 8.0 Hz, 2H), 7.24 (d, J = 8.0 Hz, 2H), 8.05–8.20 ethyl acetate and washed with HCl (0.5 M), saturated aqueous
(br, 1H); 13C NMR d 21.0, 23.7, 23.9, 25.1, 31.4, 36.7, 50.8, 94.4, NaHCO3 solution and brine. The organic layer was dried and
109.1, 117.5, 121.8, 123.1, 128.0, 129.1, 133.4, 135.3, 206.9; ESI-MS filtered. The filtrate was concentrated. The residue was crystallized
obsd 329.1852, calcd 329.1860 [(M + H)+, M = C19H24N2O3].
from hexane (20 mL) to give a light brown solid (1.95 g, 35%): mp
1
118–119 1C; H NMR (400 MHz) d 1.13 (s, 3H), 1.27 (s, 3H), 1.55
(s, 9H), 1.91 (s, 3H), 2.15 (s, 3H), 2.18 (s, 3H), 2.38–2.64 (AB, 2J =
1,3,3-Trimethyl-7-(4-methylphenyl)-2,3-dihydrodipyrrin (1-Me)
Following a standard procedure,10 a solution of 6 (256 mg, 18.3 Hz, 2H), 2.94 (ABX, 3J = 2.4 Hz, 2J = 13.2 Hz, 1H), 3.26 (ABX,
0.780 mmol) in anhydrous THF (7.8 mL) under argon was 3J = 11.4 Hz, 2J = 15.3 Hz, 1H), 5.08 (ABX, 3J = 3.3 Hz, 2J = 14.1 Hz,
treated with NaOMe (210 mg, 3.90 mmol). The mixture was 1H), 8.40–8.50 (brs, 1H); 13C NMR d 8.9, 10.7, 24.3, 24.6, 25.2, 28.7,
bubbled with argon for 10 min and then was stirred under 32.1, 37.0, 51.6, 80.6, 93.7, 118.4, 119.8, 126.3, 161.2, 207.0; ESI-MS
argon for 1 h at room temperature (first flask). In a second obsd 367.2221, calcd 367.2228 [(M + H)+, M = C19H30N2O5].
flask, TiCl3 [8.6 wt% TiCl3 in 28 wt% HCl (d = 1.2), 5.83 mL, Compound 9 has been prepared via an earlier route.32
New J. Chem.
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2016

View Article Online
NJC
Paper
2,3-Dihydro-1,3,3,7,8-pentamethyl-9-tert-butoxycarbonyldipyrrin
(10-Me)
95.1, 127.8, 133.7, 134.5, 156.4, 168.7; ESI-MS obsd 427.2850,
calcd 427.2856 [(M + H)+, M = C28H34N4]; labs(toluene) 347, 374,
490, 722 nm; e772 nm = 120 000 MÀ1 cmÀ1
.
Following a general procedure11 with modification, a solution
of 9 (1.95 g, 5.33 mmol) in THF/methanol (10 : 1, 10 mL) under
argon was treated with NaOMe (0.86 g, 15.9 mmol). The mixture
was stirred at room temperature for 30 min. In a second flask,
NH4OAc (20.0 g, 266 mmol) in distilled THF (72 mL) was
bubbled with argon for 15 min. Then TiCl3 (27 mL, 20 wt%
in 3% HCl solution) was added, and the mixture was degassed
by bubbling with argon for 30 min. Then, the contents of the
first flask mixture were transferred via cannula to the buffered
TiCl3 solution. The resulting mixture was stirred at room
temperature for 20 h under argon. The mixture was then
filtered through a pad of Celite, which was washed with ethyl
acetate. The filtrate was washed with saturated aqueous
NaHCO3 solution, brine and water. The organic layer was dried
and filtered. The filtrate was concentrated. Column chromato-
graphy (silica, CH2Cl2) afforded a light yellow solid (1.17 g,
69%): mp 143–144 1C; 1H NMR d 1.21 (s, 6H), 1.58 (s, 9H), 2.02
(s, 3H), 2.22 (s, 3H), 2.26 (s, 3H), 2.52 (s, 2H), 5.67 (s, 1H), 11.10
(brs, 1H); 13C NMR d 8.89, 10.4, 20.9, 28.6, 29.2, 41.5, 53.8, 79.6,
101.3, 118.3, 119.6, 126.2, 130.6, 161.2, 163.5, 178.4; ESI-MS
obsd 317.2218, calcd 317.2223 [(M + H)+, M = C19H28N2O2]; labs
(CH2Cl2) = 350 nm. Compound 10-Me has been prepared via an
earlier route.32
Benzyl 3,4-bis(benzyloxycarbonylmethyl)-5-methylpyrrole-2-
carboxylate (12)
Following a reported procedure,37 a solution of the trimethyl
ester 11 (7.32 g, 25.9 mmol) in benzyl alcohol (40 mL) in a
100 mL round-bottom flask was stirred under an argon stream
and heated to reflux. After a few minutes, a solution of sodium
(0.52 g) in benzyl alcohol (10.0 mL) was added dropwise;
heating was continued throughout and a few minutes later
(20 min from start of process) the hot solution was poured into
a mixture of water (50 mL), acetic acid (1.0 mL) and methanol
(100 mL). The solid product was collected, washed with water
and then dissolved in CH2Cl2 (100 mL). After washing with
water, the organic phase was dried (Na2SO4) and filtered. The
filtrate was concentrated to a yellow oil. The oil was treated with
hexanes/diethyl ether/CH2Cl2 (140 mL, 5 : 1: 1), and the resulting
mixture was refluxed whereupon a homogeneous solution was
obtained. Upon allowing to cool to room temperature, white
crystals formed. The mixture was cooled overnight at À10 1C to
promote further crystallization. Filtration afforded a white solid
(9.03 g, 68%): mp 84–85 1C; 1H NMR d 2.19 (s, 3H), 3.43 (s, 2H),
3.91 (s, 2H), 5.01 (s, 2H), 5.05 (s, 2H), 5.21 (s, 2H), 7.24–7.31 (m,
15H), 9.07 (br, 1H); 13C NMR (75 MHz) d 11.9, 30.5, 31.2, 66.1,
66.6, 66.8, 115.2, 118.0, 123.8, 128.3, 128.4, 128.7, 128.9, 131.9,
136.1, 136.3, 136.4, 161.1, 171.5, 200.6; ESI-MS obsd 512.2054,
calcd 512.2068 [(M + H)+, M = C31H29NO6]. Compound 12 has
been prepared via an earlier route.35
9-tert-Butoxycarbonyl-1-formyl-2,3-dihydro-3,3,7,8-
pentamethyldipyrrin (10-CHO)
Following a general procedure,13,28 a solution of dihydrodipyrrin
10-Me (95 mg, 0.30 mmol) in 6.0 mL of 1,4-dioxane was treated
with SeO2 (100 mg, 0.90 mmol) under argon. The reaction Benzyl 3,4-bis(methoxycarbonylmethyl)-5-methylpyrrole-2-
mixture was stirred at room temperature and monitored via carboxylate (13)
absorption spectroscopy for the characteristic bathochromic shift
Following a reported procedure,37 the triester 12 (12.78 g,
25.0 mmol) in THF (40 mL) and dry methanol (80 mL) was
that accompanies oxidation of the pyrroline a-methyl group to
the carboxaldehyde.13 After 90 min, ethyl acetate was added. The
treated with sodium methoxide (1.08 g, 20.0 mmol). The reaction
reaction mixture was washed with aqueous NaHCO3 solution and
was monitored by TLC analysis. After 3 h, glacial acetic acid
then brine. The organic layer was dried, concentrated and
(3.0 mL) was added and the solution was concentrated. Then,
chromatographed (silica, CH2Cl2) to give an orange oil (63 mg,
water (200 mL) was added, and the mixture was extracted with
CH2Cl2 (200 mL). The organic layer was washed, dried (Na2SO4)
and concentrated. The crude product was recrystallized from
63%): 1H NMR d 1.26 (s, 6H), 1.59 (s, 9 H), 2.09 (s, 3H), 2.26
(s, 3H), 2.73 (s, 2H), 6.14 (s, 1H), 9.98 (s, 1H), 10.81 (brs, 1H);
13C NMR d 9.2, 10.4, 28.6, 29.3, 41.3, 46.2, 80.6, 111.0, 122.8, 126.2,
hexanes/ether (100 mL/50 mL) to give white crystals (7.73 g,
129.6, 160.9, 162.4, 170.2, 190.3; ESI-MS obsd 331.2011, calcd
1
82%): mp 90–91 1C; H NMR (400 MHz) d 2.21 (s, 3H), 3.40 (s,
331.2016 [(M + H)+, M = C19H26N2O3]; labs(CH2Cl2) = 468 nm.
2H), 3.58 (s, 3H), 3.64 (s, 3H), 3.86 (s, 2H), 5.26 (s, 2H), 7.35 (m, 5H),
9.38 (br, 1H); 13C NMR d 11.6, 30.1, 30.9, 51.9, 52.0, 65.9, 114.9,
117.7, 123.7, 128.2, 128.3, 128.6, 131.9, 136.2, 161.0, 171.9, 172.0;
2,3,8,8,12,13,18,18-Octamethylbacteriochlorin (BC-MM)
A sample of 10-CHO (69.0 mg, 0.208 mmol) was dissolved ESI-MS obsd 360.1428, calcd 360.1442 [(M + H)+, M = C19H21NO6].
in TFA (10.0 mL) under argon and stirred for 1 h at room Compound 13 has been prepared via an earlier route.36
temperature. Then the reaction mixture was poured into saturated
aqueous NaHCO3 solution (200 mL). The resulting mixture was
extracted with CH2Cl2 (200 mL). The organic extract was washed
tert-Butyl 3,4-bis(methoxycarbonylmethyl)-5-methylpyrrole-2-
carboxylate (14)
with brine and water, dried (Na2SO4), and concentrated. Column Following a reported procedure,38 a solution of 13 (0.825 g,
chromatography [silica, hexanes/CH2Cl2 (1 : 1)] afforded a green 2.30 mmol) in 20 mL of methanol was reduced overnight with
solid (13.0 mg, 29%): 1H NMR (400 MHz) d À2.43 (brs, 2H), 1.96 hydrogen (balloon pressure) over 83 mg of 10% Pd/C. The catalyst
(s, 12H), 3.31 (s, 6H), 3.34 (s, 6H), 4.45 (brs, 4H), 8.57 (s, 2H), was removed by filtration, and the filtrate was concentrated to
8.71 (s, 2H); 13C NMR d 11.3, 11.4, 29.9, 31.3, 46.1, 51.9, 92.6, dryness. The resulting residue was dissolved in 10 mL of dry
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2016
New J. Chem.

View Article Online
Paper
NJC
tert-butyl alcohol and 10 mL of dry THF. DCC (0.473 g, were stirred together in DMF (30 mL) for 3 h. The mixture was
2.30 mmol) and a catalytic amount of DMAP were added, and then diluted with water (120 mL) and extracted with ether
the mixture was stirred for 18 h at room temperature. The (4 Â 30 mL). The combined organic extract was washed [HCl
precipitate was removed by filtration. The filtrate was concen- solution (2 M, 20 mL), saturated aqueous NaHCO3 solution
trated and chromatographed (silica, CH2Cl2) to afford a light (20 mL) and brine (20 mL)], dried (Na2SO4) and concentrated.
yellow solid (522 mg, 70%): mp 130–131 1C; 1H NMR (400 MHz) Chromatography [silica, CH2Cl2, then CH2Cl2/ethyl acetate
1
d 1.54 (s, 9H), 2.25 (s, 3H), 3.41 (s, 2H), 3.65 (s, 3H), 3.67 (s, 3H), (1 : 1)] afforded a yellow oil (393 mg, 54%): H NMR (400 MHz)
3.84 (s, 2H), 9.65 (br, 1H); 13C NMR d 11.7, 28.6, 30.3, 31.1, 52.1, d 1.13 (s, 3H), 1.25 (s, 3H), 1.53 (s, 9H), 2.17 (s, 3H), 2.43, 2.60
2
3
2
56.0, 81.1, 114.5, 120.0, 122.2, 131.2, 161.3, 172.2, 172.3; ESI-MS (AB, J = 17.6 Hz, 2H), 3.07 (ABX, J = 2.0 Hz, J = 15.2 Hz, 1H),
obsd 348.1405, calcd 348.1418 [(M + Na)+, M = C16H23NO6].
3.33 (ABX, 3J = 12.4 Hz, 2J = 16.0 Hz, 1H), 3.41 (s, 2H), 3.66
(s, 3H), 3.67 (s, 3H), 3.80 (s, 2H), 5.19 (ABX, 3J = 2.6 Hz, 3J = 12.0 Hz,
1H), 8.93 (br, 1H); 13C NMR d 24.0, 24.1, 24.7, 28.3, 29.8, 30.6,
31.8, 36.8, 51.2, 51.7, 52.0, 81.1, 93.5, 115.8, 121.2, 121.6, 128.2,
tert-Butyl 3,4-bis(methoxycarbonylmethyl)-5-(2-nitroethyl)pyrrole-
2-carboxylate (16)
Following a reported procedure,32 a solution of 14 (0.975 g, 160.3, 171.7, 171.8, 207.0; ESI-MS obsd 505.2144, calcd
3.00 mmol) in CH2Cl2 (30 mL) was stirred vigorously at 0 1C 505.2157 [(M + Na)+, M = C23H34N2O9].
with K2CO3 (4.55 g, 33.0 mmol) during dropwise addition of
9-tert-Butoxycarbonyl-2,3-dihydro-7,8-bis(methoxycarbonylmethyl)-
SOCl2 (510 mL, 6.3 mmol). The mixture was stirred for 10 min at
1,3,3-trimethyldipyrrin (18-Me)
0 1C and 10 min at 20 1C before filtration through Celite. The
filtrate was concentrated to yield the crude dichloromethylpyrrole. A Following a standard procedure,11 a solution of 17 (393 mg,
solution of this product in acetone (30 mL) and water (15 mL) was 0.815 mmol) in THF/methanol (2.0 mL/0.20 mL) under argon
kept for 1 h before dilution with water (30 mL) and extraction with was treated with sodium methoxide (221 mg, 4.08 mmol). The
CH2Cl2 (50 mL). The combined organic extract was washed with mixture was stirred at room temperature for 30 min. In a
saturated aqueous NaHCO3 solution, dried (Na2SO4) and concen- second flask, ammonium acetate (6.13 g, 81.5 mmol) in distilled
trated to give the crude formylpyrrole 15, which was used in the next THF (20 mL) was bubbled with argon for 15 min before TiCl3
step without further purification: 1H NMR (400 MHz) d 1.56 (s, 9H), solution (8.37 g of 12 wt% in 5–10% HCl solution) was added.
3.69 (s, 3H), 3.70 (s, 3H), 3.82 (s, 2H), 3.87 (s, 2H), 9.80 (s, 1H), 10.22 The mixture was degassed by bubbling with argon for 30 min.
(br, 1H); 13C NMR d 28.2, 29.5, 30.0, 52.0, 52.4, 83.0, 122.6, 125.6, Then, the solution in the first flask was transferred via cannula
126.6, 130.2, 160.0, 170.7, 171.2, 179.8; ESI-MS obsd 362.1199, calcd to the buffered TiCl3 solution. The resulting mixture was stirred
362.1210 [(M + Na)+, M = C16H21NO7].
for 20 h at room temperature under argon. The mixture was
Following a standard procedure,11 the crude formylpyrrole poured into 200 mL of saturated aqueous NaHCO3 solution and
15, nitromethane (0.48 mL, 9.0 mmol), potassium acetate (0.35 g, extracted with ethyl acetate (2 Â 200 mL). The organic extract
3.6 mmol) and methylamine hydrochloride (0.24 g, 3.6 mmol) was filtered through Celite. The filtrate was washed with brine,
were stirred together in methanol (10.0 mL) for 5 h. Then the dried (Na2SO4) and concentrated. Chromatography [silica,
mixture was diluted with water (50 mL) and extracted with CH2Cl2/ethyl acetate (3 : 1)] gave a yellow oil (65 mg, 18%):
CH2Cl2 (3 Â 10 mL). The combined organic extract was washed 1H NMR (400 MHz) d 1.21 (s, 6H), 1.55 (s, 9H), 2.23 (s, 3H),
with water, dried (Na2SO4) and concentrated to give the crude 2.53 (s, 2H), 3.52 (s, 2H), 3.64 (s, 3H), 3.66 (s, 3H), 3.90 (s, 2H),
(2-nitrovinyl)pyrrole. The residue was dissolved in methanol (40 mL) 5.71 (s, 1H), 11.46 (br, 1H); 13C NMR d 21.0, 28.5, 29.1, 30.2,
and DMF (4 mL) containing acetic acid (0.18 g, 3.0 mmol). The 30.7, 41.6, 51.9, 52.1, 53.9, 80.5, 100.8, 115.0, 120.9, 131.9, 165.0,
solution was treated with sodium borohydride (0.37 g, 10. mmol) in 172.1, 172.2, 179.5; ESI-MS obsd 433.2327, calcd 433.2333 [(M + H)+,
one portion. Then the mixture was cooled to 0 1C, acidified to pH 2 M = C23H32N2O6].
with HCl solution (2 M), diluted with water (200 mL) and extracted
2,3,12,13-Tetrakis(methoxycarbonylmethyl)-8,8,18,18-
tetramethylbacteriochlorin (BC-AmAm)
with ether (2 Â 100 mL). The combined organic extract was washed
[saturated aqueous NaHCO3 solution and brine (50 mL)], dried and
concentrated. Chromatography [silica, hexanes/ethyl acetate Following a standard procedure,13,28 a solution of 18-Me (20 mg,
(1 : 1)] gave a yellow oil (0.580 g, 50%): 1H NMR (400 MHz) d 0.046 mmol) in 1,4-dioxane (1.0 mL) was treated with SeO2
1.53 (s, 9H), 3.30 (t, J = 7.2 Hz, 2H), 3.45 (s, 2H), 3.66 (s, 3H), (15 mg, 0.139 mmol) with vigorous stirring at room temperature.
3.68 (s, 3H), 3.82 (s, 2H), 4.61 (t, J = 7.2 Hz, 2H), 10.06 (br, 1H); After 2 h, the reaction mixture was poured into saturated aqueous
13C NMR d 23.7, 28.3, 29.9, 30.7, 51.8, 52.1, 73.9, 81.5, 115.8, NaHCO3 solution and then extracted with ethyl acetate (100 mL).
121.2, 121.8, 128.5, 160.9, 171.8, 171.9; ESI-MS obsd 407.1415, The organic extract was washed with brine, dried (Na2SO4) and
calcd 407.1425 [(M + Na)+, M = C17H24N2O8].
concentrated. The residue was passed through a short silica
column [hexanes/ethyl acetate (3 : 1)] to give 18-CHO as a yellow-
orange oil. The oil was dissolved in dry CH3CN (2.50 mL) and
treated with BF3ÁOEt2 (45 mL, 0.37 mmol) under argon. The vial
4,4-Dimethyl-6-(5-tert-butoxycarbonyl-3,4-bis(methoxycarbonyl-
methyl)pyrrol–2-yl)-5-nitro-hexan-2-one (17)
Following a standard procedure,39 samples of 16 (0.580 g, was sealed with Parafilm and stirred at room temperature for
1.51 mmol), mesityl oxide (0.740 g, 7.50 mmol), TBAF (1.0 M 20 h. Then trimethylamine (50 mL) was added to quench the
in THF, 1.51 mL, 1.51 mmol) and 4 Å molecular sieves (1.5 g) reaction. The mixture was concentrated and chromatographed
New J. Chem.
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2016

View Article Online
NJC
Paper
[silica, hexanes/CH2Cl2 (3 : 1)] to give a green solid (1.0 mg, The spectra were acquired with 2 h of signal averaging
1
6.6%): H NMR (400 MHz) d À2.13 (br, 2H), 1.95 (s, 12H), 3.73 (60 Â 120 s or 120 Â 60 s scans, depending on the spectral
(s, 6H), 3.75 (s, 6H), 4.44 (s, 4H), 4.89 (s, 2H), 4.91 (s, 2H), 8.67 window). The spectral resolution was B2 cmÀ1 at a Raman shift
(s, 2H), 8.76 (s, 2H); labs(toluene) 347, 373, 495, 732 nm; ESI-MS of 200 cmÀ1. The spectral data were calibrated using the known
obsd 659.3052, calcd 659.3075 [(M + H)+, M = C36H42N4O8].
frequencies of indene60 and fenchone.61
Cu(II)-2,3,8,8,12,13,18,18-Octamethylbacteriochlorin (CuBC-MM)
Photophysical measurements
Following a reported procedure,33 BC-MM (2.0 mg, 4.7 mmol),
sodium hydride (24 mg, 0.94 mmol, 60% dispersion in mineral
oil) and copper(II) acetate (43 mg, 0.24 mmol) were placed in a
Schlenk tube and flushed with argon. Freshly distilled THF
(1.2 mL) was added, and the reaction mixture was heated at
60 1C under argon for 16 h. Then the mixture was diluted with
CH2Cl2 and washed with saturated aqueous NaHCO3 solution.
The organic layer was washed with brine, dried (Na2SO4) and
concentrated. Flash chromatography [silica, hexanes/CH2Cl2
(2 : 1), with 1% triethylamine] afforded a green solid (6.0%,
determined by absorption spectroscopy assuming equal molar
absorptivity of the respective free base and metallobacterio-
chlorins at the Qy(0,0) band): labs(toluene) 336, 385, 506, 736 nm;
LD-MS obsd 488.6; ESI-MS obsd 487.1900, calcd 487.1918
[M+, M = C28H32CuN4].
Static absorption (Shimadzu UV-1800), fluorescence (Horiba
Nanolog), and time-resolved measurements were performed
at room temperature. Measurement of the fluorescence quantum
yield (Ff) and singlet excited-state lifetime (tS) utilized dilute (mM)
Ar-purged toluene solutions. Static emission measurements
employed 2–4 nm excitation and detection bandwidths. Emission
spectra were corrected for detection-system spectral response.
Samples for Ff measurements generally had an absorbance of
o0.05 for BC-MM and BC-AmAm in the Qy band to minimize
inner-filter effects associated with the very small (1–2 nm)
fluorescence-absorption (Stokes) shifts. For analogues with larger
(up to 6 nm) Stokes shifts, the Qy absorbance could be increased
somewhat but was never 40.15. The Ff measurements employed
an integrating sphere in which the ratio of excitation and
emission bands were compared for a blank and a sample to
calculate the absolute fluorescence quantum yield (Horiba,
Quanti-Phi). Fluorescence lifetimes were obtained using several
methods and then averaged. The first method utilized a strobo-
scopic fluorescence decay technique on an apparatus with an
approximately Gaussian instrument response function with a
FWHM of B1 ns (Photon Technology International, Laser-
Strobe TM-3). Samples were excited in the blue to green spectral
regions by a dye laser pumped by a nitrogen laser at 337 nm.
The second method utilized transient absorption decay measure-
ments in which difference spectra (450–750 nm) were measured as a
function of time following a 130 fs excitation flash in the Qx or Qy
band. One setup utilized 130 fs white-light probe pulses and delay
times to 8 ns (Ultrafast Systems, Helios). The second method used a
continuum probe laser with a pulse width of B1 ns and data
acquisition in 100 ps bins to a delay time of B0.5 ms (Ultrafast
Systems, EOS). The values from the various measurements were in
good agreement with those obtained previously14 for benchmarks
using a fluorescence modulation technique (Spex Tau2). The EOS
system was also used for measurements of the yield of S1 - T1
intersystem crossing (i.e., the triplet yield). In particular, from the
transient absorption difference spectra, time profiles for bleaching
of the ground-state Q bands relative to the basically featureless
excited-state absorption were fit to an exponential (the excited-state
lifetime) and the amplitude at the asymptote of the decay (due to T1)
was compared to the value at t = 0 (due to S1).
X-ray structure determination
A single-crystal X-ray analysis of BC-MM was made on a Bruker-
Nonius X8 Apex2 CCD diffractometer at 110 K. The frame
integration was performed using SAINT+.55 The resulting raw
data were scaled and absorption-corrected by multiscan averaging
of symmetry equivalent data using SADABS.56 The structure was
solved using direct methods from SIR9257 and refined using the
NRCVAX58 crystallographic suite. The structure was refined using
the SHELXL program from the SHELX201359 package, and graphic
plots were produced using Mercury 3.5.
RR spectroscopy
The RR spectrum of CuBC-MM was obtained at room temperature
on a sample (B0.2 mM) in deoxygenated benzene solution
contained in a 5 mm diameter NMR tube which had been cut
to a length of B4 cm. The NMR tube was spun to mitigate
photodamage. Spectra were initially acquired in CH2Cl2 solution;
however, CuBC-MM was quite photolabile in this solvent and
tended to demetalate, resulting in severe interference from
fluorescence emission from the free base. CuBC-MM was much
less prone to demetalation in benzene, albeit a small amount of
demetalation occurred during the course of data acquisition, as
was evidenced by increasing fluorescence emission.
The RR spectrum was acquired with a red-optimized triple
spectrograph (Spex 1877) equipped with a holographically
etched 1200 groove per mm grating in the third state. A back-
illuminated (CCD) was used as the detector (Princeton Instruments
LN/CCD equipped with a Tektronix chip). The excitation wavelength
Acknowledgements
was provided by the discrete output of a Ti:sapphire laser (Coherent This work was supported by a grant from the Chemical
890), pumped by a diode-pumped solid-state laser (Coherent Sciences, Geosciences and Biosciences Division, Office of Basic
Verdi-V6). The laser powers were typically in the 6–7 mW range; the Energy Sciences, of the U.S. Department of Energy (DE-FG02-
beam diameter was B0.5 mm. The scattered light was collected 05ER15651). Mass spectra were obtained at the Mass Spectrometry
in a 901 configuration by using a 50 mm, f/1.2 camera lens. Laboratory for Biotechnology at North Carolina State University.
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2016
New J. Chem.

View Article Online
Paper
NJC
20 Z. Yu and M. Ptaszek, J. Org. Chem., 2013, 78, 10678–10691.
21 J. Jiang, E. Yang, K. R. Reddy, D. M. Niedzwiedzki, C. Kirmaier,
D. F. Bocian, D. Holten and J. S. Lindsey, New J. Chem., 2015,
39, 5694–5714.
Partial funding for the facility was obtained from the North Carolina
Biotechnology Center and the National Science Foundation. The
X-ray structural determination was carried out by Dr Roger Sommer.
22 K. Aravindu, M. Krayer, H.-J. Kim and J. S. Lindsey, New
J. Chem., 2011, 35, 1376–1384.
23 J. B. Paine III, in The Porphyrins, ed. D. Dolphin, Academic
Press, New York, 1978, vol. 1, pp. 101–234.
24 J. S. Lindsey, Chem. Rev., 2015, 115, 6534–6620.
25 A. R. Battersby, C. J. Dutton, C. J. R. Fookes and S. P. D.
Turner, J. Chem. Soc., Perkin Trans. 1, 1988, 1557–1567.
26 W. G. O’Neal and P. A. Jacobi, J. Am. Chem. Soc., 2008, 130,
1102–1108.
References
1 L. P. Vernon and G. R. Seely, The Chlorophylls, Academic
Press, New York, USA, 1966.
2 H. Scheer, in Chlorophylls and Bacteriochlorophylls: Biochemistry,
Biophysics, Functions and Applications, ed. B. Grimm,
R. J. Porra, W. Ru¨diger and H. Scheer, Springer, Dordrecht,
The Netherlands, 2006, pp. 1–26.
27 M. Ptaszek, J. Bhaumik, H.-J. Kim, M. Taniguchi and
J. S. Lindsey, Org. Process Res. Dev., 2005, 9, 651–659.
3 M. Kobayashi, M. Akiyama, H. Kano and H. Kise, in Chlorophylls
and Bacteriochlorophylls: Biochemistry, Biophysics, Functions and
Applications, ed. B. Grimm, R. J. Porra, W. Ru¨diger and
H. Scheer, Springer, Dordrecht, The Netherlands, 2006,
pp. 79–94.
¨
28 P. A. Jacobi, S. Lanz, I. Ghosh, S. H. Leung, F. Lower and
D. Pippin, Org. Lett., 2001, 3, 831–834.
29 H.-J. Kim and J. S. Lindsey, WO2006089122A2.
30 A. R. Battersby and L. A. Reiter, J. Chem. Soc., Perkin Trans. 1,
1984, 2743–2749.
4 Y. Chen, G. Li and R. K. Pandey, Curr. Org. Chem., 2004, 8,
1105–1134.
31 E. Bullock, A. W. Johnson, E. Markham and K. B. Shaw,
J. Chem. Soc., 1958, 1430–1440.
5 M. A. Grin, A. F. Mironov and A. A. Shtil, Anti-Cancer Agents
Med. Chem., 2008, 8, 683–697.
32 P. J. Harrison, Z.-C. Sheng, C. J. R. Fookes and A. R.
Battersby, J. Chem. Soc., Perkin Trans. 1, 1987, 1667–1678.
33 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,
Inorg. Chem., 2012, 51, 9443–9464.
6 M. Taniguchi, D. L. Cramer, A. D. Bhise, H. L. Kee, D. F. Bocian,
D. Holten and J. S. Lindsey, New J. Chem., 2008, 32, 947–958.
7 J. S. Lindsey, O. Mass and C.-Y. Chen, New J. Chem., 2011,
35, 511–516.
8 V. M. Alexander, K. Sano, Z. Yu, T. Nakajima, P. L. Choyke,
M. Ptaszek and H. Kobayashi, Bioconjugate Chem., 2012, 23,
1671–1679.
34 B. A. Gregg, M. A. Fox and A. J. Bard, J. Am. Chem. Soc., 1989,
111, 3024–3029.
35 S. E. Bari, J. Iturraspe and B. Frydman, Tetrahedron, 1995,
51, 2255–2266.
9 T. Harada, K. Sano, K. Sato, R. Watanabe, Z. Yu, H. Hanaoka,
T. Nakajima, P. L. Choyke, M. Ptaszek and H. Kobayashi,
Bioconjugate Chem., 2014, 25, 362–369.
36 I. A. Chaudhry and P. S. Clezy, Aust. J. Chem., 1982, 35, 1185–1201.
37 A. R. Battersby, E. Hunt, E. McDonald, J. B. Paine III and
J. Saunders, J. Chem. Soc., Perkin Trans. 1, 1976, 1008–1018.
38 L. Diaz, R. B. Frydman, A. Valasinas and B. Frydman, J. Am.
Chem. Soc., 1979, 101, 2710–2716.
39 H.-J. Kim, D. K. Dogutan, M. Ptaszek and J. S. Lindsey,
Tetrahedron, 2007, 63, 37–55.
40 K. M. Smith, K. C. Langry and O. M. Minnetian, J. Org.
Chem., 1984, 49, 4602–4609.
41 H. Xie, D. A. Lee, D. M. Wallace, M. O. Senge and K. M. Smith,
J. Org. Chem., 1996, 61, 8508–8517.
42 D. M. Arnott, P. J. Harrison, G. B. Henderson, Z.-C. Sheng,
F. J. Leeper and A. R. Battersby, J. Chem. Soc., Perkin Trans. 1,
1989, 265–278.
43 A. R. Battersby, C. J. Dutton and C. J. R. Fookes, J. Chem.
Soc., Perkin Trans. 1, 1988, 1569–1576.
44 E. Yang, J. R. Diers, Y.-Y. Huang, K. Aravindu, M. R. Hamblin,
J. S. Lindsey, D. F. Bocian and D. Holten, Photochem. Photobiol.,
2013, 89, 605–818.
10 H.-J. Kim and J. S. Lindsey, J. Org. Chem., 2005, 70, 5475–5486.
11 M. Krayer, M. Ptaszek, H.-J. Kim, K. R. Meneely, D. Fan,
K. Secor and J. S. Lindsey, J. Org. Chem., 2010, 75, 1016–1039.
12 C. Bru¨ckner, L. Samankumara and J. Ogikubo, in Handbook
of Porphyrin Science, ed. K. M. Kadish, K. M. Smith and
R. Guilard, World Scientific, Singapore, 2012, vol. 17, pp. 1–112.
13 K. R. Reddy, E. Lubian, M. P. Pavan, H.-J. Kim, E. Yang,
D. Holten and J. S. Lindsey, New J. Chem., 2013, 37, 1157–1173.
14 E. Yang, C. Kirmaier, M. Krayer, M. Taniguchi, H.-J. Kim,
J. R. Diers, D. F. Bocian, J. S. Lindsey and D. Holten, J. Phys.
Chem. B, 2011, 115, 10801–10816.
15 Z. Yu, C. Pancholi, G. V. Bhagavathy, H. S. Kang, J. K. Nguyen
and M. Ptaszek, J. Org. Chem., 2014, 79, 7910–7925.
16 P. Vairaprakash, E. Yang, T. Sahin, M. Taniguchi, M. Krayer,
J. R. Diers, A. Wang, D. M. Niedzwiedzki, C. Kirmaier,
J. S. Lindsey, D. F. Bocian and D. Holten, J. Phys. Chem. B,
2015, 119, 4382–4395.
17 H. S. Kang, N. N. Esemoto, J. R. Diers, D. M. Niedzwiedzki,
J. A. Greco, J. Akhigbe, Z. Yu, C. Pancholi, G. B. Bhagavathy,
J. K. Nguyen, C. Kirmaier, R. R. Birge, M. Ptaszek, D. Holten
and D. F. Bocian, J. Phys. Chem. A, 2016, 120, 379–395.
18 J. Jiang, C.-Y. Chen, N. Zhang, P. Vairaprakash and
J. S. Lindsey, New J. Chem., 2015, 39, 403–419.
45 E. Yang, N. Zhang, M. Krayer, M. Taniguchi, J. R. Diers,
C. Kirmaier, J. S. Lindsey, D. F. Bocian and D. Holten,
Photochem. Photobiol., 2016, 92, 111–125.
46 J. R. Diers, Q. Tang, C. J. Hondros, C.-Y. Chen, D. Holten,
J. S. Lindsey and D. F. Bocian, J. Phys. Chem. B, 2014, 118,
7520–7532.
19 Z. Yu and M. Ptaszek, Org. Lett., 2012, 14, 3708–3711.
New J. Chem.
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2016

View Article Online
NJC
Paper
47 M. Lutz and B. Robert, in Biological Applications of Raman 54 J. B. Birks, Photophysics of Aromatic Molecules, Wiley-
Spectroscopy, ed. T. G. Spiro, Wiley, New York, 1988, vol. 3,
Interscience, London, 1970, pp. 142–192.
pp. 347–411.
55 SAINT+, version 7.07B, Bruker-Nonius, Madison, WI, 2004.
48 V. Palaniappan, P. C. Martin, V. Chynwat, H. A. Frank and 56 SADABS, version 2.10, Bruker-Nonius, Madison, WI, 2004.
D. F. Bocian, J. Am. Chem. Soc., 1993, 115, 12035–12049.
49 N. J. Cherepy, A. P. Shreve, L. J. Moore, S. Franzen, S. G. Boxer
and R. A. Mathies, J. Phys. Chem., 1994, 98, 6023–6029.
57 A. Altomare, G. Cascarano, C. Giacovazzo, A. Guagliardi,
M. C. Burla, G. Polidori and M. Camalli, J. Appl. Crystallogr.,
1994, 27, 435.
50 J. R. Diers and D. F. Bocian, J. Am. Chem. Soc., 1995, 117, 58 E. J. Gabe, Y. Le Page, J.-P. Charland, F. L. Lee and P. S.
6629–6630. White, J. Appl. Crystallogr., 1989, 22, 384–387.
51 K. Czarnecki, J. R. Diers, V. Chynwat, J. P. Erickson, H. A. Frank 59 G. M. Sheldrick, Acta Crystallogr., Sect. A: Found. Adv., 2008,
and D. F. Bocian, J. Am. Chem. Soc., 1997, 119, 415–426. 64, 112–122.
52 N. J. Cherepy, A. P. Shreve, L. J. Moore, S. G. Boxer and 60 D. A. Carter, W. R. Thompson, C. E. Taylor and J. E. Pemberton,
R. A. Mathies, J. Phys. Chem. B, 1997, 101, 3250–3260. Appl. Spectrosc., 1995, 49, 1561–1576.
53 R. J. Donohoe, H. A. Frank and D. F. Bocian, Photochem. 61 N.-T. Yu and R. B. Srivastava, J. Raman Spectrosc., 1980, 9,
Photobiol., 1988, 48, 531–537. 166–171.
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2016
New J. Chem.