Photoinduced intramolecular tryptophan oxidation and excited-state behavior of [Re(L-AA)(CO)3(α-diimine)]+ (L = pyridine or imidazole, AA = tryptophan, tyrosine, phenylalanine)

Article abstract of DOI:10.1021/ic200252z

Re^I carbonyl-diimine complexes [Re(L-AA)(CO)₃(N,N)] ⁺ (N,N = bpy, phen) containing an aromatic amino acid (AA), phenylalanine (Phe), tyrosine (Tyr), or tryptophan (Trp), linked to Re by a pyridine-amido or imidazole-amido

Full text of DOI:10.1021/ic200252z

ARTICLE
pubs.acs.org/IC
Photoinduced Intramolecular Tryptophan Oxidation and
Excited-State Behavior of [Re(L-AA)(CO)3(r-diimine)]þ (L = Pyridine
or Imidazole, AA = Tryptophan, Tyrosine, Phenylalanine)
Ana María Blanco-Rodríguez,† Mike Towrie,‡ J. Sꢀykora,§ Stanislav Zꢀaliꢁs,§ and Antonín Vlꢁcek, Jr.*,†,§
†School of Biological and Chemical Sciences, Queen Mary University of London, Mile End Road, London E1 4NS, United Kingdom
‡Central Laser Facility, Research Complex at Harwell, STFC, Rutherford Appleton Laboratory, Harwell Oxford, Didcot,
Oxfordshire OX11 0QX, United Kingdom
§J. Heyrovskꢀy Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejꢁskova 3, CZ-182 23 Prague,
Czech Republic
S Supporting Information
b
ABSTRACT: ReI carbonyl-diimine complexes [Re(L-AA)(CO)3(N,N)]þ (N,N =
bpy, phen) containing an aromatic amino acid (AA), phenylalanine (Phe), tyrosine
(Tyr), or tryptophan (Trp), linked to Re by a pyridine-amido or imidazole-amido
ligand L have been synthesized and their excited-state properties investigated by
nanosecond time-resolved IR (TRIR) and emission spectroscopy. Near-UV optical
excitation populates a ReI(CO)3fN,N 3MLCT excited state *[ReII(L-AA)(CO)3-
(N,N•ꢀ)]þ. Decay to the ground state (50ꢀ300 ns lifetime) is the only excited-state
deactivation process observed in the case of Phe and Tyr complexes, whereas the
Trp-containing species undergo a Trp(indole)f*ReII electron transfer (ET)
producing a charge-separated (CS) state, [ReI(L-Trp•þ)(CO)3(N,N•ꢀ)]þ. The
ET occurs with a 8ꢀ40 ns lifetime depending on L, N,N, and the solvent. The CS
state is characterized by ν(CO) IR bands shifted to lower wavenumbers from their
respective ground-state positions and two bands at 1278 and 1497 cmꢀ1 tentatively
attributed to Trp•þ. The amido bridge is affected by both the MLCT excitation and the subsequent ET, manifested by the shifts and
intensity changes of the amide-I IR band at about 1680 cmꢀ1. The CS state decays to the ground state by a N,N•ꢀf Trp•þ back-ET
the rates of which are comparable to those of the forward ET, 30ꢀ60 ns. This study independently demonstrates that Trp can act as
an electron-hopping intermediate in photodriven ET systems based on Re-labeled proteins and supramolecules. Photoinduced ET
in Trp-containing Re complexes also can be used to generate Trp•þ and investigate its spectral properties and reactivity.
’ INTRODUCTION
chromophores that become strong oxidants upon optical excita-
tion (ReI) or photochemical oxidation by an external quencher
(RuII, OsII).7 Very fast excitation of the metallolabel triggers
electron-transfer from the protein, whose rate and intermediates
are investigated by following spectroscopic features of the metal-
lolabel and/or protein functional groups as a function of time. This
approach is well-suited to obtain kinetics data on long-range
electron transfer,7ꢀ9 as well as to investigate the electron hopping
mechanism involving tryptophan7,10,11 or proton-coupled electron
transfer reactions of tyrosine.12 For several reasons, ReI carbonyl-
diimine complexes13,14 are favored ET phototriggers: (i) they can
be easily appended to biomolecules, (ii) their excited states are
strongly oxidizing and long-lived (thus avoiding the necessity to
use external quenchers), and (iii) their excited-state lifetime and
the redox potential can be finely tuned by variations of the diimine
and/or the axial ligand. For example, a functional “photoRNR” has
Redox-active aromatic amino acids (AA) tryptophan (Trp) and
tyrosine (Tyr) play an important role in biological electron
transfer (ET), acting as redox-intermediates along electron-trans-
fer pathways in proteins. Temporary Trp and Tyr oxidation splits
long tunneling pathways into shorter steps. Resulting sequential
tunneling (hopping) mechanism can accomplish very fast charge
separation over long distances, often longer than 2 nm. Out of
many examples, we may single out the enzyme DNA photolyase,
where an arrangement of three tryptophans mediates a 30 ps
electron transfer across 1.5 nm.1ꢀ3 Class 1 ribonucleotide reduc-
tase (RNR) provides another example,4,5 incorporating a chain of
tyrosines and tryptophans to transfer charge from an iron-tyrosyl
cofactor over two protein subunits to an active site 3.5 nm apart.
Whereas Trp mostly acts as an electron (hole) transfer mediator,
Tyr oxidation is often coupled with proton transfer, as is the case of
RNR, photosystem II,5,6 and many other enzymes.
Kinetics and mechanism of biological electrontransfer reactions
often are investigated by labeling proteins with metalꢀdiimine
Received: February 6, 2011
Published: June 08, 2011
r
2011 American Chemical Society
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Inorganic Chemistry
ARTICLE
been constructed,12 in which a fluorinated tyrosine group (FnTyr)
of a [Re(CN)(CO)3(bpy-FnTyr)] complex occupies the site of
one of the natural Tyr residues of the RNR enzyme. Photo-
excitation of the Re(CN)(CO)3(bpy) chromophore generates
FnTyr• by a FnTyrf*Re ET coupled with FnTyrþ• deprotona-
tion, initiates radical hopping through the protein molecule,
and accomplishes high turnover deoxynucleotide formation,
while kinetics and mechanistic information are unraveled
through spectroscopic detection of the FnTyr• intermediate.12
Recently, a dramatic acceleration of a long-range (∼2 nm) ET
from a CuI center in the Pseudomonas aeruginosa protein azurin to
excited ReI(CO)3(4,7-dimethylphenanthroline)þ chromophore
appended to the protein close to a Trp residue was demonstrated.10
No ET occurs when Tyr, lysine, or Phe occupies the Trp site.
Kinetics analysis revealed several Trpf*Re ET steps whose rates
range from hundreds of femtoseconds to units of nanoseconds.10,11
Thus, the CuIf*Re ET is accelerated by splitting the 2 nm
pathway into much shorter Trpf*Re and CuIfTrpþ• steps,
over which electron tunneling occurs with ∼500 ps and 30 ns
lifetimes, respectively.10,11 Detailed spectroscopic and computa-
tional study of this system has revealed that the peptide segment
containing the Re label and the proximal Trp residue behaves as a
single redox-active unit with πꢀπ interaction between the
Trp(indole) ring and the phenanthroline ligand.11
Figure 1. Pyridine and imidazole amino acid ligands L-AA and their
abbreviations. Donor atoms are indicated in bold. Bottom row: sche-
matic structures of the Re complexes. (Actual ligand structures are most
likely trans with respect to the peptide bond; see Figure 2.)
’ EXPERIMENTAL SECTION
The propensity of ReI chromophores to photooxidize aro-
matic amino acids in biomolecules (as well as supramolecular
constructs15) presents an interesting mechanistic challenge and
commands investigations of the photobehavior of ReI carbonyl-
diimine complexes containing aromatic amino acids. So far,
Tyrf*Re ET and its coupling with a proton transfer (PCET)
were studied upon MLCT excitation of [Re(PPh2Ph-Tyr)-
(CO)3(phen)]þ and [Re(CN)(CO)3(bpy-FnTyr)] (phen =1,
10-phenanthroline, bpy =2,20-bipyridine).12,16,17 A theoretical
analysis of this reaction in an aqueous phosphate buffer solution
has identified a close interaction between the tyrosine ꢀOH
group and the proton acceptor HPO43ꢀ as the prerequisite for
fast Tyr oxidation.18 A detailed PCET mechanistic understand-
ing has been obtained on RuIIꢀbipyridine complexes with
covalently attached Tyr.19ꢀ21 Using a RuIIꢀbpy assembly with
appended Trp has shown that Trp photooxidation at acidic or
neutral pH occurs as an ET process, producing Trpþ•, followed
by a relatively slow (130 ns at pH = 8) deprotonation to Trp•.20
Another stream of research has produced a series of ReI carbonyl-
phenanthroline complexes of Trp analogues [Re(py-indole)-
(CO)3(R-phen)]þ, where indole is covalently attached to a
pyridine ligand through an amide link and aliphatic chains of
variable lengths.22ꢀ24 These complexes are very promising
luminescent labels and probes for indole-binding proteins.22ꢀ25
Aiming at a better understanding of aromatic amino acid
oxidation by electronically excited ReI chromophores, we have
prepared a series of complexes [Re(L-AA)(CO)3(N,N)]þ (L =
py or imidazole, N,N = bpy, phen, AA = Trp, Tyr, Phe; Figure 1)
and investigated their excited-state behavior in several aprotic
solvents using emission and picosecondꢀnanosecond time-
resolved IR spectroscopy (TRIR). This approach allowed us to
characterize the excited states and redox intermediates involved
and to determine the ET kinetics. We have found that excitation
of the ReI(CO)3(N,N) chromophore triggers an intramolecular
nanosecond Trp oxidation, followed by a back-electron-transfer
of a comparable rate. The relatively slow forward ET rates can be
understood in view of the lack of a direct contact between the
indole group and the polypyridine ligand.
Materials. Starting materials were obtained from Aldrich and used
as received. All measurements were performed in solvents of spectro-
scopic purity.
4-(Amino acid)pyridine Ligands (pyaa). Isonicotinic acid
chloride hydrochloride (1 g, 5.62 mmol) was suspended in DCM
(40 mL), and triethylamine (∼3 mL) was added until the solid
dissolved. The amino acid methyl ester (hydrochloride salt in the case
of Trp and Phe) (1.14 mol equiv) was added, turning the solution
orange. The reaction mixture was stirred at room temperature for
10 min. The resulting precipitate (Et3N HCl) was filtered off and the
3
DCM solution was washed with water (4 ꢁ 50 mL). The solvent was
removed by rotary evaporation to give the desired ligands in yields of
50ꢀ60%
3-(4-Hydroxyphenyl)-2-[(pyridine-4-carbonyl)amino]propionic
1
Acid Methyl Ester (pytyr). White solid. H NMR (DMSO, 270
MHz): 2.93 (1H, dd, 3J = 9.4 Hz, 2J = 14 Hz, CH methylene), 3.06 (1H,
dd, 3J = 5.5 Hz, 2J = 14 Hz CH methylene), 3.75 (3H, s, CH3), 4.82 (1H,
3
3
m, CH), 6.9 (2H, d, J = 8.5 Hz, phenol), 7.5 (2H, d, J = 8.5 Hz,
phenol), 7.69 (2H, dd, 3J = 6.2 Hz, 5J = 1.7 Hz, py), 8.73 (2H, dd, 3J = 6.2
Hz, 5J = 1.7 Hz, py), 9.09 (1H, d, 3J = 7.9 Hz, NH), 9.21 (1H, s, OH). 13
C
NMR (CD3OD, 67.9 MHz): 172.1 (CO), 166.6 (CO), 156.2 (C,
phenol), 149.6 (CH ꢁ 2, py), 142.1 (C, phenol), 129.9 (CH ꢁ 2,
phenol), 127.5 (C, py), 121.7 (CH ꢁ 2, py), 115.0 (CH ꢁ 2, phenol),
54.8 (CH), 51.6 (OCH3), 36.1 (CH2). MS (ES) m/z: calcd for
C16H16N 2O4 (Mþ þ Na) 323.1008, found 323.0998.
3-(1H-Indol-3-yl)-2-[(pyridine-4-carbonyl)amino]propionic
1
acid methyl ester (pytrp). Yellow/brown oil. H NMR (DMSO,
270 MHz): 3.23 (1H, dd, 3J = 5 Hz, CH methylene), 3.31 (1H, dd, 3J =
7.4 Hz, CH methylene), 3.64 (3H, s, Me), 4.72 (1H, m, CH), 7.01 (2H,
m, indole), 7.21 (1H, s, indole), 7.35 (1H, d, 3J = 7.9 Hz, indole), 7.57
3
(1H, d,
J = 7.9 Hz, indole), 7.71 (2H, dd, 3J = 6.2 Hz,
5J = 1.5 Hz, py), 8.71 (2H, dd, J = 6.2 Hz, J = 1.5 Hz, py), 9.15
(1H, d, 3J = 7.4 Hz, NH), 10.85 (1H, s, NH). 13C NMR (CD3OD, 67.9
MHz): 172.4 (CO), 166.6 (CO), 149.5 (CH ꢁ 2, py), 142.1 (C, py),
136.7 (C, indole), 127.4 (C, indole), 123.1 (CH, indole), 121.7 (CH ꢁ
2, py), 121.2 (CH, indole), 118.6 (CH, indole), 117.8 (CH, indole),
111.1 (CH, indole) 109.6 (C, indole), 54.3 (CH), 51.5 (OCH3), 26.9
(CH2). MS ES m/z: calcd for C18H17N3O3 (Mþ þ Na) 346.1168, found
346.1158.
3
5
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3-Phenyl-2-[(pyridine-4-carbonyl)amino]propionic Acid
Methyl Ester (pyphe). Brown oil. H NMR (CDCl3, 270 MHz):
136.2 (CH, im), 134.4 (C, im), 127.4 (C, indole), 123.4 (CH, indole),
121.3 (CH, indole), 120.9 (CH, im), 118.7 (CH, indole), 118.0 (CH,
indole), 111.2 (CH, indole) 109.1 (C, indole), 53.2 (CH), 51.6 (OCH3),
27.4 (CH2). MS ES m/z: calcd for C16H17N4O3þ (Mþ þ H) 313.1301,
found 313.1285.
1
3.20 (1H, dd, 3J = 9.9 Hz, 2J = 14 Hz, CH methylene), 3.27 (1H, dd, 3J =
5.5 Hz, 2J = 14 Hz CH methylene), 3.77 (3H, s, Me), 4.75 (1H, m, CH),
6.75 (1H, s, NH), 7.27 (5H, m, phenyl CH), 7.53 (2H, dd, 3J = 6.2 Hz,
5J = 1.7 Hz, py), 8.70 (2H, dd, 3J = 6.2 Hz, 5J = 1.7 Hz, py). 13C NMR
(CD3OD, 67.9 MHz): 171.9 (CO), 166.5 (CO), 149.6 (CH ꢁ 2, py),
142.1 (C, py), 137.0 (C, phenyl), 128.8 (CH ꢁ 2, phenyl), 128.3 (CH ꢁ
2, phenyl), 126.6 (CH, phenyl), 121.6 (CH ꢁ 2, py), 54.6 (CH), 51.6
(OCH3), 36.8 (CH2). MS ES m/z: calcd for C16H16N2O3 (Mþ þ Na)
307.1059, found 307.1051.
2-[(1H-Imidazole-4-carbonyl)amino]-3-phenylpropionic
Acid Methyl Ester (imphe). White/yellow solid. 13C NMR (CDCl3,
67.9 MHz): 172.2 (CO), 163.2 (CO), 136.1 (C, phenyl), 136 (CH, im),
135.0 (C, im) 129.2 (CH ꢁ 2, phenyl), 128.7 (CH ꢁ 2, phenyl), 127.2
(CH, phenyl), 120.3 (CH, im), 53.4 (CH), 52.5 (OCH3), 38.1 (CH2).
MS ES m/z: calcd for C14H16N3O3þ (Mþ þ H) 274.1192, found 274.1182.
[Re(L)(CO)3(N,N)]PF6 (L = pyaa, nicaa, imaa; N,N = bpy, phen).
A modified published procedure26 was used: [Re(CF3SO3)(CO)3(NN)]
(0.171 g, 0.299 mmol) was refluxed with the pyaa ligand (1 mol equiv) in
MeOH for 3 h. Upon cooling, the solvent was removed and the resulting
product was redissolved in the minimum amount of MeOH. A saturated
solution of NH4PF6 in MeOH (∼3 mL) was added to the reaction
mixture and the product was precipitated as a yellow solid by adding
water, filtered, and air-dried. The compounds were purified by pre-
cipitation from DCM with diethyl ether. The pure product is a pale
yellow solid (yield ∼70%).
3-(Amino acid)pyridine Ligands (nicaa). Nicotinic acid chlor-
ide hydrochloride (1 g, 5.62 mmol) was suspended in DCM (40 mL),
and triethylamine (∼3 mL) was added until the solid dissolved. The
amino acid methyl ester hydrochloride salt (1.14 mol equiv) was added,
turning the solution orange. The reaction mixture was stirred at room
temperature for 10 min. The resulting precipitate (Et3N HCl) was
3
filtered off and the DCM solution was washed with water (4 ꢁ 50 mL)
The mixture was washed successively with water, 0.1 N aqueous
hydrochloric acid, water, 5% aqueous sodium bicarbonate solution,
and water; dried over sodium sulfate; and evaporated. Then, 100 mL
of ether was added and the solution was evaporated again. The product
was obtained as yellow, hygroscopic crystals in ∼50ꢀ60% yield.
3-(1H-Indol-3-yl)-2-[(pyridine-3-carbonyl)amino]propio-
nic Acid Methyl Ester (nictrp). 1H NMR (CDCl3, 270 MHz): 3.23
[Re(pytyr)(CO)3(bpy)]PF6. 1H NMR (CD2Cl2, 400 MHz): 2.92
(1H, dd, 3J = 6.8 Hz, 2J = 14 Hz CH methylene), 3. 04 (1H, dd, 3J = 5.3
Hz, 2J = 14 Hz CH methylene), 3.63 (3H, s, Me), 4.74 (1H, m, CH), 5.4
(1H, s, OH), 6.59 (2H, d, 3J = 8.3 Hz, phenol), 6.65 (1H, d, 3J = 7.6 Hz,
NH), 6.84 (2H, d, 3J = 8.3 Hz, phenol), 7.43 (2H, dd, 3J = 6.6 Hz, 5J = 1.5
Hz, py), 7.71 (2H, m, bpy), 8.17 (2H, dd, 3J = 6.6 Hz, 5J = 1.5 Hz, py),
8.19 (2H, m, bpy), 8.27 (2H, d, 3J = 8.1 Hz, bpy), 9.08 (2H, d, 3J = 5.1
Hz, bpy). 13C NMR (CD2Cl2, 100.6 MHz): 195.8 (CO), 191.4 (CO),
171.6 (CO), 163.4 (CO), 156.0 (C ꢁ 2, bpy), 155.6 (C, phenol), 153.5
(CH ꢁ 2, bpy), 152.7 (CH ꢁ 2, py), 144.7 (C, phenol), 141.8 (CH ꢁ 2,
bpy), 130.7 (CH ꢁ 2, phenol), 129.5 (CH ꢁ 2, bpy), 127.8 (C, py),
125.3 (CH ꢁ 2, bpy), 124.9 (CH ꢁ 2, py), 115.9 (CH ꢁ 2, phenol) 54.6
(CH), 52.8 (OCH3), 37.8 (CH2). MS ES m/z: calcd for C29H24N4O7Reþ
(Mþ) 727.1202, found 727.1189. IR (CH2Cl2): ν(CO) = 2037 (s), 1938
3
3
(1H, dd, J = 5 Hz, CH methylene), 3.31 (1H, dd, J = 7.4 Hz, CH
methylene), 3.62 (3H, s, Me), 5.09 (1H, m, CH), 7.01 (2H, m, indole),
7.21 (1H, s, indole), 7.49 (1H, d, 3J = 7.7 Hz, indole), 7.57 (1H, d, 3J =
7.4 Hz, indole), 7.85 (2H, d, 3J = 7.7 Hz, py), 8.43 (2H, d, 3J = 4.7 Hz,
py), 8.82 (1H, s, py), 9.48 (1H, s, NH). 13C NMR (CDCl3, 67.9 MHz):
172.6 (CO), 165.9 (CO), 152.0 (CH, py), 148.2 (C, py), 136.5 (C,
indole), 135.3 (CH, py), 129.7 (CH, py), 127.4 (C, indole), 123.5 (CH,
indole), 123.2 (CH, py), 122.1 (CH, indole), 119.5 (CH, indole), 118.4
(CH, indole), 111.8 (CH, indole) 109.5 (C, indole), 54.0 (CH), 52.6
(OCH3), 27.5 (CH2). MS ES m/z: calcd for C18H18N3O3þ (Mþ þ H)
324.1348, found 324.1340.
(br) cmꢀ1
.
[Re(pytrp)(CO)3(bpy)]PF6. 1H NMR (CD2Cl2, 400 MHz): 3.21
(2H, d, 3J = 5.7 Hz, CH2), 3.55 (3H, s, Me), 4.77 (1H, m, CH), 6.80 (1H,
d, J = 7.8 Hz, NH), 6.87 (1H, t, 3J = 7.1 Hz, indole), 6.97 (1H, s, indole),
6.98 (1H, t, indole), 7.19 (1H, d, 3J = 8.1 Hz, indole), 7.31 (1H, d, 3J =
7.9 Hz, indole), 7.37 (2H, dd, 3J = 6.7 Hz, 5J = 1.4 Hz, py), 7.63 (2H, m,
bpy), 8.06 (2H, m, bpy), 8.11 (2H, m, 3J = 8.1 Hz, bpy), 8.14 (2H, dd,
3J = 6.7 Hz, 5J = 1.5 Hz, py), 8.60 (1H, s, NH), 9.03 (2H, d, 3J = 5.4 Hz,
bpy). 13C NMR (CD2Cl2, 100.4 MHz): 195.7 (CO), 191.4 (CO), 172.0
(CO), 163.5 (CO), 155.8 (C ꢁ 2, bpy), 153.6 (CH ꢁ 2, bpy), 152.8
(CH ꢁ 2, py), 144.4 (C, indole), 141.6 (CH ꢁ 2, bpy), 136.7 (C,
indole), 129.5 (CH ꢁ 2, bpy), 127.5 (C, py), 125.1 (CH ꢁ 2, bpy),
124.8 (CH ꢁ 2, py), 124.4 (CH, indole), 122.4 (CH, indole), 119.7
(CH, indole), 118.5 (CH, indole), 112.0 (CH, indole), 109.2 (C,
indole), 54.6 (CH), 52.8 (OCH3), 37.8 (CH2). MS ES m/z: calcd for
C31H25N5O6Reþ (Mþ) 750.1362, found 750.1323. IR (CH2Cl2):
3-Phenyl-2-[(pyridine-3-carbonyl)amino]propionic Acid
Methyl Ester (nicphe). 1H NMR (CDCl3, 270 MHz): 3.0 (1H, dd,
3J = 7.7 Hz, 2J = 14 Hz, CH methylene), 3.07 (1H, dd, 3J = 5.5 Hz, 2J = 14
Hz CH methylene), 3.52 (3H, s, Me), 4.88 (1H, m, CH), 6.75 (1H, s,
NH), 7.27 (5H, m, phenyl CH), 7.81 (2H, d, 3J = 7.7 Hz, py), 8.43 (2H,
3
d, J = 4.7 Hz, py), 8.82 (1H, s, py). 13C NMR (CDCl3, 67.9 MHz):
172.1 (CO), 165.7 (CO), 152.0 (CH, py), 148.3 (C, py), 136.3 (C,
phenyl), 135.2 (CH, py), 129.7 (C, py) 129.1 (CH ꢁ 2, phenyl), 128.5
(CH ꢁ 2, phenyl), 127.0 (CH, phenyl), 123.3 (CH, py), 54.0 (CH),
52.3 (OCH3), 37.5 (CH2). MS ES m/z: calcd for C16H17N2O3þ (Mþ þ
H) 285.1239, found 285.1241
4-(Amino acid)imidazole Ligands (imaa). 4-Imidazolecar-
boxylic acid (0.3 g, 2.6 mmol) and LiOH (1.1 mol equiv) were dissolved
in MeOH (∼40 mL). A few drops of water were added until the solids
went into solution. The solvent was removed to yield a white powder
(lithium imidazole carboxylate salt) that was used for the next step
without further purification. The powder was dissolved with the amino
acid methyl ester hydrochloride salt (0.67 mol equiv) and PyBOP (1 mol
equiv) in dry DMF (20 mL). The reaction mixture was stirred for 12 h at
room temperature. The DMF was removed under reduced pressure, and
the resulting oil was dissolved in DCM and washed with water (4 ꢁ
50 mL). The organic layer was dried over sodium sulfate and evaporated.
The resulting solid was purified by means of flash chromatography (Si
column and a DCM:MeOH:Et2NH 94:5:1 solvent system) to yield the
desired ligand in 36ꢀ40% yield.
ν(CO) = 2037 (s), 1937 (br) cmꢀ1
.
[Re(pytrp)(CO)3(phen)]PF6. 13C NMR (CD2Cl2, 100.4 MHz):
196.7 (CO), 192.8 (CO), 173.6 (CO), 166.3 (CO), 156.3 (CH ꢁ 2, py),
154.2 (CH ꢁ 2, phen), 151.3 (C ꢁ 2, phen), 148.0 (C, indole), 146.0 (C ꢁ
2, phen), 142.1 (CH ꢁ 2, phen), 138.4 (C, indole), 133.1 (C, py), 129.8
(CH ꢁ 2, phen), 128.9 (CH ꢁ 2, phen), 125.8 (CH ꢁ 2, py), 124.8
(CH, indole), 122.8 (CH, indole), 120.1 (CH, indole), 119.4 (CH,
indole), 112.8 (CH, indole), 111.0 (C, indole), 56.1 (CH), 53.3
(OCH3), 28.5 (CH2). MS ES m/z: calcd for C34H25N4O6Reþ (Mþ)
774.1362, found 774.1337. IR (MeCN): ν(CO) = 2037 (s), 1933
(br) cmꢀ1
.
2-[(1H-Imidazole-4-carbonyl)amino]-3-(1H-indol-3-yl)pro-
pionic Acid Methyl Ester (imtrp). White crystals. 13C NMR
(CD3OD, 67.9 MHz): 172.7 (CO), 163.3 (CO), 136.7 (C, indole),
[Re(pyphe)(CO)3(bpy)]PF6. 1H NMR (CD2Cl2, 400 MHz): 3.09
(1H, dd, 3J = 7.4 Hz, 2J = 14 Hz CH methylene), 3.23 (1H, dd, 3J = 5.5 Hz,
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ARTICLE
2J = 14 Hz, CH methylene), 3.72 (3H, s, Me), 4.87 (1H, m, J = 5.6 Hz,
CH), 6.84 (1H, d, J = 7.8 Hz, NH), 7.15 (2H, d, J = 6.7 Hz, phenyl), 7.25
(3H, m, phenyl), 7.57 (2H, dd, 3J = 6.7 Hz, 5J = 1.5 Hz, py), 7.82 (2H, m,
J = 5.4 Hz, bpy), 8.27 (2H, dd, 3J = 6.7 Hz, 5J = 1.5 Hz, py), 8.31 (2H, m,
J=8.2Hz, bpy), 8.40(2H, d, 3J= 8.2 Hz, bpy), 9.18 (2H, d, 3J=5.5Hz, bpy).
13C NMR (CD2Cl2, 100.6 MHz): 195.8 (CO), 191.4 (CO), 171.5 (CO),
163.5 (CO), 156.0 (C, bpy), 153.4 (CH ꢁ 2, bpy), 152.7 (CH ꢁ 2, py),
144.7 (C, phenyl), 141.8 (CH ꢁ 2, bpy), 136.4 (CH, phenyl), 129.5
(CH ꢁ 2, bpy), 129.4 (CH ꢁ 2, phenyl), 129.0 (CH ꢁ 2, phenyl), 127.4
(CH, py), 125.3 (CH ꢁ 2, bpy), 125.0 (CH ꢁ 2, py), 54.6 (CH), 52.8
(OCH3), 37.8 (CH2). MS ES m/z: calcd for C29H24N4O6Reþ (Mþ)
(CH ꢁ 2, bpy), 55.2 (CH), 53.2 (OCH3), 38.1 (CH2). MS ES m/z:
calcd for C27H23N5O6Re (Mþ) 700.1206, found 700.1209. IR
(CH2Cl2): ν(CO) = 2033 (s), 1930 (br) cmꢀ1
.
Physical Measurements. NMR spectra were recorded on JEOL
EX 270 (1H, 270 MHz; 13C, 67.9 MHz) or Bruker AMX400 (1H, 400
MHz; 13C[1H], 100.61 MHz) spectrometers. 1H NOESY spectra were
obtained with a 600 MHz Bruker AV600 instrument. High-resolution
electrospray mass spectrometry was performed by Kings College,
University of London, on an Apex III system.
UVꢀvis and FTIR absorption spectra were measured with HP8453
and PE1720X spectrometers, respectively. Emission spectra were ob-
tained on Jobin Yvon (Horiba) FluoroMax-3 and Fluorolog-3 instru-
ments using 405 nm excitation. Emission lifetime and time-resolved
anisotropy measurements were performed using time-correlated single-
photon counting (TCSPC) on an IBH 5000 U instrument equipped
with a cooled Hamamatsu R3809U-50 microchannel plate photomulti-
plier. The samples (aerated solutions) were excited at 405 nm with a
diode laser (IBH NanoLED-07, fwhm 80 ps, 500 kHz repetition rate).
Time-resolved IR measurements in the ν(CO) spectral region were
obtained with the PIRATE instrument.27ꢀ29 In short, for measurements
inthe0ꢀ2000 ps timedomain, the sample solutionwas excited(pumped)
at 400 nm, using frequency-doubled pulses from a Ti:sapphire laser of
∼150 fs duration (fwhm) and ca. 3 μJ energy, focused at an area ∼200 μm
in diameter. TRIR spectra were probed with spectrally broad IR pulses
(∼150 fs, ∼200 cmꢀ1) obtained by difference-frequency generation from
the same Ti:sapphire laser. In the nanosecond range, sample excitation
was performed with 355 nm, ∼0.7 ns fwhm, ∼3 μJ laser pulses generated
by an actively Q-switched AOT-YVO-20QSP/MOPA Nd:vanadate
diode-pumped microlaser that was electronically synchronized with the
femtosecond probe system.30 The ULTRA instrument31 has been used
for TRIR experiments in the fingerprint region. A titanium sapphire laser/
regenerative amplifier (Thales) produces ∼50 fs pulses at a 10 kHz
repetition rate. The laser output is split in two parts, one of which is
frequency-doubled to 400 nm and used as a pump beam. The second
pumps a TOPAS OPA, yielding signal and idler beams that are difference
frequency mixed to generate ∼400 cmꢀ1 broad mid-IR probe pulses. The
mid-IR probe spectrum is recorded at a given time delay using two 128-
element HgCdTe detectors (Infrared Associates). The sample solutions
(aerated, 1ꢀ5 mM [Re]) were flowed through a 0.1 mm CaF2 plate cell
that was at the same time scanned-rastered across the irradiated area in
two dimensions to prevent laser heating and decomposition ofthe sample.
FTIR spectra measured before and after the experiment demonstrated
sample stability. All spectral and kinetics fitting procedures were per-
formed using MicroCal Origin 7.1.
711.1253, found 711.1213. IR (CH2Cl2):ν(CO) =2037(s), 1936(br) cmꢀ1
.
[Re(nictrp)(CO)3(bpy)]PF6. 13C NMR (CD3CN, 67.9 MHz):
195.7 (CO), 191.4 (CO), 172.6 (CO), 165.9 (CO), 152.9 (C ꢁ 2,
bpy), 151.8 (CH ꢁ 2, bpy), 151.3 (CH, py), 150.2 (CH, py), 148.3
(C, py), 140.9 (CH ꢁ 2, bpy), 137.8 (CH, py), 136.1 (C, indole), 132.1
(CH, py), 128.5 (CH ꢁ 2, bpy), 127.8 (C, indole), 124.4 (CH ꢁ 2, bpy),
123.1 (CH, indole), 121.4 (CH, indole), 118.8 (CH, indole), 118.3
(CH, indole), 111.3 (CH, indole), 109.5 (C, indole), 53.6 (CH),
51.7 (OCH3), 30.7 (CH2). MS ES m/z: calcd for C31H25N5O6Reþ
(Mþ) 750.1362, found 750.1338. IR (MeCN): ν(CO) = 2037 (s), 1932
(br) cmꢀ1
.
[Re(nicphe)(CO)3(bpy)]PF6. 13C NMR (CDCl3, 67.9 MHz):
195.2 (CO), 191.3 (CO), 172.1 (CO), 163.4 (CO), 155.4 (C ꢁ 2,
bpy), 153.1 (CH ꢁ 2, bpy), 151.1 (CH, py), 149.1 (C, py), 141.5 (CH ꢁ
2, bpy), 137.8 (CH, py), 136.4 (C, phenyl), 132.5 (C, py), 129.9 (CH ꢁ
2, bpy), 129.2 (CH ꢁ 2, phenyl), 128.6 (CH ꢁ 2, phenyl), 127.9 (CH,
phenyl), 125.5 (CH ꢁ 2, bpy), 122.8 (CH, py), 54.5 (CH), 52.5
(OCH3), 37.5 (CH2). MS ES m/z: calcd for C29H24N4O6Reþ (Mþ)
711.1253, found 711.1218. IR (MeCN): ν(CO) = 2037 (s), 1933
(br) cmꢀ1
.
[Re(nictrp)(CO)3(phen)]PF6. 13C NMR (MeOD, 67.9 MHz):
196.3 (CO), 192.3 (CO), 173.5 (CO), 165.5 (CO), 155.6 (CH, py),
152.7 (C ꢁ 2, phen), 151.9 (CH, py), 147.5 (CH ꢁ 2, phen), 141.9 (CH ꢁ
2, phen), 139.5 (CH, py), 138.0 (CH, py), 137.2 (C, indole), 133.7 (C ꢁ
2, phen), 132.6 (CH, py), 129.5 (CH ꢁ 2, phen), 128.6 (CH ꢁ 2, phen),
127.7 (C, indole), 124.6 (CH, indole), 121.6 (CH, indole), 119.9
(CH, indole), 119.1 (CH, indole), 112.6 (CH, indole), 110.9
(C, indole), 55.7 (CH), 53.0 (OCH3), 28.2 (CH2). MS ES m/z: calcd
for C33H25N5O6Reþ (Mþ) 774.1362, found 774.1329. IR (MeCN):
ν(CO) = 2037 (s), 1934 (br) cmꢀ1
.
[Re(nicphe)(CO)3(phen)]PF6. 13C NMR (CDCl3, 67.9 MHz):
195.2 (CO), 191.3 (CO), 172.1 (CO), 163.4 (CO), 156.2 (CH, py),
154.2 (C ꢁ 2, phen), 150.8 (CH, py), 146.4 (CH ꢁ 2, phen), 140.6 (CH ꢁ
2, phen), 137.8 (CH, py), 136.2 (C, phenyl), 132.1 (C ꢁ 2, phen), 131.3
(C, py), 129.2 (CH ꢁ 2, phenyl), 128.6 (CH ꢁ 2, phenyl), 127.6 (CH ꢁ
2, phen), 127.1(CH, phenyl), 124.4 (CH ꢁ 2, phen), 122.9 (CH, py),
54.3 (CH), 52.5 (OCH3), 37.6 (CH2). MS ES m/z: calcd for
C29H24N4O6Reþ (Mþ) 735.1253, found: 735.1214. IR (MeCN):
Quantum Chemical Calculations. The structures of different
conformations of [Re(imtrp)(CO)3(bpy)]þ complex were calculated
by density functional theory (DFT) methods using the Gaussian 0932
program package. DFT calculations employed Perdew, Burke,
Ernzerhof33,34 (PBE0) and long-range corrected CAM-B3LYP35
hybrid functionals. Polarized triple-ζ basis sets 6-311g(d)36 were used
for H, C, N, and O atoms, together with quasirelativistic effective
core pseudopotentials and the corresponding optimized set of basis
functions37,38 for Re. Geometry optimizations were followed by vibra-
tional analysis in order to characterize stationary states. The solvent
(MeCN) was described by the polarizable calculation model (PCM).39
ν(CO) = 2037 (s), 1932 (br) cmꢀ1
.
[Re(imtrp)(CO)3(bpy)]PF6. 13C NMR (CD3CN, 67.9 MHz):
196.7 (CO), 192.7 (CO), 171.5 (CO), 162.7 (CO), 156.9 (C, im),
155.4 (C ꢁ 2, bpy), 153.4 (CH ꢁ 2, bpy), 140.6 (CH ꢁ 2, bpy), 140.2
(CH, im), 136.1 (C, indole), 129.1 (CH, im), 128.2 (CH ꢁ 2, bpy),
126.9 (C, indole), 124.4 (CH ꢁ 2, bpy), 123.3 (CH, indole), 121.3 (CH,
indole), 118.7 (CH, indole), 117.9 (CH, indole), 111.1 (CH, indole),
109.2 (C, indole), 53.0 (CH), 51.6 (OCH3), 26.7 (CH2). MS ES m/z:
calcd for C29H24N6O6Re (Mþ) 739.1315, found 739.1274. IR
’ RESULTS
(CH2Cl2): ν(CO) = 2034 (s), 1928 (br) cmꢀ1
.
Synthesis and Structures. The amino acid-bearing pyridine
or imidazole ligands (L-AA, Figure 1) were made by reacting
the pyridinecarbonyl chlorides or imidazolecarboxylic acid with
the corresponding amino acid methyl ester. The Re complexes
were prepared following the well-established26 route to cationic
Re carbonyl diimine complexes, by reacting the ligand L with
[Re(imphe)(CO)3(bpy)]PF6. 13C NMR (CD3CN, 67.9 MHz):
196.7 (CO), 192.7 (CO), 171.5 (CO), 162.7 (CO), 157.1 (C, im), 155.2
(C ꢁ 2, bpy), 153.4 (CH ꢁ 2, bpy), 142.7 (CH, im), 141.6 (CH ꢁ 2,
bpy), 136.8 (C, phenyl), 130.7 (CH ꢁ 2, bpy), 130.2 (CH, im), 129.9
(CH ꢁ 2, phenyl), 128.3 (CH ꢁ 2, phenyl), 126.3 (CH, phenyl), 124.4
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Figure 3. Emission spectra of [Re(pyaa)(CO)3(bpy)]þ in MeCN at
21 °C. Concentrations of the sample solutions were adjusted to give
identical absorbance of 0.1 at the 405 nm excitation wavelength. Spectra
are not corrected for the detector response.
symmetry plane and the indole group close to the bpy ligand,
lies at higher energy. With relatively low ΔGo differences, it is
well possible that several conformers of both isomers are
present simultaneously in the solution.
Figure 2. DFT-optimized structures of selected [Re(imtrp)(CO)3-
(bpy)]þ conformers in MeCN. Calculated Gibbs free energies relative
to the most stable structure A: B, 0.054 eV; C, 0.128 eV; D, 0.182 eV.
Emission Spectroscopy. All investigated complexes in MeCN
solutions show broad unstructured emission with a maximum at
550ꢀ585 nm (N,N = bpy) and 546ꢀ549 nm (N,N = phen), as is
[Re(OTf)(CO)3(N,N)], which was generated from [Re(Cl)(CO)3-
(N,N)] using Ag(OTf). The complexes show typical UVꢀvis
absorption spectra consisting (for N,N = bpy) of an IL band at
320 nm and a shoulder due to RefN,N MLCT transition(s) at
∼340 nm for the pyridine-based complexes, or a maximum at
∼353 nm for the imidazole-containing species (Supportimg In-
formation, Figure S1). IR spectra of all the complexes are virtually
identical, showing a sharp band at 2033ꢀ2037 cmꢀ1 and a broad
band at 1927ꢀ1933 cmꢀ1 assigned40 to the in-phase A0(1) and
quasidegenerate A0(2) þ A00 ν(CO) vibrations, respectively.
Relevant to ET reactivity are possible contacts and πꢀπ
stacking between the amino acid aromatic side chain and the
diimine ligand. Simple molecular models show that such inter-
actions are hardly possible for the pyaa and nicaa ligands. This
conclusion is supported by a 1H NOESY (tm = 350 ms, 303 K,
CD2Cl2) spectrum of [Re(pytrp)(CO)3(bpy)]þ that does not
show any cross-peaks between the indole and bpy hydrogen
atoms. On the other hand, it shows cross-peaks between the
amide and pyridine-C3 as well as between the pyridine-C2 and
bpy-C6 H-atoms. It follows that the peptide group is in a trans
configuration and the pytrp indole is oriented away from the
Re(CO)3(bpy) chromophore. [Re(imtrp)(CO)3(bpy)]þ can
exist in two isomeric forms. Figure 2 shows two characteristic
stable structures for each isomer calculated by DFT. The structure
A was calculated to have the lowest energy. The imidazole plane is
oriented perpendicularly to the symmetry plane, i.e., bisecting
the angles between the equatorial OCꢀReꢀN angles, and the
closest distance between indole and the bpy ligand is 4.53 Å,
while the respective molecular planes contain quite a large
angle. The second rotamer of structure A (the imidazole plane
coincidental with the symmetry plane, not shown) is only about
0.02 eV higher in energy. Energetically very close is the
conformer B, where the indole group points away from both
the Re center and the bpy ligand. The lowest energy structure
corresponding to less-stable second isomer is the structure C,
with the imidazole plane oriented perpendicularly to the
symmetry plane and imidazole pointing away from the bpy
ligand. The conformer D, with the im plane parallel to the
3
expected14,26,40,41 for a MLCT state of ReI carbonyl-diimines.
Emission energies depend little on the appended aa group in the
case of pyaa complexes, while a small blue-shift was observed on
going from nicphe to nictrp (3ꢀ11 nm) and from imphe to imtrp
(19 nm). Emission spectra and band-maximum wavelengths are
summarized in Figures 3 and S2 (Supportimg Information).
[Re(pyphe)(CO)3(bpy)]þ and [Re(pytyr)(CO)3(bpy)]þ emis-
sion occur with identical intensities, whereas emission from
[Re(pytrp)(CO)3(bpy)]þ is ca. 4.3 times weaker (Figure 3).
Intensity quenching was observed for all investigated Trp-con-
taining complexes.
Emission lifetimes of pyphe, pytyr, and nicphe complexes
(Supportimg Information, Figure S3) depend only slightly on L
(Table 1). Phen complexes have longer-lived emission than bpy,
270ꢀ300 and 140ꢀ150 ns, respectively. A biexponential decay
with the principal lifetime of 50 ns and a minor 2 ns component
was found for [Re(imphe)(CO)3(bpy)]þ. The amplitude of the
2 ns decay sharply decreases with increasing detection wave-
length, turning to a small early time rise at long wavelengths; see
Figure S4 (Supporting Information). This behavior suggests
that the 2 ns decay arises from structural and/or solvational
relaxation.42
Emission of Trp complexes decays bi- or triexponentially
(Table 1). The major emission decay component is significantly
faster than for the Phe analogues. Since Trp-localized excited
states lie high above 3MLCT and the complexes are photostable,
we can exclude energy transfer and bond breaking, leaving
Trp(indole)fReII ET as the likely quenching mechanism. The
short minor decay component (2ꢀ3 ns) has a wavelength-
dependent amplitude (Supporting Information, Figure S4) and
is again attributed to relaxation processes. The other minor
(<10%) component has a lifetime only a little shorter than that
of the corresponding Phe-containing species and is assigned to an
unreactive conformer. To detect the Re reduction product and
prove the ET mechanism, we have resorted to structure-sensitive
TRIR spectroscopy. (Note that visible time-resolved absorption
would not be a suitable technique in this case because of a large
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Table 1. Emission and TRIR Lifetime Data (ns) Measured in Air-Saturated MeCN Solutions at 21 °Ca
emission lifetimesb
TRIR lifetimes
MLCT decay
τ (major)
τ (minor)
CS rise
CS decay
bleach recovery
[Re(pytrp)(bpy)]þ
[Re(pytyr)(bpy)]þ
[Re(pyphe)(bpy)]þ
[Re(pytrp)(phen)]þ
27.0 (95)
30.0c
214 (5)
242c
ꢀ
19.5 ( 2
40 ( 6
29.3 ( 0.7
109 ( 3
129 ( 3
29 ( 2
42.9 ( 0.7
147
300c
ꢀ
ꢀ
ꢀ
ꢀ
117.6 ( 0.1
127 ( 3
54 ( 2
144
296c
ꢀ
26.9 (77)
2.0 (14)
221 (10)
ꢀ
14.2 ( 0.5
56 ( 2
[Re(pyphe)(phen)]þ
[Re(nictrp)(bpy)]þ
[Re(nicphe)(bpy)]þ
[Re(nictrp)(phen)]þ
[Re(nicphe)(phen)]þ
[Re(imtrp)(bpy)]þ
281
ꢀ
ꢀ
ꢀ
ꢀ
32 ( 3
ꢀ
233 ( 13
9.3 ( 0.3d
125 ( 6
5.7 ( 2d
212 ( 4
19 ( 1
255 ( 11
29.6 ( 1d
141 ( 10
8.8 (93)
114 (7)
ꢀ
6.3 ( 0.5
5.5 ( 0.8
16 ( 3
144
7.3d
ꢀ
33 ( 4
ꢀ
30 ( 6d
301
ꢀ
237 ( 3
22.0 (87)
3.0 (13)
160 ( 1
19 ( 3 (76)
144 ( 63 (14)
49 ( 2
[Re(imphe)(bpy)]þ
49.8 (86)
2.3 (14)
ꢀ
ꢀ
54 ( 5
a Percentage amplitudes in parentheses. Emission decay kinetics were measured at the wavelength corresponding to the emission band maximum.
b accuracy (0.1ꢀ1.0%. c Measured in degassed solutions. d Additional low-amplitude long-lifetime component present.
Figure 5. TRIR spectrum of [Re(pytrp)(CO)3(bpy)]þ in MeCN,
following 355 nm, 0.7 ns excitation. Experimental points are separated
by 4ꢀ5 cmꢀ1. Arrows indicate rise and decay of the transients. Inset:
Figure 4. TRIR spectra of [Re(pyphe)(CO)3(bpy)]þ (top) and [Re-
(pytyr)(CO)3(bpy)]þ (middle) in MeCN solutions. Spectra were
measured at the specified time delays (in ns) after 355 nm, 0.7 ns
excitation. The experimental points are separated by 4ꢀ5 cmꢀ1. Bottom:
Ground-state FTIR spectrum of [Re(nictrp)(CO)3(bpy)]þ in MeCN.
kinetics profile taken from band 5, as marked by the asterisk.
energy and splitting of the quasidegenerate A0(2) þ A00 vibra-
3
tions upon excitation are characteristic IR features of MLCT
excited states.13,28,40,47ꢀ51 TRIR bands decay with kinetics that
are, within the experimental accuracy, comparable to the princi-
pal kinetics component of the emission decay (Table 1). TRIR
spectra measured in the ps range show the typical29,52ꢀ57
relaxation-related dynamic upshift that is completed in the first
∼30 ps.
overlap of Trpþ and Trp• absorption, 550ꢀ600 and 520 nm,
respectively,20 with that of the excited and reduced states of Re
diimine complexes.43ꢀ46
)
Time-Resolved Infrared Spectroscopy. Nanosecond differ-
ence TRIR spectra (Figure 4) of [Re(pyphe)(CO)3(bpy)]þ and
[Re(pytyr)(CO)3(bpy)]þ show two negative bands (bleaches)
due to a depleted ground-state population and three positive
bands at 2073, 2013, and 1971 cmꢀ1, corresponding40 to the
A0(1), A0(2), and A00 excited-state vibrations, respectively. Very
similar spectra were obtained for other [Re(L-Phe/L-Tyr)-
(CO)3(N,N)]þ complexes. The shift of the A0(1) vibration by
ca. 37 cmꢀ1 (N,N = bpy) and 26 cmꢀ1 (N,N = phen) to higher
Nanosecond TRIR spectra of [Re(pytrp)(CO)3(bpy)]þ
(Figure 5) and other L-Trp complexes (Figure 6) point to a
more complicated excited-state behavior. The 3MLCT IR bands
(denoted 1ꢀ3 in Figure 5) are formed virtually instantaneously
upon excitation. Their decay is faster than for Phe and Tyr
complexes andaccompaniedbya simultaneousriseof newfeatures
4 and 5 at ∼2010 and ∼1895 cmꢀ1, respectively, whose intensities
first increase and then decay (Figure 5, inset). The two intermediate
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bands 4 and 5 are shifted to lower energies from the correspond-
ing bleach bands by 25 and 36 cmꢀ1, respectively. Such a spectral
pattern is typical for reduced [ReI(L-N)(CO)3(bpy•ꢀ/phen•ꢀ)]
complexes, where L-N is a nitrogen-coordinated ligand such as
MeCN, picoline, or pyridine, which have been generated either
electrochemically58ꢀ60 or photochemically.49,59,61 Accordingly, the
transient bands 4 and 5 are attributed to a charge-separated (CS)
state [ReI(L-Trp•þ)(CO)3(bpy•ꢀ)]þ and/or a reduced state
(RS) [ReI(L-Trp)(CO)3(bpy•ꢀ)], which cannot be distinguished
by IR spectra. The ET product(s) then decay back to the ground
state, manifested in TRIR by the decay of the bands 4 and 5 and a
concomitant bleach recovery. Figure 6 documents the qualita-
tively similar behavior of all the investigated Trp complexes. In
the case of [Re(imtrp)(CO)3(bpy)]þ, the CS bands at 1895 and
2010 cmꢀ1 are weaker and the 1895 cmꢀ1 feature shifts to higher
energies with time, probably due to the formation of a small
amount of a persistent photoproduct that is still visible in the
TRIR spectrum measured 2 ms after excitation. No CS signal was
seen in picosecond TRIR spectra measured in the 1ꢀ1000 ps
range, excluding any subnanosecond ET reactivity.
TRIR detection of the CS and/or RS proves the ET mechan-
ism of the MLCT excited-state quenching of the Trp complexes
(Scheme 1). Optical excitation produces the 3MLCT state
*[ReII(L-Trp)(CO)3(N,N•ꢀ)]þ that undergoes either intermo-
lecular or intramolecular Trp(indole)f*ReII ET shown in
Scheme 1 in blue and red, respectively. The photocycle is
completed by N,N•ꢀfTrp(indole•þ) back-electron-transfer,
BET. Further confirmation of this mechanism and informa-
tion on electronic delocalization and structural changes of the
pytrp ligand were obtained by TRIR spectra measured on
[Re(pytrp)(CO)3(bpy)]þ in the fingerprint region.
The ground-state FTIR spectrum of [Re(pytrp)(CO)3(bpy)]þ
(Figure 7) shows the CdO stretch of the ester group at
1744 cmꢀ1 in DCM (1747 cmꢀ1 in MeCN) and the amide-I
band (CdO stretching/NꢀH bending vibration) at1678 cmꢀ1 in
DCM (1676 cmꢀ1 in MeCN). The bands at 1604 (1606 cmꢀ1 in
MeCN), 1493, 1472, and 1447 cmꢀ1 correspond62 to bpy-
localized vibrations, while the strong band at 1521 cmꢀ1 most
likely belongs to overlapping Trp amide-II and indole vibrations.
The origins of the weaker bands at 1358, 1418, and 1457 cmꢀ1
are not clear.
The TRIR spectrum of the MLCT state, measured at 1 ns after
excitation (Figure 8, blue), exhibits a weak bleach of the amide-I
band, together with an overlapping transient feature whose
maximum is shifted by ca. þ9 cmꢀ1 to 1685 cmꢀ1. The CS-
state spectrum was obtained at 60 ns (Figure 8, red). The amide-I
band is bleached but without a discernible transient, suggesting
that the energy is the same as in the ground state but the molar
Figure 6. TRIR spectra of [Re(L-Trp)(CO)3(N,N)]þ in MeCN solu-
tions, following 355 nm, 0.7 ns excitation. Time delays in nanoseconds.
Experimental points are separated by 4ꢀ5 cmꢀ1
.
Scheme 1. Excited-State Electron Transfer Reactivity of [ReI(L-Trp)(CO)3(N,N)]þ: (left, blue) Bimolecular Quenching
Involving Intermolecular ET and (right, red) Intramolecular ET Pathway
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Figure 7. Ground-state FTIR spectrum of [Re(pytrp)(CO)3(bpy)]þ
in DCM in the fingerprint region. The band maxima occur at 1358, 1418,
1447, 1457, 1472, 1493, 1521, 1604, 1678, and 1744 cmꢀ1. The asterisks
denote bpy bands.62
Figure 9. TRIR spectra of [ReI(pytrp)(CO)3(N,N)]þ in the region of
aromatic-ring vibrations measured at 1 ns (blue, attributed to MLCT)
and 50 ns (red, attributed to CS) after 355 nm, 0.7 ns excitation.
Experimental points are separated by 2ꢀ3 cmꢀ1. Typical bpy•ꢀ
transient bands are denoted by arrows. (Measured in DCM using the
ULTRA TRIR instrument.31)
1472 cmꢀ1 bleach. These transient features are comparable to
those observed62 in the step-scan TRIR spectrum of MLCT-
excited [Re(4-ethylpyridine)(CO)3(bpy)]þ, supporting our as-
signment. The spectrum measured at 50 ns (Figure 9, red), when
the CS state is the predominant species in the solution, shows
two strong bands at 1278 and 1497 cmꢀ1 that are attributable to
Trp•þ, in particular to vibrations of the oxidized indole•þ. In
addition, the Trp-specific band at 1521 cmꢀ1 is bleached in the
CS but not in the MLCT spectrum. The bleached bpy bands and
corresponding bpy•ꢀ transients are still present in the CS state.
3
Altogether, the TRIR spectra confirm the CS formulation as
[Re(pytyr•þ)(CO)3(bpy•ꢀ)]þ and the mechanism proposed
in Scheme 1: The bleached bpy bands and bpy•ꢀ transients
are present immediately after excitation as well as after the
Trpf*ReII ET, whereas the bleached Trp band and Trp•þ
transients emerge only upon the CS formation.
Electron Transfer Kinetics. The rate constants kET of the
Trp(indole)fReII excited-state ET were calculated from the
MLCT excited-state lifetimes τd of the Trp complexes using the
formula
Figure 8. TRIR spectra of [ReI(pytrp)(CO)3(N,N)]þ in the
1570ꢀ1800 cmꢀ1 range measured at 1 ns (blue, attributed to MLCT)
and 60 ns (red, attributed to CS) after 355 nm, 0.7 ns excitation.
Experimental points are separated by ∼3 cmꢀ1. (Measured in MeCN
using the ULTRA TRIR instrument.31)
absorptivity is smaller. The ester CdO band is bleached very
little upon both MLCT excitation and the following ET, implying
that the ester group is almost unaffected. (A slightly more intense
bleach was observed for the CS state in DCM.)
kET ¼ 1=τd ꢀ 1=τ0
ð1Þ
The 1606 cmꢀ1 bpy band is strongly bleached in the MLCT
TRIR spectrum (Figure 8), probably due to a drop in intensity
upon excitation, rather than a band shift.62 (The weak signal at
∼1627 cmꢀ1 appears to be a high-energy wing of a weak broad
overlapping transient.) The 1606 cmꢀ1 bleach intensity de-
creases with time during the first 30 ns, becoming unobservable
after about 40 ns in MeCN, whereas it remains weakly apparent
in DCM. This behavior can be explained by assuming that the
overlapping (positive) transient band gains intensity and narrows
on going from the MLCT to the CS state, possibly due to a weak
τ0 is the inherent MLCT lifetime, for which the lifetimes of the
corresponding unreactive Phe complexes were used. The lifetime
values were determined from the decay of the emission intensity
as well as of the A0(1) and A00 TRIR bands 1 and 3. In the case of
multiexponential decay kinetics, the lifetime of the major decay
component was used (Table 1).
The BET rate constants were determined from the biexpo-
nential rise and decay kinetics of the TRIR CS/RS band 5 at
∼1895 cmꢀ1 (Figure 5, inset). The data were fitted to eq 2:
Trp•þ bpy•ꢀ interaction in the latter.
3 3 3
The bpy bands at 1315, 1448, and 1472 cmꢀ1 are strongly
bleached in the MLCT TRIR spectrum measured at 1 ns
(Figure 9, blue). The most prominent MLCT transient band
occurs at 1441 cmꢀ1, together with weaker bands at 1359 and
1426 cmꢀ1, plus a broad absorption that overlaps with the
ꢀt=τBET
Ið5Þ ¼ A1eꢀt=τ þ A2e
þ A0
ð2Þ
d
One of the time constants (τd) can be identified with the MLCT
lifetime, while the second is attributed to the back-electron-
ꢀ1
transfer; τBET = kBET
.
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Table 2. Rate Constants (sꢀ1) of Photoinduced Electron
Transfer Reactions in [Re(L-Trp)(CO)3(N,N)]þ Complexes
in MeCN at 21 °C
kET
TRIR
emissiona MLCTb
TRIR
kBET:
CS/CRc TRIR CS/CRc
[Re(pytrp)(bpy)]þ
3.0 ꢁ 107 2.6 ꢁ 107 4.4 ꢁ 107
2.5 ꢁ 107
1.8 ꢁ 107
3.1 ꢁ 107
3.0 ꢁ 107
6.3 ꢁ 106d
[Re(pytrp)(phen)]þ 3.4 ꢁ 107 3.0 ꢁ 107 6.6 ꢁ 107
[Re(nictrp)(bpy)]þ 1.1 ꢁ 108 1.0 ꢁ 108 1.5 ꢁ 108
[Re(nictrp)(phen)]þ 1.3 ꢁ 108 1.7 ꢁ 108 1.8 ꢁ 108
[Re(imtrp)(bpy)]þ
2.5 ꢁ 107 3.4 ꢁ 107 4.4 ꢁ 107d
a Determined from emission decay and eq 1, with (1% accuracy. A small
systematic error may arise from the presence of relaxation kinetics
components that affect decay fits. b From TRIR-measured decay of
MLCT IR bands and eq 1, with an accuracy between (8 and (15%.
c From biexponential fits of the CS/RS band at 1890 cmꢀ1 (eq 2); kET
determined from eq 1; kBET = τBETꢀ1. Accuracy was between (8 and
(15%. d Estimate, with an accuracy of (34%.
Figure 10. TRIR spectra of 2.0 ꢁ 10ꢀ3 M [Re(pyphe)(CO)3(bpy)]þ
and 2.0 ꢁ 10ꢀ3 tryptophan methyl ester in MeCN, following 355 nm,
0.7 ns excitation. Time delays in nanoseconds. Peaks are labeled in
accordance with Figure 5. Inset: kinetic profile of the RS(5) band peak
intensity. The experimental points are separated by 4ꢀ5 cmꢀ1
.
Table 3. Rate Constants (sꢀ1) of Photoinduced Electron
Transfer Reactions in [Re(pytrp)(CO)3(bpy)]þ in Various
Solvents at 21 °Ca
Table 4. TRIR Kinetics Data for 2.02 ꢁ 10ꢀ3 M
[Re(pyphe)(CO)3(bpy)]þ and Different Molar Equivalents
of Tryptophan Methyl Ester
MeCN
DMF
DCM
THF
[Trp]/M
0
2.02 ꢁ 10ꢀ3
50 ( 3
3.14 ꢁ 10ꢀ3
45 ( 4
k
ET/sꢀ1
2.6 ꢁ 107 4.3 ꢁ 107 6.1 ꢁ 107 7.5 ꢁ 107
RS rise/ns
ꢀ
ꢀ
kBET/sꢀ1
kBET1 2.5 ꢁ 107 1.7 ꢁ 107 2.5 ꢁ 107c 4.1 ꢁ 107
RS decay/ns
135 ( 10
70 ( 2
126 ( 15
55 ( 2
kBET2
ꢀ
0.529
ꢀ
0.463
3.7 ꢁ 106c 1.8 ꢁ 106
MLCT decay/ns
bleach recovery/ns
kET(app)/sꢀ1 MLCTa
k2,ET/Mꢀ1sꢀ1b
kBET(app)/sꢀ1c
a Calculated using eq 1 from MLCT lifetimes measured in the presence
and absence of the quencher. b k2,ET = kET(app)/[Trp]. c Calculated from
the RS decay (eq 2).
129 ( 3
1/εop ꢀ 1/εsb
0.383
0.375
124 ( 3
99 ( 1
96 ( 5
viscosity/cPd
0.35
0.92
0.45
0.55
ꢀ
ꢀ
ꢀ
6.5 ꢁ 106
3.2 ꢁ 109
7.4 ꢁ 106
1.0 ꢁ 107
3.2 ꢁ 109
7.9 ꢁ 106
a Accuracy from (8% to (14%. b From ref 63. c (30%. d From ref 64.
The rate constants are summarized in Table 2. The kET values
determined from emission and TRIR MLCT decay lifetimes are
well-comparable, whereas those obtained from the biexponential
fits of the weaker CS/CR band (third column) are subjected to a
larger error. The values obtained from the CS/CR band of [Re-
(imtrp)(CO)3(bpy)]þ are only rough estimates, since the measured
intensity changes are affected by the gradual band shift to higher
energies, which indicates a chemical reaction of the CS (or RS) state.
TRIR spectra and ET kinetics of [Re(pytrp)(CO)3(bpy)]þ
were investigated in a series of solvents with different dielectric
properties: MeCN, N,N-dimethylformamide (DMF), CH2Cl2
(DCM), and tetrahydrofuran (THF). The excited-state behavior
is qualitatively the same as described above for MeCN, although a
biexponential CS/CR decay was observed in DCM and THF.
The rate constants and lifetimes are summarized in Tables 3 and
S1 (Supporting Information), respectively. Small band-shape
changes and upshift usually accompany the CS/RS band 5 decay,
and a small amount of a CS/RS product persists at 500 ns. The
back ET is slightly slower than the forward ET in all solvents
except MeCN, leading to larger CS yields [compare Figures 5
and S5 (Supporting Information) showing TRIR spectra ob-
tained in MeCN and DMF, respectively].
[Re(pyphe)(CO)3(bpy)]þ measured in the presence of a molar
equivalent of a tryptophan methyl ester, Me-Trp. The features
due to the redox state (RS) [ReI(pyphe)(CO)3(bpy•ꢀ)] occur at
1890 and 2009 cmꢀ1, undistinguishable from the IR bands of the
CS/RS state [ReI(pytrp•þ)(CO)3(bpy•ꢀ)]þ shown in Figure 5.
The redox state is formed by a bimolecular excited-state ET:
þ
I
ꢂ
½Re ðpypheÞðCOÞ3ðbpyÞꢃ
k2
,
s
ET
þ Me-Trp
½ReIðpypheÞðCOÞ ðbpy•ꢀÞꢃ þ Me-Trp•þ
f
3
ð3Þ
followed by the corresponding back-ET. The TRIR-determined
kinetics parameters (Table 4) show that the bimolecular quench-
ing is nearly diffusion-controlled. Nevertheless, the pseudo-
monomolecular rate constant kET(app) measured at typical TRIR
concentrations is 5ꢀ3 times smaller than the kET obtained for
[ReI(pytrp)(CO)3(bpy)]þ, while BET is 6 times slower. These
observations indicate that the ET kinetics reported above for
[ReI(pytrp)(CO)3(bpy)]þ are mostly attributable to intramole-
cular processes.
Intramolecular or Intermolecular Electron Transfer? As
shown in Scheme 1, left, the tryptophan unit in one
[Re(pytrp)(CO)3(bpy)]þ molecule can quench the excited state
of another molecule. Bimolecular ET quenching by Trp(indole)
is documented in Figure 10, which shows TRIR spectra of
To assess the role of bimolecular ET in the excited-state
behavior of [ReI(pytrp)(CO)3(bpy)]þ more quantitatively, we
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Table 5. Concentration Dependence of
Scheme 2. Intramolecular Photoinduced Electron Transfer
in [ReI(L-Trp)(CO)3(bpy)]þa
[Re(pytrp)(CO)3(bpy)]þ TRIR Kinetics in MeCN
concentration/M
MLCT/ns, τd
3.7 ꢁ 10ꢀ3 2.7 ꢁ 10ꢀ3 9.6 ꢁ 10ꢀ4 4.2 ꢁ 10ꢀ4
28.2 ( 0.4 29.7 ( 0.4 31.3 ( 0.4 31.9 ( 0.8
CS/RS decay/ns
48 ( 4
46 ( 2
36 ( 5
46 ( 7
bleach recovery/ns 51 ( 2
52 ( 2
53 ( 2
47 ( 1
kET/sꢀ1 a
2.8 ꢁ 107
2.6 ꢁ 107
2.4 ꢁ 107
2.4 ꢁ 107
kBET/sꢀ1 b
2.1 ꢁ 107
2.2 ꢁ 107
2.8 ꢁ 107
2.2 ꢁ 107
a Calculated using eq 1 where τd is the MLCT lifetime of
[Re(pytrp)(CO)3(bpy)]þ at given concentration and τ0 is the
[Re(pyphe)(CO)3(bpy)]þ MLCT lifetime (129 ns). b Calculated from
the CS/RS decay (eq 2).
a The forward Trp(indole)f*ReII and back bpy•ꢀfTrp(indole•þ) ET
steps are indicated by orange and green arrows, respectively.
by the corresponding back-ET (Scheme 1). ET proceeds both
intra- and intermolecularly, the former pathway being predomi-
nant. The intermolecular quenching contributing no more than
10% to the reaction rate, we will discuss the kinetics parameters
from the intramolecular point of view (Scheme 2).
Figure 11. Concentration dependence of the reciprocal [Re(pytrp)-
(CO)3(bpy)]þ MLCT lifetime in MeCN. The linear fit (1.21 ꢁ
109 Mꢀ1 sꢀ1 slope and 3.08 ꢁ 107 sꢀ1 intercept) is shown in red.
The MLCT excited state exhibits a ν(CO) IR spectrum that is
typical of MLCT states in cationic Re complexes11,29,40,55,57,65,66
such as [Re(Etpy)(CO)3(N,N)]þ or analogous [Re(L-Phe/L-
Tyr)(CO)3(N,N)]þ. These observations exclude any significant
contribution from indolefN,N CT to the reactive excited state
that would be manifested11 by the shift of the A0(1) ν(CO) band
upon excitation of about þ10 cmꢀ1 instead of the observed 37
(bpy) or 26 cmꢀ1 (phen). Nevertheless, the indole group exerts
some influence on the MLCT excited-state energy, as is docu-
mented by small blue shifts of the emission band on going from
nicphe to nictrp and imphe to imtrp complexes (Supporting
Information, Figure S2). Despite being predominantly localized
at the Re(CO)3(N,N) chromophore, MLCT excitation shifts the
amide-I vibration by ca. þ9 cmꢀ1 (Figure 8), demonstrating that
the amide bridging group and the excited Re(CO)3(N,N)
chromophore are electronically coupled.
have investigated the dependence of the TRIR kinetics on the
[Re(pytrp)(CO)3(bpy)]þ concentration (Table 5). The data
show that the MLCT lifetime slightly increases with decreasing
concentration while kET decreases, albeit in a narrow range.
In the simultaneous presence of inter- and intramolecular
quenching (Scheme 1), the MLCT lifetime τd will depend on the
concentration [Re(pytrp)] according to eq 4:
1
1
intra
¼
þ k þ kQ ½ReðpyrtrpÞꢃ
ð4Þ
ET
τd
τ0
intra
ET
k
is the rate constant of the intramolecular ET. The corre-
sponding correlation of the τd values from Table 5 (Figure 11)
yields the kQ value of 1.21 ꢁ 109 Mꢀ1 sꢀ1, which means that, at
typically mM TRIR concentrations, the bimolecular rate will be
on the order of 106 sꢀ1. Therefore, the maximum contribution of
the bimolecular reaction to the reported ET rates is ∼10%,
comparable to the experimental error. We can thus conclude that
the reported kinetics are predominantly due to the intramole-
cular ET pathway, shown by the red arrows in Scheme 1. For
emission experiments, the bimolecular contribution will be about
10 times smaller because of lower concentrations. The BET rate
constant is concentration-independent (Table 5), in accordance
with the conclusion that most of the ET occurs intramolecularly.
The Trp(indole)f*ReII ET occurs with a driving force of
about 0.2 V,67 in the Marcus normal region. The slightly faster
rate in phen than bpy complexes (Table 2) could be due to ∼0.14
eV higher driving force.40,67 The observed increase of the ET rate
with decreasing solvent dielectric function (Table 3), caused by
decreasing outer-sphere reorganization energy λo, is expected for
a nonadiabatic reaction.68 The relatively slow ET rates, in the
(∼30 ns)ꢀ1 range, are in agreement with the absence of a direct
contact between the Trp(indole) and the Re(CO)3(N,N) chro-
mophore. For comparison, a much faster (∼500 ps) Trp-
(indole)f*ReII ET has been observed10 in a Re-labeled azurin
mutant Re124W122Az containing a Trp residue placed next to
the ReI(histidine)(CO3)(4,7-Me2-phen) label. The fast ET in
this case is attributable10,11 to an indoleꢀphen πꢀπ stacking,
proven crystallographically. Picosecond ET to the MLCT-excited
’ DISCUSSION
Rhenium(I) carbonyl-diimine complexes with appended tryp-
tophan undergo photoinduced electron transfer from the Trp-
(indole) group to the MLCT-excited Re chromophore, followed
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ReI chromophore also was observed in a chromophoreꢀ
quencher complex [Re(py-CH2-PTZ)(CO)3(bpy)]þ (PTZ =
phenothiazine)69 with a short bent ꢀCH2ꢀ linker, and [Re(py-
azacrown)(CO)3(bpy)]þ, where the electron-donating azacrown
N-atom is linked to the pyridine ligand through a ꢀN(H)ꢀC-
(dO)ꢀphꢀ bridge.70
its spectroscopic properties and reactivity, limited only by the
30ꢀ60 ns back-electron-transfer. The [Re(L-Trp•þ)(CO)3(N,
N•ꢀ)]þ transient yield can be maximized by (i) prolonging the
inherent MLCT lifetime (τ0) by using phen instead of bpy and
linking Trp to Re through pyridine instead of imidazole, (ii)
maximizing the forward ET rate either by ligand engineering
(nictrp is superior to pytrp and imtrp) or using nonpolar solvents
with small dielectric functions, and (iii) minimizing the back-ET
rate that, however, appears to be the least tunable factor. The
present study also independently demonstrates the feasibility of
Trp to act as an electron-hopping intermediate in Re-labeled
proteins and photodriven supramolecular ET systems.
The photoinduced ET in [ReI(L-Trp)(CO)3(N,N)]þ can
occur either through space, mediated by intervening solvent
molecules, or through σ bonds.9,71,72 The former looks unlikely
since both NMR and DFT indicate that the Trp(indole) donor
group in the most stable conformation points away from the
Re(CO)3(N,N) chromophore. However, the L-Trp ligands
are flexible, and conformational movements may influence the
electronic coupling7 by changing the bond angles and the
Reꢀindole distance. Therefore, the Trp(indole)f*ReII ET
could be gated by fluctuations of solvated tryptophan whereby
through-space ET would occur (ultra)fast, but only in favorable
structural arrangements. However, there is no experimental
evidence for this proposition so far. On the other hand, the
through-bond mechanism is supported by the observed 4-fold
ET acceleration on going from pytrp to nictrp complexes, i.e., upon
shortening the ET path by one CꢀC bond.73
’ ASSOCIATED CONTENT
S
Supporting Information. UVꢀvis absorption spectra of
b
[Re(L-AA)(CO)3(bpy)]þ, emission spectra of [Re(L-AA)(CO)3-
(N,N)]þ, emission decay profiles of [Re(L-AA)(CO)3(bpy)]þ
and their emission-wavelength dependences, TRIR spectra of
[Re(pytrp)(CO)3(bpy)]þ in DMF, transient lifetimes of
[Re(pyphe)(CO)3(bpy)]þ and [Re(pytrp)(CO)3(bpy)]þ as a
function of solvent, and the full reference for the Gaussian 2009
software. This material is available free of charge via the Internet at
http://pubs.acs.org.
The CS state [ReI(L-Trp•þ)(CO)3(N,N•ꢀ)]þ is analogous to
interligand CT states observed in Re-based chromophoreꢀ
quencher complexes.61,69,70 The CS term is preferred in the
context of L-Trp complexes because the interaction between the
two redox centers is rather weak. The oxidized tryptophan
moiety is assumed to be in the protonated form (Trp•þ) for
two reasons: (i) no proton acceptor is present in the system, and
(ii) Trp•þ deprotonation is slower (∼130 ns in H2O)20 than the
CS-state decay. The absence of a suitable proton acceptor also
explains why a Tyrf*ReII ET does not take place upon irradia-
tion of the tyrosine complex [ReI(pytyr)(CO)3(bpy)]þ: The
Tyr•þ/Tyr potential is higher than þ1.35 V (vs NHE),74
apparently above the *ReII/ReI excited-state potential of about
1.2ꢀ1.3 V,67 making the ET thermodynamically unfavorable.
On the other hand, Tyr photooxidation has been observed in
[ReI(Ph2PꢀphꢀTyr)(CO)3(phen)]þ even at acidic pH due to a
much higher *ReII/ReI potential, þ1.78 V.16
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: a.vlcek@qmul.ac.uk.
’ ACKNOWLEDGMENT
This research was supported by STFC Rutherford Appleton
Laboratory (CMSD 43), QMUL, European collaboration pro-
gram COST D35, and the Czech Ministry of Education grant
LD11082. Dr. H. Toms and Prof. G. E. Hawkes (QMUL) are
thanked for measuring and interpreting the 1H NOESY spectra.
’ REFERENCES
(1) Byrdin, M.; Eker, A. P. M.; Vos, M. H.; Brettel, K. Proc. Natl.
Acad. Sci. U. S. A. 2003, 100, 8676–8681.
(2) Lukacs, A.; Eker, A. P. M.; Byrdin, M.; Brettel, K.; Vos, M. H.
J. Am. Chem. Soc. 2008, 130, 14394.
(3) Byrdin, M.; Lukacs, A.; Thiagarajan, V.; Eker, A. P. M.; Brettel,
K.; Vos, M. H. J. Phys. Chem. A 2010, 114, 3207–3214.
(4) Stubbe, J.; Nocera, D. G.; Yee, C. S.; Chang, M. C. Y. Chem. Rev.
2003, 103, 2167.
(5) Reece, S. Y.; Hodgkiss, J. M.; Stubbe, J.; Nocera, D. G. Philos.
Trans. R. Soc. B 2006, 361, 1351.
(6) Loll, B.; Kern, J.; Saenger, W.; Zouni, A.; Jacek Biesiadka, J.
Nature 2005, 438, 1040.
(7) Gray, H. B.; Winkler, J. R. Chem. Phys. Lett. 2009, 483, 1.
(8) Winkler, J. R.; Di Bilio, A. J.; Farrow, N. A.; Richards, J. H.; Gray,
H. B. Pure Appl. Chem. 1999, 71, 1753.
(9) Gray, H. B.; Winkler, J. R. Proc. Natl. Acad. Sci. U.S.A. 2005,
102, 3534.
(10) Shih, C.; Museth, A. K.; Abrahamsson, M.; Blanco-Rodriguez,
A. M.; Di Bilio, A. J.; Sudhamsu, J.; Crane, B. R.; Ronayne, K. L.; Towrie,
M.; Vlꢁcek, A., Jr.; Richards, J. H.; Winkler, J. R.; Gray, H. B. Science 2008,
320, 1760.
The BET rate does not show any systematic dependence on
the L-Trp and N,N ligands or the solvent, although it is slightly
faster for nictrp than pytrp. BET is a highly inverted process,
occurring with a 2.0ꢀ2.2 eV driving force.67 The observed BET
rates (Table 2) are comparable to those reported for more
strongly coupled [Re(py-CH2-PTZ)(CO)3(bpy)]þ and
[Re(py-azacrown)(CO)3(bpy)]þ, 4 ꢁ 107 and 5.3 ꢁ 107 sꢀ1
,
respectively,69,70 as well as for the Re-labeled azurin mutant
Re124W122Az, 2 ꢁ 107 sꢀ1 10
.
The fast BET rates in [Re(L-
Trp)(CO)3(N,N)]þ suggest the possibility of a through-space
interaction in the CS state between the negatively charged
diimine and positively charged indole rings. Emergence of such
an interaction would require a conformational change of the CS
state, whose occurrence seems to be manifested by the small shift
and a shape change of the IR band 5 on a tens of nanoseconds
time scale. Moreover, the TRIR behavior in the region of the
amide-I band (Figure 8) suggests that the amide bridge geometry
changes on going from the MLCT to the CS state.
In conclusion, we have demonstrated intramolecular photo-
oxidation of tryptophan (but not tyrosine) by appended ReI-
(CO)3(N,N) chromophores in a variety of aprotic solvents that
occurs with a lifetime of 8ꢀ40 ns. A ligand-appended tryptophan
radicalꢀcation is generated in a sufficient concentration to study
(11) Blanco-Rodríguez, A. M.; Di Bilio, A. J.; Shih, C.; Museth, A. K.;
Clark, I. P.; Towrie, M.; Cannizzo, A.; Sudhamsu, J.; Crane, B. R.;
Sꢀykora, J.; Winkler, J. R.; Gray, H. B.; Zꢀaliꢁs, S.; Vlꢁcek, A., Jr. Chem.—Eur.
J. 2011, 17, 5350–5361.
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Inorganic Chemistry
ARTICLE
(12) Reece, S. Y.; Seyedsayamdost, M. R.; Stubbe, J.; Nocera, D. G.
J. Am. Chem. Soc. 2007, 129, 13828.
(46) El Nahhas, A.; Consani, C.; Blanco-Rodríguez, A. M.; Lancaster,
K. M.; Braem, O.; Cannizzo, A.; Towrie, M.; Clark, I. P.; Zꢀaliꢁs, S.; Chergui,
M.; Vlꢁcek, A., Jr. Inorg. Chem. 2011, 50, 2932–2943.
(13) Vlꢁcek, A., Jr. Top. Organomet. Chem. 2010, 29, 73.
(14) Kumar, A.; Sun, S.-S.;Lees, A. J. Top. Organomet. Chem. 2010, 29, 1.
(15) Pan, J.; Xu, Y.; Benk€o, G.; Feyziyev, Y.; Styring, S.; Sun, L.;
(47) George, M. W.; Johnson, F. P. A.; Westwell, J. R.; Hodges,
P. M.; Turner, J. J. J. Chem. Soc., Dalton Trans. 1993, 2977.
(48) Gamelin, D. R.; George, M. W.; Glyn, P.; Grevels, F.-W.;
Johnson, F. P. A.; Klotzb€ucher, W.; Morrison, S. L.; Russell, G.;
Schaffner, K.; Turner, J. J. Inorg. Chem. 1994, 33, 3246.
(49) Dattelbaum, D. M.; Omberg, K. M.; Hay, P. J.; Gebhart, N. L.;
Martin, R. L.; Schoonover, J. R.; Meyer, T. J. J. Phys. Chem. A 2004,
108, 3527.
ꢀ
Åkermark, B.; Polívka, T.; Sundstr€om, V. J. Phys. Chem. B 2004, 108,
12904.
(16) Reece, S. Y.; Nocera, D. G. J. Am. Chem. Soc. 2005, 127, 9448.
(17) Reece, S. Y.; Seyedsayamdost, M. R.; Stubbe, J.; Nocera, D. G.
J. Am. Chem. Soc. 2006, 128, 13654.
(18) Ishikita, H.; Alexander V. Soudackov, A. V.; Hammes-Schiffer,
S. J. Am. Chem. Soc. 2007, 129, 11146.
(19) Sj€odin, M.; Styring, S.; Åkermark, B.; Sun, L.; Hammarstr€om, L.
(50) Dattelbaum, D. M.; Martin, R. L.; Schoonover, J. R.; Meyer,
T. J. J. Phys. Chem. A 2004, 108, 3518.
ꢀ
J. Am. Chem. Soc. 2000, 122, 3932.
(51) Vlꢁcek, A., Jr.; Zꢀaliꢁs, S. Coord. Chem. Rev. 2007, 251, 258.
(52) Asbury, J. B.; Wang, Y.; Lian, T. Bull. Chem. Soc. Jpn. 2002,
75, 973.
(53) Lenchenkov, V. A.; She, C.; Lian, T. J. Phys. Chem. B 2004,
108, 16194.
(20) Sj€odin, M.; Styring, S.; Wolpher, H.; Xu, Y.; Sun, L.;
Hammarstr€om, L. J. Am. Chem. Soc. 2005, 127, 3855.
(21) Irebo, T.; Johansson, O.; Leif Hammarstr€om, L. J. Am. Chem.
Soc. 2008, 130, 9194.
(22) Lo, K. K.-W.; Tsang, K. H.-K.; Hui, W.-K.; Zhu, N. Inorg. Chem.
2005, 44, 6100.
(54) Liard, D. J.; Busby, M.; Matousek, P.; Towrie, M.; Vlꢁcek, A., Jr.
J. Phys. Chem. A 2004, 108, 2363.
(23) Lo, K. K.-W.; Lee, T. K.-M.; Zhang, K. Y. Inorg. Chim. Acta
2006, 359, 1845–1854.
(24) Lo, K. K.-W.; Sze, K.-S.; Tsang, K. H.-K.; Zhu, N. Organome-
tallics 2007, 26, 3440.
(25) Lo, K. K.-W.; Louie, M.-W.; Zhang, K. Y. Coord. Chem. Rev.
2010, 254, 2603–2622.
(26) Hino, J. K.; Della Ciana, L.; Dressick, W. J.; Sullivan, B. P. Inorg.
Chem. 1992, 31, 1072.
(27) Towrie, M.; Grills, D. C.; Dyer, J.; Weinstein, J. A.; Matousek,
P.; Barton, R.; Bailey, P. D.; Subramaniam, N.; Kwok, W. M.; Ma, C. S.;
Phillips, D.; Parker, A. W.; George, M. W. Appl. Spectrosc. 2003, 57, 367.
(28) Vlꢁcek, A., Jr.; Farrell, I. R.; Liard, D. J.; Matousek, P.; Towrie,
M.; Parker, A. W.; Grills, D. C.; George, M. W. J. Chem. Soc., Dalton
Trans. 2002, 701.
(55) Blanco-Rodríguez, A. M.; Busby, M.; Grꢂadinaru, C.; Crane,
B. R.; Di Bilio, A. J.; Matousek, P.; Towrie, M.; Leigh, B. S.; Richards,
J. H.; Vlꢁcek, A., Jr.; Gray, H. B. J. Am. Chem. Soc. 2006, 128, 4365.
(56) Blanco-Rodríguez, A. M.; Towrie, M.; Collin, J.-P.; Zꢀaliꢁs, S.;
Vlꢁcek, A., Jr. Dalton Trans. 2009, 3941.
(57) Blanco-Rodríguez, A. M.; Busby, M.; Ronayne, K. L.; Towrie,
M.; Sꢀykora, J.; Hof, M.; Zꢀaliꢁs, S.; Grꢂadinaru, C.; Di Bilio, A. J.; Crane,
B. R.; Gray, H. B.; Vlꢁcek, J., A. J. Am. Chem. Soc. 2009, 131, 11788.
(58) Johnson, F. P. A.; George, M. W.; Hartl, F.; Turner, J. J.
Organometallics 1996, 15, 3374.
(59) Gabrielsson, A.; Hartl, F.; Zhang, H.; Lindsay Smith, J. R.; Towrie,
M.; Vlꢁcek, A., Jr.; Perutz, R. N. J. Am. Chem. Soc. 2006, 128, 4253.
(60) Stor, G. J.; Hartl, F.; van Outersterp, J. W. M.; Stufkens, D. J.
Organometallics 1995, 14, 1115.
(29) Blanco-Rodríguez, A. M.; Ronayne, K. L.; Zꢀaliꢁs, S.; Sꢀykora, J.;
Hof, M.; Vlꢁcek, A., Jr. J. Phys. Chem. A 2008, 112, 3506.
(30) Towrie, M.; Parker, A. W.; Vlꢁcek, A., Jr.; Gabrielsson, A.; Blanco
Rodriguez, A. M. Appl. Spectrosc. 2005, 59, 467.
(31) Greetham, G.; Burgos, P.; Cao, Q.; Clark, I. P.; Codd, P.;
Farrow, R.; George, M. W.; Kogimtzis, M.; Matousek, P.; Parker, A. W.;
Pollard, M.; Robinson, D.; Xin, Z.-J.; Towrie, M. Appl. Spectrosc. 2010,
64, 1311.
(61) Lewis, J. D.; Towrie, M.; Moore, J. N. J. Phys. Chem. A 2008,
112, 3852.
(62) Omberg, K. M.; Schoonover, J. R.; Treadway, J. A.; Leasure,
R. M.; Dyer, R. B.; Meyer, T. J. J. Am. Chem. Soc. 1997, 119, 7013.
(63) Liard, D. J.; Busby, M.; Farrell, I. R.; Matousek, P.; Towrie, M.;
Vlꢁcek, A., Jr. J. Phys. Chem. A 2004, 108, 556.
(64) Handbook of Chemistry and Physics; CRC Press: Boca Raton,
FL, 2002.
(32) Frisch, M. J. et al. Gaussian 09, Revision A.02; Gaussian, Inc.:
Wallingford CT, 2009.
(65) Gabrielsson, A.; Matousek, P.; Towrie, M.; Hartl, F.; Zꢀaliꢁs, S.;
Vlꢁcek, A., Jr. J. Phys. Chem. A 2005, 109, 6147.
(33) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996,
77, 3865.
(66) Busby, M.; Matousek, P.; Towrie, M.; Clark, I. P.; Motevalli, M.;
Hartl, F.; Vlꢁcek, A., Jr. Inorg. Chem. 2004, 43, 4523.
(34) Adamo, C.; Barone, V. J. Chem. Phys. 1999, 110, 6158.
(35) Yanai, T.; Tew, D. P.; Handy, N. C. Chem. Phys. Lett. 2004,
393, 51.
(36) Raghavachari, K.; Binkley, J. S.; Seeger, R.; Pople, J. A. J. Chem.
Phys. 1980, 72, 650.
(67) The driving force of the forward reaction is estimated as
*E(ReII/ReI) ꢀ Eo(Trp•þ/Trp), where the excited-state potential
*E(ReII/ReI) = E00 ꢀ Eo(N,N/N,N•ꢀ). A value of þ1.1 V (vs NHE)
is assumed for Eo(Trp•þ/Trp).74,75 The reduction potentials and excited-
state energy E00 of pytrp and nictrp complexes are assumed to be similar
to those of the corresponding [Re(Etpy)(CO)3(N,N)]þ complexes40,
giving *E(ReII/ReI) of þ1.29 and þ1.48 V for N,N = bpy and phen,
respectively. Reduction of [Re(im)(CO)3(bpy)]þ is expected (by
analogy with the phen complex76) to be slightly more negative while
emission occurs at lower energies than in the pytrp and nictrp complexes.
Hence, we estimate the *E(ReII/ReI) potential of [Re(imtrp)(CO)3-
(bpy)]þ as þ1.2 V. The BET driving force is estimatedas Eo(Trp•þ/Trp)
þ Eo(N,N/N,N•ꢀ).
(37) Andrae, D.; H€aussermann, U.; Dolg, M.; Stoll, H.; Preuss, H.
Theor. Chim. Acta 1990, 77, 123.
(38) Martin, J. M. L.; Sundermann, A. J. Chem. Phys. 2001, 114, 3408.
(39) Tomasi, J.; Mennucci, B.; Cammi, R. Chem. Rev. 2005,
105, 2999.
(40) Dattelbaum, D. M.; Omberg, K. M.; Schoonover, J. R.; Martin,
R. L.; Meyer, T. J. Inorg. Chem. 2002, 41, 6071.
(41) Stufkens, D. J.; Vlꢁcek, A., Jr. Coord. Chem. Rev. 1998, 177, 127.
(42) Horng, M. L.; Gardecki, J. A.; Papazyan, A.; Maroncelli, M.
J. Phys. Chem. 1995, 99, 17311.
(68) Marcus, R. A.; Sutin, N. Biochim. Biophys. Acta 1985, 811, 265.
(69) Chen, P.; Westmoreland, T. D.; Danielson, E.; Schanze, K. S.;
Anthon, D.; Neveux, P. E., Jr.; Meyer, T. J. Inorg. Chem. 1987, 26, 1116.
(70) Lewis, J. D.; Bussotti, L.; Foggi, P.; Perutz, R. N.; Moore, J. N.
J. Phys. Phem. A 2002, 106, 12202.
(43) Zꢀaliꢁs, S.; Consani, C.; El Nahhas, A.; Cannizzo, A.; Chergui, M.;
Hartl, F.; Vlꢁcek, A.Jr.; Inorg. Chim. Acta 2011, in press, doi:10.1016/
j.ica.2011.02.084.
(44) Kalyanasundaram, K. J. Chem. Soc. Faraday Trans. 2 1986, 82, 2401.
(45) El Nahhas, A.; Cannizzo, A.; van Mourik, F.; Blanco-Rodríguez,
A. M.; Zꢀaliꢁs, S.; Vlꢁcek, A., Jr.; Chergui, M. J. Phys. Chem. A 2010,
114, 6361.
(71) Wenger, O. S.; Leigh, B. S.; Villahermosa, R. M.; Gray, H. B.;
Winkler, J. R. Science 2005, 307, 99.
(72) Kaplan, R.; Napper, A. M.; Waldeck, D. H.; Zimmt, M. B.
J. Phys. Chem. A 2002, 106, 1917.
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ARTICLE
(73) This argument seems to be countered by ET in [Re(imtrp)-
(CO)3(bpy)]þ being slower than in [Re(nictrp)(CO)3(bpy)]þ, despite
involving the same number of σ bonds. However, this difference could
be caused by at least ∼50 mV smaller driving force in [Re(imtrp)(CO)3-
(bpy)]þ.
(74) Tommos, C.; Skalicky, J. J.; Pilloud, D. L.; Wand, A. J.; Dutton,
P. L. Biochemistry 1999, 38, 9495.
(75) Harriman, A. J. Phys. Chem. 1987, 91, 6102.
(76) Connick, W. B.; Di Bilio, A. J.; Hill, M. G.; Winkler, J. R.; Gray,
H. B. Inorg. Chim. Acta 1995, 240, 169.
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