Photoinduced Electron Transfer of Flavin Derivatives
FULL PAPER
(kdiff =5.6ꢀ109 mꢀ1 sꢀ1).[34] The transient absorption spectrum
of the radical cation of bis(ethylenedithiol)tetrathiafulvalene
(BEDT-TTF) (Eox =0.40 V vs. SCE; lmax =1000 nm) is also
observed by the one-electron oxidation of BEDT-TTF with
Methylisoalloxazine (MeFl) was purchased from Aurora Fine Chemicals,
Ltd. and used without further purification. Benzonitrile and butyronitrile
were distilled from P2O5 in vacuo.[63] Tris(2,2’-bipyridine)iron
ACHTUNGTRENNUNG
fluorophosphate [FeACTHNUGERTN(NUNG bpy)3]ACHTUNGTRENNUNG
iron(II) sulfate heptahydrate and 2,2’-bipyridine followed by oxidation of
+
+
C
C
Cꢀ
the resulting iron(II) complex by ceric sulfate in aqueous H2SO4.[64]
the DMA moiety of DMA –Fl in PhCN (see Support-
ing Information S21), when ET is thermodynamically feasi-
Tris(2,2’-bipyridine)ruthenium
was prepared by oxidizing RuACTHGUNTRNE(UNG bpy)3
(III) hexafluorophosphate [Ru
E
3ACHTUNGTRENNUNG(PF6)3]
2+
Cꢀ
ble. In contrast, no ET from the Fl moiety to nitrobenzene
H2SO4 followed by the addition of KPF6.[65] Thin-layer chromatography
(TLC) and column chromatography were performed with silicagel 60 F254
(Merck) and silica gel 60, respectively.
(Ered =ꢀ1.17 V) occurred, when the laser excitation of
DMA–Fl with nitrobenzene results in the same transient ab-
sorption spectra as the CS state of DMA–Fl (see Supporting
Synthesis of DMA–Fl
Information S22). Similarly, the one-electron reduction was
4’-(N,N-Dimethylamino)-2-nitrodiphenylamine (1): A mixture of 2-chlor-
onitrobenzene (8.1 g, 52 mmol), the N,N-dimethyl-p-phenylenediamine
(7.0 g, 52 mmol) and DBU (7.88 g, 52 mmol) were heated at 1408C for
10 h. The reaction mixture was allowed to cool to room temperature and
partitioned between dichloromethane and water. The organic layer was
dried over anhydrous sodium sulfate and the residue was purified via
silica gel column chromatography with chloroform/hexane 2:1 as an
eluent (Rf =0.4) to afford dark red color solid (2.3 g, 8.8 mmol, 17%).
1H NMR (300 MHz, CD3CN): d=9.27 (s, 1H), 8.13 (d, 1H, J=9.0 Hz),
7.37 (t, 1H, J=7.7 Hz), 7.14 (d, 2H, J=8.7 Hz), 6.95 (d, 1H, J=8.4 Hz),
6.80 (d, 1H, J=8.7 Hz), 6.71 (t, 3H, J=8.0 Hz), 2.90 ppm (s, 6H); ele-
mental analysis calcd (%) for C14H15N3O2 + 1/5 H2O: C 64.45, H 5.95, N
16.11; found: C 64.52, H 5.79, N 16.00; MS (FAB): m/z: 257.2 [M]+.
+
C
not observed from DMA moiety to 1,4-dimethoxybenzene
(Eox =1.21 V) (see Supporting Information S23). Thus, the
+
Cꢀ
C
Fl and DMA moieties of the CS state act as a strong re-
ductant and an oxidant, respectively.
Conclusion
We have discovered that the rate constants of intermolecu-
lar back electron-transfer (BET) reactions from the radical
Cꢀ
4’-(N,N-Dimethylamino)-2-diphenyldiamine (2):
A propanol solution
anion of a flavin analogue (MeFl ) to the radical cations of
electron donors (D +) following intermolecular photoin-
(100 mL) of 1 (2.3 g, 8.8 mmol) was added to a well stirred refluxing
aqueous solution (200 mL) containing ammonium chloride (1.5 g,
27 mmol) and reduced iron (1.5 g), and the reflux maintained for 3 h.
The hot solution was filtered through celite and the residue was washed
well with chloroform. The filtrate was extracted with chloroform and the
combined extracts were washed with water, dried (MgSO4,), and evapo-
rated. The brown oil was distilled to afford 4’-(N,N-dimethylamino)-2-ni-
trodiphenylamine (1.9 g, 8.5 mmol, 96%). 1H NMR (300 MHz, CD3CN):
d=7.09 (s, 1H), 6.93 (d, 1H, J=7.8 Hz), 6.84 (t, 1H, J=7.5 Hz), 6.65–
6.74 (m, 6H), 4.95 (s, 1H, NH), 3.55 (s, 1H, NH2), 2.81 ppm (s, 6H).
C
duced electron transfer (ET) from D to MeFl decrease re-
markably with increasing the driving force of BET. Among
examined electron donors N,N-dimethylaniline (DMA),
which afforded the slowest rate of BET, was selected to be
linked at the 10-position of flavin to synthesize N,N-dime-
thylaniline–flavin (DMA–Fl). DMA–Fl undergoes efficient
intramolecular photoinduced ET to afford the charge-sepa-
rated (CS) state that has the longest CS state lifetime
(2.1 ms) ever reported for electron donor–acceptor linked
molecules by preventing the intermolecular charge recombi-
nation in solution at 298 K. Such a long CS lifetime results
from an extremely small reorganization energy for electron
Cꢀ
10-[4’-(N,N-Dimethylamino)phenyl]–isoalloxazine (DMA–Fl): Alloxane
tetrahydrate (1.8 g, 8.5 mmol) and boric acid (0.56 g, 9.0 mmol) were
added to an acetic acid solution (75 mL) of 2 (1.9 g, 8.5 mmol) and the
mixture was heated at 608C for 1 h under a nitrogen atmosphere. After
evaporation to dryness the residue was washed with a small volume of
water and ether and recrystallized from MeOH to give DMA–Fl (0.63 g,
22%). 1H NMR (300 MHz, [D6]DMSO): d=11.37 (s, 1H, NH), 8.16 (d,
1H, J=8.4 Hz), 7.75 (t, 1H, J=7.2 Hz), 7.60 (t, 1H, J=7.2 Hz), 7.18 (d,
2H, J=9.0 Hz), 6.92 (d, d, 3H, J=12.0 Hz, 11.4 Hz), 3.02 ppm (s, 6H,
NMe2); HRMS (FAB+): m/z: calcd for: 333.1226; found: 333.1233 [M]+;
elemental analysis calcd (%) for C18H15N5O2·0.17H2O: C 64.28, H 4.59,
N 20.82; found: C 64.55, H 4.47, N 20.60.
self-exchange between DMA–Fl/DMA–Fl
(0.26 eV),
which was determined by the ESR line width alternation.
The long-lived CS state acts as both a strong electron donor
and an acceptor, not only reducing electron acceptors with
Ered <ꢀ0.83 V versus SCE but also oxidizing electron
donors with Eox <0.94 V versus SCE.
Laser flash photolysis: For nanosecond laser flash photolysis experiments,
deaerated PhCN solutions of DMA–Fl or MeFl and electron donors
were excited by a Panther OPO pumped by Nd/YAG laser (Continuum,
SLII-10, 4–6 ns fwhm) at l=440 nm. The photodynamics was monitored
by continuous exposure to a xenon lamp (150 W) as a probe light and a
photomultiplier tube (Hamamatsu 2949) as a detector. The transient ab-
sorption spectra were recorded using fresh solutions in each laser excita-
tion. The solution was deoxygenated by argon purging for 15 min prior to
measurements. The experiments at various temperatures were performed
using a Unisoku thermostated cell holder. Femtosecond transient absorp-
tion spectroscopy experiments were conducted using an ultrafast source:
Integra-C (Quantronix Corp.), an optical parametric amplifier: TOPAS
(Light Conversion Ltd.) and a commercially available optical detection
system: Helios provided by Ultrafast Systems LLC. The instrumental de-
tails are described in Supporting Information S24.
Experimental Section
General procedures: 1H NMR spectra were measured on a JEOL JNM-
AL300 using tetramethylsilane as an internal standard. Fast atom bom-
bardment (FAB) mass spectra were obtained on
a JEOL EX-270.
Matrix-assisted laser desorption/ionization (MALDI) time-of-flight
(TOF) mass spectra were measured on a Kratos Compace MALDI I
(Shimazu). Steady-state absorption spectra were measured on a Shimad-
zu UV3100 spectrometer. Fluorescence and phosphorescence spectra
were taken using a Shimadzu RF5300PC fluorospectrophotometer.
Materials: All commercially available solvents and chemicals were of re-
agent grade quality and used without further purification unless other-
wise noted. Tetra-n-butylammonium perchlorate (TBAP) used as a sup-
porting electrolyte for electrochemical measurements was purchased
from Tokyo Chemical Industry Co., Ltd. and recrystallized from EtOH/
water, and dried in vacuo according to the standard procedure.[63] 10-
Fluorescence lifetime measurements: Fluorescence decays were mea-
sured by a Photon Technology International GL-3300 with a Photon
Technology International GL-302 and a nitrogen laser/pumped dye laser
system equipped with a four-channel digital delay/pulse generator (Stan-
dard Research System Inc. DG535) and a motor driver (Photon Technol-
Chem. Eur. J. 2010, 16, 7820 – 7832
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
7829