Single- and two-photon properties of a dye-derivatized roussin's red salt ester (Fe2(μ-RS)2(NO)4) with a large TPA cross section

Article abstract of DOI:10.1021/ic0607336

The synthesis, characterization, photochemistry, and two-photon photophysical properties of a new dye-derivatized iron sulfur nitrosyl cluster Fe₂(μ-RS)₂(NO)₄ (AFX-RSE, RS = 2-thioethyl ester of N-phenyl-N-(3-(2-ethoxy)phenyl)-7-(benzothiazol-2-yl)-9,9-diethyl- fluoren-2-yl-amine) were investigated. Under continuous photolysis, AFX-RSE decomposes with modest quantum yields (Φ_diss = (4.9 ± 0.9) × 10^-3 at λ_irr = 436 nm) as measured from the loss of the nitrosyl bands in the IR absorbance spectrum. Nitric oxide (NO) was qualitatively demonstrated to be photochemically produced via single-photon excitation through the use of an NO-specific electrode. Steady-state luminescence measurements have shown that AFX-RSE fluorescence is about 88% quenched relative to the model compound AF-tosyl. This is attributed to a relatively efficient energy transfer from the excited states of the antenna chromophores to the dinuclear metal center, with the subsequent production of NO. In addition, the two-photon absorption (TPA) cross sections (δ) were measured for the AF-chromophores via the two-photon excitation (TPE) photoluminescence technique using a femtosecond excitation source. The TPA cross section of AFX-RSE was found with this technique to be δ = 246 ± 8 GM (1 GM = 10^-50 cm⁴ s photon^-1 molecule ^-1).

Full text of DOI:10.1021/ic0607336

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Inorg. Chem. 2007, 46, 395−402
Single- and Two-Photon Properties of a Dye-Derivatized Roussin’s Red
Salt Ester (Fe2( -RS)2(NO)4) with a Large TPA Cross Section
µ
Stephen R. Wecksler,† Alexander Mikhailovsky,† Dmitry Korystov,† Fabian Buller,†,2
Ramamurthi Kannan,‡ Loon-Seng Tan,*,‡ and Peter C. Ford*,†
Department of Chemistry and Biochemistry, UniVersity of California, Santa Barbara, California
93106-9510, and Polymer Branch, Materials and Manufacturing Directorate, Air Force Research
Laboratory, Wright-Patterson Air Force Base, Ohio 45433-7750
Received April 28, 2006
The synthesis, characterization, photochemistry, and two-photon photophysical properties of a new dye-derivatized
iron sulfur nitrosyl cluster Fe2( -RS)2(NO)4 (AFX-RSE, RS 2-thioethyl ester of N-phenyl-N-(3-(2-ethoxy)phenyl)-
µ
)
7-(benzothiazol-2-yl)-9,9-diethyl-fluoren-2-yl-amine) were investigated. Under continuous photolysis, AFX-RSE
3
decomposes with modest quantum yields (Φdiss
)
(4.9
±
0.9)
×
10- at λirr ) 436 nm) as measured from the loss
of the nitrosyl bands in the IR absorbance spectrum. Nitric oxide (NO) was qualitatively demonstrated to be
photochemically produced via single-photon excitation through the use of an NO-specific electrode. Steady-state
luminescence measurements have shown that AFX-RSE fluorescence is about 88% quenched relative to the model
compound AF-tosyl. This is attributed to a relatively efficient energy transfer from the excited states of the antenna
chromophores to the dinuclear metal center, with the subsequent production of NO. In addition, the two-photon
absorption (TPA) cross sections (
δ) were measured for the AF-chromophores via the two-photon excitation (TPE)
photoluminescence technique using a femtosecond excitation source. The TPA cross section of AFX-RSE was
50
1
found with this technique to be δ ) 246
± 8 GM (1 GM )
10- cm4 s photon- molecule-1).
Introduction
tumors.4 These properties have generated considerable inter-
est in developing drugs for the delivery of NO for the
treatment of disease, with several in clinical use.5 One
limitation of such drugs is that thermally activated NO release
is nonspecific, and large localized concentrations cannot be
achieved without undesired cardiovascular implications. An
alternative strategy to address this issue would be one in
which a thermally stable precursor is triggered photochemi-
cally to release NO on demand.6 In these contexts, we have
been concerned with the development of thermally stable metal
Nitric oxide (NO, nitrogen monoxide) has important roles
in mammalian biology, including functions in the cardio-
vascular system, neurotransmission, and immune response.3
Furthermore, NO is known to be a γ-radiation sensitizer and,
as such, may be useful in the treatment of certain hypoxic
* To whom correspondence should be addressed. E-mail: ford@
chem.ucsb.edu (P.C.F.), Loon-Seng.Tan@wpafb.af.mil (L.-S.T.).
† University of California, Santa Barbara.
‡ Wright-Patterson Air Force Base.
(1) Taken in part from the Ph.D. dissertation of S. R. Wecksler, University
of California, Santa Barbara, 2006.
(2) Visiting undergraduate researcher from the University of Muenster,
Germany.
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10.1021/ic0607336 CCC: $37.00
Published on Web 12/30/2006
© 2007 American Chemical Society
Inorganic Chemistry, Vol. 46, No. 2, 2007 395

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Wecksler et al.
nitrosyl and nitrito complexes that are capable of the photo-
chemical generation of NO for therapeutic applications.7-10
A property of considerable importance for the use of
photoactivated agents in ViVo is the ability to absorb
wavelengths of light where tissue penetration is maximized
and that are sufficiently energetic to labilize the metal-
nitrosyl bond selectively. For mammalian tissue, near-
infrared (NIR; λ ) 800-1100 nm) light has the greatest
transmission.11 Therefore, one focus of our current research
is the development of compounds that can be subjected to
single- or two-photon excitation (SPE or TPE) by NIR light,
resulting in the photochemical production of NO. TPE is
especially interesting given that this allows access to excited
states that would require near-ultraviolet wavelengths if
activation were to be achieved by SPE. Another key
advantage of TPE for pro-drug activation is spatial selectively
visible wavelengths, thus limiting potential in ViVo applica-
tions. In this context, dye-derivatized compounds, such as
Fluor-RSE (RS ) 2-thioethyl ester of fluorescein)17,18 and
PPIX-RSE (RS ) bis(2-thioethyl)diester protoporphyrin
IX),15,16 were prepared with the goal of imparting specific
optical properties to improve the light harvesting ability and
the photochemical response to longer wavelength light. Both
PPIX-RSE and Fluor-RSE proved to be much more
photochemically effective than the simple RSE complexes
at longer wavelengths, since the dye chromophores dramati-
cally improve the light collecting efficiency for longer visible
wavelengths under continuous photolysis conditions where
single-photon excitation (only) would be expected.15,17
Although attaching antennae to the iron sulfur nitrosyl
cluster improved the light harvesting efficiency, the quantum
yield for photodecomposition for these clusters decreases at
longer irradiation wavelengths.15,17 An alternative strategy
to address this issue is to use TPA in the NIR to achieve
up-conversion to higher energy state(s) from which the
desired photochemistry results. This was first achieved
qualitatively with PPIX-RSE, for which TPE with 810 nm
femtosecond pulses leads both to weak emission from the
PPIX chromophore and to photochemical NO labilization.16
While this provided “proof of concept”, the poor solubility
of PPIX-RSE and the recognition that porphyrins are very
modest TPA chromophores (δ ) 2 GM for PPIX, 1 GM )
10-50 cm4 s photon-1 molecule-1)20 led us to develop other
compounds such as Fluor-RSE with fluorescein antennae.17
Fluorescein has well-characterized photophysical behavior,
2
achieved from the intensity-squared dependence (A ∼ I0 ,
where I0 is the intensity of the incident light and A is the
absorption rate) of the two-photon absorption (TPA) process,
since the highest probability of TPA is at the focal point of
a focused excitation beam.12 For analogous reasons, TPE has
drawn attention in strategies for photodynamic therapy
involving singlet oxygen generation in tissue via organic dye
photosensitization and for the photochemical “uncaging” of
bioactive substances.13
Recent studies at the University of California at Santa
Barbara (UCSB) have been concerned with the photochemi-
cal properties of Roussin’s red salt esters (Fe2(µ-RS)2(NO)4;
RSEs), which can be prepared from Roussin’s red salt (RRS)
(eq 1).14-19 The simple esters (R ) -CH3, -CH2CH3, -CH2-
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CH2OH, -CH2CH2SO3) undergo photochemical decomposi-
tion with moderate quantum yields (Φd ) 0.02-0.13) when
irradiated with near-UV light and release ∼4 mol of NO
per mole of cluster decomposed.14 However, the absorptivity
of the iron sulfur nitrosyl chromophore is quite low at longer
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396 Inorganic Chemistry, Vol. 46, No. 2, 2007

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Single- and Two-Photon Properties of Fe2(µ-RS)2(NO)4
Figure 1. Formulas for AFX-RSE and AF-tosyl.
is water soluble, and has a moderate TPA cross section
(δ ) 38 GM at 782 nm for fluorescein in pH 11 aqueous
solution).21
transition dipole moment resulting from an intramolecular
charge-transfer excitation.23 This has driven the development
of structural motifs, such as paracyclophane derivatives,22d
fused diporphyrins,22e bis(styryl)benzene/oligo-stilbene deriv-
atives,22g-i and the diphenylaminofluorene-based compounds
known as the AF chromophores,22b,24,25 having donor-π-
acceptor-π-donor or acceptor-π-donor-π-acceptor proper-
ties that may enhance dipole-moment changes, thus giving
larger δ values.23 The AF chromophores (e.g., Figure 1)
demonstrate synthetic versatility owing to the nature of the
attached linker functionality and show significant promise
for photonic applications. In the present manuscript, we
describe the synthesis, photochemistry, and two-photon
photophysical properties of a new dye-derivatized RSE for
which the light harvesting antennae are such AF chro-
mophores.
Like the case of PPIX-RSE, the excitation of Fluor-RSE
with pulsed femtosecond light at 800 nm results in frequency
up-conversion and weak emission from the antennae along
with the photochemical production of NO.18 The TPA cross
section measured for Fluor-RSE is δ ) 63 (( 7) GM at
800 nm, about twice that of the free chromophore. Further-
more, intensity dependence studies showed the quantity of
NO generated using 100 femtosecond pulses of 800 nm light
to follow a quadratic relationship. This result confirmed the
operation of a two-photon mechanism under these conditions,
leading to the generation of higher energy excited states from
which nonradiative processes populate the reactive states
responsible for NO labilization. These findings represent
significant progress toward the goal of using NIR two-photon
excitation for photochemical NO production but remain
limited by the need to develop systems with larger TPE cross
sections δ.
(23) Albota, M.; Beljonne, D.; Bredas, J. L.; Ehrlich, J. E.; Fu, J. Y.; Heikal,
A. A.; Hess, S. E.; Kogej, T.; Levin, M. D.; Marder, S. R.; McCord-
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G. S.; Lin, T.-C.; Prasad, P. N. Proc. SPIEsInt. Soc. Opt. Eng. 2003,
4797, 171-178. (b) He, G. S.; Lin, T.-C.; Dai, J.; Prasad, P. N.;
Kannan, R.; Dombroskie, A. G.; Vaia, R. A.; Tan, L.-S. J. Chem.
Phys. 2004, 120, 5275-5284. It should be pointed out that inherent
in the nonlinear absorption (transmission) measurement using either
nanosecond or picosecond laser pulses is the assumption that the two-
photon process is the predominant mechanism responsible for the
observed intensity-dependent nonlinear absorption. However, it is now
well recognized that the strong TPA process increases molecular
populations considerably in excited states, and thus a secondary
process, i.e., the cascaded one-photon absorption from the excited
states, may greatly enhance the observed nonlinear absorption of the
input laser beam. Since it is difficult to separate these two contributions
through the nonlinear absorption measurement in these time regimes,
the term “effective TPA cross section” is used to describe the apparent
δ values under these excitation conditions for the two-photon
chromophore. The effective cross section of a TPA chromophore can
be more than 2 orders of magnitude larger than the intrinsic cross
section obtained via a femtosecond masurement, depending on the
type of technique (transmission or fluorescence method) used.
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Sankaran, B.; Kannan, R.; Tan, L.-S.; Fleitz, P. A. J. Phys. Chem. A
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Soc. Am. B 2005, 22, 1939-1948.
There has been significant progress in recent years in
developing compounds with large TPA cross sections,22,23
and it has been shown that δ is largely a function of the
(21) Webb, W. W.; Xu, C. J. Opt. Soc. Am. B 1997, 13, 481-491.
(22) (a) Meyers, F.; Marder, S. R.; Pierce, B. M.; Bre´das, J.-L. J. Am.
Chem. Soc. 1994, 116, 10703-10714. (b) Baur, J. W.; Alexander, M.
D., Jr.; Banach, M.; Denny, L. R.; Reinhardt, B. A.; Vaia, R. A. Chem.
Mater. 1999, 11, 2899-2906. (c) Marder, S. R.; Gorman, C. B.;
Meyers, F.; Perry, J. W.; Bourhill, G.; Bre´das, J.-L.; Pierce, B. M.
Science 1994, 265, 632-635. (d) Woo, H. Y.; Hong, J. W.;
Mikhailovsky, A.; Korystov, D.; Bazan, G. C. J. Am. Chem. Soc. 2005,
127, 820-821. (e) Kim, D. Y.; Ahn, T. K.; Kwon, J. H.; Kim, D.;
Ikeue, T.; Aratani, N.; Osuka, A.; Shigeiwa, M.; Maeda, S. J. Phys.
Chem. A 2005, 109, 2996-2999. (f) Rumi, M.; Ehrlich, J. E.; Heikal,
A. A.; Perry, J. W.; Barlow, S.; Hu, Z.; McCord-Maughon, D.; Parker,
C. T.; Rockel, H.; Thayumanavan, S.; Marder, S. R.; Beljonne, D.;
Bre´das, J.-L. J. Am. Chem. Soc. 2000, 122, 9500-9510. (g) Belfield,
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V.; Porres, L.; Pons, T.; Mertz, J.; Tretiak, S.; Blanchard-Desce, M.
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Inorganic Chemistry, Vol. 46, No. 2, 2007 397

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Wecksler et al.
108.72, 110.21, 117.10, 119.29, 119.64, 121.24, 121.66, 121.75,
123.09, 123.37, 123.76, 124.68, 125.21, 125.49, 126.51, 127.50,
128.23, 128.42, 129.23, 129.51, 130.01, 130.11, 131.67, 132.93,
135.04, 135.79, 144.64, 145.13, 147.77, 148.03, 149.33, 150.91,
152.31, 154.29, 159.09, 169.04 (sp2 C). Anal. Calcd for C45H40-
N2O4S2: C, 73.34; H, 5.47; N, 3.80; S, 8.70. Found: C, 73.46; H,
5.54; N, 3.64; S, 8.58. Also recovered from this chromatographic
separation were (i) unreacted starting alcohol, 0.2 g (6%), mp
138.6-142.1 °C, and (ii) chloroethoxy AF-341, 0.15 g (4% yield),
mp 140.8-142.2 °C, MS m/z 600, 602 (M+). The latter appears to
have been generated from the reaction between AF-tosyl and the
byproduct chloride ion.
Experimental Section
Materials. Tetrahydrofuran (THF) used for synthesis and
photochemical measurements was distilled under dinitrogen from
sodium and benzophenone. Dichloromethane (DCM) was distilled
under N2 from calcium hydride (CaH2). Silica gel 60 (Geduran 60
Å) was purchased from EMC sciences. Gases were purchased from
Praxair Inc. and used as received unless specified. High grade argon
used for inert atmosphere preparations was passed through a
Drierite/molecular sieve (4 Å) column to remove water. Deoxy-
genated solutions were prepared by entraining with dinitrogen or
argon for ∼1 min/mL. Inert atmosphere work was performed on a
Schlenk line or in an argon-filled glove box (VAC Nexus model).
Deaerated solutions were prepared via three freeze-pump-thaw
cycles.
The synthesis and characterization of AF-341,24 AF-343 (AF-
Br),24 and Roussin’s red salt (Na2[Fe2(µ-S)2(NO)4], RRS)10 are
described elsewhere. AF-343-b (AF-tosyl) was made in two steps
from AF-341 (N-phenyl-N-(3-hydroxyphenyl)-7-(benzothiazol-2-
yl)-9,9-diethylfluoren-2-ylamine). AF-341 was first alkylated with
ethylene carbonate to produce the phenoxy-ethanol intermediate,
which was then reacted with p-toluenesulfonyl chloride (tosyl
chloride) in pyridine to get the tosylate, AF-343-b (AF-tosyl).
Fluorescein dye was purchased from Acros Organics and used as
received.
Bis[(µ-S,µ-S′)-N-phenyl-N-(3-(2-ethoxy)phenyl)-7-(benzothia-
zol-2-yl)-9,9-diethylfluoren-2-ylamine-thioethylester]tetrani-
trosyl diiron (AFX-RSE). A three-neck flask (100 mL) covered
in aluminum foil and equipped with a stir bar and rubber septa
was charged with a solution containing 0.5 g (0.00068 mol) of
N-phenyl-N-(3-(2-tosylethoxy)phenyl)-7-(benzothiazol-2-yl)-9,9-
diethylfluoren-2-ylamine (AF-tosyl) in approximately 50 mL of dry,
distilled deoxygenated THF. This solution was blanketed with argon
and stirred until the compound was fully dissolved, and then 0.165
g (0.00034 mol) of solid recrystallized Na2[Fe2S2(NO)4]‚8H2O
(RRS) was added under inert gas. This mixture was refluxed under
argon for approximately 4 h, after which the solution was checked
by IR and TLC to ensure that the reaction had gone to completion.
At this point, the solution was allowed to cool to room temperature,
and then the solvent was removed via rotary evaporation. The
remainder was washed with deoxygenated water and was then
evaporated to complete dryness on the rotary evaporator.
N-Phenyl-N-(3-(2-hydroxyethoxy)phenyl)-7-(benzothiazol-2-
yl)-9,9-diethylfluoren-2-ylamine. A mixture of 10.76 g of AF-
341 (20 mmol), 2.76 g of potassium carbonate (20 mmol), and
dimethylformamide (DMF, 100 mL) was heated to 60 °C, and then
3.54 g of ethylene carbonate (40 mmol) was added. After 24 and
30 h, two additional portions each of potassium carbonate (1.38 g,
10 mmol) and ethylene carbonate (1.8 g, 20 mmol) were added.
After a total of 48 h at 60 °C, the mixture was diluted with water,
and the separated greenish yellow solids (11.48 g) were collected.
These solids were dissolved in a 5% ethyl acetate-toluene mixture
and passed through a column of silica gel. The residue that was
left behind on evaporation of the solvents was recrystallized from
a mixture of isopropanol and methanol to afford 10.91 g (94% yield)
of the product, mp 143-145 °C. Another recrystallization raised
the mp to 145.1-146.5 °C. MS: m/z 582 (M+). 1H NMR (CDCl3)
δ ppm: 0.38 (t, 6H), 1.19 (1H), 1.90-2.15 (m, 4H), 3.88-3.99
(m, 4H), 6.57-8.10 (m, 19H). 13C NMR (CDCl3) δ ppm: 8.61,
30.93, 32.63, 56.41, 61.34, 69.03 (sp3C), 108.68, 110.01, 116.63,
119.05, 119.39, 121.01, 121.41, 121.52, 122.88, 123.08, 123.57,
124.49, 124.98, 126.28, 127.28, 129.27, 129.90, 131.46, 134.86,
135.52, 144.45, 147.61, 147.87, 149.14, 150.67, 152.05, 154.15,
159.48, 168.84. Anal. Calcd for C38H34N2O2S: C, 78.32; H, 5.88;
N, 4.81; S, 5.50. Found: C, 77.96; H, 6.03; N, 4.55; S, 5.56.
The product was further purified via chromatography on silica
gel with a DCM/hexanes mixture as the eluent. The first band
collected from the column was evaporated to dryness and found to
contain not only the desired material but also a small amount of
unreacted AF-tosyl precursor. This material was further purified
by flash chromatography on Geduran 60 Å silica gel (63 µm) with
a DCM/hexanes mixture as the eluent. The first band collected from
the column no longer contained the unreacted precursor, only the
desired AFX-RSE. The fraction contained 420 mg (0.000294 mol)
of material, giving a yield of 43.3%. The compound was further
purified by recrystallization via diethyl ether diffusion into DCM
under an inert atmosphere. The compound was characterized by
1
UV-vis, IR, low-resolution ESI-MS, H NMR, and C, H, and N
elemental analysis. UV-vis (THF) (λmax in nm (ꢀ in M-1 cm-1)):
242 (5.52 × 104), 308 (4.93 × 104), 394 (7.81 × 104). IR (THF)
(νNO in cm-1 (ꢀ in M-1 cm-1)): 1752 (5.16 × 103), 1777 (4.97 ×
103). ESI-MS (20% CH2Cl2 in CH3OH): m/z 1427 ([(C38H33N2-
OS)2Fe2S2(NO)4] + H+)+, 714 ([(C38H33N2OS)2Fe2S2(NO)4] +
1
2H+)2+. H NMR (in d-THF, δ ) ppm, J ) Hz): 0.40 (m, 12H),
1.90-2.25 (m, 8H), 3.42 (t, 5.6 Hz, 4H), 4.27 (m, 4H), 6.65-8.15
(m, 38H). Anal. Calcd for (C38H33N2OS)2Fe2S2(NO)4: C, 63.96;
H, 4.63; N, 7.85. Calcd for (C38H33N2OS)2Fe2S2(NO)4‚CH2Cl2: C,
61.45; H, 4.46; N, 7.35. Found (Marine Science Institute analy-
tical lab at UCSB): C, 61.38; H, 4.65; N, 6.77. Our experience
with other RSEs is that they typically give low N values despite
considerable other experimental support for their overall purity.
Instruments. UV-vis absorption spectra were recorded using
a HP8572 diode array spectrophotometer or a Shimadzu dual beam
UV-2401 PC spectrophotometer and using high quality quartz UV-
vis cells (NSG Precision Cells Inc., Starna Cells) with 1 cm path
lengths. IR spectra were obtained on a BioRad FTS 60 SPC 3200
FTIR spectrometer. Solution IR spectra were recorded in sealed
cells (Reflex Analytical Inc, International Crystal Labs) with a fixed
path length of 0.05 or 0.02093 cm and CaF2 windows with Teflon
N-Phenyl-N-(3-(2-tosylethoxy)phenyl)-7-(benzothiazol-2-yl)-
9,9-diethylfluoren-2-ylamine AF-343-b (AF-tosyl). To a solution
of (3.5 g, 6 mmol) N-phenyl-N-(3-(2-hydroxyethoxy)phenyl)-7-
(benzothiazol-2-yl)-9,9-diethylfluoren-2-ylamine in pyridine (21
mL), cooled in an ice-bath, 4-toluenesulfonyl chloride (2.2 g, 12.1
mmol) was added. The mixture was then allowed to warm to room
temperature. After 6 h, the mixture was diluted with water, and
the separated orange solids (4.74 g) were collected. The product
mixture was chromatographed over silica gel and eluted with a 3:1
toluene-heptane mixture. The desired product was obtained in 73%
yield (3.24 g) after recrystallization from a 1:3 toluene-heptane
mixture, mp 153.4-155.7 °C. MS: m/z 736 (M+). 1H NMR
(CDCl3) δ ppm: 0.37 (t, 6H), 1.90-2.35 (m, 4H), 2.40 (s, 3H),
4.03-4.06 (m, 2H), 4.28-4.31 (m, 2H), 6.45-8.10 (m, 23H). 13
C
NMR δ ppm: 8.82, 21.85, 32.81, 56.63, 65.80, 68.24 (sp3C),
398 Inorganic Chemistry, Vol. 46, No. 2, 2007