Photophysical properties of fullerene-dendron-pyropheophorbide supramolecules

Chemical Physics 301 (2004) 27–31 www.elsevier.com/locate/chemphys

Photophysical properties of fullerene-dendronpyropheophorbide supramolecules €der E.A. Ermilov a, S. Al-Omari a, M. Helmreich b, N. Jux b, A. Hirsch b, B. Ro a

a,*

Photobiophysik, Institut f€ ur Physik, Humboldt-Universit€at zu Berlin, Newtonstr. 15, D-12489 Berlin, Germany Institut f€ur Organische Chemie, Universit€ at Erlangen-N€urnberg, Henkestr. 42, D-91054 Erlangen, Germany

b

Received 21 November 2003; accepted 24 February 2004 Available online 19 March 2004

Abstract Two novel monofullerene-bis(pyropheophorbide a) complexes were synthesized and their photophysical properties were studied by using both steady-state and time-resolved techniques. It was revealed that in the pyropheophorbide a (pyroPheo)-C60 molecular system (FP1) strong quenching of the first excited singlet state of the pyroPheo and, as result, dramatically decreasing of photosensitized singlet oxygen generation occurs by efficient photoinduced electron transfer to the fullerene molecule with a rate constant of 2.5
109 s1 . In contrast, the fullerene hexaadduct-bis(pyroPheo) system (FHP1), which possesses five diethyl malonate addends in the remaining octahedral positions, shows a high singlet oxygen quantum yield which is due to the reduced fullerene chromophore which is not a good electron acceptor anymore. Ó 2004 Elsevier B.V. All rights reserved. Keywords: Photosensitizer; Pyropheophorbide a; Fullerene; Photoinduced electron transfer; Singlet molecular oxygen

1. Introduction Since the initial discovery of C60 and the development of a method for its preparation, the fullerenes have been used in the design of novel complex molecular systems for different applications [1–5]. The high capability of C60 to act as an electron acceptor or even as an electron accumulator has led to the synthesis of a large number of compounds in which the fullerene is covalently linked to photoactive groups serving as potential donors. Such compounds are of interest as model systems for artificial photosynthetic reactions centers [4–9]. It was also shown that C60 can be used as versatile building block for the construction of globular dendrimers [10–12]. According to these facts, it should be additionally mentioned that the special properties of the fullerenes make them very promising molecular system for biomedical applications, like, e.g., in photodynamic therapy (PDT). For example, C60 is able to efficiently generate singlet oxygen due to a *

Corresponding author. Tel.: +49-30-2093-7625; fax: +49-30-20937666. E-mail address: [email protected] (B. R€ oder). 0301-0104/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.chemphys.2004.02.015

practically unity value of intersystem crossing from excited singlet state to the first excited triplet state. Moreover, biologically active molecules can be linked to fullerenes [13,14]. The aim of the present work is to prepare novel C60 dendron-dye supramolecules and to investigate their photophysical properties. We focused on the development of two different types of complexes: one exhibiting efficient electron transfer and another showing high energy transfer rates to molecular oxygen. As fluorophore, the well-known photosensitizer pyropheophorbide a (pyroPheo) which can also act as an electron donator [15] was chosen.

2. Materials and methods 2.1. Chemicals and preparation of the samples The syntheses and spectroscopic data of P1, P2, FP1 and FHP1 are described elsewhere [16]. PyroPheo was obtained by in situ hydrolysis and decarboxylation of pheophorbide a according to the

28

E.A. Ermilov et al. / Chemical Physics 301 (2004) 27–31

literature protocol [17,18]. Due to the inherent sensitivity of pyroPheo to light and oxygen all reactions needed to be carried out in the dark and under an inert atmosphere. Therefore, it also seemed reasonable to introduce the dye at the very end of the syntheses to avoid loss of material. For the synthesis of FP1 C60 was reacted with a malonate unit connected to a tert-butyldimethylsilyl (TBDMS) protected octane-1,8-diol. Utilizing well-known fullerene chemistry this malonate was added to C60 and deprotected to give a fullerene diol compound. PyroPheo was subsequently attached to this material by a modified Sheehan-coupling [19] to give compound FP1 (see Fig. 1(a), top). The octyl chain serves as a spacer unit. P1 and P2 were synthesized as reference compounds using similar coupling conditions (Fig. 1(a), bottom). All compounds were completely characterized by 1 H/13 C NMR, UV/Vis and IR spectroscopy and FAB mass spectrometry [16]. All samples were dissolved in DMF at dye concentrations of 106 –107 M to reach an absorbance of 0.2 at the excitation wavelength.

citation and a polychromator with a CCD matrix as a detector system (Lot-Oriel, Instaspec IV) [20]. 2.3. Fluorescence decay Fluorescence lifetimes were measured by time-correlated single photon counting technique, using the frequency doubled pulses of a Ti:Sapphire laser (Coherent Mira 900, 405 nm, FWHM 200 fs) for excitation. The instrument response function was 100 ps, as measured at excitation wavelength with Ludox. The set up was previously described in [21]. 2.4. Singlet oxygen generation Photosensitized generated time-resolved singlet oxygen luminescence was measured at 1270 nm. A nanosecond Nd–YAG laser (BMI) equipped with an OPO (BMI) was used to excite the samples at 510 nm and luminescence signal was recorded by the germanium pin diode (Northcoast) [22]. To calculate the singlet oxygen quantum yield, D, the solution of the pheophorbide a in DMF was used as reference (UD ¼ 0:52 [22]).

2.2. Absorption and steady-state fluorescence The ground state absorption spectra were recorded using a commercial spectrophotometer Shimadzu UV 160A at room temperature. Emission spectra were measured in 1 cm
1 cm quartz optical cells using a combination of a cw-Xenon lamp (XBO 150) and a monochromator (Lot-Oriel, bandwidth 10 nm) for ex-

Si

malonyl dichloride,

O

OH

O Si

O 7

O Si

pyridine, CH2Cl2

O 7

O O Si 7

O

HO HCl, EtOH

DBU, toluene

O

O

O

7

The absorption spectra of P1, P2 and FP1 in DMF are shown in Fig. 2(a). The shape of the absorption spectra of all samples is very similar. Absorption max-

C60, CBr4,

O Si 7

O

O

O

O

O

3. Results and discussion

OH 7

O

pyroPheo, EDC, 1-HOBT,

Si

O 7

O

O

O

O

O Si 7

Si

O 7

diethylmalonate, CBr4, DMAP, DMF, 0˚C

X

O

O

X

X

N

H N

O

N

O O

7

O

O

O

N H N

O 7 O

O

HO N

7

H N

DMAP, DMF, 0˚C X

O

X=

N H H N

octan-1,8-diol, DCC, DMAP

N

N H

N

OH

H N

O O

malonyl dichloride, pyridine, CH2Cl2

(a)

N

N

O

N

N H H N

O

N

O

P1

O

7

O 7 O

N

P2 O

O

O 7

O X

O N H

O

O

O 7

O

X

X=

O O

N O

H N

O

X

X

X N

H N

O

O

O

N

O

N H

O O

O

OH

O O

N H H N

O

pyroPheo, EDC, 1-HOBT,

X

X

O

FP1

N

O

OH 7

O

X

X

O O

O

O

HCl, EtOH

O

X

X

DBU, DMA,CH2Cl2

X= N H

O Si 7

O

O O

FHP1

(b)

Fig. 1. (a) Top: synthesis of FP1; bottom: syntheses of reference compounds P1 and P2. (b) Synthesis of FHP1.

N

E.A. Ermilov et al. / Chemical Physics 301 (2004) 27–31

Table 1 Fluorescence lifetime ðsÞ, relative (to P1) fluorescence quantum yield ðuÞ and singlet oxygen quantum yield ðUD Þ of P1, P2, FP1 and FHP1 in DMF

200000 2

ε, M-1cm-1

150000

100000 4 50000

29

Sample

s (ns)

u

UD

P1 P2 FP1

7.0  0.1 5.2  0.1 4.7  0.1 0.5  0.1 5.2  0.1

1 0.77  0.02

0.50  0.05 0.43  0.05

0.09  0.02 0.77  0.02

0.03  0.05 0.43  0.05

FHP1

3 1 300

(a)

400

500 600 Wavelength, nm

800

1

1.0

0.8 Relative intensity

700

2

0.6

0.4

0.2 3 0.0 600

(b)

650

700

750

800

Wavelength, nm

Fig. 2. (a) Electronic absorption spectra in DMF at room temperature of P1 – 1, P2 – 2, FP1 – 3 and FHP1 – 4. (b) Fluorescence emission spectra of P1 – 1, P2 and FHP1 – 2, FP1 – 3 in DMF. kexc ¼ 408 nm, OD ¼ 0.2 for all samples.

ima are at 320, 413, 509, 538, 611 and 667 nm for all samples, but in case of FP1 an additional shoulder at the blue edge of the absorption spectrum appears as a result of the fullerene chromophore contribution. However, the covalent coupling between P2 and C60 results in strong changes in the absorbance abilities of the FP1 compound. It is seen, that FP1 has practically two times lower absorbance than P2. The difference between the absorption spectra of P1 and P2 can be very easily explained by taking into account the different number of the pyroPheo molecules in these compounds (one and two, respectively). The fluorescence of all samples originates from the pyroPheo moiety (Fig. 2(b)). The fluorescence intensity of P2 is slightly lower than that of P1. In contrast to P1 and P2 samples, for FP1 a strong quenching of the fluorescence signal was observed. The estimations have shown that the fluorescence quantum yield of FP1 is 8.6 and 11.2 times smaller than the fluorescence quantum yields of P2 and P1, respectively (see Table 1).

For P1 and P2 time-resolved measurements of the fluorescence decay have shown a single exponential depopulation of the first excited singlet states with a lifetime of 7.0 and 5.2 ns, respectively (see Fig. 3). For FP1 fluorescence decay showed a higher order. A double exponential fit led to two decay times: s1 ¼ 4:7 ns and s2 ¼ 0:5 ns. The amplitude of the slow component is approximately two times higher than that of the fast one. This fast deactivation of the first excited singlet state of FP1 should result in a dramatic decrease of the ISC quantum yield and a reduction of the photoinduced singlet oxygen generation. Using the time-resolved experiments with detection of the singlet oxygen luminescence we obtained a quantum yield of singlet oxygen generation of UD ¼ 0:03 for FP1 in DMF whereas for P1 and P2 compounds this value was 0.5 and 0.43, respectively. Weak interactions between the two pyroPheo molecules linked to the one chain in P2 may explain the difference in the experimental data observed for P2 and P1 compounds. As it was mentioned above, C60 is a very good electron acceptor whereas pyroPheo can act as an electron donor. Therefore, the observed quenching of FP1 fluorescence and the very low value of singlet oxygen quantum yield might originate from a photoinduced electron transfer (ET) process from the pyroPheo

1

Intensity (normalized)

0

1 2

0.1

3

0

3

6

9

12

Time, ns Fig. 3. Fluorescence decay of P1 – 1, P2 and FHP1 – 2, FP1 – 3 in DMF. kexc ¼ 405 nm, kreg ¼ 680 nm. OD ¼ 0.2 for all samples at excitation wavelength.

E.A. Ermilov et al. / Chemical Physics 301 (2004) 27–31

moieties to the fullerene molecule. Another possibility to explain the experimental data for FP1 is to take into account the presence of the singlet–singlet electronic energy transfer (EET) process between pyroPheo and C60 molecules. However, our investigations have shown that the last of the above-mentioned processes does not occur in FP1. The reason for this is that the energy of the first excited singlet state of C60 is higher than that of the S1 –S0 transition of a pyroPheo molecule (2.0 eV for C60 [23] and 1.84 eV for pyroPheo – estimated from absorption and fluorescence spectra). Moreover, an EET should result in a very efficient occupation of the first excited triplet state of the fullerene chromophore followed by photosensitized singlet oxygen generation. To prove the presence of the ET between excited pyroPheo molecules and C60 the forward free energy for charge separation, DG0 , in the polar solvent DMF has been computed using the well-known Rehm–Weller equation [24–26] e2 ox red  DG0 ¼ E1=2 ð D=Dþ Þ  E1=2 ð A=A Þ  E0;0  4pe0 es R

 e2 1 1 1 1  þ  ; 8pe0 rd þ ra eref es

ð1Þ

ox ðD=Dþ Þ is the half-wave oxidation potential where E1=2 red of the donor molecules (pyroPheo); E1=2 ðA=A Þ is the half-wave reduction potential of the acceptor (fullerene);  E0;0 is the first excited singlet state–ground state transition energy of the donor; e is the charge of the transferred electron; e0 is the vacuum permittivity; es and eref are the dielectric constants of the solvent used for photophysical studies (DMF) and for the measurements of the redox potential, respectively; R is the separation distance between donor and acceptor molecules; rd þ and ra are effective radii of the cation and the anion radical. According to Eq. (1) and using the following data: ox red  E1=2 ¼ 0:42 V [15], E1=2 ¼ 0:52 V [2,27,28], E0;0 ¼ 1:84


 þ eV, ra ¼ 4:4 A [26], rd ¼ 4 A and R ¼ 12 A (these last two values were estimated using the HyperChem program package [29], es ¼ 37 we have calculated the value of DG0 ¼ 0:9 eV. This negative value of DG0 shows that the excited donors undergo exothermic ET reactions with the fullerene. The Marcus first order ET rate constant kET was computed from [2] " # 2 2p J2 ðDG0 þ kÞ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi exp  kET ¼ ; ð2Þ h 4pkB T k  4kB T k

where J is the transfer integral, which exponentially depends on the separation distance R and may be expressed as [30] J 2 ¼ J02 exp ½  bð R  ra  rd þ Þ;

ð3Þ

b is attenuation coefficient with R; kB and  h are the Boltzmann and Planck constants, respectively; T is the

absolute temperature; k is the reorganization energy, which can formally be divided into an intramolecular kI , part and a contribution from inertial solvent reorganization, kS [31] k ¼ kI þ kS :

ð4Þ

As shown in [32,33] kI , can be chosen as 0.3 eV, whereas the kS value was calculated by using the usual approximation [34,35]    e2 1 1 1 1 1 kS ¼  ; ð5Þ  þ 2ra 2rd þ R 4pe0 n2 es where n is the refractive index of the solvent (n ¼ 1:43 for DMF). According to Eqs. (1)–(5) and using J0 ¼ 100 cm1 ,
1 [36] we have obtained a value of kET ¼ b¼1 A 2:5
109 s1 , which is in good agreement with the experimentally observed fast component of the fluorescence decay of FP1 in DMF (1=s2 ¼ 2
109 s1 ). We also calculated the energy of charge-separated state,  ECS ¼ E0;0  DG0 . It turned out that this energetic level (ECS ¼ 0:94 eV) is lower than all excited states both of the pyroPheo units and the fullerene moiety (see Fig. 4). These results indicate the primary role of the ET process in depopulating the first excited singlet state of pyroPheo molecules in FP1 and can explain the very low value of the photoinduced singlet oxygen generation quantum yield of FP1 in DMF. Therefore, FP1 cannot be applied as a modular carrier system for PDT. Obviously, it is necessary to prevent the ET processes between dye and fullerene moieties. It was shown [12] that addition of six malonates to the fullerene molecule causes a dramatic decrease of the electron-accepting properties of C60 . Due to the octahedral hexa-addition pattern, the fullerene p-system is broken up into a system of eight separate benzene rings which does not act as an electron acceptor anymore. Therefore, the new compound (FHP1, see Fig. 1(b)) was designed, synthesized and its photophysical properties were examined. The aforementioned TBDMS-protected fullerene-diol was reacted with diethyl malonate and

2.0

P-S1C60

S1

P-C60 P-T1C60 kISC

T1

Energy, (eV)

30

P-C60

kCS P•–-C60• +

1.0

kexc kT

kS kCR P-C60

0.0

Fig. 4. Scheme of energy levels of the FP1 molecular system solved in DMF and transitions between them.

E.A. Ermilov et al. / Chemical Physics 301 (2004) 27–31

carbon tetrabromide under conditions optimized for a C60 hexaaddition to give the fullerene hexaadduct. After deprotection and coupling with pyroPheo, the desired FHP1 molecule was obtained. All compounds were completely characterized. The shape of the absorption spectrum of FHP1 in DMF is similar to that of FP1 in the same solvent (Fig. 2(a)), but the intensities of the absorption bands are much higher and nearly reach those of P2. Steadystate and time-resolved fluorescence experiments have shown that there is no difference in the intensity of the fluorescence signal as well as the lifetime of the first excited singlet state between P2 and FHP1 in DMF (Figs. 2(b) and 3). Moreover, the quantum yield of photosensitized singlet oxygen generation for FHP1 has the same value as was found for P2 (UD ¼ 0:43, see Table 1). These results reflect the complete absence of any ET or EET processes between excited pyroPheo and fullerene molecules in FHP1.

4. Conclusions The results presented in this study have shown that it is possible to prepare pyroPheo-fullerene supermolecules which have good solubility in DMF. In FP1, the photosensitized generation of singlet oxygen is strongly reduced due to ET processes between the pyroPheo and the C60 units resulting in a fast degradation of the first excited singlet state of pyroPheo. If the p-system of the fullerene is broken up by hexaaddition, the C60 moiety only acts as a neutral attachment and therefore, the FHP1 dyad shows the same photoactivity as P2. We believe that these fullerene-dendron-dye structures can be very promising molecular systems for different applications such as synthesis of efficient photoconducting and biomedical materials. Currently, we are expanding the FHP1 motif to larger systems utilizing dendrimers as multiplying units.

Acknowledgements The authors would like to thank the DFG (E.A.E., B.R.: Grant No. RO 1042/11-1; B.R., S. al O.: Grant No. RO 1042/8-3; N.J., A.H.: Grant No. HI 468/11-1) and Fonds der Chemischen Industrie (M.H.: doctoral stipend) for financial support. They also thank Mrs. Gisela Woehlecke for technical assistance. References [1] H. Imahori, Y. Sakata, Adv. Mater. 9 (1997) 537. [2] H. Imahori, Z. Mori, Y. Matano, J. Photochem. Photobiol. C 4 (2003) 51.

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