fullerene interactions studied by spectral methods

Chemical Physics 305 (2004) 277–284 www.elsevier.com/locate/chemphys

Supramolecular porphyrin/fullerene interactions studied by spectral methods Andrzej Łapin´ski a, Andrzej Graja a,*, Iwona Olejniczak a, Andrzej Bogucki a, Hiroshi Imahori b b

a Institute of Molecular Physics, Polish Academy of Sciences, 60-179 Poznan´, Poland Department of Molecular Engineering, Graduate School of Engineering, Kyoto University, PRESTO, Japan Science and Technology Agency (JST), 615-8510 Kyoto, Japan

Received 25 May 2004; accepted 6 July 2004 Available online 31 July 2004

Abstract Solid-state electronic (UV–Vis) and vibrational (IR) spectra of the fullerene–zinc porphyrin dyad (1) as well as 5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)porphyrinatozinc(II) (2) and modified fullerene (3) were investigated in the frequency range between 400 and 50,000 cm 1. Variable temperature IR studies were also performed. Molecular geometry of the compounds and their IR theoretical absorption spectra were calculated using AM1 method. It was stated, that redistribution of the charges occurs in both fullerene and porphyrin moieties upon covalent linkage. This effect is mainly observed as shifts of both electronic and vibrational bands in comparison with the respective free porphyrin and fullerene features. Ó 2004 Elsevier B.V. All rights reserved. Keywords: Fullerene–porphyrin dyad; Chromophoric interactions; Vibrational spectra; Electronic spectra

1. Introduction Functionalized fullerenes linked to particular electro- or photoactive species have attracted much attention in the last few years. The unique shape of the fullerene C60 [1] combined with its distinct physical properties [2] make it a good candidate for the preparation and functionalization of large, supramolecular aggregates. The C60 is a very attractive electron acceptor with its ability to accept up to six electrons per molecule and unique three dimensional delocalized p-electron system. Due to the electron-accepting character combined with interesting photophysical properties, C60 is a very promising molecular block for building chromophoric species. *

Corresponding author. Tel.: +48-61-86-95-275; fax: +48-61-8684-524. E-mail address: [email protected] (A. Graja). 0301-0104/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.chemphys.2004.07.002

Design of organic molecules, that mimic photoinduced electron-transfer processes characteristic of photosynthesis, has recently involved cyclic tetrapyrrolic species, e.g., porphyrins or phthalocyanines as chromophores. After the first report on a C60 derivative covalently linked to a porphyrin [3], a significant number of chromophore–fullerene dyads and more complex molecules have been reported [4–10]. The nature and kinetics of intramolecular interactions resulting in photoexcitation of porphyrin–fullerene hybrids has been studied using fluorescence quenching and transient absorption measurements [11]. In one of the earlier works of the authors [12], a strong evidence for a high affinity between fullerene and porphyrin molecules was noticed. As a consequence, a new family of fullerene–porphyrin dyads linked via a pyrrolidine spacer has been synthesized [13]. In order to provide information on electronic and vibrational properties of selected fullerene–porphyrin

A. Łapin´ski et al. / Chemical Physics 305 (2004) 277–284

278

systems, we have performed spectral investigations of the fullerene–porphyrin dyad (1) consisting of the meso-tetraphenyl zinc porphyrin scaffold (2) with C60 moiety attached to by short pyrrolidine single linker (Scheme 1). Our overall goal is to understand the correlation between the structure of new fullerene-derived systems and their spectral properties recorded in the solid state. Results of the studies will be discussed and compared with similar investigations of the bis-linked tetrathiafulvalenes (TTFs) to C60 systems [14–16] and meso,meso-linked oligoporphyrin bearing one or two fullerene moieties [17].

with FT IR Bruker Equinox 55 spectrometer equipped with helium cryostat. The samples were dispersed in KBr pellets with concentration 1:1000–1:8000, depending on the band intensities. The PeakFit computer program was used for analysis of the complex absorption in the selected spectral regions. We focused on two spectral regions: the first one between 200 and 800 nm (50,000– 12,500 cm 1), where electronic excitations of the dyads are observed, and the second one between 400 and 1700 cm 1, where the intramolecular vibrations occur. Between these two regions only weak absorption bands typical for –CH groups are observed. With the help of the Gaussian 03 program [18], we performed normal mode analysis for 2–4 and 3,5-ditert-butylphenyl group (Scheme 1). Molecular geometry was optimized by the energy minimalization at the AM1 method with the default Gaussian parameters. The IR absorption spectra were calculated at the same level of approximation. The level of calculations used for smaller fragments of molecules was HF and DFT(B3LYP) with the 6-31G(d) basis set.

2. Experimental Fullerene–porphyrin dyad (1), consisting of porphyrin scaffold bearing C60 moiety, have been synthesized following the procedure previously reported by one of us [5,9,12,13]. For this dyad, the electronic (UV–Vis– NIR) and vibrational (IR) spectra were investigated in the solid state. Complementary spectral studies were performed on reference meso-tetraphenyl zinc porphyrin (2) and modified fullerene with a linker (3) (Scheme 1). Molecular geometry and normal vibrations of the compounds were calculated and compared to the experimental data. Pristine C60 have been also investigated as reference compound. Electronic absorption spectra between 200 and 2500 nm were recorded with Perkin–Elmer UV–Vis–NIR Lambda 19 spectrometer, at room temperature. Vibrational spectra in the frequency range 400–7000 cm 1 for temperatures ranging from 300 to 5 K were recorded

3. Results and discussion 3.1. Electronic spectra The UV–Vis absorption spectra of the compounds 1– 3 are shown in Fig. 1; the C60 spectrum is also shown for comparison. For 3, two strong and overlapping optical absorption bands at 262 and 328 nm are observed. They correspond to bands identified as the dipole-allowed transitions observed in pristine C60 at 264 and 340 nm,

CH3 N

Ar H3C(H2C)21O Ar N Ar

N

N CH3

N

Zn N

N Zn

Ar N

Ar N

N Ar

Ar

1

2

t-Bu Ar =

3

t-Bu

CH3 N H3C(H2C)21O

4 Scheme 1. Chemical structure of fullerene–porphyrin dyad (1), tetra Ar–zinc porphyrin (2), modified fullerene with pyrrolidine linker (3), and molecule 4.

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Absorbance

3

C60 1

2

300

400

500

600

700

800

Wavelength (nm)

Fig. 1. Absorption spectra in the UV spectral range of the dyad 1, reference compounds 2, 3 and C60, recorded in KBr pellets at room temperature.

respectively [19]. The bands of the fullerene moiety in 1 undergo hipsochromic shifts by 5 and 14 nm, respectively. The fullerene band at 340 nm is strongly blue-shifted and also broadened as in other fullerene adducts, e.g., in the bis-linked TTF–[60] fullerene derivatives [14–16] and meso,meso-linked oligoporphyrin bearing one or two fullerene moieties [17]. In the former derivatives, the shift between 2 and 20 nm depends on the number of linked TTF-derived groups. The electronic absorption spectrum of 2 is dominated by the very strong Soret band located at 428 nm, which corresponds to the allowed transition from the ground state to the two singlet upper states. This band results from a superposition of two electronic transitions with the reinforcing transition dipoles and it is therefore very intense [20,21]. Symmetrical Soret band suggests an absence of molecular aggregates observed, e.g., in meso,meso-linked oligoporphyrins [17]. Another band, commonly referred to as Q band, appears in metal porphyrins with D4h symmetry, like in 2, on the red side of the spectrum. The Q states are degenerate and one band at 550 nm is observed, usually with its vibronic satellite (very week band at 590 nm). According to Gouterman [20], the Q(0,0) band results from transition between the ground state and the lowest electronic state and the Q(1,0) is related to the transition between the ground state and the vibronic state involving the most active vibrations. Both Q bands result from transition dipoles nearly canceling each other, and therefore show relatively weak absorbance. In the spectrum of 2, there is also a very broad and weak band centered at about 295 nm. The wavelengths of the Soret and Q bands correspond to that of the bands typical for Zn(II) porphyrins [20,21]. The counterpart of the band at 295 nm in toluene solution is observed at about 305 nm and it is probably related to an electronic transition of the porphyrin monomer [17].

279

The very narrow and symmetrical Soret absorption band in spectrum 1 is bathochromically shifted to 437 nm (from 428 nm in 2), suggesting an electronic interaction between the porphyrin and fullerene moieties. Several covalently linked porphyrin–fullerene systems have shown comparable red shifts in organic solution [5,13,22] and in solid state [17]. The Q(1,0) band in this dyad also undergoes medium bathochromic shift by about 9 nm. Our earlier observations on both porphyrin- and TTF-containing fullerene-derived adducts [14–17] confirm, that the spectra of compounds derived from C60, have some common features, virtually not dependent on the attached molecular groups [23]. In spite of the covalent linkage and vicinity of both fullerene and porphyrin p-systems, the observed spectral differences can be probably ascribed to both through-bond and throughspace electronic interaction between the two chromophores [17]. However, it seems that through-bond interaction is more important because changes in the fullerene-related spectral features do not depend on the substituent and its spatial structure. 3.2. Vibrational spectra We start our investigations of the vibrational spectra of the fullerene–porphyrin dyads from an analysis of possible conformations of the porphyrin and its normal vibrations. The phenyl groups are perpendicular to the porphyrin plane in the spatially most convenient conformation D4h of the zinc tetraphenylporphyrin (TPP). However, a low symmetric arrangement can be also possible. For example, for a free TPP molecule in a solvent independent rotations of the phenyl groups within the range of 90 ± 25° can be expected [24]. Therefore, we take into consideration three different conformers with different angles / between the benzene ring plane in aromatic group of Ar and zinc porphyrin planes (Table 1). For the conformers with D4h and S4 symmetry, at semiempirical AM1 level the imaginary frequencies were obtained. It means that the system is in transition structure. In the case of C1 symmetry, the independent motion of the Ar group can stabilize the structure.

Table 1 List of conformers of the 5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)porphyrinatozinc(II) (2) Symmetry

/ 1a

D4h S4 C1

90 62 65

/2a 90 62 65

/3a 90 62 63

/4a 90 62 62

Energy (kcal/mol) 245.12b 244.81b 243.12

a Angles between the benzene ring plane in Ar group and zinc porphyrin planes (small deviations from planarity in the chromophore was neglected), see Scheme 1. b Transition structure, with two imaginary frequencies.

A. Łapin´ski et al. / Chemical Physics 305 (2004) 277–284

In order to analyze the experimental spectrum of 2 we calculated normal vibrations for the molecule 2 and also for its fragments: Ar and zinc meso-tetraphenylporphyrin (ZnTPP). We include Ar group into our calculation, because presumably it does not strongly interact with the core of the molecule 2 and hence do not significantly influence its optical activity. The frequencies and IR intensities of the normal modes of Ar group (C2v symmetry) were calculated at the AM1, HF/6-31G(d), and B3LYP/6-31G(d) levels of theory. For HF/6-31G(d) [25] and AM1, the scale factor of 0.8929 was used in order to match the experimental data. The calculated and experimental spectra are plotted in Fig. 2. The normal mode assignment for 2 is based on the comparison of calculated and experimental IR absorption spectra (Fig. 2, spectra (b) and (a), respectively). Because of the large number of normal modes (3N 6 = 513), vibrational assignment was based on comparison of simulated and experimental spectral peaks rather than on individual modes. Positions of bands and their shape were taken into our consideration. Dotted lines in Fig. 2 mark corresponding vibrations. In the mid-frequency region, quite realistic resemblance of both experimental and calculated spectra is observed. Comparison between the experimental spectrum Fig. 2(a) with the simulated spectra of Ar group Fig. 2(c) and ZnTPP Fig. 2(d) give supplementary information. Most of the modes in this region can be related to the deformation of porphyrin chromophore, as can be verified by the dynamic displacement visualization. The calculated vibrations below 800 cm 1 are observed at lower frequencies than in experiment, as observed for such ab initio modeling [24]. Fig. 3 shows the experimental (a) and theoretical (b) spectra of 3. The calculated spectrum Fig. 3(c) was

(c) Intensity

280

(b)

(a) 400

600

800

1000

1200

1400

1600

-1

Wavenumber (cm )

Fig. 3. Experimental IR absorption spectrum of 3 (a) and calculated spectra of 3 (b) and 4 (c).

obtained for the molecule 4. The normal mode analysis was performed using AM1 level of theory. The geometrical structure of 3 and 4 was fully optimized. The simulated spectra of 3 and 4 show similar vibrational features at almost the same frequencies. For example, such coincidence we observed for the following bands: 658, 782, 794, 1216, 1359, 1393, 1404, 1492, 1579, 1591, 2717, 2732, 2770, 2833 and 2855 cm 1. Based on their dynamic visualization we could identify bands, which are related mostly to the vibrations of benzene and/or pyridine rings (e.g., 599, 655, 792, 796, 876, 919, 981, 987, 1216, 1235, 1294, 1359, 1578, 1591, 2671, 2732 and 2855 cm 1) and to the vibrations of the deformed fullerene (e.g., 401, 512, 568, 697, 698, 758, 1341, 1516, 1549, 1553 and 1570 cm 1). The number of the IR active modes of modified C60 increase

Absorbance

Intensity

(d)

(c)

(b)

(a) 400

3

2

1 600

800

1000

1200

1400

1600

Wavenumber (cm-1)

400

600

800

1000

1200

1400

1600

Wavenumber (cm-1)

Fig. 2. Experimental IR absorption spectrum of 2 (a) and simulated spectra of 2 (b), Ar group (c), and Zn–TPP (d). Dotted lines between a and b mark corresponding vibrations.

Fig. 4. Infrared absorption spectra of the investigated compounds 1–3 recorded in KBr pellets at room temperature.

Table 2 Normal mode analysis of 1 based on the experimental and calculated spectra of 2 and 3 Experimenta (cm 1) 1 402vw 413vw 420w 430vw 464vw 479w 485vw 527vs 532sh

Calculationc (cm 1) 2

419w

532vw

3

C60b

402vw 413vw 420vw 430vw 464vw 479w 485vw 527vs 531sh

403

618vw 637vw 668vw 719m

553w 575w 583vw 598w 618vw 637vw 668w 719w

747w 767w 798vs

765vw 800s

746w 766w 797w

583vw

824m 840vw

824m

882w 900w 1003s 1067m 1122w 1130vw 1179w 1188w 1218w 1231w

882m 900w 1004m 1071w

1247m

441

485 526 563

441 459 479 488 512 518 544 568

575 595

597 610 613 668 712

692

764 796

741 773

655 722

760

796 799

794

744

1068vw 1122w 1129vw 1178w 1187w

1061 1121

1232sh

1237

1246m

1242

832 858 1020 1066 1113 1129

1175 1182

1219m

1247m

401 417

431

840w

1130vw

3

1145 1158 1210 1204 1241

1216

def., C60 def., C60; def., benzene ring in 4; def., N-methylopiridine in 4 in-plane def., zinc porphyrin def., C60; def., benzene ring in 4; def., N-methylopiridine in 4 def., C60; def., benzene ring in 4; def., N-methylopiridine in 4 def., C60; def., benzene ring in 4; def., N-methylopiridine in 4 def., C60; def., N-methylopiridine in 4; CH out of plane, benzene ring in 4 def. of C60 CH out of plane, phenyls groups; def., C60; def., N-methylopiridine in 4; CH out of plane, benzene ring in 4 def., C60; def., N-methylopiridine in 4; CH out of plane, benzene ring in 4 def., C60 CH out of plane, phenyls groups; in-plane def., zinc porphyrin def., N-methylopiridine in 4; def., benzene ring in 4 CH out of plane, phenyls groups CH out of plane, phenyls groups def., N-methylopiridine in 4; def., benzene ring in 4; def., C60 CH out of plane, phenyls groups; CH out of plane, zinc porphyrin; def., C60; def., N-methylopiridine in 4; def., benzene ring in 4 def., C60; def., N-methylopiridine in 4; CH out of plane, benzene ring in 4 in-plane def., zinc porphyrin CH out of plane, phenyls groups; in-plane def., zinc porphyrin; def., C60; def., N-methylopiridine in 4 in-plane def., zinc porphyrin CH out of plane, zinc porphyrin; CH out of plane, benzene ring in 4; def. of N-methylopiridine in 4 and C60 CH out of plane, phenyls groups; in-plane def., zinc porphyrin CH out of plane, phenyls groups; in-plane def., zinc porphyrin CH in-plane, zinc porphyrin CH in-plane, phenyls groups; breathing def., zinc porphyrin CH in-plane, benzene ring in 4; def., C60 CH in-plane, phenyls groups; CH bend, t-Butyl groups CH in-plane, benzene ring in 4; def., C60 CH in-plane, benzene ring in 4; def., C60 CH rocking, zinc porphyrin; CH in-plane, phenyls groups CH rocking, N-methylopiridine in 4; CH in-plane, benzene ring in 4; def., C60 CH in-plane, phenyls groups; CH bend, t-Butyl groups; in-plane def., zinc porphyrin; CH rocking, N-methylopiridine in 4; CH in-plane, benzene ring in 4; def., C60 (continued on next page)

A. Łapin´ski et al. / Chemical Physics 305 (2004) 277–284

553w 575w 583vw 599w 617vw 632vw 668w 718m

2

Approximate descriptiond

281

282

Table 2 (continued) Experimenta (cm 1)

Calculationc (cm 1) 2

3

C60b

2

3

1261m

1265w

1265vw

1260

1266

1234

1289w

1289m

1303

1258

1306vw 1338w 1362m 1393w 1425m

1306vw 1341w 1362m 1393w 1424m

1279

1392w 1428w

1394 1429

1325 1348 1379 1403 1420

1323 1341 1359

1447w 1463m

1447w 1463m

1463m

1470

1454 1469

1393

1476m 1495w 1506vw 1524vw 1540vw 1558vw 1569vw 1576w 1591m 2782w 2866m 2904m 2926s 2952vs 2960vs

1476m 1497w

a b c d

1487 1503 1510m

1525vw 1540vw

1509

1540vw 1559vw 1570vw 1577vw

1539 1556 1572 1576

2778w

2778

1592m

1516 1522 1544 1549 1553 1570

1607

2866m 2904m

2732 2740 2835

2920vs 2953vs 2962vs

1545 1566

2914, 2930

2855 2910 2921

Intensities: vs, very strong; s, strong; m, medium; w, weak; vw, very weak; sh, shoulder. IR data taken from [26]. Scaled by 0.8929. def. means deformation.

CH in-plane, phenyls groups; CH bend, t-Butyl groups; CH rocking, N-methylopiridine in 4; CH in-plane, benzene ring in 4; def., C60 CH in-plane, phenyls groups; CH bend, t-Butyl groups; CN stretching, zinc porphyrin CN and ZnN stretching, zinc porphyrin CN and ZnN stretching, zinc porphyrin CC stretching, zinc porphyrin CC stretching, zinc porphyrin; def., C60 CC stretching, phenyls groups; def., t-Butyl groups; CN and CH stretching, N-methylopiridine in 4; def., C60; def., benzene ring in 4 CC stretching, phenyls groups; def., zinc porphyrin CC stretching, zinc porphyrin; def., phenyls groups; CC stretching, benzene ring in 4; CC stretching, N-methylopiridine in 4; def., C60 CC stretching, zinc porphyrin CC stretching, zinc porphyrin def., C60 in-plane def., zinc porphyrin; def., C60 in-plane def., zinc porphyrin; def., C60 def., C60 def., C60 def., C60 CC stretching, phenyls groups CH stretching, N-methylopiridine in 4 CH stretching, t-Butyl groups CH stretching, phenyls groups CH stretching, benzene ring in 4 CH stretching, zinc porphyrin CH stretching, zinc porphyrin

A. Łapin´ski et al. / Chemical Physics 305 (2004) 277–284

1

Approximate descriptiond

A. Łapin´ski et al. / Chemical Physics 305 (2004) 277–284

Wavenumber (cm-1)

dyad 1. The bands at 527 and 575 cm 1, which also appear for pristine C60 and 3, correspond to T1u modes denoted as T1u(1) and T1u(2), respectively; they are related to radial deformation of the fullerene moiety [27]. The frequencies of these vibrations are unchanged upon formation of the dyad. The two remaining IR-active bands of C60, T1u(3) and T1u(4), which are related to its tangential deformations [27], are located at 1182 and 1428 cm 1, respectively. The former band has its counterpart at 1179 cm 1 in 1. On the other hand, the vicinity of the latter mode in C60 and the normal vibrational mode of the porphyrin scaffold at 1424 cm 1 (2) makes difficult to assign the T1u(4) mode in 1, which is probably located at about 1425 cm 1. So that, the tangential modes of the fullerene would undergo small, about 3 cm 1 shift and line shape deformation after functionalization of the C60 molecule. Similar effects also observed for other such dyads as [60]fullerene–TTF [14–16], meso,meso-linked oligoporphyrin bearing one or two fullerene moieties [17] and [60]fulleropyrrolidines [28]. It seems that tangential deformations of C60 in opposite to radial deformations are distinctly perturbed in modified fullerenes. Variable temperature IR spectra of the dyad 1 were measured at temperatures between 300 and 10 K. The bands chosen for analysis were decomposed in two or three components and their band parameters were estimated. Using Gaussian function, a couple of absorption bands were decomposed. Based on the temperature dependencies of band parameters, it was stated that all investigated bands keep their shape independent on temperature and their positions show very weak temperature dependence. A typical complex band recorded at 10 K and its decomposition is shown in Fig. 5. The wave numbers of the component bands a1, a2 and a3 show a very low temperature dependent shift to higher wave

Absorbance

strongly in comparison with four IR active vibrations of free fullerene molecule. Deformation of the spherical shape of C60 molecule caused by interactions with environment or chemical bonding with substituent is responsible for activation of so-called ‘‘silent’’ modes of the fullerene [26]. Infrared absorption spectra of the investigated compound 1 with its reference compounds 2 and 3 are shown if Fig. 4. Most of the bands attributed to vibrations of 2 can be found in the spectra of the dyad 1 at the same or slightly shifted frequencies. For example, the strongest and well-separated bands of 2 at 532, (719), 800, 824, 1004, 1071, (1247), (1265), 1362, 1424, (1463), 1476, 1592, 2866, 2904, 2953 and 2962 cm 1 (the wave numbers of the bands which coincide also with the bands of 3 are given in parentheses) have their counterpart in 1 at 532, 718, 798, 824, 1003, 1067, 1247, 1261, 1362, 1425, 1463, 1476, 1591, 2866, 2904, 2952 and 2960 cm 1. Many frequencies of the bands for 1 coincide not only with some normal vibrations of 2 but also with 3. Such strongest coinciding bands are located at 527, 575 and 2920 cm 1 (for other coinciding bands see also Table 2). Therefore, the assignment of the IR bands for 1 has been done following the analysis of 2 and 3 compounds. Because of the large number of normal modes, this assignment was based on comparison of simulated and experimental spectral peaks. The description of the normal modes based on their dynamic visualization is given in Table 2. The scale factor of 0.8929 was used for all simulated frequencies. Detailed computer output is available upon request. Similar vibrational spectra of various porphyrins and the band assignment have been reported in literature [24,29–33]. The shape of some bands of dyad 1 is changed in comparison with those in the adequate region of 2. For example, there is a complex, not very strong band centered at 767 cm 1 in 1, which has no counterpart in the spectra of 2 and 3. This mode is assigned to inplane deformation of Zn porphyrin and CH out of plane deformation of the Ar rings. For 1, we observe the band 1261 cm 1, which is related to the band at 1265 cm 1 for 2 and 3. In this region one can find the CH in-plane deformations, stretching of CN groups and deformation of hexagonal rings of both porphyrin and fullerene moieties. For stretching vibrations of CN groups in Zn porphyrin, a small frequency shift from 1341 cm 1 (2) to 1338 cm 1 (1) is observed. The C‚C and C–C stretching modes of 2 occurring at 1476 and 1497 cm 1 appear as a double band with maxima at 1476 and 1495 cm 1 in 1. We conclude that the formation of the fullerene–porphyrin dyad causes small changes in the electronic distribution on the porphyrin moiety, which affect its vibrational force constants observed as small shifts of the band position. Four bands at 527 (very strong and narrow), 575, 1179 and 1425 cm 1 were observed in the investigated

283

1348

a3

1344

a2

1340

a1

1336 1332

0

100

200

300

Temperature (K)

a1

a2

a3

1324

1328

1332

1336

1340

1344

1348

1352

-1

Wavenumber (cm ) Fig. 5. The band at 1338 cm 1 for 1 and its decomposition; the temperature dependence of the component wavenumbers is shown in the insert.

284

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numbers (1 or 2 cm 1/300 K) upon temperature rising (see inset in Fig. 5).

4. Conclusions Interpretation of the vibrational spectrum of the fullerene–porphyrin dyad 1 was performed based on the ab initio calculations. UV–Vis and IR studies of 1 performed in the solid state show the redistribution of charges in both fullerene and porphyrin moieties. Electronic and vibrational spectra of fullerene moiety are not significantly affected by the dyad formation; more distinct effect is observed for tangential than for radial vibrations of C60 moiety. On the other hand, a distinct evolution of the porphyrin spectra in the dyad 1 compared to porphyrin 2 suggests appreciable reorganization of the electronic structure of the porphyrin part. We observe that the electronic absorption bands undergo some shifts after dyad formation. The modifications in vibrational features are rather small. These spectral changes suggest a chromophoric interaction between the two moieties. IR spectra of the dyad show, that the investigated fullerene–porphyrin dyad 1 does not undergo temperature anomalies within the investigated range of temperatures.

Acknowledgements The work was supported by the Centre of Excellence for Magnetic and Molecular Materials for Future Electronics within the European Commission (Contract No. G5MA-CT-2002-04049). H.I. also thanks Grant-in-Aid from MEXT, Japan (21st Century COE on Kyoto University Alliance for Chemistry) for financial support.

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