photo-induced spin alignment utilizing the excited molecular field between the excited triplet state...

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Polyhedron 20 (2001) 1163 – 1168 Photo-induced spin alignment utilizing the excited molecular field between the excited triplet state of phenyl- or diphenylanthracene and the dangling nitroxide radicals: theoretical investigation of the mechanism for the intramolecular spin alignment Yoshio Teki Department of Material Science, Graduate School of Science, PRESTO JST, 3 -3 -138 Sugimoto, Sumiyoshi -ku, Osaka 558 -8585, Japan Received 17 September 2000; accepted 6 November 2000 Abstract In the previous paper [J. Am. Chem. Soc., 122 (2000) 984], we reported the first observation of the photo-excited quartet (S =3/2) and quintet (S =2) states arising from the radical-triplet pairs on the purely organic -conjugated spin systems. 9-Anthracen(4-phenyliminonitroxide) (1) and 9,10-anthracen-bis(4-phenyliminonitroxide) (2) were designed and synthesized. The time-resolved ESR (TRESR) experiments were carried out in order to study the photo-induced spin alignments on the excited states. In this paper, we report the ab initio molecular orbital calculations based on the density functional theory for the photo-excited quartet (S =3/2) state of 1 and the quintet (S =2) state of 2 as well as the brief summary of our TRESR experiments. The mechanism of the photo-induced spin alignment has been clarified based on the spin distribution obtained by the MO calculations. © 2001 Elsevier Science Ltd. All rights reserved. Keywords: Photo-induced spin alignment; Excited quartet state; Excited quintet state; Time-resolved ESR www.elsevier.nl/locate/poly 1. Introduction The studies on the spin alignment between the metastable excited triplet state and the stable radicals will give very important insights into the novel spin alignment, and this leads to a new strategy for the photo-induced magnetic spin systems. However, there are only few examples of the direct observation of excited high-spin states arising from the radical – triplet pair in the solid phase [1–5]. We have recently reported the first observation of the photo-excited quartet (S = 3/2) and quintet (S =2) states on the purely organic -conjugated spin systems [1]. In this work we have studied9-[4-(4,4,5,5-tetramethyl-1-yloxyimidazolin-2-yl)- phenyl]anthracene (1), 9,10-bis[4-(4,4,5,5-tetramethyl-1- yloxyimidazolin-2-yl)phenyl]anthracene (2) (Fig. 1) in which 9-phenylanthracene and 9,10-diphenylanthracene are chosen as the spin coupler and iminonitroxide is chosen as the dangling stable radical. In 1 and 2 the stable radicals have coupled strongly with the excited triplet state of the phenyl- or diphenylanthracene moi- ety upon photo-excitation, and their triplet moieties have played as a suitable photo-coupler which provided a ferromagnetic exchange interaction as reported in our previous paper [1]. In order to clarify the mechanism of the photo-induced ferromagnetic spin alignment, a de- tailed study of the electronic structures of the photo-ex- cited high-spin states will be necessary. Fig. 1. Molecules studied in this work. The photo-excited quartet and quintet states were observed for 1 and 2, respectively, by our previous TRESR experiments. E-mail address: [email protected] (Y. Teki). 0277-5387/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved. PII:S0277-5387(01)00589-7

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Polyhedron 20 (2001) 1163–1168

Photo-induced spin alignment utilizing the excited molecular fieldbetween the excited triplet state of phenyl- or diphenylanthraceneand the dangling nitroxide radicals: theoretical investigation of the

mechanism for the intramolecular spin alignment

Yoshio TekiDepartment of Material Science, Graduate School of Science, PRESTO JST, 3-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan

Received 17 September 2000; accepted 6 November 2000

Abstract

In the previous paper [J. Am. Chem. Soc., 122 (2000) 984], we reported the first observation of the photo-excited quartet(S=3/2) and quintet (S=2) states arising from the radical-triplet pairs on the purely organic �-conjugated spin systems.9-Anthracen(4-phenyliminonitroxide) (1) and 9,10-anthracen-bis(4-phenyliminonitroxide) (2) were designed and synthesized. Thetime-resolved ESR (TRESR) experiments were carried out in order to study the photo-induced spin alignments on the excitedstates. In this paper, we report the ab initio molecular orbital calculations based on the density functional theory for thephoto-excited quartet (S=3/2) state of 1 and the quintet (S=2) state of 2 as well as the brief summary of our TRESRexperiments. The mechanism of the photo-induced spin alignment has been clarified based on the spin distribution obtained bythe MO calculations. © 2001 Elsevier Science Ltd. All rights reserved.

Keywords: Photo-induced spin alignment; Excited quartet state; Excited quintet state; Time-resolved ESR

www.elsevier.nl/locate/poly

1. Introduction

The studies on the spin alignment between themetastable excited triplet state and the stable radicalswill give very important insights into the novel spinalignment, and this leads to a new strategy for thephoto-induced magnetic spin systems. However, there

are only few examples of the direct observation ofexcited high-spin states arising from the radical–tripletpair in the solid phase [1–5]. We have recently reportedthe first observation of the photo-excited quartet (S=3/2) and quintet (S=2) states on the purely organic�-conjugated spin systems [1]. In this work we havestudied 9-[4-(4,4,5,5-tetramethyl-1-yloxyimidazolin-2-yl)-phenyl]anthracene (1), 9,10-bis[4-(4,4,5,5-tetramethyl-1-yloxyimidazolin-2-yl)phenyl]anthracene (2) (Fig. 1) inwhich 9-phenylanthracene and 9,10-diphenylanthraceneare chosen as the spin coupler and iminonitroxide ischosen as the dangling stable radical. In 1 and 2 thestable radicals have coupled strongly with the excitedtriplet state of the phenyl- or diphenylanthracene moi-ety upon photo-excitation, and their triplet moietieshave played as a suitable photo-coupler which provideda ferromagnetic exchange interaction as reported in ourprevious paper [1]. In order to clarify the mechanism ofthe photo-induced ferromagnetic spin alignment, a de-tailed study of the electronic structures of the photo-ex-cited high-spin states will be necessary.

Fig. 1. Molecules studied in this work. The photo-excited quartet andquintet states were observed for 1 and 2, respectively, by our previousTRESR experiments.

E-mail address: [email protected] (Y. Teki).

0277-5387/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.PII: S 0277 -5387 (01 )00589 -7

Y. Teki / Polyhedron 20 (2001) 1163–11681164

Fig. 2. Typical TRESR spectra of 1 at 30 K in EPA rigid glassmatrix: (a) the observed spectrum of 1 at 1.0 �s after laser excitation;and (b) the simulation spectrum of the excited quartet state using thespin Hamiltonian parameters described in the text.

2. Method of calculations

The ab initio molecular orbital calculations werecarried out based on the density functional theory(DFT). In order to save the computing time, the energyminimum structures for the low-spin ground states andthe first excited high-spin states of 1 and 2 were fullyoptimized by semi-empirical molecular orbital calcula-tions (MNDO/AM1 method). The electronic structuresand the spin distributions were calculated using GAUS-

SIAN-98 for the optimized molecular structures [6]. Inthe ab initio calculations, the STO 6-31G basis sets andUHF Becke 3LYP (UBecke 3LYP) hybrid method wereemployed. The validity of the basis sets and the em-ployed method (UBecke 3LYP) for these calculationswere tested using whether or not the observed hyperfinesplitting due to the two nitrogen atoms in the groundstate of 1 are well reproduced.

3. Brief summary of our TRESR experiments [1]

TRESR spectra of the first excited states with re-solved fine-structure splitting have been observed for 1and 2 in EPA or 2 MTHF rigid glass matrix. Thetypical TRESR spectra of 1 and 2 were shown in Figs.2 and 3 together with simulations. The observedTRESR spectra for 1 and 2 have been unambiguouslyassigned as an excited quartet (S=3/2) spin state (Q)and the excited quintet (S=2) state (Qu), respectively.The g value and fine-structure parameters for the quar-tet state of 1 have been determined to be g(Q)=2.0043,D(Q)=0.0235 cm−1, E(Q)=0.0 cm−1. For the quin-tet state of 2, the spin Hamiltonian parameters havebeen determined to be g=2.0043, D=0.0130 cm−1,and E=0.0 cm−1. These are the first observations ofthe photo-excited quartet and quintet high-spin stateson the �-conjugated triplet-radical pair systems. Themagnitude of the fine-structure parameter, D, clearlyshows that the observed high-spin excited states areconstructed from the ��* excited metastable tripletstate of the spin coupler and the doublet spin of thedangling radical. Since a weak antiferromagnetic ex-change interaction was observed in the ground state of2,1 the clear detection of the excited high-spin quintetstate shows that the effective exchange coupling be-tween the two dangling radicals through the dipheny-lanthracene spin coupler has been changed fromantiferromagnetic to ferromagnetic by photo-excitation.Thus, the photo-induced spin alignment utilizing theexcited triplet molecular field has been realized for thefirst time on the purely organic �-conjugated spin sys-

Fig. 3. Typical TRESR spectra of 2 at 30 K in 2 M THF rigid glassmatrix: (a) the observed spectrum at 30 K and 1.0 �s after laserexcitation; and (b) the simulation of the excited quintet spectrumusing the spin Hamiltonian parameters described in text.

In this paper, we report the ab initio molecularorbital calculation of the photo-excited quartet andquintet states as well as the brief summary of ourprevious time-resolved electron spin resonance(TRESR) experiments. The photo-induced spin align-ment of 1 and 2 has been well interpreted based on thespin distribution obtained by the MO calculations.

1 From the temperature dependence of the ESR signal intensity ofthe ground state of 2, Jintra/kB values was determined to be −5.8�0.2 K (antiferromagnetic).

Y. Teki / Polyhedron 20 (2001) 1163–1168 1165

tem. The results obtained by the TRESR experimentsare summarized in Fig. 4.

4. Results and discussion

Fig. 5(a) shows the calculated total spin densities onthe carbon sites for the doublet ground state and theexcited quartet state of 1. The one-electron molecularorbital energies of NHOMO, SOMO, LUMO, etc. aregiven in Fig. 5(b). The total energies of the optimizedstructures are also given in Fig. 5(b). The schematicillustrations of the spin alignment, clarified on the basisof spin distribution are also shown. In the doubletground state, the unpaired spin is almost localized onthe iminonitroxide radical moiety. In the excited quar-tet state the anthracene moiety of phenylanthracene hasnearly symmetrical spin distribution. The almost twonet unpaired spins exist on the anthracene moiety andthe remaining one unpaired spin localizes on the dan-gling iminonitroxide. Thus, in the quartet state theexcited triplet state of the anthracene moiety couplesweakly to the dangling radical spin. Fig. 5(b) showsthat the excited quartet state is generated by the one-electron transition from the �-NHOMO (�) to the�-LUMO (�*) in the orbital-energy diagram of theground doublet state. These results obtained from thecalculation are consistent with our experimental results[1], in which the magnitude of the observed fine-struc-ture splitting has been well interpreted as the exchangecoupled system constructed from the ��* metastable

excited triplet state of anthracene and the danglingradicals. In the anthracene moiety, positive spin densi-ties overcome the negative spin densities. Only 11, 12,13, and 14 positions have negative spin densities andother carbon sites have positive spin densities. This spindistribution shows that spin delocalization overcomesthe spin polarization effect on the anthracene moiety.In contrast, the alternating sign of the spin density isrealized on the dangling phenyl groups of phenylan-thracene as shown in Fig. 5(a). This sign alternation ofthe spin densities shows that the spin polarizationmechanism overcomes the spin delocalization withinthe dangling phenyl groups even in the excited state.Thus, through the up-spin and down-spin networkestablished in the phenyl group, the unpaired spin onthe dangling radical couples ferromagnetically to theexcited triplet state localized on the anthracene moiety,leading to the excited quartet high-spin state. The en-ergy separation between the excited quartet state andthe doublet ground state was calculated to be 2.17 eVfrom the total energies of these states shown in Fig. 5.This magnitude is close to the energy separation be-tween the excited triplet state and the ground state ofanthracene.

The calculated spin distributions of 2 and the sche-matic illustrations of the spin alignment are shown inFigs. 6 and 7 for the excited quintet state and theground triplet state. The one-electron molecular orbitalenergies of NHOMO, SOMO, LUMO, etc. are alsogiven in Fig. 6(b). In this molecule the triplet statelocates very closely above the singlet ground state. The

Fig. 4. Brief summary of the results obtained by our previous TRESR experiments [1]. The photo-excited quartet and quintet states were observedfor 1 and 2 as the lowest photo-excited spin states, respectively.

Y. Teki / Polyhedron 20 (2001) 1163–11681166

Fig. 5. The spin distributions and the schematic illustrations of the spin alignment in the ground state and the excited quartet state of 1: (a) thespin distributions and the schematic illustrations (arrows) of the spin alignment based on the spin distribution; and (b) the one-electron molecularorbital energy diagrams and the total energies (Etot). The occupied �- and �-orbitals are shown by the up and down arrows, respectively.

Fig. 6. The spin distributions and the schematic illustrations of the spin alignment in the low-lying triplet ground state and the excited quintet stateof 2: (a) the spin distributions; and (b) the one-electron molecular orbital energy diagrams and the total energies (Etot). The occupied �- and�-orbitals are shown by the up and down arrows, respectively.

Y. Teki / Polyhedron 20 (2001) 1163–1168 1167

Fig. 7. The numerical spin distributions and the schematic illustrations (arrows) of the spin alignment in the low-lying triplet ground state closelylocated above the singlet ground state and the excited quintet state of 2: (a) the low-lying triplet ground state; and (b) the excited quintet state.

total energies of the optimized structures were alsogiven in Fig. 6(b). The energy separation between thelow-lying triplet state and the singlet ground state isdetermined experimentally to be approximately 8 cm−1

from the temperature dependence of cw-ESR intensity.1

Therefore, the energy separation between the excitedquintet state and the singlet ground state was calculatedto be 1.34 eV from the total energies of these statesshown in Fig. 6(b). Fig. 6(b) shows that the excitedquintet state is generated by the one-electron transitionfrom the �-NHOMO (�) to the �-LUMO (�*) in theorbital-energy diagram of the ground state. Similar tothe excited quartet state of 1, Figs. 6(a) and 7 show thatthe spin delocalization overcomes the spin polarizationeffect on the anthracene moiety in the excited quintetstate of 2. The sign alternation of the spin densities isalso realized within the dangling phenyl groups in thediphenylanthracene spin coupler, showing that the spinpolarization mechanism overcomes the spin delocaliza-tion within the dangling phenyl groups even in theexcited state. The large positive spin densities at the 9and 10 positions are realized by the spin delocalizationin the anthracene moiety. Each dangling radical spin(S=1/2) couples ferromagnetically to the large positivespin of the excited triplet state localized on the anthra-cene moiety, through the spin polarization in thephenyl. This mechanism leads to the ferromagneticexchange between excited triplet state and radical spinsin 1 and 2. As a result of such a ferromagnetic couplingbetween the triplet state and each dangling radicals, theeffective ferromagnetic spin coupling between two dan-gling radicals was realized in this system. Therefore, thesign of the effective spin coupling of the dangling

radicals can be changed from antiferromagnetic to fer-romagnetic by the exchange coupling through thetriplet excited molecular field generated by the photo-excitation. These findings from the ab initio molecularorbital calculations are consistent with all the experi-mental results and the schematic diagrams of the spinalignment shown in Fig. 4.

5. Conclusions

The mechanism of the spin alignment on the excitedstates of 1 and 2 has been clarified in details by the abinitio molecular orbital calculations based on DFT. Inthe anthracene moiety of the spin couplers, the spindelocalization effect overcomes the spin polarizationeffect on the photo-excited high-spin states of thesemolecules. This leads to the sign inversion of the effec-tive exchange interaction between two dangling radicalsin the quintet state of 2 induced by the photo-excitation.

References

[1] Y. Teki, S. Miyamoto, K. Iimura, M. Nakatsuji, Y. Miura, J.Am. Chem. Soc. 122 (2000) 984.

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[3] (a) K. Ishii, J. Fujiwara, Y. Ohba, S. Yamauchi, J. Am. Chem.Soc. 118 (1996) 13079. (b) K. Ishii, J. Fujisawa, A. Adachi, S.Yamauchi, N. Kobayashi, J. Am. Chem. Soc. 120 (1998) 3152.

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[4] P. Ceroni, F. Conti, C. Corvaja, M. Maggini, F. Paolucci, S.Roffia, G. Scorrano, A. Toffoletti, J. Phys. Chem. A 104 (2000)156.

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