Photophysical and Photochemical Processes of Excited Singlet and

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Photophysical and Photochemical Processes of Excited Singlet and Triplet [3]Cyclophanes (n = 2–6) Studied by Emission Measurements, Steady-state and Laser Flash Photolyses n

Minoru Yamaji, Hideki Okamoto, Yuhko Hakoshima, and Teruo Shinmyozu J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/jp511105v • Publication Date (Web): 16 Feb 2015 Downloaded from http://pubs.acs.org on February 18, 2015

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The Journal of Physical Chemistry

Photophysical and Photochemical Processes of Excited Singlet and Triplet [3n]Cyclophanes (n = 2–6) Studied by Emission Measurements, Steady-State and Laser Flash Photolyses Minoru Yamaji,*†Hideki Okamoto,§ Yuhko Hakoshima‡ and Teruo Shinmyozu‡,††

† Division of Molecular Science, Graduate School of Science and Technology, Gunma University, Kiryu, Gunma 376-8515, Japan, § Division of Earth, Life, and Molecular Sciences, Graduate School of Natural Sciences and Technology, Okayama University, Okayama 700-8530, Japan ‡ Department of Chemistry, Graduate School of Sciences, Kyushu University, Fukuoka 812-8581 †† Institute for Materials Chemistry and Engineering (IMCE), Kyushu University, Fukuoka 8128581, Japan RECEIVED DATE (to be automatically inserted after your manuscript is accepted if required according to the journal that you are submitting your paper to) Abstract Photophysical and photochemical features of [3n]cyclophanes (3nCPs) (n = 2–6) in solution were investigated by emission and transient absorption measurements. The studied 3nCPs show excimer fluorescence without locally excited fluorescence while some of them emit excimer phosphorescence in rigid glass at 77 K. Probability of excimeric phosphorescence from transannular π-electron systems was shown to strictly depend on the symmetric molecular structures. A feature of * To whom correspondence should be addressed. E-mail: [email protected] (MY) ACS Paragon Plus Environment

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intersystem crossing from an excimeric fluorescence state to the excimeric triplet state was 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

observed. Transient absorption spectra obtained upon laser flash photolysis of 3nCP revealed formation of the triplet excimer states. Triplet sensitization of 33CP using xanthone as the sensitizer demonstrated formation of triplet 33CP via triplet energy transfer while from the xanthone ketyl radical formation, it was inferred that triplet xanthone undergoes H-atom abstraction from 32CP, providing a benzylic 32CP radical as the counter species. Based on kinetic and spectroscopic data obtained upon laser flash photolysis, differences in photochemical reactions of triplet xanthone between 32CP and 33CP were discussed

Key words; triplet excimer, excimer fluorescence, transient absorption, hydrogen abstraction, triplet energy transfer

Introduction A large number of spectroscopic investigations have been carried out to understand the nature of singlet excimer1 since the first report of excimer fluorescence from pyrene in solution.2 The preferred molecular configuration of the singlet excimer has been revealed to be parallel-arranged chromophores by using diarylalkanes, known as the Hirayama rule.3 It seems that the structural requirements and dynamics of singlet excimers have been established. Conversely, in these four decades, presence of triplet excimer in intramolecular and intermolecular systems has been alleged.454

Lim et al. have published a review on theoretical and experimental works to characterize triplet

excimer of naphthalene derivatives in solution.16 One of their main conclusions on triplet excimer is that the geometry of the naphthalene triplet excimer is completely different from that of the corresponding singlet excimer where the two naphthalene moieties are arranged in parallel to each other. The favored geometry of triplet excimer is suggested by employing, so called, Agosta dimer25 and dinaphthylmethane to be an L-shaped arrangement of the two naphthalenes whose long axes are parallel and short axes make an angle of about 109˚. Nickel and Rodrigez Prieto, however, could not find any excimeric phosphorescence of naphthalene and 1,n-di-α-naphthalenes in solution.14,17

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Negative results on triplet excimer formation in solutions of 1-bromonaphthalene16 and 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

naphthalene18 were successively reported. Recently, a number of reports have appeared on inter- and intramolecular triplet excimers of carbazolyl chromophores and dinaphthyl compounds containing heteroatoms in fluid and rigid media.19–24 It seems that the triplet excimer problem is still far from being settled. Recently, triplet exciplexes and excimers including charge transfer interactions are of great importance in current industrial and academic developments toward organic photovoltaic materials.55,56 For instance, perylene-bisimide dimers have been put under attention in relation to the excimer research fields.57-60 Most of the intramolecular systems subjected for triplet excimeric emission research have been designed to be bichromophoric. The aromatic moieties are tethered with one oligomethylene chain in order to take the conformation appropriate for the triplet excimeric interactions by molecular motion in solution at room temperature where phosphorescence measurement is unfavorable. In a rigid matrix at low temperature, detecting triplet excimer emission from such molecular systems is substantially less expected due to rigidity of media that does not allow for aromatic moieties to set for the triplet excimer interaction. In some cases, transient absorption measurements by laser photolysis techniques at room temperature have been carried out to characterize triplet excimeric states11–13,19–23 since the absorption is not affected by the presence of small quantities of strongly luminescent impurities. The transient absorption spectra of excimer triplets have been assigned by subtracting that of a monomeric triplet from the whole spectrum observed. To understand the nature of triplet excimer, it is desirable to simultaneously elucidate both the emission and absorption by using well-designed molecular systems. In this context, we have synthesized and subjected cyclophanes having two aromatic chromophores linked with two oligomethylene chains to study the deactivation processes in the photoexcited singlet and triplet states by emission and transient absorption measurements.61-63 We have been investigating photochemical reactions of cyclophanes tethering two benzene rings with various numbers of the oligomethylene chain, -(CH2)3- that enables to keep the chromophores parallel (Scheme 1).64-68


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In the present study, we have performed spectroscopic investigations of 3nCPs by measurements 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

of emission and transient absorption, and revealed the photophysical properties of singlet and triplet excimeric states. Photochemical processes of 32CP and the related intermediates are studied by laser photolysis techniques and DFT calculations.

Scheme 1. Structures of [3n]cyclophanes (3nCP; n = 2–6).

Experimental Section [3n]Cyclophanes (3nCP; n = 2–6) were prepared according to the literature reported previously.64-72 Xanthone (XT) was recrystallized from ethanol before use. Acetone (Ac) was purified by distillation while cyclohexane (CH, Aldrich, Spectroscopic grade), dichloromethane (Dojin, Uvasol), methylcyclohexane (MCH, Dojin, Uvasol) and isopentane (IP, Merck, Spectroscopic grade) were used as supplied. CH, CH2Cl2 and Ac were used as the solvent at room temperature whereas a mixture of MCH and IP (3:1 v/v) was used for phosphorescence measurement at 77 K. Sample solutions were freshly prepared and degassed on a high vacuum line. Absorption and emission spectra were recorded on a JASCO model U-best 50 spectrophotometer and a Hitachi model F-4010 fluorescence spectrophotometer, respectively. The fluorescence quantum yields (Φf) of 3nCPs were determined by comparing with the corrected fluorescence spectrum of mesitylene in CH (Φf = 0.088).73 Nanosecond fluorescence lifetimes were determined by using a time-correlated single-photon counting fluorimeter (Edinburgh Analytical Instrument, FL-900CDT). Fourth (266 nm) and third harmonics (355 nm) of a Nd3+:YAG laser (JK Lasers HY-500; pulse width 8 ns) were used as the light sources for the direct excitation and triplet sensitization, respectively. The details of the


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detection system for the time profiles of the transient absorption upon laser flash photolysis have 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

been reported elsewhere.74 Cyclic voltammograms were recorded by a BAS-100B/W electron chemical analyzer. A Pt wire counter electrode and a Ag/0.01 M AgNO3 reference electrode were used. The measurements were carried out in 0.1 M 1,1,2,2-tetrachloroethanes solution of a substrate using Bu4NPF6 as supporting electrolyte, and the oxidation potential values were calibrated with ferrocene.

Results and discussion Absorption and emission spectra of 3nCP. Figure 1 shows absorption and emission spectra of 3nCP.

Figure 1. Absorption (solid) and fluorescence (blue color) spectra of [3n]CP (32CP (a), 33CP (b), 34(1235)CP (c), 34(1245)CP (d), 35CP (e) and 36CP (f)) in CH at 295 K and phosphorescence spectra (red color) in a mixture of MCH and IP (3:1 v/v) at 77 K. The absorption band in the wavelength region from 250 nm to 300 nm and week one in the region from 280 nm to 350 nm are seen in the studied cyclophanes. The former band corresponding to the “cyclophane band” gradually shifts to a longer wavelength region with an increase of the number of ACS Paragon Plus Environment

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trimethylene bridges. The weak absorption band is due to the transannular π−π* interaction between the two benzene rings as a benzene dimer in the ground state.1,66 The appearance of this band is characteristic to [3n]CPs whereas the [2n]CPs (n =2–6) do not show this band.66 All the fluorescence spectra obtained have no vibrational structures, indicating that the fluorescence is excimeric. No locally excited (LE) fluorescence was seen in the wavelength region shorter than the excimer fluorescence wavelength region. With an increase of the number (n) of the oligomethylene chain in 3nCP, the wavelength maximum of the excimer fluorescence band increases. This finding indicates that the transannular interaction between the benzene rings increases in the excited singlet state due to a decrease of the distance between the benzene rings as an increase in the n number.75 The quantum yields (Φf), lifetimes (τf) and rates (kf) of the fluorescence, and phosphorescence lifetimes (τp) are listed in Table 1. The estimated kf values are in the same order of the magnitude (105 s−1) except for 36CP, plausibly, for the similar excimer fluorescence. Conversely, observation of phosphorescence depends on the variety of 3nCPs. Phosphorescence was absent from 33CP and 36CP with our instrument while the others appreciably provided phosphorescence without vibrational structures. These phosphorescence spectra are, thus, due to triplet excimer, indicating the presence of intersystem crossing from the excimeric excited singlet states. The absence of phosphorescence indicates two possibilities of photophysical pathways. One is the absence of intersystem crossing from the excimeric excited singlet state, and the other is the triplet state being non-phosphorescent. For the purpose of clearing the issue, laser photolysis studies were carried out.

Table 1. Photophysical and photochemical parameters obtained in the present work.

a

Compound

Φf a / 10−2

τf b / ns

kf c / 105 s−1

ET d / kcal mol−1

τp e / s

32CP

1.2

30

4.0

60.8

6.0

33CP

1.3

32

4.1

-

-

34(1235)CP

1.9

62

3.1

58.0

1.7

34(1245)CP

2.0

67

3.0

56.0

3.4

35CP

1.4

116

1.2

55.6

1.2

36CP

Photophysical and Photochemical Processes of Excited Singlet and

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