canadian journal of chemistry · 2018. 11. 28. · also undergoes intersystem crossing to give an...

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Photochemical properties of enediyne-cored dendrimers bearing naphthalenes at the periphery

Journal: Canadian Journal of Chemistry

Manuscript ID cjc-2018-0128.R2

Manuscript Type: Article

Date Submitted by the Author: 10-Jul-2018

Complete List of Authors: Ichino, Rina; University of Tsukuba, Department of ChemistryMomotake, Atsuya; University of Tsukuba, Department of ChemistryArai, Tatsuo; University of Tsukuba, Department of Chemistry

Is the invited manuscript for consideration in a Special

Issue?:Not applicable (regular submission)

Keyword: Dendrimers, Energy transfer, Photoisomerization

https://mc06.manuscriptcentral.com/cjc-pubs

Canadian Journal of Chemistry

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Photochemical properties of enediyne-cored dendrimers bearing naphthalenes at the

periphery

Rina Ichino, Atsuya Momotake, Tatsuo Arai*

Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan

E-mail: [email protected]; Fax: +81-298-53-6503; Tel: +81-298-53-4315

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Abstract

　A novel series of trans and cis enediyne-cored dendrimers bearing naphthalenes at the periphery were

synthesized and their photochemical properties were examined. The trans/cis isomer ratio in the

photostationary state was dependent on the excitation site in the dendrimers. When the enediyne core was

selectively excited, the trans/cis isomer ratio in the photostationary state was either around 50/50 or a cis-

rich mixture in all dendrimers due to the larger molar extinction coefficient of the trans-enediynes. On the

other hand, when naphthalene was excited, a trans-rich mixture was unexpectedly obtained in higher

generation dendrimers even though the energy transfer efficiency was almost quantitative in the trans

dendrimers. These results could be explained by the energy transfer process, which was different

depending on the geometric isomerism of the enediyne core.

Key words: Dendrimers, Energy transfer, Photoisomerization

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Introduction.

Light harvesting and energy transfer in dendrimers have been studied extensively. 1, 2 In particular, energy

transfer between the chromophoric units within dendrimers has been the focus of many researchers

because tree-like dendritic structures are suitable for transferring photon energy harvested at the peripheral

chromophores.3 For example, Fréchet et al. reported light harvesting followed by cascade singlet-singlet

energy transfer in multichromophoric dendrimers.4 Triplet-triplet energy transfer in dendrimers has also

been reported by Vögtle et al.5 However, in many cases, the initial excitation energy was eventually

consumed as fluorescence or phosphorescence emission, whereas photoisomerization through energy

transfer has been reported less frequently.6

We intended to develop photoresponsive dendrimers wherein photon energy is efficiently

transferred to produce an excited state of the core, leading to structural changes of the dendrimer as a

whole. For example, we prepared stilbene-cored dendrimers bearing benzophenones at the periphery,

where the excitation energy of the dendrimer core was transferred to the peripheral benzophenones and

back to the core almost quantitatively, resulting in trans-cis isomerization in the excited triplet state of the

stilbene core.7 In naphthalene-terminated stilbene-cored dendrimers, the excitation energy of the

naphthalenes was transferred efficiently to the stilbene core leading to trans-cis isomerization.6 In this

case, the energy transfer efficiency was found to be higher in the trans-isomer compared to the cis-isomer,

resulting in a cis-rich isomer ratio in the photostationary state.

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Enediyne compounds have been the topic of intense studies in the areas of biological activities

because they undergo Bergman cyclization to produce a highly reactive aromatic 1,4-biradical

intermediate which causes the cutting of DNA chains. In addition, the research interest have recently

expanded to the photochemical version of this process because photochemical activation provides both

temporal and spatial control over drug activation.8,9

Aromatic enediyne compounds, e.g., bisphenylethynylethene (BEE) (Figure 1), are unique

photoresponsive molecules that exhibit a large structural change associated with trans-cis isomerization

and can be used as isomerizable cores of dendrimers. However, unlike stilbenoid compounds, energy

transfer in enediyne-cored dendrimers has only rarely been reported. Aromatic enediynes have two

characteristic carbon-carbon triple bonds directly connected to a central double bond that eliminates steric

hindrance, and thereby terminal phenyl rings of cis-isomers can adopt a coplanar structure. Accordingly,

the absorption spectra of cis-BEE10 appears in a similar wavelength range to that of trans-BEE, suggesting

similar ground state and excited state energies between the isomers. BEE emits fluorescence efficiently

from both the trans and cis isomers in addition to mutual trans-cis photoisomerization. Furthermore, BEE

also undergoes intersystem crossing to give an excited triplet state, where the planar triplet state (T1 state

of trans and cis isomers) and the perpendicular triplet state are in equilibrium10 (Figure 1).

In the field of photochemistry of aromatic enediynes, we have newly synthesized aromatic

enediynes and studied their photochemical properties. For example, “push-pull” aromatic enediynes

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exhibited solvatochromic fluorescence in the less-polar region.11 The ortho- and meta-substituted

enediynes were prepared to investigate the steric hindrance of phenyl rings in cis-enediynes.12 In addition,

a series of linear-shaped phenylacetylenyl- and (phenylacetylenyl)phenylacetylenyl-substituted aromatic

enediynes were synthesized as pure trans and cis isomers. With expansion of the -electron system by

introduction of the phenylacetylenyl units, the absorption spectra red-shifted and the molar extinction

coefficients increased up to 122,000 M-1cm-1.13 Furthermore, we prepared enediyne-cored dendrimers

having lipophilic14a or amphiphilic14b dendrons as pure trans and cis isomers, where all dendrimers were

fluorescent and underwent mutual trans-cis isomerization with relatively high quantum yields of 0.18-

0.50.

In our ongoing investigations of dendrimers and aromatic enediynes, we focused our attention on

the photoisomerization of enediyne-cored dendrimers where photons are harvested by terminal

naphthalenes. In this paper, first, we discuss the spectroscopic properties, energy transfer from the

naphthalene periphery to the enediyne core, and photoisomerization by direct enediyne-core excitation.

We then describe the photoisomerization via energy transfer by excitation of the naphthalenes, where the

higher generation dendrimers exhibited unexpected results that differed from the case of stilbene-cored

dendrimers having naphthalene termini.6

Experimental　

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Solvents and commercially available compounds were purchased from standard suppliers and purified by

standard methods. 1H NMR spectra were measured with a Bruker ARX-400 (400 MHz for 1H) NMR

spectrometer in CDCl3 solution with tetramethylsilane as an internal standard. UV absorption and

fluorescence spectra were recorded on a Shimadzu UV-1600 UV-visible spectrophotometer and on a

Hitachi F-4500 fluorescence spectrometer, respectively. MALDI-TOF MS measurements were taken

using G2-(4-hydroxyphenylazo)benzoic acid (HABA) as a matrix without adding any salts. Quantum

yield of fluorescence emission and quantum yield of photoisomerization were determined by a procedure

previously reported.15

cis-2 and trans-2 have been reported previously.14

cis-G1 (typical procedure). A mixture of cis-1 (40 mg, 0.053 mmol), G1-Br (81.6 mg, 0.37 mmol),

K2CO3 (84.2 mg, 0.61 mmol), and 18-crown-6-ether (17 mg, 0.064 mmol) in THF (10 mL) was refluxed

under nitrogen for 6 h. After the reaction was complete, the mixture was filtered and evaporated. The

residue was purified by silica gel chromatography (eluent: hexane/dichloromethane = 1/2) followed by

GPC chromatography (CHCl3) to give cis-G1 as a white solid (32 mg, 71%). 1H NMR (CDCl3, 400 MHz):

7.81-7.75 (m, 16H), 7.48-7.40 (m, 12H), 6.89 (d, J = 2.7 Hz, 4H), 6.85 (t, J = 2.7 Hz, 2H), 6.11 (s, 2H),

5.09 (s, 8H). MALDI-TOF-MS (m/z) [M+Na]+ calcd for C62H44O4Na, 875.32; Found 876.30.

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cis-G2. 1H NMR (CDCl3, 400 MHz): 7.79-7.71 (m, 32H), 7.42-7.40 (m, 24H), 6.77 (s, 4H), 6.60-6.65

(m, 14H), 6.11 (s, 2H), 5.04 (s, 16H), 4.80 (s, 8H). MALDI-TOF-MS (m/z) [M+Na]+ calcd for

C134H100O12Na, 1923.72; Found 1925.83.

cis-G3. 1H NMR (CDCl3, 400 MHz): 7.73-7.70 (m, 64H), 7.40-7.35 (m, 46H), 6.76 (d, J = 2.7 Hz, 4H),

6.60 (d, J = 2.7 Hz, 16H), 6.55 (t, J = 2.7 Hz, 8H), 6.47 (d, J = 2.7 Hz, 12H), 6.40 (t, J = 2.7 Hz, 2H), 6.01

(s, 2H). MALDI-TOF-MS (m/z) [M+Na]+ calcd for C278H212O28Na, 4020.52; Found 4025.40.

trans-G1. 1H NMR (CDCl3, 400 MHz): 7.86-7.84 (m, 16H), 7.52-7.47 (m, 12H), 6.77 (d, J = 2.7 Hz,

4H), 6.69 (t, J = 2.7 Hz, 2H), 6.26 (s, 2H), 5.00 (s, 8H). MALDI-TOF-MS (m/z) [M+Na]+ calcd for

C62H44O4Na, 875.32; Found 877.23.

trans-G2. 1H NMR (CDCl3, 400 MHz): 7.83-7.80 (m, 32H), 7.50-7.43 (m, 24H), 6.68-6.63 (m, 12H),

6.55 (s, 2H), 6.25 (s, 2H), 5.17 (s, 16H), 4.94 (s, 8H). MALDI-TOF-MS (m/z) [M+Na]+ calcd for

C134H100O12Na, 1923.72; Found 1924.47.

trans-G3. 1H NMR (CDCl3, 400 MHz): 7.79-7.76 (m, 64H), 7.46-7.40 (m, 48H), 6.69-6.58 (m, 40H),

6.51 (t, J = 2.7 Hz, 2H), 6.19 (s, 2H), 5.12 (s, 32H), 4.92 (s, 16H), 4.86 (s, 8H). MALDI-TOF-MS (m/z)

[M+Na]+ calcd for C278H212O28Na, 4020.52; Found 4024.06.

Results and Discussion

Synthesis. Enediyne-cored dendrimers with benzyl ether dendrons having naphthyl groups16 at the

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periphery, trans- and cis-Gn (n = 1-3), were newly synthesized as illustrated in Scheme 1. Their structures

were identified by NMR and MALDI-TOF MS spectroscopies.

Absorption spectra. Figures 2a and 2b show the absorption spectra of trans and cis dendrimers G1-G3

in THF at room temperature under Ar. Those of standard compounds (trans- and cis-2) are also shown for

comparison. The absorption band around 280 nm is higher in higher generations. In G2 and G3 dendrimers,

this band includes the one for terminal naphthyl groups, benzyl ether dendrons, and enedyine-core. The

ratio of the absorbance at 280 nm for each site can be estimated as described below. The bands around

300-370 nm, which are well overlapped among all compounds, are mainly due to the aromatic enediyne

cores. The spectral shapes of the cores were very similar to those of the standard compounds, indicating

that the electronic interaction between the core and naphthalene units was negligible. In contrast to cis-

stilbene or cis-azobenzene, steric hindrance in the cis-enediynes may be disregarded as mentioned above.

Thus, the ground-state energy gap between the trans and cis isomers is smaller in the enediynes, and the

spectral shapes between the isomers become similar. The molar extinction coefficient values of the cis

isomers around 300-370 nm are about half that of the trans isomers (Table 1), which is also typical of

other aromatic enediyne compounds.10,14

Estimation of the ratio of the absorbance at 280 nm

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On irradiation at 280 nm, the peripheral naphthalene is mainly excited in higher generation dendrimers.

In this case, however, enediyne core and benzyl ether dendrons also absorb the light. The ratio of the

absorbance at 280 nm for each site (Table 2) can be calculated as follows;

(1)𝐴𝑛𝑎 =𝜀𝐺𝑛𝑥 ‒ 𝜀𝐺𝑛𝑦

𝜀𝐺𝑛𝑥× 100

(2)𝐴𝑑𝑒 =𝜀𝐺𝑛𝑦 ‒ 𝜀2


(3)𝐴𝑒𝑛 =𝜀2


where Ana, Ade, Aen is the ratio of the absorbance at 280 nm for peripheral naphthalene units, benzyl ether

dendrons, and the enedyine core, respectively, Gnx, Gny, 2 is the molar extinction coefficient at 280 nm

of Gn (n = 1, 2, 3) enedyine-cored naphthalene-labeled dendrimers, Gn (n = 1, 2, 3) enedyine-cored

dendrimers without naphthalene labels14a (Figure S3), and 2, respevctively.

Fluorescence spectra. Figures 3a and 3b show the fluorescence spectra of trans- and cis-2 and their

dendrimers. The excitation wavelength was 330 nm, indicating that the enediyne core was selectively

excited and therefore the spectra can be assigned to the enedyines. The fluorescence spectra of the

dendrimers did not change depending on the dendrimer generation, and well overlapped that of 2. In

addition, the fluorescence quantum yield values for both trans and cis dendrimers were similar among

G1-G3 (Table 1), suggesting the dendrimer effect was negligible for the decay process of the excited

singlet state of the enediyne core. There was no excitation wavelength dependence in the fluorescence

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spectra of all dendrimers G1-G3 (Figure S1). For example, when excited at 280 nm, where 78%, 13%,

9% of excitation light is absorbed by terminal naphthyl groups, benzyl ether dendrons, and enedyine core,

respectively in trans-G3 (Table 2), the fluorescence spectra almost completely overlapped with those

excited at 330 nm (Figure S1d), where the enediyne core was selectively excited. In addition, the

fluorescence, including naphthalene excimer, from the naphthalene moiety, was not detected in all

dendrimers even in cis-isomers (Figure S2) where naphthalene excimer formation seems likely to occur

due to bringing the naphthalene moieties close to each other. This suggests that the excitation energy at

the peripheral naphthalene was transferred prior to other deactivation processes such as fluorescence

emission or excimer formation to give the excited singlet state of the enediyne core.

Spectral overlapping of standard compounds.

One advantage of the singlet energy transfer via the Förster process is to overlap the fluorescence

spectrum of the energy donor and the absorption spectrum of the energy acceptor.17 Figure 4 shows the

fluorescence spectra of standard compound 2-methylnaphthalene (donor) and the absorption spectra of

trans-2 and cis-2 (acceptors). This significant spectral overlap suggests that efficient singlet energy

transfer can occur from the naphthalene periphery to the enediyne core in both trans and cis dendrimers.

Singlet state energy transfer efficiency.

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Singlet energy transfer efficiency from the naphthalene periphery to the enediyne core can be calculated

from the absorption and excitation spectra (Figure 5) and by the following equation,15

(4)Φ𝐸𝑇 =𝐹𝐺𝑛 ‒ 𝐴𝑠𝑡

𝐴𝐺𝑛 ‒ 𝐴𝑠𝑡

where ΦET is the energy transfer efficiency from the naphthalene periphery to the enediyne core, FGn is

the fluorescence intensity of trans- or cis-Gn dendrimers at 280 nm in the fluorescence excitation spectra,

Ast is the absorption intensity of standard compound trans- or cis-2 at 280 nm, and AGn is the absorption

intensity of the dendrimers at 280 nm. The calculated energy transfer efficiencies are almost 100% for all

trans dendrimers, whereas those of the cis dendrimers decreased in higher generations: 100%, 69% and

49% for cis-G1, G2, and G3, respectively (Table 1). Another fundamental parameter for estimation of

singlet energy transfer efficiency is the Förster radius R0 (Table 3), which is calculated from equation 5.18

(5)𝑅60 =

9000Φ𝑑κ2𝑙𝑛10

128π5𝑛4𝑁𝐴𝐽(𝜈)

where d is the fluorescence quantum yield of donor 2-methyl naphthalene19 (d = 0.27) in the absence

of acceptor enedyine, is the orientation factor. In this case, the averaged value of is calculated as 2/3,

corresponding to the average of all possible orientation, because of the random relative alignment of

chromophoric dipoles in dendrimers with the flexible backbone,20 n is the refraction index of THF (n =

1.4), NA is Avogadro’s number, J is the overlap integral which is given by

(6)J(ν) = ∫𝐹𝐷(𝜈)𝜀𝐴(𝜈)

𝜈4 𝑑𝜈

where FD() is the intensity of the donor (2-methyl naphthalene) fluorescence, A() is the extinction

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coefficient of the acceptor (trans- or cis-2), the integral is calculated over whole donor fluorescence and

acceptor absorption spectrum.

The energy transfer efficiency can be expressed using R0 as follows:

(7)Φ𝐸𝑇 =𝑅6

0

𝑅60 + 𝑅6

where R is the center-to-center distance between donor naphthalene and acceptor enedyine which is

estimated by MM2 (Table 3). The distance R ranges between 14.6 and 25.0 Å depending on their construct

and is much smaller than R0 for all dendrimers. The equation 7 indicates that when R is smaller than R0,

the energy transfer efficiency is high. Accordingly, the calculated energy transfer efficiency values by

equation 7 (Table 3) are high for all dendrimers. In addition, it has been reported that when D-A distance

is much smaller than R0, energy transfer is expected to occur much faster than excimer formation.3d In our

case, naphthalene excimer emission did not be detected for all dendrimers that is consistent with an earlier

report. The calculated energy transfer efficiency decreased with increasing generation of both trans- and

cis-dendrimers, which is likely to simply follow D-A distance calculated by MM2 and is slightly different

from the observed values (Table 1) estimated from the absorption and excitation spectra. The disregard

for solvent effects in molecular modeling might cause differences between the calculated and observed

values.

Rate constant for energy transfer from periphery to core.

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To gain more insight into the difference in energy transfer between the trans and cis dendrimers, both

theoretical and observed rate constants for energy transfer from naphthalene to the enediyne core (kET)

were summarized in Table 3. The theoretical energy transfer rate constants can be estimated by equation

8,21

(8)𝑘𝐸𝑇 =9000Φ𝑑κ

2𝑙𝑛10

128π5𝑛4𝑁𝐴𝑅6τ𝐷𝐽(𝜈)

Where the parameters d, n, NA, R, and J are described as above, D is the lifetime of the donor 2-

methyl naphthalene (D = 59 ns) in the absence of the acceptor.

The theoretical values of the energy transfer rate constants (Table 3) decreased significantly with

increasing the generation of dendrimer because it is sensitive to donor-acceptor distance, whereas small

differences of the theoretical values between trans- and cis-isomers are not sufficient to explain the large

differences of energy transfer efficiency between trans- and cis-isomers.

The observed energy transfer rate constant is linked to the energy transfer efficiency by the

following relationship,

(9)Φ𝐸𝑇 =𝑘𝐸𝑇

𝑘𝐸𝑇 + 𝑘𝐹𝑙 + 𝑘𝑛𝑟

where kFl and knr are the rate constants of fluorescence emission and nonradiative decay, respectively, in

the absence of energy acceptor, and thus can be represented as follows,

(10)𝑘𝐹𝑙 + 𝑘𝑛𝑟 = τ ‒ 1𝐷

In this case, the fluorescence lifetime of 2-methylnaphthalene (τD = 59 ns) was employed.18 From

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equations 8 and 9, the observed rate constant of energy transfer kET can be calculated as follows,

(11) 𝑘𝐸𝑇 =Φ𝐸𝑇 × 𝜏 ‒ 1

𝐷

1 ‒ Φ𝐸𝑇

In the case of the trans dendrimers, the ΦET values (Table 1) are approximated as 0.99, because kET cannot

be calculated if ΦET is 1. The observed kET values, showing a significant decrease in cis-G2 and cis-G3

compared to those of the trans isomers (Table 3), are consistent with the significant differences in energy

transfer efficiency between trans- and cis-isomers.

Photoisomerization by enediyne-core excitation.

All dendrimers showed mutual photoisomerization both by direct enediyne excitation and by naphthalene

excitation. First, we discuss photoisomerization by enediyne core excitation. Figure 6 shows the three

absorption lines corresponding to trans-Gn and cis-Gn and the photostationary state. The trans/cis isomer

ratio in the photostationary state can be estimated from these three spectra. For photoirradiation at 334

nm, where the enediyne core is selectively excited, the trans/cis isomer ratios in the photostationary state

were 37/63, 40/60, and 51/49 for G1, G2 and G3, respectively. According to the reported

photoisomerization mechanism for aromatic enediynes,10 the trans-cis isomerization requires twisting of

the central double bond in the excited state, followed by deactivation to the perpendicular ground state

conformation. Since the perpendicular conformation is quite unstable in the ground state, further twisting

takes place immediately to give either another conformation or to return to the starting isomer (Figure 7).

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In this mechanism, the trans/cis ratio in the photostationary state is expected to be either around 50/50 or

a cis-rich mixture due to the larger molar extinction coefficient of the trans isomer.

Photoisomerization via energy transfer by peripheral naphthalene excitation.

Upon irradiation at 280 nm, where the peripheral naphthalene is mainly excited, the absorption spectra

also changed in a similar manner as in irradiation at 330 nm, indicating that energy transfer from the

naphthalene periphery to the enediyne core occurred, followed by trans-cis mutual photoisomerization.

However, the obtained isomer ratios in the photostationary state were different from those with excitation

334 nm, i.e., at 35/65, 49/51, and 74/26, for G1, G2 and G3, respectively (Figure 5 d-f). The trans-rich

isomer ratio for the G3 dendrimer (Figure 5f) was unexpected because the energy transfer efficiency from

naphthalene to the enediyne core was almost 100% in trans-G3, while that in cis-G3 was 49%. In other

words, upon irradiation at 280 nm, the excited singlet state of the enediyne core in trans-G3 should be

more efficiently produced compared to cis-G3, and therefore, we initially expected a cis-rich mixture in

the photostationary state.

The trans-rich isomer ratio for the G3 dendrimers cannot be explained only by assuming that

isomerization proceeds in the excited singlet state without any specific effects such as steric hindrance of

only either one of geometric isomer. We therefore assume two possible mechanisms to explain the trans-

rich isomer ratio for the G3 dendrimers. First, this result can be attributed to the increased steric strain

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in higher generation dendrimers based on the cis-enediyne. As we mentioned above, enediynes have two

carbon-carbon triple bonds connected to a central double bond that eliminates steric hindrance, and

thereby terminal phenyl rings of cis-isomers are free of strain. However, in cis-isomers, the substituents

are placed only on one side of the central core and the bulk of substitution significantly increase due to

dendritic structures in higher generations. Such crowded substituents can change the geometry of the

twisted intermediate in the excited state placing it closer to the trans-isomer, resulting in trans-rich isomer

ratio. Another possible mechanism is that cis-G3 undergoes isomerization in the excited triplet

state as well as in the singlet state. In order to isomerize cis-G3 in the triplet state with irradiation at 280

nm, the intersystem crossing of excited naphthalenes must be faster or at least comparable in time scale

to other deactivation processes. A plausible photochemical reaction diagram of cis-G3 is depicted on the

left side of Figure 7. When naphthalene is excited by 280 nm irradiation, the possible decay paths are

singlet energy transfer, fluorescence emission, intersystem crossing, and non-radiative decay. As already

mentioned, the efficiency of singlet energy transfer is 49% in cis-G3 and its rate constant kET is 1.5 × 107

s-1. On the other hand, the fluorescence emission rate constant of 2-methylnaphthalene is lower and

reported to be 4.6 × 106 s-1. In fact, fluorescence of the naphthalene periphery was not detected in all

dendrimers. Assuming that the rate constant of intersystem crossing of 2-naphthalene (0.95 × 107 s-1)18 is

applicable to the naphthalene periphery in cis-G3, the intersystem crossing and the singlet energy transfer

are comparable. Once the excited triplet state of naphthalene is produced, an intramolecular triplet-triplet

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energy transfer occurs to give an excited triplet enediyne, of which the main deactivation process is

reported to be trans-cis isomerization. However, unlike in cis-G3, singlet energy transfer is dominant in

trans-G3, because its rate constant (kET = 1.9 × 109 s-1) is much larger than that of intersystem crossing

(right side of Figure 7). According to this mechanism, upon 280 nm excitation, the excited triplet state of

the enediyne core in cis-G3 is produced more efficiently than that in trans-G3, and therefore,

isomerization from cis-G3 takes place more efficiently compared to that from trans-G3, resulting in the

trans-rich photostationary ratio in the higher generation dendrimers.

Conclusions.

A novel series of trans and cis enediyne-cored dendrimers bearing naphthalenes at the periphery have

been synthesized and their photochemistry, especially the energy transfer effect on isomerization, was

investigated. In trans dendrimers, singlet energy transfer from the peripheral naphthalene to the enediyne

core occurred quantitatively even in G3, whereas that of cis dendrimers decreased with increasing

generations. When the enediyne core was selectively excited, the trans/cis isomer ratio in the

photostationary state was either around 50/50 or a cis-rich mixture in all dendrimers, due to the larger

molar extinction coefficient of the trans enediynes. On the other hand, when naphthalene was excited, a

trans-rich mixture was unexpectedly obtained in G3 even though the energy transfer efficiency was higher

in the trans dendrimers. These results could be explained by the increased steric strain in G3 based on the

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cis-enediyne, where crowed substituents can change the geometry of the twisted intermediate in the

excited state placing it to the trans-isomer, resulting in trans-rich isomer ratio. Another possible

mechanism may be the energy transfer process, which was different depending on the geometric

isomerism of the enediyne core as follows. Excited naphthalene in cis-G3 may undergo intersystem

crossing competitively, probably because the singlet energy transfer to the enediyne is partially suppressed

in the crowded dendron of cis-G3, even though D-A distance is within the Förster radius. Next, the

energy of triplet naphthalene transfers to the enediyne core, followed by cis-to-trans isomerization, which

is the major deactivation process of triplet enediynes. On the other hand, although singlet energy transfer

takes place quantitatively in trans-G3, the enediyne core undergoes fluorescence emission and

isomerization comparably. These results indicate that the photochemical behaviors of dendrimers may be

controlled by regulating the rate of energy transfer from their peripheral chromophores. These insights

may be helpful in establishing photocontrol of dendrimer-based materials.

References

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Table 1. Photochemical and photophysical properties of 2 and G1-G3 dendrimers.

abs/nm /M-1cm-1 fl ETa iso

trans-2 332, 355 41100, 27500 0.55 - 0.12

trans-G1 332, 357 43100, 28400 0.54 ~1 0.12

trans-G2 334, 358 42100, 28000 0.55 ~1 0.12

trans-G3 335, 359 41000, 28000 0.59 ~1 0.14

cis-2 332, 358 19500, 12000 0.14 - 0.16

cis-G1 334, 359 18600, 10600 0.29 ~1 0.23

cis-G2 335, 361 19400, 11900 0.29 0.69 0.26

cis-G3 336, 361 17400, 11600 0.28 0.49 0.25

a The values of energy transfer efficiency were estimated from equation 4.

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Table 2. Molar extinction coefficients and the ratio of absorbance at 280 nm for each site of 2 and G1-G3 dendrimers.

280nm/104 M-1 cm-1 The ratio of absorbance at 280 nm/%

Gna Gny 2 Ana Ade Aen

trans-2 - - 0.96 - - 100

trans-G1 2.6 - 0.96 63 - 37

trans-G2 5.3 1.4 0.96 74 8 18

trans-G3 10.4 2.3 0.96 78 13 9

cis-2 - - 0.77 - - 100

cis-G1 2.3 - 0.77 66 - 34

cis-G2 5.3 1.3 0.77 75 10 15

cis-G3 10.0 2.1 0.77 79 13 8

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Table 3. The Förster radius, the theoretical energy transfer efficiency, and the theoretical and the

observed energy transfer rate constants for G1-G3 dendrimers.

D-A distance R(Å) J() (cm3 M-1) Förster radius R0(Å) ETa

(s-1) 𝑘𝑡ℎ𝑒𝑜𝑟𝐸𝑇 (s-1) 𝑘𝑜𝑏𝑠𝑒𝑣𝑑

𝐸𝑇

trans-G1 14.7 3.04×10-14 32.9 0.99 2.1 ×109 > 1.7 ×109

trans-G2 19.8 3.04×10-14 32.9 0.95 3.5 ×108 > 1.7 ×109

trans-G3 24.4 3.04×10-14 32.9 0.86 9.9 ×107 > 1.7 ×109

cis-G1 14.6 1.97×10-14 30.6 0.99 1.4 ×109 > 1.7 ×109

cis-G2 19.9 1.97×10-14 30.6 0.93 2.2 ×108 3.6 ×107

cis-G3 25.0 1.97×10-14 30.6 0.78 5.5 ×107 1.5 ×107

a The values of energy transfer efficiency were estimated from equation 7.

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Scheme 1. Synthesis and chemical structures of trans and cis enediyne-cored dendrimers with

naphthalene periphery, and their standard compounds trans- and cis-2. (a) 18-crown-6, K2CO3, Gn-Br

(n = 1, 2, 3), THF.

Figure 1. Photoisomerization and potential energy surface of BEE.

Figure 2. Absorption spectra of trans form (a) and cis form (b) of 2 and enediyne-cored dendrimers Gn

(n = 1-3) in THF; trans- and cis-forms of 2 (black lines), G1 (blue lines), G2 (green lines) and G3 (red

lines), respectively.

Figure 3. Normalized fluorescence spectra of trans form (a) and cis form (b) of 2 and enediyne-cored

dendrimers Gn (n = 1-3) in THF excited at 330 nm; trans form and cis form of 2 (black lines), G1 (blue

lines), G2 (green lines) and G3 (red lines), respectively.

Figure 4. Absorption spectra of trans-2 (solid line) and cis-2 (open circles) and fluorescence spectra of

2-methylnaphthalene (dotted line).

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Figure 5. Absorption (solid lines) and fluorescence excitation (dotted lines) spectra of trans-G3 (a) and

cis-G3 (b). The spectra were normalized at 330 nm for calculation of the energy transfer efficiency ΦET.

Figure 6. Absorption spectra of G1 (a and d), G2 (b and e), and G3 (c and f) dendrimers. Blue lines, red

lines and black lines indicate the absorption spectra of the trans-isomer, cis-isomer and the

photostationary state, respectively. Irradiation wavelength is 334 nm (a-c) or 280 nm (d-f).

Figure 7. Plausible energy diagram of intramolecular energy transfer and photoisomerization of trans-

and cis-G3.

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T1 T1

S1 S1

S0

S0

Ener

gy

Angle of Twist/Degree0 18090

trans-BEE cis-BEE

hn

S1

T1S0

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DraftWavelength/nmWavelength/nm

e/104 M

-1cm

-1

e/104 M

-1cm

-1

a b

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a b

Wavelength/nm Wavelength/nm

Nor

mal

ized

fluo

resc

ence

inte

nsity

Nor

mal

ized

fluo

resc

ence

inte

nsity

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DraftWavelength/nm

Fluorescence intensity e/

104 M

-1cm

-1

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DraftWavelength/nm Wavelength/nm

Fluorescence intensity

Fluorescence intensity A

bsor

banc

e

Abs

orba

nce a b

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Wavelength/nmWavelength/nm Wavelength/nm

Abs

orba

nce

Abs

orba

nce

Abs

orba

nce

a b c

d e f

334 nmirradiation

334 nmirradiation

334 nmirradiation

280 nmirradiation

280 nmirradiation

280 nmirradiation

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Cis-form Trans-form

ISC ISC

SET SET

~100%49%

FL FLhn280 nm

hn280 nm

TET

S1

S0

T1 T1T1 T1

S1

S0

S1 S1

S0

S0

FL FLhn334 nm

hn334 nm

Isomerization

Enediyne coreNaphthalene periphery Naphthalene periphery

Energy

ISC ISC

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canadian journal of chemistry · 2018. 11. 28. · also undergoes intersystem crossing to give an...

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