Systematic studies on photoluminescence of oligo(arylene-ethynylene)s: Tunability of excited states and derivatization as luminescent labeling probes for proteins

FULL PAPER

DOI: 10.1002/ejoc.200600103

Systematic Studies on Photoluminescence of Oligo(arylene-ethynylene)s:

Tunability of Excited States and Derivatization as Luminescent Labeling

Probes for Proteins

Yong-Gang Zhi,^[a]Siu-Wai Lai,*^[a]Queenie K.-W. Chan,^[a]Yuen-Chi Law,^[a]

Glenna S.-M. Tong,^[a]and Chi-Ming Che*^[a]

Keywords: Oligo(phenylene-ethynylene) / π-Conjugation / Fluorescence / Solvatochromism / Protein-labeling

Functionalized oligo(phenylene-ethynylene)s (OPEs) with

different conjugation lengths, p-X(C₆H₄CϵC)_nSiMe₃(n = 1–

4; X = NH₂, NMe₂, H) were synthesized by Sonogashira

coupling of (phenylene-ethynylene)s and 1-iodo-4-(trime-

thylsilylethynyl)benzene, followed by desilylation of the p-

substituted (trimethylsilylethynyl)benzenes with potassium

hydroxide. The photoluminescent properties for the OPE

series with different chain lengths and their solvatochromic

responses were examined. The absorption maxima were red-

shifted with increasing numbers of –(C₆H₄CϵC)– units (n),

and a linear plot of the absorption energy maxima vs. 1/n

was obtained for each series. The emission spectra in dichlo-

romethane showed a broad and structureless band, the ener-

gies of which (in wavenumbers) also fit linearly with 1/n.

Both the absorption and emission wavelength maxima of the

NH₂- and NMe₂-substituted OPEs exhibited significant sol-

vent dependence, whereas the parent OPEs (X = H) showed

only minor shifts of the λ_maxvalues in different solvents. Sub-

stituent effects upon the photoluminescent characteristics of

the OPEs and the tunability of the excited states were exam-

ined with the p-X(C₆H₄CϵC)_nSiMe₃(n = 2, 3; X = NH₂,

NMe₂, H, SMe, OMe, OH, and F) series. The H- and F-sub-

stituted counterparts exhibited high-energy vibronically

structured emissions attributed to the (ππ*) excited states of

3

the (arylene-ethynylene) backbone. For compounds bearing

NH₂and NMe₂groups, a broad red-shifted emission with a

remarkable Stokes shift from the respective absorption maxi-

mum was observed, which can be assigned to an n Ǟ π*

transition. The n Ǟ π* assignment was supported by MO cal-

culations on the model compounds p-X(C₆H₄CϵC)₂SiH₃(X

= NH₂, H). Functionalization of the oligo(arylene-ethynylene)s

with the N-hydroxysuccinimidyl (NHS) moiety enabled co-

valent attachment of the fluorophore to HSA protein mole-

cules. A series of fluorescent labels, namely p-X(C₆H₄CϵC)_n-

C₆H₄NHS, (n = 1, X = NH₂, NMe₂, SMe, OMe, OH, F; n = 2,

X

=

NH₂, NMe₂) and p-Me₂NC₆H₄CϵC(C₄H₂S)-

CϵCC₆H₄NHS were synthesized, and their conjugates with

HSA (human serum albumin) were characterized by MALDI-

TOF mass spectrometry, UV/Vis absorption spectroscopy,

and gel electrophoresis.

Germany, 2006)

properties and high quantum yields,^[5–7]and accordingly,

OPE materials have been employed in organic electrolumi-

nescent light-emitting devices,^[8]liquid-crystal displays,^[9]

and chemosensors^[10]for electron-poor aromatics,^[11]fluo-

ride ions,^[12]biological targets,^[13]and metal cations.^[14]

The photophysical characteristics of OPEs can be signifi-

cantly perturbed by different end-capping substituents and

the extent of conjugation, and the latter enables electronic

communication across a linear OPE chain over a long dis-

tance.^[15,16]Herein is described the synthesis and photo-

physical properties of conjugated OPE materials with pro-

gressively increasing chain length and different terminal

groups. The correlation between photoluminescent charac-

teristics and conjugation length is presented, and through

studying the electronic excited states of the conjugated

oligomers, the effect of remote terminal electron-donating

substituents upon the nature of the excited state is eluci-

dated. Through derivatization of the oligo(arylene-ethynyl-

ene)s with the lysine-reactive N-hydroxysuccinimidyl (NHS)

Introduction

Oligo(phenylene-ethynylene)s (OPEs) with extended con-

jugated aromatic systems have shown applications in molec-

ular electronics^[1]and nonlinear optics^[2]because of their

intriguing optical and electronic properties. General syn-

thetic procedures of OPEs involve Pd/Cu-catalyzed coup-

ling and desilylation reactions, which enable high degrees of

modification in conjugation length,^[1f,1g]terminal substitu-

ents,^[3]and sensory pendants.^[4]Recent photophysical in-

vestigations of OPEs have revealed intriguing luminescent

[a] Department of Chemistry, HKU-CAS Joint Laboratory on

New Materials and Open Laboratory of Chemical Biology of

the Institute of Molecular Technology for Drug Discovery and

Synthesis, The University of Hong Kong,

Pokfulam Road, Hong Kong SAR, China

Fax: +852-2857-1586

E-mail: cmche@hku.hk

swlai@hku.hk

Supporting information for this article is available on the

WWW under http://ww w .eurjoc.org or from the author.

Eur. J. Org. Chem. 2006, 3125–3139

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Y.-G. Zhi, S.-W. Lai, Q. K.-W. Chan, Y.-C. Law, G. S.-M. Tong, C.-M. Che

FULL PAPER

moiety, we have been able to covalently attach the non-toxic

Substituent effects were also investigated by preparing

OPEs as strongly fluorescent labels to the HSA (human se- two series of compounds p-X(C₆H₄CϵC)_nSiMe₃[n = 2, 3;

rum albumin) biomolecules used in this work.

X = NH₂(2a, 3a), NMe₂(2b, 3b), H (2h, 3h), SMe (2c, 3c),

OMe (2d, 3d), OH (2e, 3e), F (2f, 3f)]. N-Hydroxysuccin-

imidyl (NHS) ester derivatives of diphenylacetylenes, 5a–5f,

were synthesized by reaction of succinimidyl 4-iodobenzo-

ate with p-X(C₆H₄CϵC)H (X = NH₂, NMe₂, SMe, OMe,

OH, F) (Scheme 2). Similarly, p-X(C₆H₄CϵC)₂C₆H₄NHS

Results and Discussion

Synthesis and Characterization

(X

= NH₂, NMe₂), and p-Me₂NC₆H₄CϵC(C₄H₂S)-

CϵCC₆H₄NHS were obtained using p-X(C₆H₄CϵC)₂H

and p-Me₂NC₆H₄CϵC(C₄H₂S)CϵCH, respectively. The

NHS moiety is capable of reacting with lysine and α-amino

groups of proteins, such as HSA in this work, which has

resulted in the covalent attachment of fluorescent oligo-

(arylene-ethynylene)s to biomolecules.

Figure 1 depicts the oligo(arylene-trimethylsilylethynyl-

ene)s containing –(C₆H₄CϵC)_n– chains (n = 1–4) with the

terminal para substituents NH₂(1a–4a) and NMe₂(1b–4b).

For comparison, the unsubstituted parent derivatives (1h–

4h) have also been prepared. Scheme 1 outlines the syn-

thetic routes for 1a–4a and 1b–4b, which involve Sonoga-

shira^[17]coupling of (arylene-ethynylene)s with 1-iodo-4-

(trimethylsilylethynyl)benzene in the presence of CuI and

Pd(PPh₃)₂Cl₂as catalysts and NHEt₂as base, followed by

the desilylation of the p-substituted (trimethylsilylethynyl)-

benzenes with potassium hydroxide. These procedures have

been adopted for the preparation of OPEs and their oligo-

mers.^{[1b,1e,18–23]}

Scheme 2. Synthesis of NHS-capped diphenylacetylenes bearing

different para substituents, 5a–5f, and coordinative attachment of

fluorophores to protein molecules.

Compounds 3a, 4a, 2b–4b, 2c–2f, 3c–3f, and 5a–5f were

prepared by high-yielding reactions, and have been charac-

1

terized by H and ¹³C NMR and high-resolution electron

ionization (EI) mass spectrometry. All compounds are

stable in the solid state and in solution. For p-X(C₆H₄-

CϵC)_nSiMe₃(X = NH₂, 1a–4a; X = NMe₂, 1b–4b), the

first three compounds (n = 1–3) of each series exhibit good

solubility in common solvents. However, the p-

X(C₆H₄CϵC)₄SiMe₃compounds are barely soluble in

dichloromethane, chloroform, DMF and DMSO, and only

sparingly soluble in n-hexane, methanol, and acetonitrile.

Similarly, for H(C₆H₄CϵC)_nSiMe₃(1h–4h), compounds 1h

and 2h are soluble in most solvents, whereas 4h is only solu-

ble in toluene and dichloromethane.

Figure 1. Structures of oligo(arylene-trimethylsilylethynylene)s and

NHS-capped diphenylacetylenes.

Phase Transition, Thermo- and Photostability

Samples for differential scanning calorimetric (DSC)

studies and thermogravimetric analysis (TGA) were dried

under vacuum for 24 h. The DSC thermograms of 2a, 3a

and 1b–3b are shown in Figures S1 and S2 of the Support-

ing Information. For 2a, an endothermic maximum at

153 °C was observed, but upon increasing the chain length

for 3a, a higher transition temperature of 234 °C (with de-

creased magnitude of the melting peak) was observed. Sim-

ilar results were found for 1b–3b, with the peak temperature

progressively increasing from 96 (1b) to 132 (2b) and 216 °C

(3b). The values of the enthalpy of transition (∆H) for 1b

and 2b are 19.4 and 18.0 kJ/mol, respectively, whereas that

of 3b is increased to 36.8 kJ/mol. The TGA thermograms

Scheme 1. Synthesis of 1a–4a and 1b–4b: a) 1.1 equiv. of KOH in

MeOH/THF, 25 °C, 4 h; b) Pd(PPh₃)₂Cl₂(2 mol-%), CuI (1 mol-

%), 1-iodo-4-(trimethylsilylethynyl)benzene (0.91 equiv.), and

NHEt₂in THF, 25 °C, 24 h.

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Eur. J. Org. Chem. 2006, 3125–3139

Photoluminescence of Oligo(arylene-ethynylene)s

FULL PAPER

of 3a, 4a, and 2b–4b (Figures S3 and S4) reveal that they the excitation spectra of 2a–4a are identical to their respec-

are stable up to 245, 273, 241, 299, and 295 °C, respectively. tive absorption spectra. The emission quantum yield in

The photostability of 5b in acetonitrile was examined: no dichloromethane varies from 0.023 (1a) to 0.84 (3a).

detectable changes in absorption and emission spectra in

For the p-Me₂N(C₆H₄CϵC)_nSiMe₃(1b–4b; n = 1–4)

acetonitrile solution were observed under ambient light and series, the red-shifted π-π* absorption band in CH₂Cl₂solu-

air for a week. However, irradiation with a high-power, tion appears at λ_max= 298, 356, 369, and 370 nm [ε = (3.53,

broad-band mercury arc lamp (500 W) for 2.5 h under air 4.04, 6.16, and 7.88)×10⁴dm³mol^–1cm^–1] for 1b–4b,

resulted in blue-shifted absorption maxima, from 277 and respectively (Figure S5). The ε values at λ_maxfor 1b–4b in-

378 nm to 271 and 351 nm, accompanied by a gradual de- crease along the series, like for 1a–4a. The absorption spec-

crease in absorbance, by 28 and 42%, respectively.

trum of 2b in dichloromethane exhibits well-resolved ab-

sorption peak maxima at 275–291 nm plus a 356 nm ab-

sorption maximum. The absorption spectrum of 4b in vari-

ous solvents features one broad band at λ_max= 364–

371 nm.

Absorption and Emission Spectroscopy

The UV/Vis absorption and emission data of 1a–4a,

Emission data of 1b–4b in various solvents at 298 K are

summarized in Table 1. Emission spectra of 2b–4b in hex-

ane exhibit vibronically structured bands with peak maxima

at 366–401 nm. The emission spectra of 1b–4b in other sol-

vents are broad and structureless. In CH₂Cl₂, the emission

λ_maxred-shifts from 362, to 440, 488, and 510 nm for 1b–4b,

respectively. A plot of the emission energy vs. 1/n is linear

(Supporting Information, Figure S6), and the emission ap-

proaches the limit of 586 nm as n Ǟ ϱ. In CH₂Cl₂, the

1b–4b,

1h–4h,

2c–2f,

3c–3f,

5a–5f

and

p-

Me₂NC₆H₄CϵC(C₄H₂S)CϵCC₆H₄NHS in various sol-

vents are listed in Table 1 (details in the Supporting Infor-

mation). Our intention is to examine the effect of (A) chain

length, (B) solvent polarity, and (C) para substituent upon

the electronic excited states of (arylene-ethynylene)s.

(A) Effect of Increasing Chain Length

Figure 2 shows the absorption spectra of 1a–4a in dichlo- emission quantum yields show a similar trend to 1a–4a, and

romethane at 298 K. Upon lengthening of the conjugation differ between 0.025 for 1b and 0.72 for 3b, while the life-

of p-H₂N(C₆H₄CϵC)_nSiMe₃(1a–4a) chain (from n = 1 to time measurement varies from 0.35 (1b) to 1.95 ns (3b).

4), the π-π* absorption band^[24]of 1a at λ_max= 278 nm (ε

For the parent H(C₆H₄CϵC)_nSiMe₃(1h–4h; n = 1–4)

= 2.81×10⁴dm³mol^–1cm^–1) in CH₂Cl₂shows a batho- series, the π-π* absorptions in several solvents show vibron-

chromic shift with progressively increased ε value to λ_maxically structured bands (Figure 4), which become less re-

331, 345, and 356 nm [ε (3.62, 5.88, and solved with increasing n. The absorption maxima of 2h in

=

8.89)×10⁴dm³mol^–1cm^–1] for 2a–4a, respectively. The red- dichloromethane appear at 303 and 323 nm [ε = (4.69 and

shifted absorption maximum for increasing n is consistent 4.48)×10⁴dm³mol^–1cm^–1], and are red-shifted from those

with greater π-conjugation upon lengthening of the arylene- of 1h at 248 and 260 nm [ε = (2.82 and 2.50)×10⁴

ethynylene chain.^[25]Interestingly, a plot of the absorption dm³mol^–1cm^–1]. The λ_maxvalues of 3h and 4h are further

maximum (in wavenumber) vs. 1/n affords a linear fit (inset red-shifted to 332 (ε = 6.95×10⁴dm³mol^–1cm^–1) and

of Figure 2), and as n Ǟ ϱ, the absorption maximum ap- 345 nm (ε = 9.39×10⁴dm³mol^–1cm^–1), respectively. The

proaches the limiting value of 25330 cm^–1(λ_max= 395 nm). magnitude of the red shift in λ_maxfor n = 1 to 4, from 1h

The molar extinction coefficients (ε) of the lowest-energy to 4h, is 11340 cm^–1, which is larger than the corresponding

transition of 1a–4a also increase from n = 1 to 4. In the shift of 7880 and 6530 cm^–1from 1a to 4a and from 1b to

absorption spectrum of 2a in dichloromethane, additional 4b, respectively. A plot of the absorption energies for 1h–4h

well-resolved vibronic absorption bands at λ_max= 276 and in CH₂Cl₂vs. 1/n is linear, and gives a limiting energy of

288 nm are observed besides the major peak maximum at 25200 cm^–1(397 nm) at n Ǟ ϱ (inset of Figure 4), whereas

331 nm. These high-energy absorption bands become broad the ε_maxvalues are observed to increase with n (Supporting

and less resolved as the conjugation length increases, as in Information, Figure S7).

the cases of 3a and 4a.

The emission spectra of 1h–4h in various solvents are

Excitation of 1a–4a at 280–360 nm in dichloromethane vibronically structured. The emission maximum of 1h in

at 298 K produces a blue-green or green emission (Table 1). dichloromethane occurs at the 0–1 transition of 299 nm,

Upon increasing of the chain length, the emission λ_maxred- whereas those of 2h–4h gradually shift to the 0–0 transition

shifts from 342 nm for 1a, to 415 nm for 2a, 457 nm for 3a, at 328, 364 and 383 nm, respectively. Hence, the relative in-

and 471 nm for 4a (Figure 3). This is reminiscent of the tensity of the 0–0 transition increases with n, while that of

bathochromic shift in the corresponding absorption spectra the 0–1 transition decreases. This is indicative of an increase

(Figure 2). A plot of the emission maximum (in wave- in intrachain coupling interaction(s) with greater conjuga-

number) vs. 1/n shows a linear fit (inset of Figure 3), and tion.^[26]The 0–0 emission peak (in wavenumbers) of 1h–4h

the emission maximum apparently reaches a limit of ca. in dichloromethane follows a linear relationship with 1/n

540 nm as n Ǟ ϱ. When monitoring the emission wave- (Supporting Information, Figure S8), and the λ_maxvalue

length at 342 nm, the excitation spectrum of 1a in dichloro- approaches 416 nm as n Ǟ ϱ. The quantum yield of 1h in

methane exhibits an intense band at 278 nm, which matches dichloromethane is relatively low (0.04) compared to those

the ground-state absorption depicted in Figure 2. Likewise, of 2h–4h (0.55–0.78).

Eur. J. Org. Chem. 2006, 3125–3139

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Y.-G. Zhi, S.-W. Lai, Q. K.-W. Chan, Y.-C. Law, G. S.-M. Tong, C.-M. Che

Table 1. Photophysical data for oligo(arylene-ethynylene)s in various solvents at 298 K.

FULL PAPER

Compound

Medium

λ_abs/nm (ε/dm³mol^–1cm^–1)^[a]

λ_em/nm; Φ^[a]

∆ν [cm^–1]^[b]

p-H₂N(C₆H₄CϵC)SiMe₃(1a)

n-hexane

Et₂O

CH₂Cl₂

CH₃OH

CH₃CN

273 (28000)

280 (32720)

278 (28100)

279 (26400)

281 (31600)

332; 0.28

344; 0.20

342; 0.023

351; 0.013

346; 0.075

6510

6645

6732

7352

6686

p-H₂N(C₆H₄CϵC)₂SiMe₃(2a)

n-hexane

273 (24600), 285 (28300), 322

(47900), 333 (sh, 42500), 343

(41300)

351, 372 (max); 0.54

4174

C₆H₅CH₃

Et₂O

332 (34800), 349 (sh, 31000)

274 (23900), 286 (24400), 337

(35800)

276 (19700), 288 (21900), 331

(36200), 343 (sh, 34800)

274 (25100), 286 (26700), 335

(42500)

392; 0.47

408; 0.28

4610

5164

CH₂Cl₂

CH₃OH

MeCN

415; 0.34; τ^[c]= 1.14 ns

440; 0.008

6115

7124

8177

275 (21500), 286 (22600), 339

(37400)

469; 0.071

p-H₂N(C₆H₄CϵC)₃SiMe₃(3a)

n-hexane

C₆H₅CH₃

Et₂O

CH₂Cl₂

CH₃OH

CH₃CN

342 (53300), 366 (sh, 32300)

347 (57700)

377 (max), 397; 0.83

416; 0.88

2715

4780

5814

7104

9870

9505

351 (58900)

441; 0.81

345 (58800)

457; 0.84

514; 0.047

520; 0.14

341 (60600)

348 (54100)

p-H₂N(C₆H₄CϵC)₄SiMe₃(4a)

p-Me₂N(C₆H₄CϵC)SiMe₃(1b)

C₆H₅CH₃

Et₂O

356 (88600)

275 (31000), 356 (80910)

275 (27400), 356 (88850)

417; 0.79

462; 0.68

471; 0.63

4268

6445

6859

CH₂Cl₂

n-hexane

C₆H₅CH₃

Et₂O

CH₂Cl₂

CH₃OH

CH₃CN

289 (35000), 297 (sh, 32600)

294 (42200), 307 (sh, 34300)

291 (35500), 299 (sh, 33600)

298 (35300), 305 (sh, 34100)

294 (33400)

346; 0.24

5700

5845

6036

5933

6159

6198

355; 0.14

353; 0.16

362; 0.025; τ^[d]= 0.35 ns

359; 0.020

297 (34400)

364; 0.16

p-Me₂N(C₆H₄CϵC)₂SiMe₃(2b)

n-hexane

273 (26400), 278 (27100), 289

(28600), 339 (48300), 361

(45300)

366 (max), 384; 0.69; τ^[c]

2176

= 0.88 ns

C₆H₅CH₃

Et₂O

354 (42700)

402; 0.62; τ^[c]= 0.94 ns

411; 0.60; τ^[c]= 1.13 ns

3373

4322

273 (30100), 279 (30300), 288

(30000), 349 (48600)

275 (25000), 283 (sh, 25500),

291 (26300), 305 (sh, 19600),

327 (30200), 356 (40400)

273 (26300), 279 (26400), 288

(26200), 350 (44800)

274 (28600), 281 (28800), 289

(28800), 324 (sh, 29500), 356

(47100)

CH₂Cl₂

440; 0.44; τ^[c]= 1.88 ns

5363

CH₃OH

CH₃CN

482; 0.014

7825

7514

486; 0.14; τ^[c]= 2.21 ns

p-Me₂N(C₆H₄CϵC)₃SiMe₃(3b)

n-hexane

259 (15500), 290 (25100), 298

(25900), 312 (sh, 30800), 322

(33100), 341 (sh, 44000), 359

(51200)

392 (max), 412, 429 (sh); 2345

0.77

C₆H₅CH₃

Et₂O

319 (47200), 326 (47200), 345

(sh, 52000), 368 (65800)

259 (20300), 291 (33900), 297

(34900), 317 (41200), 324

(41500), 341 (sh, 50200), 362

(59900)

262 (20400), 293 (sh, 35900),

301 (sh, 38900), 320 (46600),

327 (46300), 344 (sh, 50100),

369 (61600)

259 (17600), 290 (sh, 30700),

299 (sh, 32800), 316 (38400),

325 (sh, 37600), 339 (sh, 42700),

361 (53300)

260 (23300), 298 (sh, 42000),

316 (47600), 323 (sh, 46200),

340 (sh, 49600), 368 (66700)

427; 0.84

447; 0.81

3755

5253

CH₂Cl₂

CH₃OH

CH₃CN

488; 0.72; τ^[c]= 1.95 ns

522; 0.084

6609

8544

8825

545; 0.11

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Photoluminescence of Oligo(arylene-ethynylene)s

FULL PAPER

Table 1. (Continued)

Compound

Medium

λ_abs/nm (ε/dm³mol^–1cm^–1)^[a]

λ_em/nm; Φ^[a]

∆ν [cm^–1]^[b]

p-Me₂N(C₆H₄CϵC)₄SiMe₃(4b)

C₆H₅CH₃

323 (sh, 51500), 346 (66100),

371 (73000)

342 (sh, 68200), 364 (75900)

347 (sh, 69800), 370 (78800)

439; 0.75

5311

Et₂O

CH₂Cl₂

468; 0.72

6105

7419

510; 0.58; τ^[c]= 1.58 ns

387; 0.70; τ^[d]= 0.82 ns

p-MeS(C₆H₄CϵC)₂SiMe₃(2c)

CH₂Cl₂

292 (sh, 31700), 324 (55500),

342 (51100)

5024

p-MeO(C₆H₄CϵC)₂SiMe₃(2d)

p-HO(C₆H₄CϵC)₂SiMe₃(2e)

CH₂Cl₂

313 (51800), 332 (45100)

292 (sh, 27300), 310 (37500),

329 (32600)

371; 0.39; τ^[d]= 0.59 ns

367; 0.38; τ^[d]= 0.55 ns

4995

5010

p-F(C₆H₄CϵC)₂SiMe₃(2f)

CH₂Cl₂

261 (sh, 13300), 289 (sh, 35300), 328, 345 (max); 0.38; τ^[d]

4127

302 (45500), 311 (sh, 37700),

321 (43400), 340 (sh, 3350)

= 0.56 ns

p-H₂N(C₆H₄CϵC)C₆H₄–NHS (5a)

CH₂Cl₂

CH₃CN

THF

271 (12400), 354 (19400)

269 (19800), 358 (26500)

273 (19100), 366 (26800)

271 (20300), 359 (29000)

260 (14500), 320 (15100)

260 (14300), 320 (15400)

275, 328

495; 0.095

490; 0.001

500; 0.058

504; 0.034

476; 0.0021

475; 0.0017

475; 0.061

8047

7525

7322

8014

10242

10197

9435

EA

DMF/H₂O

DMF/PBS

PBS

5a–HSA conjugate

p-Me₂N(C₆H₄CϵC)C₆H₄–NHS (5b)

5b–HSA conjugate

DMF/PBS

PBS

263 (16200), 343 (17100)

278, 345

515; 0.0036

480; 0.13

9737

8152

p-MeS(C₆H₄CϵC)C₆H₄–NHS (5c)

5c–HSA conjugate

DMF/PBS

252 (18900), 278 (12800), 311

(21400)

269 (sh), 286, 322

451; 0.041

9981

PBS

419; 0.29

7190

p-MeO(C₆H₄CϵC)C₆H₄–NHS (5d)

5d–HSA conjugate

DMF/PBS

PBS

265 (9200), 307 (18900)

259, 275 (sh), 287 (sh), 308

418; 0.059

409; 0.25

8650

8018

p-HO(C₆H₄CϵC)C₆H₄–NHS (5e)

5e–HSA conjugate

DMF/PBS

PBS

280 (10800), 327 (14800)

260 (sh), 278, 286 (sh), 308

420; 0.0017

409; 0.020

6772

8018

p-F(C₆H₄CϵC)C₆H₄–NHS (5f)

5f–HSA conjugate

DMF/PBS

PBS

309 (18300)

293, 313 (sh)

419; 0.11

364; 0.065

8496

6657

p-Me₂NC₆H₄CϵC(C₄H₂S)CϵCC₆H₄–

NHS

DMF/PBS

267 (sh, 14600), 307 (11300),

344 (13400), 383 (10900)

271, 362

572; 0.001

11587

p-Me₂NC₆H₄CϵC(C₄H₂S)CϵCC₆H₄–

PBS

511; 0.10

8055

NHS–HSA conjugate

[a] Numbers in italic indicate the most intense bands. [b] ∆ν is the difference between the absorption and emission maximum in wave-

number. [c] The lifetime measurement was performed using a 373 nm laser source. [d] The lifetime measurement was performed using a

295 nm laser source.

Figure 3. Emission spectra of 1a–4a in dichloromethane at 298 K

(concentration = 2×10^–5moldm^–3, λ_ex= 280–360 nm). Inset: plot

Figure 2. UV/Vis absorption spectra of 1a–4a in dichloromethane

[cm^–1] vs. 1/n.

at 298 K. Inset: plot of ν

[cm^–1] vs. 1/n.

of ν

˜

max

˜

max

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3129

Y.-G. Zhi, S.-W. Lai, Q. K.-W. Chan, Y.-C. Law, G. S.-M. Tong, C.-M. Che

FULL PAPER

length solvatochromic absorption band of the pyridinium

N-phenolate betaine dye), and the emission quantum yield

of 2b, as depicted in Figure 5. Plots of emission maximum

(in wavenumber) and lifetime for 2b against E_Talso show

reasonably linear relationships (Figures S9 and S10, respec-

tively).

Figure 4. UV/Vis absorption spectra of 1h–4h in dichloromethane

at 298 K. Inset: plot of absorption maximum in ν

for H(C₆H₄CϵC)_nSiMe₃(1h–4h).

[cm^–1] vs. 1/n

˜

max

(B) Effect of Solvent Polarity

The absorption spectra of 1a–4a are moderately sensitive

to solvent polarity. For example, the absorption band of

1a at 273 nm in hexane red-shifts to 281 nm in acetonitrile,

whereas that of 3a shows a solvatochromic shift from

342 nm in hexane to 351 nm in diethyl ether. p-

H₂N(C₆H₄CϵC)₂SiMe₃(2a) exhibits the largest solvato-

Figure 5. Plot of emission quantum yield for 2b in various solvents

against the E_Tvalue.

The π Ǟ π* transitions in the absorption spectra of 1b–

chromic effect; the π-π* absorption maximum (λ_max) shifts 4b show moderate sensitivity toward solvent polarity; p-

from 322 nm (ε = 4.79×10⁴dm³mol^–1cm^–1) in hexane to Me₂N(C₆H₄CϵC)₂SiMe₃(2b) exhibits the largest solva-

339 nm (ε = 3.74×10⁴dm³mol^–1cm^–1) in acetonitrile. The tochromic shift and its peak maximum at 339 nm in hexane

low solubility of 4a limits the solvents that can be employed red-shifts to 356 nm in dichloromethane. The highest en-

for solvatochromic studies; this compound is only sparingly ergy absorption maximum of 1b (289 nm) and 3b (359 nm)

soluble in toluene, diethyl ether and dichloromethane.

in hexane are red-shifted only slightly to 298 and 369 nm,

The emission spectra of 2a–4a in hexane show vibron- respectively, in dichloromethane. In contrast, the emission

ically structured bands. For 3a, the peak maxima appear at spectra of 2b–4b show more significant solvatochromic

377 and 397 nm, with vibrational progression of ca. shifts. The emission spectra of 2b–4b in hexane show vib-

1340 cm^–1, which corresponds to a combination of phenyl ronic bands with peaks at 366 and 384 nm for 2b, 392 and

ring deformation and symmetric phenyl ring and CϵC 412 nm for 3b, and 404 and 426 nm for 4b. The vibrational

stretches.^[23]Likewise, p-H₂N(C₆H₄CϵC)₄SiMe₃(4a) in spacings of 1240–1280 cm^–1can be attributed to a combina-

hexane shows vibronically structured bands with peak max- tion of phenyl ring deformation and symmetric phenyl ring

ima at 388 and 404 nm, with vibrational progression of ca. and CϵC stretches.^[23]However, the emissions become

1020 cm^–1. Like 1h–4h, the relative intensity of the 0–0 tran- broad and structureless in toluene and acetonitrile. The

sition increases with n, while that of the 0–1 transition de- 439 nm emission for 4b in toluene shows a bathochromic

creases. Red-shifted emission maxima from 372 for 2a to shift to 510 nm in dichloromethane. The largest red-shift of

377 and 388 nm for 3a and 4a, respectively, have been ob- the emission maximum was observed for 3b, changing from

served in hexane. In other solvents, the emission of 1a–4a λ_max= 392 nm in hexane to 545 nm in acetonitrile (∆ν =

is usually broad and structureless. In acetonitrile, 3a shows 7160 cm^–1); the emission of 2b also changes significantly

a broad emission with λ_max= 520 nm, which is red-shifted from 366 nm in hexane to 486 nm in acetonitrile (∆ν =

by 7290 cm^–1from its 377 nm emission in hexane, affording 6750 cm^–1). The quantum yields of 2b and 3b in hexane,

the greatest solvatochromic shift. The quantum yields of 3a toluene, diethyl ether and dichloromethane lie in the range

in hexane, toluene, diethyl ether and dichloromethane are of 0.44–0.69 and 0.72–0.84, respectively, but in methanol

comparable (0.81–0.88), whereas those in methanol (0.047) and acetonitrile, the quantum yields decreased to 0.014–

and acetonitrile (0.14) are substantially lower.

0.14. This highlights the remarkable effects of solvent po-

Effect of solvent upon the emission of 2a (concentration: larity upon the spectroscopic properties of these com-

2×10^–5moldm^–3) has been examined in detail (Table 1). pounds. Self-quenching experiments on the emission of 2b

Both the emission maximum (λ_max) and quantum yield (Φ) were performed in the concentration range of 4.1×10^–7–

are sensitive to solvent polarity; λ_maxvaries from 372 nm in 3.2×10^–4moldm^–3in CH₂Cl₂at 298 K, but no detectable

hexane to 469 nm in acetonitrile, while Φ decreases from quenching effect was found.

0.54 in hexane to 0.008 in methanol. We have observed a

The absorption maxima of 1h–4h show relatively small

linear correlation between the solvent polarity, defined by bathochromic shifts with solvent polarity (Supporting In-

the E_Tvalue (classified with respect to the longest-wave- formation) compared to 1a–4a and 1b–4b. The maximum

3130

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Photoluminescence of Oligo(arylene-ethynylene)s

FULL PAPER

solvatochromic shift is only 4 nm for 3h (λ_max= 328 nm in their absorption energies; NH₂- (2a) and NMe₂-substituted

diethyl ether to 332 nm in dichloromethane). The emission (2b) compounds emit at λ_max= 415 and 440 nm, respec-

spectra of 1h–4h also show little variation in peak maxi- tively, which are red-shifted from those observed for 2c–2f

mum and quantum yield with solvent; 3h shows emission and 2h. The quantum yields of 2a–2f and 2h lie in the range

spectra with λ_max= 358–364 nm and a quantum yield of of 0.34–0.70, with the highest value found for the SMe

0.69–0.86 despite the solvent changing from hexane to counterpart (2c), while their fluorescent lifetimes measured

dichloromethane and toluene.

in CH₂Cl₂range from 0.55 to 1.88 ns.

(C) Effect of the para Substituent in the Oligo(arylene-

ethynylene)s

We have examined the effect of different para substitu-

ents upon the photoluminescent properties of p-

X(C₆H₄CϵC)_nSiMe₃[n = 2, 3; X = NH₂(2a, 3a), NMe₂

(2b, 3b), H (2h, 3h), SMe (2c, 3c), OMe (2d, 3d), OH (2e,

3e), F (2f, 3f)]. Figure 6 depicts the absorption spectra of

2a–2f and 2h in dichloromethane at 298 K. Compounds 2h

and 2c–2f exhibit vibronically structured π-π* absorption

bands with λ_max

=

302–324 nm [ε

=

(3.8–5.6)×

10⁴dm³mol^–1cm^–1]. For compounds bearing NH₂(2a) and

NMe₂(2b) substituents, there is a broad low-energy π-π*

absorption band with λ_max= 331 and 356 nm [ε = (3.62 and

4.04)×10⁴dm³mol^–1cm^–1], respectively, in addition to the

vibronically structured bands at λ_max= 275–291 nm [ε =

(1.97–2.63)×10⁴dm³mol^–1cm^–1]. Among the p-substituted

derivatives, the SMe congener (2c) has the highest molar

extinction coefficients.

Figure 7. Emission spectra of 2a–2f and 2h in dichloromethane at

298 K (concentration = 2×10^–5moldm^–3, λ_ex= 280–360 nm).

The absorption spectra of p-X(C₆H₄CϵC)₃SiMe₃[X =

NH₂(3a), NMe₂(3b), H (3h), SMe (3c), OMe (3d), OH

(3e), F (3f)] in dichloromethane are shown in the Support-

ing Information (Figure S11). Compounds 3h and 3f ex-

hibit a vibronically structured π-π* absorption band at λ_max

Ϸ 332 nm [ε = (6.95 and 6.02)× 10⁴dm³mol^–1cm^–1, respec-

tively], while 3a–3e show a broad π-π* absorption band at

336–369 nm [ε = (5.88–8.08)×10⁴dm³mol^–1cm^–1] in

dichloromethane. The emission spectra of 3a–3f and 3h in

dichloromethane are shown in Figure S12. Upon excitation

at 330 nm, the emission of 3h and 3f are vibronically struc-

tured with peak maxima at ca. 362 nm and quantum yields

of 0.78–0.80. The Stokes shifts (around 2600 cm^–1) for 3h

3

and 3f are consistent with the (π-π*) excited states of aryl-

ene-ethynylenes. For other para-substituted congeners (3c–

3e), single emission bands were observed at λ_max= 394–

410 nm with quantum yields of 0.83–0.85. The intensely

emissive SMe-substituted 3c displays the highest Φ value of

0.85. For amino- and dimethylamino-substituted derivatives

(3a and 3b), emissions in dichloromethane solutions appear

at λ_max= 457 (Φ = 0.84) and 488 nm (Φ = 0.72), with

Stokes shifts of 7100 and 6610 cm^–1, respectively. Hence, an

Figure 6. UV/Vis absorption spectra of 2a–2f and 2h in dichloro-

methane at 298 K.

The emissions of 2a–2f and 2h in dichloromethane at

298 K have also been examined (Figure 7). The emission

spectrum of H(C₆H₄CϵC)₂SiMe₃(2h) exhibits vibronically

structured bands, which are attributed to the ³(π-π*) excited

states of the diphenylacetylene. The Stokes shift between

the emission maximum of 2h at 328 nm from its absorption

maximum at 303 nm in dichloromethane is 2520 cm^–1,

which matches those for typical ¹(π-π*) excited states.^[24]

Similar spectral features have also been found for 2f with

the Stokes shift being 4130 cm^–1. The emissions of 2a–2e in

dichloromethane are broad, with λ_maxranging from 367

(2e) to 440 nm (2b) (Table 1). The Stokes shifts between the

3

assignment of (π-π*) excited states is not favored, and we

tentatively assign the emission of 3a and 3b to originate

from n Ǟ π* transitions of the (arylene-ethynylene) mole-

cules.

Theoretical Calculations

To rationalize the differences in photoluminescent prop-

π-π* absorption and emission energies for 2a–2e are larger erties between p-H₂N(C₆H₄CϵC)₂SiMe₃(2a) and

than those for 2f and 2h. We tentatively assign the emission H(C₆H₄CϵC)₂SiMe₃(2h), the electronic structures of the

of 2a–2e to originate from n Ǟ π* transitions. The emission ground and excited states of p-H₂N(C₆H₄CϵC)₂SiH₃(2aЈ)

maxima (λ_max) for 2a–2f and 2h follow a similar trend to and H(C₆H₄CϵC)₂SiH₃(2hЈ) have been examined through

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FULL PAPER

TD-DFT calculations. Previous reports have shown that ro- absorption energy blue-shifts from 323 nm for the coplanar

tation about the phenyl–acetylene single bond is essentially conformation to 281 nm when the two phenyl rings are per-

frictionless due to the cylindrical symmetry of the triple pendicular to each other. From Table 4, this transition is π

bond, which is able to maintain conjugation between the Ǟ π* in nature. As the calculated absorption peaks at 323

phenyl rings irrespective of the relative orientations of the and 312 nm with the relative phenyl ring orientations of 0°

aromatic planes.^[27–29]This leads to the rapid equilibration and 45° are consistent with the experimental values of 321

and coexistence of the coplanar and twisted conformations and 309 nm, respectively, for 2h in n-hexane, we assign that

in the ground state. Upon electronic excitation, on the other these two peaks are derived from the π Ǟ π* transition.

hand, changes in bond order along the C(phenyl)–CϵC

fragment, from alternate single-triple bonds in the ground

state (S₀) to a cumulene-type (C=C=C) bonding situation in

Table 4. % Contribution of the ground state frontier molecular or-

bitals (FMO) of 2hЈ.

the first singlet excited state (S₁), may afford larger energy

differences between the coplanar and twisted conforma-

tions. In order to investigate how these geometric changes

can affect the absorption and fluorescence spectra, three

relative orientations of the phenyl rings (θ) were sampled,

viz. 0°, 45°, and 90° for both the ground and the first singlet

excited states of 2aЈ and 2hЈ, and the calculation results are

reported in Tables 2 and 3, respectively.

Table 2. Computed relative energies E [eV], absorption λ_abs[nm],

and fluorescence energies λ_F[nm] of the gas phase optimized

ground and the first singlet excited state structures for 2aЈ at vari-

ous relative orientations of the two phenyl rings θ [°]. The oscillator

strengths (f) are in the parentheses.

S₀

S₁

θ

E

0

45

0.027

347

90

0.047

350

0

0.00

45

0.047

90

[a]

0.000

λ_abs348

(1.4521) (0.8047) (0.0000)

[a]

λ_F

371

378

(1.6239) (0.8745)

[a] Not converged.

Table 3. Computed relative energies E [eV], absorption λ_abs[nm],

and fluorescence energies λ_F[nm] of the gas phase optimized

ground and the first singlet excited state structures for 2hЈ at vari-

ous relative orientations of the two phenyl rings θ [°]. The oscillator

strengths (f) are in the parentheses.

S₀

S₁

θ

E

0

45

0.021

312

90

0.043

281

0

0.00

45

0.010

90

0.063

For 2aЈ, the frontier orbitals are less delocalized com-

pared with 2hЈ at the angles studied (Table 5). Thus, the

changes in the HOMO–LUMO energy gap are smaller for

2aЈ than 2hЈ along the rotation coordinate, θ. As the lowest

optically active transition is derived from a HOMO Ǟ

LUMO transition [except for θ = 90° where this transition

is calculated to be optically inactive (f = 0)], the calculated

0.000

λ_abs323

(1.4804) (1.1585) (1.2006)

λ_F

350

337

302

(1.5445) (1.2072) (1.2439)

As expected, the rotational barrier of the phenyl rings absorption energy blue-shifts only very slightly from 348 to

about the triple bond is very small (less than 0.047 and 347 nm at θ = 0° and 45°, respectively. Here, the character

0.043 eV for the ground states of 2aЈ and 2hЈ, respectively). of the HOMO is dominated by the second phenylacetylene

At 298 K, an ensemble of rotamers should coexist in the unit [CC(2) and Ph(2)] and the NH₂group while the

sample. For 2hЈ, as the phenyl rings rotate from a fully LUMO is mainly composed of the first phenylacetylene

planar geometry to a perpendicular conformation, the ex- unit [CC(1) and Ph(1)]. This is in direct contrast to the case

tent of frontier orbital delocalization decreases (Table 4). of 2hЈ, where both the HOMO and LUMO contain major

Hence, the energy gap between the HOMO and LUMO contributions from the first phenylacetylene unit [Ph(1) and

(∆E_H–L) increases from coplanar to perpendicular confor- CC(1)]. Since the calculated absorption wavelengths are in

mations. We note that the lowest optically active (oscillator good agreement with the experimental value of 343 nm for

strength f Ͼ 0) excitation corresponds to the HOMO Ǟ 2a in n-hexane, this peak is assigned to an n Ǟ π* (mixed

LUMO transition for all angles studied in this work. The with some π Ǟ π*) charge transfer transition.

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Photoluminescence of Oligo(arylene-ethynylene)s

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Table 5.% Contribution of the ground state frontier molecular or-

bitals (FMO) of 2aЈ.

Table 6.% Contribution of the first singlet excited state frontier mo-

lecular orbitals (FMO) of 2hЈ.

Table 7.% Contribution of the first singlet excited state frontier mo-

lecular orbitals (FMO) of 2aЈ.

Upon excitation of 2aЈ (and 2hЈ) to the first singlet ex-

cited state, the molecule relaxes to a cumulene-like structure

with respect to the C(phenyl)–CϵC unit, making the tor-

sional barrier between the coplanar and perpendicular con-

formations higher. For instance, the rotational barrier is cal-

culated to be 0.063 eV for the first singlet excited state

(compared with 0.043 eV for the ground state) of 2hЈ. The

character of the HOMO and LUMO in the first singlet (S₁)

excited state for 2hЈ and 2aЈ (Tables 6 and 7, respectively)

are essentially the same as in the ground state. As the calcu-

lated emission energies of 350 and 337 nm are in good

agreement with the experimental values of 347 and 324 nm

measured for 2h in n-hexane, these two peaks are assigned

to originate from the π Ǟ π* transition. On the other hand,

the calculated emission energies at 371 and 378 nm are in

excellent agreement with the experimental emission of 2a at

372 nm in n-hexane, and hence we assign this peak to an n

Ǟ π* (mixed with some π Ǟ π*) charge transfer excited

state. The proposed charge transfer nature of the fluores-

cence band for 2a may explain the prominent observed sol-

vatochromism (see above). On the contrary, 2h displays only

small spectral shifts upon changes in solvent polarity be-

cause the transition is localized in nature.

Application of Oligo(arylene-ethynylene)s for

Bioconjugation

The N-hydroxysuccinimidyl (NHS) ester is a commonly

used functional group for covalent attachment of chromo-

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FULL PAPER

phoric labels to primary amino moieties (usually lysine resi- cent fragments per HSA ranges from 7.5 to 10.6, whereas

dues) of biomolecules such as albumins, enzymes, and im- that of 5b (5.2) is lower (Figure S13).

munoglobulins.^[30]In this work, a series of fluorescent oli-

The degree of labeling for (5a–5f)–HSA can also be de-

go(arylene-ethynylene)s were functionalized with amine-re- termined by absorption spectroscopy^[32,33]with the assump-

active succinimidyl 4-iodobenzoate, p-X(C₆H₄CϵC)_n- tion of additive absorption characteristics (see Supporting

C₆H₄NHS [n = 1, X = NH₂(5a), NMe₂(5b), SMe (5c), Information). The values for (5a–5c)–HSA are 6.4, 4.4 and

OMe (5d), OH (5e), F (5f); n = 2, X = NH₂, NMe₂] and p- 8.1, respectively, which approach the calculated ones from

Me₂NC₆H₄CϵC(C₄H₂S)CϵCC₆H₄NHS. We have per- mass spectrometric analysis (7.6, 5.2, and 8.1, respectively).

formed cytotoxicity studies of 5b and p-Me₂N(C₆H₄- However, for (5d–5f)–HSA, the value obtained by the UV/

CϵC)₂C₆H₄NHS on lung cancer cells, revealing that their Vis spectroscopic method (4.3, 5.3, and 2.7, respectively)

inhibitory concentrations (IC₅₀) are higher than 100 µ, shows a larger discrepancy from that using mass spectrome-

suggesting the benign nature of this class of fluorescent try (9.3, 7.5, and 10.6, respectively). The degree of labeling

OPE labels toward cells. The optimal fluorophore/HSA (F/ for p-Me₂NC₆H₄CϵC(C₄H₂S)CϵCC₆H₄NHS–HSA was

P) ratio for labeling has been determined by comparing the determined by MALDI-TOF and absorption spectrometry

emission intensity of the conjugates with the intensity of a to be 6.8 and 7.4, respectively.

66.6 kDa stained band in gel electrophoresis using F/P ra-

tios of 1:1, 2.5:1, 5:1, 7.5:1 and 10:1. The emission spectra

Photoluminescence of Oligo(arylene-ethynylene)–HSA

of the 5b–HSA conjugate labeled with F/P ratios of 7.5:1

Conjugates

and 10:1 reveal the 480 nm emission band at the highest

intensity. Gel electrophoresis of conjugates with different F/

The photophysical properties of 5a–5f and their HSA

P ratios was performed, and the bands were stained with conjugates have been investigated (Table 1 and Supporting

Coomassie Blue. Based on the brightness of the 66.6 kDa Information). The absorption spectra of 5a and 5b in

bands, the conjugation saturated at the F/P ratio of 10:1. CH₂Cl₂, MeCN, THF and ethyl acetate consist of two

Hence, we subsequently employed the F/P labeling ratio of distinct peaks at 269–280 [ε

=

(1.24–2.42)×

10:1 to enable complete conjugation for characterization 10⁴dm³mol^–1cm^–1] and 354–385 nm [ε = (1.94–3.46)×

and photophysical studies. Further increases of the F/P ra- 10⁴dm³mol^–1cm^–1], whereas the emission spectra show

tio to 20:1, 50:1 and 100:1 were performed in the protein broad peaks at λ_max= 490–504 (5a) and 522–566 nm (5b),

conjugation; however, precipitation of fluorescent dyes was respectively. Both the absorption and emission bands of 5a

observed.

and 5b are blue-shifted in protic solvents (4% DMF/H₂O

Characterization of (5a–5f)–HSA and p-Me₂NC₆- and 4% DMF/PBS). The emission quantum yields of both

H₄CϵC(C₄H₂S)CϵCC₆H₄NHS–HSA conjugates and their compounds in CH₂Cl₂, THF, and ethyl acetate are in the

degree of labeling have been investigated by MALDI-TOF range of 0.034–0.096, whereas those in MeCN, 4% DMF/

mass spectrometry^[31]and UV/Vis absorption spec- H₂O and 4% DMF/PBS are greatly diminished (Ͻ 0.004).

troscopy.^[32,33]The difference in molecular mass between the Upon conjugation with HSA, the emission quantum yields

(5a–5f)–HSA conjugates and HSA (66.6 kDa) was calcu- of 5a–HSA and 5b–HSA in PBS solution show a dramatic

lated and divided by the mass of the p-XC₆H₄CϵCC₆H₄- increase to 0.061 (λ_max= 475 nm) and 0.13 (λ_max= 480 nm),

(O)C labeling fragment to afford the mean number of flu- respectively.

orophores per HSA molecule (see Experimental Section).

Similarly, the emissions of 5c–5e in aqueous medium (4%

Figure 8 depicts the MALDI-TOF mass spectra of 5a–HSA DMF in H₂O and 4% DMF in PBS) are weak (0.0017–

and HSA. The calculated number of 5a and 5c–5f fluores- 0.059) with diminished Φ values compared to those in non-

aqueous solutions. Upon conjugation with HSA, the emis-

sion exhibits a higher-energy band with enhanced Φ value

(0.02–0.29). Perturbation of the photoluminescent proper-

ties therefore confirms their protein-binding capabilities.

Photoluminescent properties of the p-X(C₆H₄CϵC)₂-

C₆H₄NHS–HSA (X = NH₂, NMe₂) conjugates have also

been examined; their absorption spectra in 4% DMF/PBS

show a 278 nm band with broad shoulders at 316–358 nm,

and they show less intense emissions at λ_max= 455 and

478 nm, respectively, compared to 5a–HSA and 5b–HSA.

This general method for modifying OPEs into N-hydroxy-

succinimidyl derivatives can be applied to a broader range

of fluorescent dyes; for example, the thiophene counterpart

has also been prepared. The absorption and emission max-

ima of the p-Me₂NC₆H₄CϵC(C₄H₂S)CϵCC₆H₄NHS–

HSA conjugate in PBS lie at 362 and 511 nm, respectively.

These values show a red shift in energy compared to the p-

Me₂N(C₆H₄CϵC)₂C₆H₄NHS–HSA congener.

Figure 8. MALDI-TOF mass spectra of 5a–HSA conjugate (

and HSA (-----) (∆m/z gives a labeling ratio of 7.6:1).

)

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Photoluminescence of Oligo(arylene-ethynylene)s

FULL PAPER

Acrylamide Gel Electrophoresis

sion maximum and quantum yield with solvent polarity; 3a

and 3b show vibronically structured emissions in hexane at

λ_max= 377 and 392 nm, which red-shift to 520 and 545 nm,

respectively, in acetonitrile. On the contrary, very minor sol-

vatochromic shifts are observed for 1h–4h, which is consis-

tent with the π-π* emissive excited state assignment. Using

theoretical calculations, the emission of the electron-donat-

ing NH₂derivative (2a) is attributed to an n Ǟ π* (mixed

with some π Ǟ π*) charge transfer excited state. The nature

of this distinctive transition explains the difference in solva-

tochromic behavior of NH₂(1a–4a) and NMe₂derivatives

(1b–4b) compared to unsubstituted (1h–4h) counterparts.

The interchange between (π-π*) and (n-π*) excited states

have been achieved by varying the para substituent in 2c–

2f and 3c–3f. Systematic studies on the physical (melting

point increases with increasing the conjugation length) and

spectroscopic properties of these OPE molecules are useful

for a fundamental understanding in the area of conducting

polymers.

These oligo(arylene-ethynylene)–HSA conjugates exhibit

green fluorescence under UV irradiation. Using native gel

and sodium dodecyl sulfate polyacrylamide gel electropho-

resis (SDS-PAGE), (5a–5f)–HSA are observed to display

fluorescent bands under excitation with UV light (Support-

ing Information) and upon staining with Coomassie Blue

(Figure 9). The brightness of the fluorescent bands for the

(5a–5f)–HSA conjugates was found to correlate with their

emission quantum yields in PBS (Table 1). For instance, the

fluorescence of the SMe-substituted conjugate 3c–HSA is

the most intense, which agrees with the highest Φ value of

0.29 in PBS.

The versatile functionalization of conjugated polymers

and oligomers enables the application of biologically rel-

evant pendants for binding DNA,^[34]proteins,^[35]or other

biomolecules.^[36]In this work, our strategy entails the esteri-

fication of oligo(arylene-ethynylene)s with the N-hydroxy-

succinimidyl group, which reacts with lysine moieties in

proteins and enables the covalent attachment of fluoro-

phores to biomolecules. We have demonstrated that the

high quantum efficiencies, large Stokes shifts (maximum

11530 cm^–1) and strong solvatochromism (except H- and F-

substituted derivatives) of the oligo(arylene-ethynylene)s

can confer rich photophysical characteristics upon the re-

sultant bioconjugates. The absorption and emission ener-

gies could also be red-shifted by derivatizing with thiophene

at the oligo(arylene-ethynylene) chain.

The (5a–5f)–HSA, p-X(C₆H₄CϵC)₂C₆H₄NHS–HSA (X

= NH₂, NMe₂) and p-Me₂NC₆H₄CϵC(C₄H₂S)CϵCC₆H₄-

NHS–HSA conjugates were characterized by MALDI-TOF

mass spectrometry and UV/Vis absorption spectroscopy.

The optimal (5a–5f)/HSA ratio for labeling was determined

to be 10:1, whereas the degree of labeling, as estimated by

MALDI-TOF mass spectrometry and UV/Vis spec-

troscopy, were compared. The gel electrophoresis reveals

green fluorescent bands for the oligo(arylene-ethynylene)–

HSA conjugates under UV irradiation. The conjugates ex-

hibit molecular masses that are consistent with those of

HSA and a 66 kDa marker standard, thus confirming the

Figure 9. Native gel electrophoresis of (5a–5f)–HSA (marked as a–

f, respectively) in 10% (w/v) polyacrylamide gel after Coomassie

Blue staining.

The mobilities of the (5a–5f)–HSA conjugates were ex-

amined by performing the electrophoresis in parallel with

HSA and a 66 kDa marker (S). The native gel and SDS-

PAGE of (5a–5f)–HSA were stained with Coomassie Blue

after electrophoresis (Supporting Information), and the

(5a–5f)–HSA bands showed similar mobilities to unlabeled

HSA. The total mass of conjugated fluorophores is insig-

nificant in comparison to that of HSA, according to the

degree of labelling described earlier, and this indicates that

the mobilities of (5a–5f)–HSA should be close to HSA.

Conjugation of 5a–5f with HSA have also been verified by

the coherent mobilities of (5a–5f)–HSA against that of the

66 kDa marker of a high-range SDS-PAGE molecular

weight standard (S) (Supporting Information). The gel elec-

trophoresis of p-X(C₆H₄CϵC)₂C₆H₄NHS–HSA (X = NH₂,

NMe₂) and p-Me₂NC₆H₄CϵC(C₄H₂S)CϵCC₆H₄NHS–

HSA conjugates reveals similar results to those of 5a–HSA

and 5b–HSA, although the bands are less intense.

Concluding Remarks

The photophysical properties and solvatochromic re- protein-binding ability of 5a–5f.

sponses of series of oligo(arylene-ethynylene)s, p-

a

X(C₆H₄CϵC)_nSiMe₃with different chain lengths and para

substituents have been examined. For a given para substitu-

ent [1a–4a (X = NH₂), 1b–4b (X = NMe₂), and 1h–4h (X

= H)], the absorption and emission maxima show batho-

chromic shifts with increasing phenylene units (n), and lin-

ear correlations were observed for absorption or emission

energies (in wavenumber) vs. 1/n. For compounds with NH₂

(1a–4a) and NMe₂(1b–4b) termini, the π-π* absorptions

are moderately sensitive to solvent polarity. The emission

Experimental Section

[19]

General Procedures: p-H₂N(C₆H₄CϵC)₂SiMe₃

and p-

Me₂NC₆H₄CϵCH^[20]were prepared as described in the literature.

H(C₆H₄CϵC)₂SiMe₃,^[19b,21]H(C₆H₄CϵC)₃SiMe₃,^[22]and H(C₆H₄-

[23]

CϵC)₄SiMe₃

were prepared by reaction of [(4-iodophenyl)eth-

ynyl]trimethylsilane with H(C₆H₄CϵC)_nH (n = 1, 2 and 3, respec-

tively). Dichloromethane used for photophysical studies was

washed with concentrated sulfuric acid, 10% sodium hydrogencar-

data show more discernible evidence for variations in emis- bonate, and water, dried with calcium chloride, and distilled from

Eur. J. Org. Chem. 2006, 3125–3139

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3135

Y.-G. Zhi, S.-W. Lai, Q. K.-W. Chan, Y.-C. Law, G. S.-M. Tong, C.-M. Che

FULL PAPER

calcium hydride. Acetonitrile used for photophysical measurements

was distilled from potassium permanganate and calcium hydride.

All other solvents were of analytical grade and purified according

to literature methods.^[37]Human serum albumin (HSA) was pur-

chased from Sigma Chemical Co. Protein labeling experiments were

performed in a 50 m hydrogencarbonate buffer of pH = 9.0,

which contained 1.59 g of Na₂CO₃and 2.93 g of NaHCO₃in 1 L

of doubly distilled water. A 20 m phosphate buffered saline (PBS)

solution of pH = 7.2 was prepared by dissolving 1.25 g of

Labeling of Protein: Protein labeling experiments were carried out

using a label/protein molar ratio of 10:1 in a 50 m hydrogencar-

bonate buffer of pH = 9.0.^[30a]To a solution of 0.77 mmol of NHS-

capped fluorescent labels [5a–5f, p-X(C₆H₄CϵC)₂C₆H₄NHS (X =

NH₂, NMe₂), or p-Me₂NC₆H₄CϵC(C₄H₂S)CϵCC₆H₄NHS] in

50 µL of anhydrous dimethylformamide was added a stirred protein

solution (5 mg of HSA in 1 mL of the hydrogencarbonate buffer).

The mixture was incubated at 298 K for 5 h and was then diluted

to 2.5 mL with PBS solution (20 m, pH = 7.2). Purification of

Na₂HPO₄, 0.35 g of NaH₂PO₄, and 8.0 g of NaCl in 1 L of doubly the protein conjugates was performed using gel permeation

distilled water. Gel permeation chromatography (GPC) was carried chromatography (GPC) in a 1×10 cm column filled with Sephadex

out using Sephadex G-25 (medium) chromatography resin from

Amersham Biosciences as the stationary phase in a PD-10 desalting

column (1×10 cm) and a 20 m PBS solution (pH = 7.2) as eluent.

High-resolution electron ionization (EI) mass spectra were ob-

G-25 and ca. 3.5 mL PBS (20 m, pH = 7.2) as the eluent. A solu-

tion of ca. 3.5 mL purified label–HSA conjugate was collected.

Gel Electrophoresis: Native polyacrylamide gel electrophoresis

(PAGE) analysis was performed using a 4% stacking gel and 10%

resolving gel, which were prepared according to the Bio-Rad Mini-

PROTEAN^®3 Cell Instruction Manual. The 10% resolving gel was

prepared by mixing deionized water (6.7 mL), 40% acrylamide/bis

solution (3.3 mL), and lower gel buffer [1.5  tris(hydroxymethyl)-

aminomethane, pH = 8.8, 3.3 mL]; 10% ammonium persulfate (AP,

200 µL) and N,N,NЈ,NЈ-tetramethylethylenediamine (TEMED,

10 µL) solutions were added, the mixture was stirred gently, and

the solution was poured between glass plates for casting. The 4%

stacking gel was prepared by mixing deionized water (3.25 mL),

40% acrylamide/bis solution (0.5 mL), upper gel buffer [0.5  tri-

s(hydroxymethyl)aminomethane, pH = 6.8, 1.25 mL], 10% AP

(100 µL), and TEMED (10 µL). The stacking gel solution was

poured between the glass plates above the resolving gel and a 10-

well comb was inserted. The stacking gel was allowed to polymerize

and the comb was then gently removed. The gel cassette sandwich

1

tained with a Finnigan MAT 95 mass spectrometer. H (500 MHz)

and ¹³C (126 MHz) NMR spectra were recorded with a DPX 500

Bruker FT-NMR spectrometer with chemical shifts (in ppm) rela-

tive to tetramethylsilane. Elemental analyses were performed by the

Institute of Chemistry at the Chinese Academy of Sciences, Beijing.

Infrared spectra were recorded with a BIO RAD FTIR spectropho-

tometer. Thermal analyses were performed with a Perkin–Elmer

TGA 7 thermogravimetric analyzer and a Perkin–Elmer DSC 7

differential scanning calorimeter (heating rate 15 °C/min, under

N₂). UV/Vis spectra were recorded with a Perkin–Elmer Lambda

19 UV/Vis spectrophotometer or with a Hewlett–Packard HP8453

spectrophotometer. Matrix-assisted laser desorption ionization

time-of-flight (MALDI-TOF) mass spectrometry was performed

with a Finnigan Lasermat with a 337 nm nitrogen laser as the en-

ergy source.

was assembled into the inner chamber of the Mini-PROTEAN^®

3

Emission and Lifetime Measurements: Steady-state emission spectra

were recorded with a SPEX 1681 Fluorolog-2 series F111AI spec-

trophotometer. Solution samples for measurements were degassed

with at least four freeze-pump-thaw cycles. The emission spectra

were corrected for monochromator and photomultiplier efficiency

and for xenon lamp stability. Emission lifetime measurements were

performed with an IBH FluoroCube 5000U fluorescence lifetime

system (laser source with peak nominal at 295 or 373 nm and op-

tical pulse durations Ͻ200 ps). The emission quantum yields were

determined using the method of Demas and Crosby^[38]with quinine

sulfate in degassed 0.1  sulfuric acid as a standard reference solu-

tion (Φ_r= 0.546). Errors for λ values ( 1 nm), τ ( 10%), Φ ( 10%)

are estimated.

Cell system and lowered into the Mini tank. Running buffer was

prepared by dissolving 3.03 g of Tris and 14.4 g of glycine in 1 L

of deionized water and used to fill up the inner chamber and the

Mini tank. Sample solutions were prepared by mixing solutions

of labeled protein [(5a–5f)–HSA, p-X(C₆H₄CϵC)₂C₆H₄NHS–HSA

(X = NH₂, NMe₂), or p-Me₂NC₆H₄CϵC(C₄H₂S)CϵCC₆H₄NHS–

HSA] (7 µL) with Laemmli sample buffer (3 µL), and were then

loaded onto the sample wells. The electrophoresis was carried out

using a constant current of 70–90 mA at 200 V, and the running

time was approximately 60 min. Coomassie Blue was used as a

staining agent. For the denaturing sodium dodecyl sulfate poly-

acrylamide gel electrophoresis (SDS-PAGE), the gel preparation

was similar to that of PAGE except that reducing SDS denaturant

was added to the lower and upper gel buffers (0.4%, w/v), and the

running buffer (0.1%).

Computational Details: Calculations on the electronic structures of

p-H₂N(C₆H₄CϵC)₂SiMe₃(2a) and H(C₆H₄CϵC)₂SiMe₃(2h), with

the methyl groups attached to Si replaced by H atoms (herein the

two model compounds are 2aЈ and 2hЈ), were performed using the

Gaussian 03 software package.^[39]A 6-31G* basis set^[40,41]was used

for all atoms in our calculations. Ground state geometries were

fully optimized using the mPW1PW91 (one-parameter modified

Perdew-Wang exchange and Perdew-Wang correlation) func-

tional.^[42]To calculate the fluorescence energies, the geometries of

singlet excited-state would have to be adopted. In this regard, we

used the CIS (configuration interaction with single excitations)^[43]

method for optimization of the geometry of the first singlet excited

state. Absorption and fluorescence energies were computed at the

optimized ground state and first singlet excited-state geometries,

respectively, using time-dependent density functional theory (TD-

DFT)^[44,45]and the mPW1PW91 functional. For simplicity, no sol-

vent has been included in our calculations; only gas phase results

are given, which are expected to be compatible with the experimen-

tal results reported in non-polar solvents, such as n-hexane.

Synthesis of p-H₂N(C₆H₄CϵC)_nSiMe₃(n = 1–4; 1a–4a): 3a and 4a

were synthesized according to a procedure similar to that for p-

H₂N(C₆H₄CϵC)₂SiMe₃(2a).^[19b]p-H₂N(C₆H₄CϵC)_nH (1 mmol; n

=

2, 3) and 1-iodo-4-(trimethylsilylethynyl)benzene (0.35 g,

1.16 mmol) were dissolved in dry THF (40 mL) under nitrogen,

and Et₂NH (10 mL), Pd(PPh₃)₂Cl₂(0.014 g, 0.02 mmol) and CuI

(7.62×10^–3g, 0.04 mmol) were added. Upon stirring the reaction

mixture for 24 h, the solvent was removed under reduced pressure.

The product was extracted with dichloromethane (3×40 mL) and

washed with water (2×30 mL). The organic phase was dried with

anhydrous MgSO₄, concentrated under reduced pressure, and the

crude product was purified by chromatography on a silica gel col-

umn using hexane/CH₂Cl₂(2:1, v/v) as eluent.

General Procedures for Desilylation: The trimethylsilyl derivatives

(1a–3a) were dissolved in CH₃OH/THF (1:1, v/v), and an equi-

molar amount of potassium hydroxide was added. The reaction

3136

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Eur. J. Org. Chem. 2006, 3125–3139

Photoluminescence of Oligo(arylene-ethynylene)s

FULL PAPER

mixture was stirred at room temperature for 4 h, and the solvent

was evaporated under reduced pressure. The product was washed

with water (2×20 mL) and extracted with dichloromethane

(3×40 mL). The organic phase was washed with brine (40 mL) and

dried with anhydrous MgSO₄. The solvent was removed under re-

duced pressure.

0.26 (s, 9 H, SiMe₃), 3.00 (s, 6 H, NMe₂), 6.67 (d, J = 9.0 Hz, 2 H,

C₆H₄), 7.41 (d, J = 8.9 Hz, 2 H, C₆H₄), 7.46–7.50 (m, 12 H, C₆H₄)

ppm. ¹³C{¹H} NMR (CDCl₃): δ = –0.07 (SiMe₃), 40.2 (NMe₂),

87.2, 90.5, 90.9, 91.0, 91.4, 93.0, 96.4, 104.6, 109.7, 111.8, 121.8,

122.9, 123.1, 123.2, 123.3, 124.4, 131.2, 131.4, 131.5, 131.6, 131.9,

132.8, 150.3 ppm. IR (KBr disc): ν = 2206, 2151 (m, CϵC) cm^–1.

˜

TGA (N₂, 15 °C/min) showed 5% weight loss at 295 °C.

3a: Pale yellow solid. Yield 0.35 g, 90%. M.p. (DSC) 234 °C.

Synthesis of H(C₆H₄CϵC)_nSiMe₃(n

=

1–4, 1h–4h):

C₂₇H₂₃NSi (389.57): calcd. C 83.24, H 5.95, N 3.60; found C 82.88,

H 5.93, N 3.46. EI-MS: m/z = 389 [M⁺], 374 [M⁺– Me], 344 [M⁺

–

C₆H₅CϵCSiMe₃(1h) was obtained from Aldrich Chemical Co. and

used as received. H(C₆H₄CϵC)_nSiMe₃(n = 2–4, 2h–4h) were pre-

pared according to literature methods.^[21–23]p-X(C₆H₄CϵC)₂SiMe₃

[X = SMe (2c), OMe (2d), OH (2e), F (2f)] was similarly obtained

by the reaction of p-X(C₆H₄CϵC)H (1 mmol) with 1-iodo-4-(tri-

methylsilylethynyl)benzene (0.35 g, 1.16 mmol).

Me₃]. HRMS: m/z (%) = 389.1603 (100), 390.1633 (39.38). ¹H

NMR (CDCl₃): δ = 0.26 (s, 9 H, SiMe₃), 3.84 (br. s, 2 H, NH₂),

6.64 (d, J = 8.5 Hz, 2 H, C₆H₄), 7.34 (d, J = 8.5 Hz, 2 H, C₆H₄),

7.46 (d, J = 7.6 Hz, 8 H, C₆H₄) ppm. ¹³C{¹H} NMR (CDCl₃): δ

= –0.09 (SiMe₃), 87.2, 90.6, 91.2, 92.4, 96.4, 104.6, 112.3, 114.7,

122.0, 123.0, 123.2, 124.1, 131.3, 131.4, 131.5, 131.9, 133.0,

p-MeS(C₆H₄CϵC)₂SiMe₃(2c): White solid. Yield 0.30 g, 94%.

C₂₀H₂₀SSi (320.52): calcd. C 74.95, H 6.29; found C 74.58, H 6.18.

EI-MS: m/z = 320 [M⁺], 305 [M⁺– Me], 290 [M⁺– Me₂], 275 [M⁺–

Me₃]. HRMS: m/z (%) = 320.1052 (100), 321.1078 (25.47). ¹H

NMR (CDCl₃): δ = 0.26 (s, 9 H, SiMe₃), 2.50 (s, 3 H, SMe), 7.21

(d, J = 8.4 Hz, 2 H, C₆H₄), 7.42–7.44 (m, 6 H, C₆H₄) ppm. ¹³C{¹H}

NMR (CDCl₃): δ = –0.08 (SiMe₃), 15.3 (SMe), 89.1, 91.2, 96.2,

104.7, 119.2, 122.8, 123.4, 125.5, 125.9, 131.3, 131.9, 139.7 ppm.

146.9 ppm. IR (KBr disc): ν = 2202, 2141 (m, CϵC) cm^–1. TGA

˜

(N₂, 15 °C/min) showed 5% weight loss at 245 °C.

4a: Pale orange solid. Yield 0.44 g, 90%. Decomposition tempera-

ture (T_d) 275 °C. C₃₅H₂₇NSi (489.69): calcd. C 85.85, H 5.56, N

2.86; found C 85.47, H 5.78, N 3.09. EI-MS: m/z = 489 [M⁺], 474

[M⁺– Me], 393 [M⁺– CϵCSiMe₃]. HRMS: m/z (%) = 489.1920

(100), 490.1951 (42.31). ¹H NMR ([D₈]THF): δ = 0.24 (s, 9 H,

SiMe₃), 4.92 (br. s, 2 H, NH₂), 6.55 (d, J = 8.5 Hz, 2 H, C₆H₄),

7.20 (d, J = 8.5 Hz, 2 H, C₆H₄), 7.43–7.53 (m, 12 H, C₆H₄) ppm.

¹³C{¹H} NMR ([D₈]THF): δ = –0.08 (SiMe₃), 87.0, 91.1, 91.6, 91.7,

92.1, 94.3, 96.6, 105.5, 110.8, 114.6, 122.6, 123.9, 124.2, 124.3,

124.4, 125.9, 131.8, 131.9, 132.2, 132.3, 132.4, 132.7, 133.6,

IR (KBr disc): ν = 2224, 2158 (m, CϵC) cm^–1.

˜

p-MeO(C₆H₄CϵC)₂SiMe₃(2d): White solid. Yield 0.27 g, 89%.

C₂₀H₂₀OSi (304.46): calcd. C 78.90, H 6.62; found C 78.32, H 6.25.

EI-MS: m/z = 304 [M⁺], 289 [M⁺– Me], 274 [M⁺– Me₂]. HRMS:

1

m/z (%) = 304.1283 (100), 305.1304 (21.23). H NMR (CDCl₃): δ

150.4 ppm. IR (KBr disc): ν = 2206, 2152 (m, CϵC) cm^–1. TGA

˜

= 0.25 (s, 9 H, SiMe₃), 3.83 (s, 3 H, OMe), 6.88 (d, J = 8.7 Hz, 2

H, C₆H₄), 7.43 (s, 4 H), 7.46 (d, J = 8.7 Hz, 2 H, C₆H₄) ppm.

¹³C{¹H} NMR (CDCl₃): δ = –0.09 (SiMe₃), 55.3 (OMe), 84.8, 91.4,

96.0, 104.7, 114.0, 115.1, 122.5, 123.7, 131.2, 131.8, 133.1,

(N₂, 15 °C/min) showed 10% weight loss at 273 °C.

Synthesis of p-Me₂N(C₆H₄CϵC)_nSiMe₃(n = 1–4, 1b–4b): The pro-

cedure for p-H₂N(C₆H₄CϵC)_nSiMe₃(1a–4a) was adopted except

for using p-Me₂N(C₆H₄CϵC)_nH (1 mmol, n = 1–3 for 2b–4b) in

the reaction with 1-iodo-4-(trimethylsilylethynyl)benzene (0.35 g,

1.16 mmol), which was followed by desilylation.

159.8 ppm. IR (KBr disc): ν = 2217, 2158 (m, CϵC) cm^–1.

˜

p-HO(C₆H₄CϵC)₂SiMe₃(2e): Pale yellow solid. Yield 0.27 g, 93%.

C₁₉H₁₈OSi (290.44): calcd. C 78.57, H 6.25; found C 78.21, H 6.05.

EI-MS: m/z = 290 [M⁺], 275 [M⁺– Me], 245 [M⁺– Me₃]. HRMS:

2b: Yellow solid. Yield 0.31 g, 98%. M.p. (DSC) 132 °C. C₂₁H₂₃NSi

(317.51): calcd. C 79.44, H 7.30, N 4.41; found C 79.09, H 7.48, N

1

m/z (%) = 290.1110 (100), 291.1147 (15.15). H NMR (CDCl₃): δ

4.53. EI-MS: m/z = 317 [M⁺], 302 [M⁺– Me], 286 [M⁺– Me₂

–

= 0.25 (s, 9 H, SiMe₃), 5.03 (s, 1 H, OH), 6.80 (d, J = 8.4 Hz, 2 H,

C₆H₄), 7.25–7.42 (m, 6 H, C₆H₄) ppm. ¹³C{¹H} NMR (CDCl₃): δ

= –0.08 (SiMe₃), 87.8, 91.2, 96.1, 104.7, 115.4, 115.6, 122.6, 123.7,

1

H]. HRMS: m/z (%) = 317.1604 (100), 318.1615 (24.81). H NMR

(CDCl₃): δ = 0.25 (s, 9 H, SiMe₃), 3.00 (s, 6 H, NMe₂), 6.66 (d, J

= 9.0 Hz, 2 H, C₆H₄), 7.38–7.41 (m, 6 H, C₆H₄) ppm. ¹³C{¹H}

NMR (CDCl₃): δ = –0.07 (SiMe₃), 40.2 (NMe₂), 87.2, 92.8, 95.7,

104.9, 109.7, 111.8, 121.9, 124.3, 131.0, 131.8, 132.8, 150.2 ppm.

IR (KBr disc): ν = 2208, 2152 (m, CϵC) cm^–1. TGA (N , 15 °C/

131.2, 131.9, 133.3, 155.9 ppm. IR (KBr disc): ν = 3467 (m, OH),

˜

2206, 2143 (m, CϵC) cm^–1.

p-F(C₆H₄CϵC)₂SiMe₃(2f): White solid. Yield 0.28 g, 96%.

C₁₉H₁₇FSi (292.43): calcd. C 78.04, H 5.86; found C 78.08, H 5.51.

EI-MS: m/z = 292 [M⁺], 277 [M⁺– Me]. HRMS: m/z (%) =

˜

2

min) showed 5% weight loss at 241 °C.

1

292.1087 (100), 293.1104 (22.98). H NMR (CDCl₃): δ = 0.26 (s, 9

3b: Yellowish green solid. Yield 0.39 g, 93%. M.p. (DSC) 216 °C.

H, SiMe₃), 7.06 (t, J = 8.7 Hz, 2 H, C₆H₄), 7.49–7.54 (m, 6 H,

C₆H₄) ppm. ¹³C{¹H} NMR (CDCl₃): δ = –0.08 (SiMe₃), 88.9, 90.2,

96.3, 104.6, 115.6, 115.9, 123.0, 131.3, 131.5, 131.9, 133.5 (d, J =

C₂₉H₂₇NSi (417.63): calcd. C 83.40, H 6.52, N 3.35; found C 83.01,

H 6.64, N 3.36. EI-MS: m/z = 417 [M⁺], 402 [M⁺– Me], 386 [M⁺

–

Me₂– H], 372 [M⁺– NMe₂– H], 345 [M⁺– SiMe₃– H]. HRMS:

8.2 Hz), 164.3 ppm. IR (KBr disc): ν = 2219, 2156 (m, CϵC) cm^–1.

˜

1

m/z (%) = 417.1912 (100), 418.1940 (37.15). H NMR (CDCl₃): δ

= 0.26 (s, 9 H, SiMe₃), 3.00 (s, 6 H, NMe₂), 6.66 (d, J = 9.0 Hz, 2

H, C₆H₄), 7.41 (d, J = 8.9 Hz, 2 H, C₆H₄), 7.44–7.47 (m, 8 H,

C₆H₄) ppm. ¹³C{¹H} NMR (CDCl₃): δ = –0.09 (SiMe₃), 40.2

(NMe₂), 87.3, 90.5, 91.3, 93.0, 96.3, 104.7, 109.7, 111.8, 121.8,

123.0, 123.3, 124.4, 131.2, 131.4, 131.5, 131.9, 132.8, 150.3 ppm.

p-X(C₆H₄CϵC)SiMe₃[X = NH₂(a), NMe₂(b), SMe (c), OMe (d),

OH (e), F (f)]: These compounds were synthesized by coupling re-

actions of 1-iodo-4-X-benzene with (trimethylsilyl)acetylene.^[19b,46]

Desilylation of p-X(C₆H₄CϵC)SiMe₃according to the procedure

as described in the previous section gave p-X(C₆H₄CϵC)H.

IR (KBr disc): ν = 2209, 2152 (m, CϵC) cm^–1. TGA (N , 15 °C/

˜

2

Synthesis of p-X(C₆H₄CϵC)C₆H₄(O)CO–N(C=OCH₂)₂(5a–5f):

To a solution of succinimidyl 4-iodobenzoate (0.17 g, 0.50 mmol)

and p-X(C₆H₄CϵC)H (0.50 mmol) in THF (50 mL) under nitrogen

in the presence of Et₃N (20 mL) were added Pd(PPh₃)₂Cl₂

(14.0 mg, 0.02 mmol) and CuI (7.62 mg, 0.04 mmol). The reaction

mixture was stirred at room temperature under nitrogen for 24 h.

The solvent was evaporated, and the residue was purified by col-

min) showed 5% weight loss at 299 °C.

4b: Yellowish green solid. Yield 0.50 g, 97%. T_d= 306 °C.

C₃₇H₃₁NSi (517.75): calcd. C 85.84, H 6.03, N 2.71; found C 85.43,

H 6.05, N 2.84. EI-MS: m/z = 517 [M⁺], 502 [M⁺– Me], 486 [M⁺

–

Me₂– H], 445 [M⁺– SiMe₃– H]. HRMS: m/z (%) = 517.2228

(100), 518.2244 (24.74), 518.2280 (18.12). ¹H NMR (CDCl₃): δ =

Eur. J. Org. Chem. 2006, 3125–3139

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3137

Y.-G. Zhi, S.-W. Lai, Q. K.-W. Chan, Y.-C. Law, G. S.-M. Tong, C.-M. Che

FULL PAPER

umn chromatography (hexane/ethyl acetate, 4:1) to afford a green-

ish-yellow solid, 5a–5f.

5f: Yellow solid. Yield: 0.15 g, 89%. C₁₉H₁₂NO₄F (337.31): calcd.

C 67.66, H 3.59, N 4.15; found C 67.29, H 3.99, N 4.50. EI-MS:

m/z = 337 [M⁺], 223 [M⁺– ON(C=OCH₂)₂], 195 [M⁺– (O=C)-

5a: Greenish-yellow solid. Yield: 0.16 g, 96%. C₁₉H₁₄N₂O₄

(334.33): calcd. C 68.26, H 4.22, N 8.38; found C 67.90, H 4.20, N

8.03. EI-MS: m/z = 334 [M⁺], 220 [M⁺– ON(C=OCH₂)₂], 192

[M⁺– (O=C)ON(C=OCH₂)₂]. ¹H NMR (CDCl₃): δ = 2.92 (s, 4 H,

CH₂), 3.90 (br. s, 2 H, NH₂), 6.66 (d, J = 8.5 Hz, 2 H, C₆H₄), 7.37

(d, J = 8.5 Hz, 2 H, C₆H₄), 7.60 (d, J = 8.5 Hz, 2 H, C₆H₄), 8.09

(d, J = 8.4 Hz, 2 H, C₆H₄) ppm. ¹³C{¹H} NMR ([D₈]THF): δ =

26.3 (CH₂), 86.8, 97.0 (CϵC), 110.0, 114.6, 124.8, 131.0, 132.0,

132.2, 133.9, 150.9, 162.3, 169.9 ppm. 5a–HSA: MS (MALDI-

TOF): m/z = 67895 [MS (MALDI-TOF for HSA): m/z = 66103].

Mean number of p-H₂N(C₆H₄CϵC)C₆H₄(O)C per HSA molecule

= 7.6.

1

ON(C=OCH₂)₂]. H NMR (CDCl₃): δ = 2.92 (s, 4 H, CH₂), 7.08

(t, J = 8.7 Hz, 2 H, C₆H₄), 7.53–7.57 (m, 2 H, C₆H₄), 7.64 (d, J =

8.5 Hz, 2 H, C₆H₄), 8.12 (d, J = 8.5 Hz, 2 H, C₆H₄) ppm. ¹³C{¹H}

NMR (CDCl₃): δ = 25.6 (CH₂), 88.0, 92.8 (CϵC); 115.7, 118.5,

124.3, 130.0, 130.5, 131.8, 133.8, 161.3, 164.6, 169.1 ppm. 5f–HSA:

MS (MALDI-TOF): m/z = 68641 [MS (MALDI-TOF for HSA):

m/z = 66103]. Mean number of p-F(C₆H₄CϵC)C₆H₄(O)C per HSA

molecule = 10.6.

Supporting Information (see footnote on the first page of this arti-

cle): Characterizations and photophysical data, DSC and TGA

thermograms (Figures S1–S4); absorption and emission spectra,

and correlation of absorption and emission characteristics with

chain length (n) and solvent polarity (E_T) (Figures S5–S12).

MALDI-TOF mass spectrometry (Figure S13), gel electrophoresis

(Figures S14–S16), degree of labeling by UV/Vis spectroscopy.

5b: Greenish yellow solid. Yield: 0.17 g, 94%. C₂₁H₁₈N₂O₄

(362.38): calcd. C 69.60, H 5.01, N 7.73; found C 69.22, H 5.00, N

8.13. EI-MS: m/z = 362 [M⁺], 248 [M⁺– ON(C=OCH₂)₂], 220

[M⁺– (O=C)ON(C=OCH₂)₂]. ¹H NMR (CDCl₃): δ = 2.91 (s, 4 H,

CH₂), 3.02 (s, 6 H, NMe₂), 6.67 (d, J = 9.0 Hz, 2 H, C₆H₄), 7.43

(d, J = 8.9 Hz, 2 H, C₆H₄), 7.59 (d, J = 8.6 Hz, 2 H, C₆H₄), 8.08

(d, J = 8.6 Hz, 2 H, C₆H₄) ppm. ¹³C{¹H} NMR (CDCl₃): δ = 25.7

(CH₂), 40.1 (NMe₂), 86.9, 96.1 (CϵC); 108.8, 111.7, 123.1, 130.4,

Acknowledgments

We are grateful for financial support from the Research Grants

Council of the Hong Kong SAR, China (HKU 7039/03P), The

University of Hong Kong (URC-administered Seed Funding Grant

200411159082), and Area of Excellence Scheme (AoE/P-10/01) of

the University Grants Committee of the Hong Kong SAR, China.

131.3, 133.1, 150.6, 161.5, 169.2 ppm. IR (KBr disc): ν = 2211 (m,

˜

CϵC), 1764, 1741, 1597 (s, C=C, C=O) cm^–1. 5b–HSA: MS

(MALDI-TOF): m/z = 67476 [MS (MALDI-TOF for HSA): m/z =

66103]. Mean number of p-Me₂N(C₆H₄CϵC)C₆H₄(O)C per HSA

molecule = 5.2.

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5c: Yellow solid. Yield: 0.16 g, 88%. C₂₀H₁₅NO₄S (365.40): calcd.

C 65.74, H 4.14, N 3.83; found C 65.52, H 4.12, N 3.62. EI-MS:

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1

251 [M⁺– ON(C=OCH₂)₂], 223 [M⁺– (O=C)ON(C=OCH₂)₂]. H

NMR (CDCl₃): δ = 2.51 (s, 3 H, SMe), 2.92 (s, 4 H, CH₂), 7.23 (d,

J = 8.5 Hz, 2 H, C₆H₄), 7.47 (d, J = 8.5 Hz, 2 H, C₆H₄), 7.63 (d,

J = 8.6 Hz, 2 H, C₆H₄), 8.11 (d, J = 8.6 Hz, 2 H, C₆H₄) ppm.

¹³C{¹H} NMR (CDCl₃): δ = 15.2 (SMe), 25.7 (CH₂), 88.4, 93.9

(CϵC); 118.5, 124.1, 125.8, 130.3, 130.5, 131.8, 132.1, 140.6, 161.4,

169.2 ppm. 5c–HSA: MS (MALDI-TOF): m/z = 68262 [MS

(MALDI-TOF for HSA): m/z = 66103]. Mean number of p-

MeS(C₆H₄CϵC)C₆H₄(O)C per HSA molecule = 8.1.

5d: Yellow solid. Yield: 0.16 g, 92%. C₂₀H₁₅NO₅(349.34): calcd. C

68.76, H 4.33, N 4.01; found C 68.76, H 4.39, N 4.33. EI-MS:

m/z = 349 [M⁺], 235 [M⁺– ON(C=OCH₂)₂], 207 [M⁺– (O=C)-

1

ON(C=OCH₂)₂]. H NMR (CDCl₃): δ = 2.92 (s, 4 H, CH₂), 3.85

(s, 3 H, OMe), 6.91 (d, J = 8.9 Hz, 2 H, C₆H₄), 7.50 (d, J = 8.9 Hz,

2 H, C₆H₄), 7.62 (d, J = 8.6 Hz, 2 H, C₆H₄), 8.10 (d, J = 8.6 Hz,

2 H, C₆H₄) ppm. ¹³C{¹H} NMR (CDCl₃): δ = 25.6 (CH₂), 55.4

(OMe), 87.3, 94.3 (CϵC); 114.2, 114.4, 123.8, 130.5, 130.7, 131.6,

133.4, 160.3, 161.5, 169.2 ppm. 5d–HSA: MS (MALDI-TOF): m/z

= 68450 [MS (MALDI-TOF for HSA): m/z = 66103]. Mean

number of p-MeO(C₆H₄CϵC)C₆H₄(O)C per HSA molecule = 9.3.

5e: Yellow solid. Yield: 0.12 g, 72%. C₁₉H₁₃NO₅(335.32): calcd. C

68.06, H 3.91, N 4.18; found C 68.46, H 4.08, N 3.79. EI-MS:

m/z = 335 [M⁺], 221 [M⁺– ON(C=OCH₂)₂], 193 [M⁺– (O=C)-

1

ON(C=OCH₂)₂]. H NMR ([D₆]DMSO): δ = 2.91 (s, 4 H, CH₂),

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3138

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Eur. J. Org. Chem. 2006, 3125–3139

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Systematic studies on photoluminescence of oligo(arylene-ethynylene)s: Tunability of excited states and derivatization as luminescent labeling probes for proteins

DOI: 10.1002/ejoc.200600103

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