Jürgen H. Gross, Markus Sailer, Bianca Flock, Thomas J. J. Müller

Organisch-Chemisches Institut der Universität, Im Neuenheimer Feld 270, D-69120 Heidelberg

Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry of Linear Chain and Cyclic Phenothiazine Oligomers

Motivation

The major motivation for the synthesis1,2 of the oligophenothiazines (PTn) lies in their potential as electroactive materials (hole conductors). Phenothiazines are highly interesting building blocks for rigid rod- and wire-like molecular modules for a future single molecule electronics. The ultimate goal is the design of phenothiazine-based single molecule devices. These properties are due to their tendency to form stable and planar radical cations.3 The selective synthesis of well-defined monodisperse oligomers also allows reliable structure-property correlations and the determination of effective conjugation lengths in PTn. Further studies are directed towards polymerizations of phenothiazines and require reliable characterization, preferably by matrix-assisted laser desorption/ionization.4,5

[1] M. Sailer, R.-A. Gropeanu, T. J. J. Müller, J. Org. Chem. 2003, 68, 7510. [2] C. S. Krämer, T. J. Zimmermann, M. Sailer, T. J. J. Müller, Synthesis 2002, 9, 1163.[3] D. Pan, D. L. Phillips, J. Phys. Chem. A 1999, 103, 4737. [4] Mass Spectrometry of Polymers, G. Montaudo, R. P. Lattimer, eds., CRC Press, Boca

Raton, 2001.[5] H. Pasch, W. Schrepp, MALDI-TOF Mass Spectrometry of Synthetic Polymers, Springer-

Verlag, Heidelberg, 2003.

Cyclic PT Oligomers

Linear PT Oligomers

Interestingly, all PT oligomers form molecular ions, M+•, with all matrices. Even the strongly protonating SA and DHB only yield minor portions of [M+H]+ ions. This behavior can be explained by the easy formation of positive PTn radical ions: PTn radical cations are planar and aromatic, while the neutrals are not. Resulting from the selective generation of radical ions with DI, the experimental isotopic patterns do not suffer from superimposition with other ionic species and thus, are in very good agreement with those calculated.

Radical Ions Only

6. Igler MS-Tage, Igels, Feb. 16-18, 2005

References

Experimental

Positive-ion MALDI-TOF spectra have been measured in the reflector mode of a BrukerBiflex instrument equipped with Pulsed Ion Extraction. The instrument was calibrated with a protein mixture covering the m/z 1000-3500 range. This external calibration yielded a mass accuracy of normally better than ± 0.3u.SA was employed as a saturated solution in acetonitrile. DHB and DI were each dissolved in acetone at 10 mg ml–1. Linear and cyclic phenothiazine oligomers were dissolved in chlorobenzene (PhCl) or tetrahydrofurane (THF). Analyte solutions were admixed to the matrix solutions as to obtain analyte peaks of 1:1 to 1:5 intensity ratio relative to the most abundant matrix ions. About 0.5 µl of the solution was applied to a stainless steel sample holder and allowed to crystallize upon gentle heating.

200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 3000 3200 3400m/z 0

2000

4000

6000

8000

10000

12000

14000

16000

18000

20000a.i.

2000

4000

6000

8000

1910 1950 1990 m/z

a.i.

1887.31895.4 1987.5

1971.5

PT7 DI

S/N = 150

189019101930195019701990 m/z

50

100

150

200

250

300

350

400

450

500

550

600a.i.

200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 3000 3200 3400m/z 0

200

400

600

800

1000

1200

1400

1600

1800

2000

2200

2400

2600

2800a.i.

1887.9

1972.0

1998.0

PT7 DHB

S/N = 40

1880 1900 1920 1940 1960 1980m/z

100

200

300

400

500

600

700

800

900

1000

1100

1200

a.i.

200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 3000 3200 3400 m/z 0

200

400

600

800

1000

1200

1400

1600

1800

2000

a.i. PT7 SA

S/N = 60

1960 1970 1980 1990 m/z

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

5500

6000

6500

7000

7500

8000

8500a.i.

1960 1970 1980 1990 m/z

50

100

150

200

250

300

350

400

450

500

550

600a.i.

1960 1970 1980 1990 m/z

100

200

300

400

500

600

700

800

900

1000

1100

1200a.i.

PT7 DI PT7 DHBPT7 SA1972.0

1970.0

1988.0

1970.2

1971.21972.2

1987.5

1971.5

1970.5

Calculated C126H135N7S7

rel. abund.m/z

0.71977.888

3.21976.888

8.41975.888

19.31974.887

39.41973.887

67.51972.887

95.21971.886

100.01970.885

66.71969.882N

S

RN

S

R

oxidation .

Conjugation effects ease of oxidative radical cation formation.

non-planar planar and aromatic

S

N

S

N

S

N

S

N

S

N

S

N

S

N

S

N

S

N

S

N

S

N

S

N

S

N

S

N

SN

SN

S

N

SN

SN

S

N

2250 2260 2270 2280 2290 m/z

50

100

150

200

250

300

350

400a.i.

600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 3000 3200 m/z

50

100

150

200

250

300

350

400

450

a.i.

DHB (PhCl)8mer

10mer

921.3997.3

1202.4

1688.5 1703.5

1576.5

2250.62266.6

2813.82829.8

3375.912mer

2250.62266.6

2282.6

600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 3000 3200 m/z

50

100

150

200

250

300

350

400

450

500

a.i. DI (THF)2249.9

2266.9

2282.9

1687.7

1704.7

1219.5

6mer

8mer

1560.1

600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 3000 3200 m/z

20

40

60

80

100

120

140

160

180

200

220

240

260

280

300

320

340

380

a.i. DI (PhCl)2250.0

2812.7

2828.7

2845.7

2686.7

2265.6

2281.6

1703.5

1687.5

1719.5

1219.4

6mer

8mer 10mer

2000 2100 2200 2300 m/z

50

70

90

110

130

150

a.i.

600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 3000 3200 m/z

100

200

300

400

500

600

700

800

900

1000

1100

1200

1300

1400a.i. SA (PhCl) 2251.1

2166.0

2080.8

- hexyl

- hexyl

1687.8 2251.12813.4

6mer 8mer10mer

2250 2260 2270 20

40

60

80

100

120

140

160

180

200

220

240

260

280

300

320

340a.i.

2250.6

2266.6

cyclic 8merDI

Whereas THF commonly serves as a versatile solvent for low-polarity oligomers, in case of cyclic PTn it caused the suppression of higher-mass compounds. In particular, the 10mer contained in a reaction mixture of larger cyclic PT oligomers, was not detected from THF solution. Furthermore, oxide impurities are underestimated from MALDI spectra when the oligomer is dissolved in THF. Though less volatile, chloro-benzene performed much better, i.e., effected no suppression of components.

In general, the combination of DI in acetone with sample solutions in chlorobenzene is recommended for use with linear as well as cyclic oligophenothiazines. It yields signals exhibiting correct isotopic patterns due to pure radical ion formation.

DHB still yielded useful MALDI spectra but gave weaker signals than DI even at higher laser power. Oxides were clearly visible in the spectra.While SA can yield useful spectra of small PT oligomers, it requires too much laser power and presents problems with larger rings. In addition, a comparatively strong fragmentation by successive losses of hexyl groups was occasionally observed.

S

N

S

N

S

N

C6H13

n

n = 0-5

Upper formula: cyclic 6mer, examined 4 to 10mer; lower formula: linear oligomers.

The comparison of sinapinic acid (SA), 2,5-dihydroxy-benzoic acid (DHB), and dithranol (DI) revealed that at first sight any of these matrices might be employed for MALDI of the linear PTn.Due to its advantageous crystallization characteristics DI offered best reproducibility. The comparatively low laser power requirements to provide high signal intensities made DI become the most versatile among the matrices studied (cf. signal-to-noise ratios, S/N).Although offering somewhat higher absolute intensities than DHB, SA turned out as the least suitable matrix because it formed layers exhibiting an inhomogeneous spatial distribution of the analyte.In-source decay of the molecular ions by loss of C6H13

. (85 u) is observed for all matrices. However, one advantageous feature of DHB was the reduction and in some cases almost suppression of the metastable decay (also known as post-source decay, PSD) of the molecular ions. Thus, the peak at m/z 1895 is missing in case of PT7.

calc.

calc.calc.

MALDI-TOF spectra of a reaction mixture of cyclic PTn as obtained from

different matrices and solvents.

MALDI-TOF spectra of a sample of the linear PT7 as obtained from

different matrices.

Contact: J. H. Gross, OCI, Im Neuenheimer Feld 270, D-69120 Heidelberg, juergen.gross@urz.uni-heidelberg.de, http://www.rzuser.uni-heidelberg.de/~bl5