21 July 20000 Ž . Chemical Physics Letters 325 2000 196–202 www.elsevier.nlrlocatercplett Intrinsic conformer jumps observed by single molecule spectroscopy in real time Frank Stracke a , Christian Blum a , Stefan Becker b , Klaus Mullen b , ¨ Alfred J. Meixner a, ) a Physikalische Chemie, UniÕersitat Siegen, Adolf Reichwein Strasse 2, 57068 Siegen, Germany ¨ b Max Planck Institut f ur Polymerforschung, Ackermannweg 10, 55128 Mainz, Germany ¨ Received 3 April 2000; in final form 16 May 2000 Abstract Ž. Ž. Single molecule spectral dynamics originate from changes 1 within the chromophore and 2 in the chromophoric environment, hence are intrinsic and extrinsic. To interpret spectral phenomena of intrinsic origin, assignment of fluorescence behaviour to distinct molecular states is required. We realized this by means of a perylene based chromophore with an amino substituent. The fluorescence spectrum of this dye depends strongly on the amino group conformation relative to the chromophore. Thus different conformers of the chromophore can be distinguished. Here we evidence the occurrence of spectrally defined conformers and report transitions between them, revealed by single molecule spectroscopy. q 2000 Elsevier Science B.V. All rights reserved. 1. Introduction Spectral dynamics and intensity fluctuations are characteristic for single molecule fluorescence, whereas the underlying mechanics are rarely under- w x stood and subject to speculation 1–12,14 . Except intersystem crossing, the so far explainable processes are related to the chromophore’s environment and w x are thus extrinsic 7–9 . Conformational changes ob- served by single molecule fluorescence resonance Ž . energy transfer FRET techniques take place in a w x macromolecule linking two chromophores 13 . The ) Corresponding author. Fax: q 49-271-7402-805; e-mail: meixner@mail.pc.chemie.uni-siegen.de chromophores’ conformations remain unaffected; consequently the observed effect is of extrinsic na- ture. Spectral effects due to intrinsic conformational changes have not yet been described, as ensembles only permit to determine averaged conformer proper- ties. To distinguish between conformers on single molecule level the conformational changes must af- fect the emission spectra of the fluorophores suffi- ciently. Furthermore the surrounding matrix has to allow these processes. We recorded fluorescence as well as absorbance Ž . bulk spectra of 9-amino-N- 2,6-diisopropylphenyl - Ž perylen-3,4-dicarboximid in the following: API, Fig. . Ž . 1 and its derivative N, N-di- tert-butoxycarbonyl - Ž . API in the following: DAPI, Fig. 1 ; Fig. 2a,b. In 0009-2614r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. Ž . PII: S0009-2614 00 00633-3