ULTRAFAST ELECTRONIC ENERGY FLOW IN A BICHROMOPHORIC MOLECULE Vlastimil FIDLER a1 , Peter KAPUSTA a2 , Milos NEPRAS b , Jorg SCHROEDER c , Igor V. RUBTSOV d1, * and Keitaro YOSHIHARA d2 a Department of Physical Electronics, Czech Technical University, 180 00 Prague 8, Czech Republic; e-mail: 1 fidler@dec1.fjfi.cvut.cz, 2 kapusta@dec1.fjfi.cvut.cz b Department of Organic Technology, University of Pardubice, 532 10 Pardubice, Czech Republic; e-mail: milos.nepras@upce.cz c Institute of Physical Chemistry, University of Gottingen, Tammannstrasse 6, D-37077 Germany; e-mail: jschroe2@gwdg.de d School of Materials Science, Japan Advanced Institute of Science and Technology,1-1 Asahidai, Tatsunokuchi, Ishikawa, 923-12 Japan; e-mail: 1 ivr@jaist.ac.jp, 2 yosihara@jaist.ac.jp Received June 29, 1998 Accepted July 9, 1998 Dedicated to Professor Rudolf Zahradnik on the occasion of his 70th birthday. The intramolecular electronic excitation energy flow was investigated in a specially designed bichro- mophoric molecule, 2-(3-benzanthronylamino)-4-(1-pyrenylamino)-6-chloro-1,3,5-triazine (1) and was compared with the behaviour of two relevant component model compounds that closely mimic the photophysical properties of acceptor and donor sub-units in the bichromophore. Electronic ab- sorption and fluorescence spectroscopy was applied (including fluorescence anisotropy and decay kinetics measurements with nanosecond to femtosecond time resolution) in order to resolve the en- ergy relaxation process on a real time. An unambiguous piece of evidence is reported for an ultrafast process which leads to practically instantaneous population of the emitting state of the acceptor sub- unit after selective ≈200-fs-excitation of the donor sub-unit. This first direct observation of extremely fast energy transfer in a stiff bichromophore is significant for further development of relevant theory. Two conceptually different approaches to explaining such fast energy flow are discussed. Key words: Intramolecular electronic energy transfer; Bichromophore; Ultrafast excited states relax- ation; Vibronic coupling of electronic states; Time-resolved fluorescence anisotropy; Electronic ab- sorption spectroscopy; Fluorescence spectroscopy. Advance in understanding of the mutual interactions among chromophores under a situ- ation, when minimally one of them is electronically excited, is crucial both to gaining insight into some elementary processes in nature (e.g. primary steps in photosynthesis, dynamics of photochemical reactions, etc.) 1 , and to designing new molecular materials for technical application (e.g. non-linear optical materials for optoelectronics 2 ). 1460 Fidler, Kapusta, Nepras, Schroeder, Rubtsov, Yoshihara: Collect. Czech. Chem. Commun. (Vol. 63) (1998) * Permanent address: Institute for Chemical Physics Problems, Chernogolovka, Moscow Region 142-432, Russia.