Femto- to Microsecond Excited State Relaxation of 9-(4-(N,N-Dimethylamino)phenyl)phenanthrene and 4-(9-Phenanthryl)-3,5-N,N-tetramethylaniline A. Onkelinx, G. Schweitzer, F. C. De Schryver,* H. Miyasaka, and M. Van der Auweraer Department of Chemistry, Katholieke UniVersiteit LeuVen, Celestijnenlaan 200F, BE-3001 HeVerlee, Belgium T. Asahi, H. Masuhara, and H. Fukumura Department of Applied Physics, Faculty of Engineering, Osaka UniVersity, Suita, Osaka 565, Japan A. Yashima and K. Iwai Department of Chemistry, Faculty of Science, Nara Women’s UniVersity, Nara 630, Japan ReceiVed: January 17, 1997; In Final Form: March 11, 1997 X This paper discusses how the solvent-induced rapid relaxation of the initial delocalized excited state of 9-(4- N,N-dimethylaminophenyl)phenanthrene (9DPhen), obtained immediately after picosecond pulsed excitation, can be resolved by means of femtosecond transient absorption experiments. The results obtained for 9DPhen are compared to the results of a sterically hindered compound 4-(9-phenanthryl)-3,5-N,N-tetramethylaniline (3,5Me9DPhen) in order to get more information about the possible conformational relaxation process suggested for these compounds. From the results of the femtosecond transient absorption experiments, a possible model is proposed to characterize the kinetic behavior of these molecules. After photoexcitation of 9DPhen and 3,5Me9DPhen, the distribution of higher excited states shows a fast transition within a femtosecond timescale to a “hot” charge transfer state. This state looses excess energy by a relaxation process (electronic and/or vibrationally and/or conformationally relaxation) on picosecond timescale. From this relaxed excited charge transfer state, fluorescence and intersystem crossing to a triplet state originate simultaneously and in competition. From the comparison of the steady state absorption spectrum of 9DPhen and 3,5Me9DPhen, as well as the transient absorption spectra of the triplet state, one can distinguish the quite different nature of the ground- and the triplet state in both compounds. The bathochromic shift of the emission spectrum of both compounds suggests a larger excited-state dipole moment for 3,5Me9DPhen compared to 9DPhen. The lower values of the radiative rate constant 〈k f 〉 and the longer decay times of 3,5Me9DPhen correlate with a less allowed radiative transition compared to that of 9DPhen. It is suggested that for 3,5Me9DPhen, the emissive state mixes to a smaller extent with a state with a strongly allowed transition and/or that the average angle between the phenyl and phenanthrene moieties of the excited state is larger (farther away from 0) than in the unsubstituted molecule, leading to a less allowed transition and a smaller value of the rate constant of fluorescence. Introduction Conformational folding bringing donor and acceptor π-elec- tron systems into parallel planes, within a distance allowing through-space interactions and Coulomb stabilization of the charge-transfer (CT) state, 1-6 is excluded for molecules consist- ing of an electron donor and an electron acceptor directly connected by a single σ-bond. It has been shown 7-30 that primary excited singlet states of p-(9-anthryl)-N,N-dimethyl- aniline (ADMA) and related compounds undergo solvent- assisted femto- or picosecond relaxation to a polar fluorescent CT state. The geometry and electronic structure of the emitting CT state, however, are still points of discussion. 5,6,11,23 Okada and co-workers 7-9 originally explained their results in terms of an abrupt structural change of the excited ADMA when the solvent was changed from diethylether to more polar solvents. More recent picosecond transient absorption studies by the same authors, however, show a difference between the electronic structure of the excited state of ADMA or structurally similar molecules and that of molecules with strongly orthogonal π-systems of the donor and the acceptor (e.g., 4-(9-anthryl)- N,N-2,6-tetramethylaniline). 23,26,32 They showed that the tran- sient absorption spectra of ADMA in medium polar solvents cannot be presented by a linear combination of the anthracene- like band in weakly polar solvents and the anthracene anion radical band in strongly polar solvents. They therefore ex- pressed doubts as to the validity of the simple two-state TICT (twisted intramolecular charge transfer) model for ADMA and related compounds 11,31 according to which the excited singlet state undergoes an adiabatic intramolecular electron transfer and where two metastable states are assumed to interconvert by a torsion motion, which provides a possible reaction coordinate for the electron transfer. It seems to be necessary to invoke “multiple states” with different degrees of charge transfer and twisting angle, depending on the interaction with solvent. A similar conclusion was also reached in recent quantum chemical calculations, 33 where twist angle dependent values of transition energies, oscillator strengths, and dipole moments were calculated. Also, here the authors expressed doubts as to the validity of the simple two-state TICT model. Recently, 34 a CT absorption band in the spectrum of 9-(4-N,N-dimethyl- aminophenyl)phenanthrene (9DPhen; Figure 1) has been de- tected, which indicates an interaction between the dimethyl- anilino- and the phenanthrene subunits in the ground state. * To whom correspondence should be addressed. X Abstract published in AdVance ACS Abstracts, June 15, 1997. 5054 J. Phys. Chem. A 1997, 101, 5054-5062 S1089-5639(97)00256-9 CCC: $14.00 © 1997 American Chemical Society