Femtosecond broad-band pump−probe spectroscopy has been used to study intramolecular bichromophoric coupling and structural relaxation in pyrene and aryl pyrene derivatives in solution. The influence of aryl substituents on the S2 → S1 internal conversion process, which occurs with a time constant of ∼75 fs in pyrene, has been investigated. While in 1-phenylpyrene the internal conversion is faster than 50 fs, it is slower in 1-biphenyl-4-yl-pyrene (105 fs). The temporal evolution of the transient absorption spectrum indicates strong mixing of several “zero-order” electronic configurationswhich evolve separately with timein the S1 and the S2 states. The time-resolved spectra are interpreted within the framework of an adiabatic state model which includes interchromophoric electronic coupling. In this paper we give a full description of the experimental setup, the data acquisition procedure, and several experimental details about the characterization of the broad-band femtosecond white light source.

The photophysical properties of several acceptor substituted 1-arylpyrene derivatives were investigated. The fluorescence spectra strongly depend on the nature of the aryl moiety and the position and number of methoxycarbonyl acceptor groups. Dual fluorescence, originating from a locally excited and a charge transfer state, was observed for the diester derivatives. The solvent dependence of the dual fluorescence and the slightly curved solvatochromic plots indicate a change of the character of the excited states from solvents of low to high polarity. The rate constants for fluorescence and nonradiative decay were calculated to reveal the nature of the excited-state relaxation, that is, the increase of the mesomeric interactions by geometrical flattening or stabilization of the CT state by further twisting toward perpendicularity. Flattening is the major relaxation pathway of the diester phenylpyrene derivative in nonpolar solvents whereas in highly polar solvents the low value for the fluorescence transition dipole moment (M f ) indicates stabilization of the charge transfer state by further twisting. The fluorescence of the diester biphenylpyrene derivative originates from a locally excited state (LE) in nonpolar solvents. The low value for M f in polar solvents and the change of the relative intensity of the dual fluorescence signals with the temperature indicate that the red-shifted fluorescence can be assigned to a twisted intramolecular charge transfer state.

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Efficient syntheses of the novel, pyrene containing branched building blocks 10, 13, 22, 25 and of a pyrene based core molecule 16 for the construction of dendrimers are reported. The main tool is a Suzuki cross-coupling. The functional groups for further growth are amines and carboxylic acids, which were used in an orthogonally protected fashion. It was proven that the building blocks could be assembled to a low generation dendrimer 17.The spherically shape, dendrimers, 1 are unlike practically all other chemical compounds and can potentially be used to generate and investigate a directed charge transfer. 2,3 If the interior of dendrimers is equipped with electron acceptors at defined positions (generations), an electron transfer should be possible from acceptor to acceptor if there is a gradient, which provides the necessary driving force. We are presently pursuing a project to construct dendrimers with a polar gradient and to use them as components to investigate a photochemically induced energy and charge transfer. 4 The polarity gradient should serve as a driving force for either of these transfers and the large distance between the dendrimers' core and surface ought to increase the life-time of an eventual charge separated state. Both an outline of the concept 4 as well as first spectroscopical results 5 have been preliminarily published. We here report in detail the basic synthetic sequences involved. The aim is to construct pyrenes containing branched building blocks like compounds 10 and 13 for dendron and dendrimer synthesis and therefore carry an AB 2 functionality pattern (A: carboxylic acid; B: amine) to allow for application of repetitive growth schemes. The coupling between individual dendritic blocks is based on peptide derived amide bond formation, which has been proven to be very efficient in dendrimer chemistry. 6The concept involves the use of fluorescence probes and dummies, 7 whereby the latter should structurally resemble the former as closely as possible. This is why in the following some of the dendrons carry a methylene spacer between pyrene and acceptor-substituted branching unit (potential dummies) and others do not (potential probes). Tests on model compounds have shown that the incorporation of just a methylene spacer is sufficient for that purpose. 5The whole strategy rests upon compounds 4a and 4b (Scheme 1), which besides the ester (as the protected A of an AB 2 building block), carry two bromo functionalities and one iodo functionality. It was anticipated that, by utilizing the high iodo selectivity 8 of Suzuki cross-coupling (SCC) 9 that the required functionality could be incorporated into the molecule. If both iodinated and brominated sp 2 -hybridized C-atoms are present in the same molecule, the pyrene unit and two N-protected amine-terminated arms (as the B's of an AB 2 building block) could be incorporated at C-I and C-Br, respectively. Scheme 1 Reagents and conditions: (a) EtOH, toluene, H 2 SO 4 , reflux, 5 h, 78% (b) concd HOAc, Br 2 , 0 °C, 12 h, 87% (c) i. H 2 O, NaNO ...

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Definition von BAI: BAl =100(1 ~ ViV,), wobei Vk = 35.0 gewihlt wird[11.12] und I' = 1 0 0 /~[ ( z ( N , -N)'/n]'.' ist. N ist dabei das arithmetische Mittel der n Ringbindungsordungen N,. Eine Bindungsordnung N wird hierfur aus der entsprechenden Bindungslinge R gemaS N = a / R 2b bestimmt, wobei u und b Konstanten sind[ll].

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