Franz et al.
absorptions.5 Thus, phenothiazine derivatives have become
important spectroscopic probes in molecular and supramolecular
arrangements for photoinduced electron transfer (PET) studies6
and as motifs in organic materials.7 The prospect of integrating
strongly coupled redox fragments like phenothiazines into
conjugated chains could constitute a so far unknown class of
redox addressable molecular wires, in particular, for a redox
manipulation of single molecules with nanoscopic scanning
techniques.8,9 As part of our program to synthesize and
investigate phenothiazinyl based molecular wires,10 we have
communicated syntheses, structures, and first cyclic voltammetry
measurements of directly linked phenothiazinyl dyads and
triads11 that can be regarded as models for polymer-based
coupled electrophores. The oxidation potentials of phenothiaz-
ines can be influenced by electronic substituent effects in the
3- or 7-position10-12 and by steric effects of substituents at the
nitrogen atom. The latter directly affect the butterfly conforma-
tion as indicated by the folding angle.13 Here, we report studies
on the correlation of the folding angle of phenothiazines and
the electronic properties by introducing several sterically
demanding aromatic substituents in the 10-position. In some
cases, although under standard conditions, rather unusual
structures are formed that have been unambiguously assigned
by X-ray structure analyses.
Results and Discussion
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Computations. For several series of phenothiazines with
extended π-conjugation the first reversible oxidation potential
of phenothiazines can be sharply fine-tuned by introducing
suitable substituents in the 3- or 7-position.10-12 Since substit-
uents in the 10-position can either adopt an extra (quasi-axial)
or intra (quasi-equatorial) position14 and, therefore, alter the
folding angle in the butterfly structure13 the oxidation potential
can also be influenced to a large extent. Alkyl substituents,
giving rise to lower oxidation potentials, rather prefer an extra
configuration whereas hydrogen and aromatic substituents favor
the intra orientation. Hence, the extra configuration shifts the
oxidation potential anodically as a consequence of diminished
interaction of the phenothiazine benzo π-electrons with the
nitrogen lone pair. However, for planar phenothiazine structures,
ensuring a coplanar orientation of all interacting orbitals, only
few examples with highly electron withdrawing groups like
fluorine or nitro have been reported.15 As a consequence of the
pronounced electron deficiency the desired oxidation to the
phenothiazine radical cation is extremely hampered. Therefore,
we decided first to explore the geometrical and electronic effects
of sterically demanding N-aryl substituents on phenothiazines
by DFT calculations on selected representatives 1 (Figure 1,
Table 1).16 The extent of the butterfly conformation is quantified
by the folding angle θ of the intersecting planes of the benzo
rings, or the tilt angle R which describes the deviation of these
planes from coplanarity, i.e., R ) 0°.
The structures were optimized on the level of density
functional theory with the B3LYP and the Becke-Perdew
model, using increasing numbers of basis sets. In comparison
to N-phenylphenothiazine17 (R ) 29.3°) the computed tilt angles
R of the model structures 1 are expectedly smaller, due to the
increase in steric demand of the N-aryl substituents. Within the
series of N-aryl phenothiazines 1 the steric interaction of the
orthogonalized aryl fragment with their di-o-alkyl moieties with
the peri-hydrogen atoms in the 1- and 9-benzo positions of the
phenothiazine core gradually increases and causes a reduction
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