Strongly Fluorescent Push-Pull Substituted Carbostyrils Absorbing in the Visible

Previous work on differently substituted carbostyrils [1-3] has shown that only some especially substituted molecules of this substance class show interesting fluorescence properties. However, in contrast to the widely used oxa analogue coumarins strong fluorophors are scarce. Therefore principle strategies have been investigated, which substituent properties are important. What makes a carbostyril to be a good fluorophor [3]

As known for our model 2(1H)-quinolinone CS370 1 , the newly described N-methyl derivative 2 also shows no significant difference in absorption and emission maxima to the parent compound. The observed solvent dependence is consistent with previous observations leading to significant blue shifts (about 10-15 nm in water compared with DMSO). However, O-Methylation and hence formation of a quinoline structure 3 leads to significant shorter wavelength absorption and especially small Stoke´s shifts. This is a very good indicator for a quick analysis if O or Nalkylation has occurred.
A comparison of the UV-spectra of 6-amino 4 vs methylamino 6 and dimethylaminoderivative 7 shows surprisingly the dimethyl derivative 7 having the lowest longwave absorption maximum. As expected, N-acetylamino derivative 6 is most blue shifted. This is also reflected in the calculation of the lowest energy structure showing the sterically more demanding dimethylamino group out of plane. However, it is very interesting also from a theoretical point of view, that the Stoke shifts of 173 nm or 214 nm as well as the emission maxima of 557 or 580 nm (in DMSO or water)are the highest values ever observed on a carbostyril. Excitation and emission spectra of this interesting compound are displayed below.
Surprisingly, our compounds believed to be of structure 8 and 9 show too low wavelength absorption maxima as evidenced by comparison with analogues 7 and 8 (see yellowish fields in table). The calculated values support strongly, that we should think about revision of these structures, because they could be unexpected isomers. We are continuing to get a confirmation about this subject.

Calculations
The geometries of all investigated compounds were completely optimized by the semiempirical AM1 Hamiltonian [9] using the eigenvector following routine [10] (keyword PRECISE) as implemented in the Vamp program package [11]. Electronic excitation energies were obtained by the ZINDO program [12,13]. Bulk solvent effects (DMSO, H2O) were included by the self-consistent reaction field (SCRF) approximation [14,15]

Conclusion
As shown in our recent publication, push-pull substituted carbostyrils in position 6,7 and 4, respectively, have absorption maxima close to the visible region.
The large bathochromic shifts are coupled mainly due to the influence of electron donating groups in position 6 of the ring system. Monoalkylated 6-amino groups in combination with a 7-methoxy group give the largest red shifts (UV maximum: 414 nm for compound 6 in DMSO), whereas dialkylated 6-amino groups give the largest Stokes shifts (173 nm in DMSO respectively 214 nm in water for compound 7). It is interesting to note that acetylated 6-amino groups have about the same impact on photophysical properties as methoxy groups (compare 1 with 5).
Quantum yields are typically at least 0.1. As expected, non acylated amino substituents in position 7 exhibit the strongest electron donating effects increasing quantum yields significantly. In most of the investigated cases intensities are lower in water as solvent compared with polar-aprotic DMSO.
From our previous experience [3,4] one can expect quite good agreement between calculated and experimental absorption maxima. There is, however, one notable exception: the donor properties of amino groups, and hence the bathochromic shift induced thereby, is greatly underestimated by the ZINDO procedure (see e.g., results for 4, 6, 10, 11). This seems to be a quite general shortcoming of this semiempirical quantum chemical method. For the other derivatives the agreement between experiment and calculation is quite satisfying. From the present results one can conclude that an acylamino group is comparable in its effect on absorption maxima to an alkoxy function.
According to this set of data we can suggest that 6,7-diamino substituted carbostyrils, especially the ones with monoalkylated amino groups should provide more excellent photophysical properties compared with the compounds presented in this paper. This new fluorophors can serve as potent competitors to the widely used coumarins especially because of the advantage of a better photochemical and thermal stability.
In short we have prepared highly fluorescent carbostyrils with absorption maxima up to 414 nm and it is evident that this range can still be extended preparing 6,7-bis-N-alkyl derivatives.