Fluorescent aryl naphthalene dicarboximides with large Stokes shifts and strong solvatochromism controlled by dynamics and molecular geometry †

A series of highly fluorescent 4-aryl substituted naphthalene dicarboximides were efficiently prepared via metal organic C–C-coupling reactions. The obtained push–pull fluorophores display a distinct positive solvatochromism of the fluorescence. These optical properties are shown to be significantly dependant on the molecular geometry. Corresponding to TICT, a twist between the donor and the acceptor moiety enhances the intramolecular charge transfer resulting in such pronounced solvatochromism. Complete orthogonalisation inhibits the fluorescence. An intentional skew arrangement leads to solvent-adjustable chromophores with high fluorescence quantum yields and Stokes shifts of more than 1.6 eV.


Introduction
peri-Naphthylcarboximides 1 are well known fluorescent dyes which find broad applications like in white light-emitting diodes. 2  Naphthalimides with donor groups in position 4 such as 4-aminonaphthalene-1,8-dicarboximides 3 and 4-alkoxy-naphthalene-1,8carboximides 4 are of special interest since comparably large Stokes shifts and positive solvatochromism 5 of the fluorescence are observed.A photo-induced shift of electron density from the donor to the carboximide is responsible for inducing a large dipole moment.Lowering of the energy of the excited state by solvation with polar solvents causes a bathochromic shift of the fluorescence.This process corresponds to the positive solvatochromism of 4-amino-N-methylphthalimide applied for Zelinskii's 6 solvent polarity S scale.An even more pronounced solvatochromism of naphthalimides should be obtained through the introduction of extended electron rich aryl moieties.

Results and discussion
The fluorescent solvatochromism is expected to increase with the photo-induced dipole moment depending on the distance of the separated charges.Therefore, we inserted aryl groups as conjugating spacers between the donor groups and the naphthalimide acceptor moiety to achieve such a prolongation.However, little is known about such 4-aryl naphthalimides. 7We targeted their synthesis by means of transition metal-mediated arylations.To obtain highly soluble dyes, we started with a condensation of readily available 4-bromonaphthalic anhydride 1 with tridecan-7amine 8 giving the highly soluble key intermediate 2 (Scheme 1).The Suzuki cross-coupling reaction of 2 with various aryl dioxaborolanes gave the corresponding arylated derivatives 3a-d and 3g-j, respectively.
Substitution with the sterically hindered 2,6-dimethyl phenyl boronate failed where synthesis could be alternatively realised by the stepwise peripheral introduction of the sterically demanding methyl groups.The easily accessible trimethoxy derivative 3d was brominated with N-bromosuccinimide to give 3e and then further treated with methylzinc chloride under typical Negishi cross-coupling conditions 9 to provide 3f (Scheme 2).Since also the borylation of more complex aryl halides proved to be difficult, we converted the trimethylsilyl derivative 3i to the corresponding aryl iodide 3k in order to extend the conjugated system.A typical Negishi cross-coupling of 3k with p-anisylzinc chloride 10 (prepared from the reaction of 4-iodoanisole with iPrMgClÁLiCl, followed by ZnCl 2 ) gave the methoxy biphenyl derivative 3l in very good yield.Further, the cyanation of 3k led to the corresponding arylnaphthyl cyanide 3j in an improved yield (Scheme 2).
From UV/Vis and fluorescence measurements it can be clearly recognized that our new dyes 3 are moderately solvatochromic in absorption (Fig. S24 and S26-S34, ESI †) and strongly solvatochromic in fluorescence as shown for the simple phenyl derivative 3a (Fig. 1a and Fig. S25, ESI †).This indicates an optical excitation-induced increase of the dipole moment and was subject of further investigations.The molar energies of fluorescence light of various carboximides were calculated by means of eqn (1) where l max is the fluorescence maximum of the individual dye in the tested solvent (E T values 11 are in kcal mol À1 for comparison with previously reported values in the literature to avoid confusion; these may be multiplied by 4.2 to obtain SI units).The solvatochromism of the carboximides was analysed according to various theoretical approaches.Those of Kawski, 12  Kamlet, Taft and Abboud 13 or Catala ´n14 fitted our experimental results well.The respective analyses are found in the supporting information in Chapter 4. Furthermore, we investigated the fluorescent solvatochromism in more detail using the concepts of Brooker's w R scale 15 and Dimroth and Reichardt's E T (30) polarity scale 11 which delivered the best results.The first represents mainly the polarisability of the solvent whereas the second indicates mostly the effect of dynamic solvation.The spectroscopic data of dyes 3 were compared to reported data to evaluate the solvent sensitivity of the fluorescence.The highly solvatochromic 4-amino-N-methylphthalimide (4) as the basis of Zelinskii's universal S solvent polarity scale 6 served as reference as well as the simple donor substituted 4-amino-N-methylnaphthalimide (5) (Fig. 2).The E T values of 4 were calculated from literature data for various solvents.A linear free energy relation (LFER) 16 of these E T values with the E T (30) polarity scale according to eqn (2) gave appreciably better results (correlation number r = 0.95 for n = 14 solvents) than with Brooker's w R scale (r = 0.90 for n = 14). (1) Similar results were obtained for the solvatochromism of the fluorescence of 3 (Table 1).As a consequence, we conclude that the solvent effects by polar dynamic orientation of the solvent molecules dominate for the reported carboximides and agreed with the E T (30) scale as most appropriate comparison.
We investigated the solvents n-tetradecane, n-hexane, toluene, chloroform, N,N-dimethyl formamide (DMF), 1-undecanol and 1-butanol for an overview of solvent effects where the two protic solvents were applied for studying the influence of hydrogen bonds.Linear correlations of the E T values with the E T (30)  values were obtained.Larger deviations to higher E T were observed for hydrogen bond-donating solvents such as 1-butanol and 1-undecanol indicating the specific influence of such interactions; the solvent viscosity seems to have a minor influence (compare hexane with tetradecane and 1-butanol with 1-decanol) and large Stokes shifts are even observed in a solid glassy matrix of PMMA.As a consequence, the further discussion was concentrated on the non-hydrogen bond-donating solvents for better comparability between the dyes 3-5.A slope a of À0.60 is found for 4-amino-N-methylphthalimide (dye 4, Table 1) and characterises the sensitivity of this highly solvatochromic fluorescent dye to polar solvent effects.In comparison, this interaction is appreciably lower for the methoxynaphthalimide 5 (a = À0.27) and indicates a smaller alteration of the molecular dipole moment with optical excitation.An extension of the conjugated system of the naphthalimide with a phenyl group in 3a increases the slope slightly to a = À0.34 (Fig. 1b).Further introduction of a donor group into the p-position of the phenyl substituent to obtain 3b establishes a donor acceptor system between the methoxy-and the carbonyl groups and enhances the sensitivity (a = À0.74) to exceed the solvatochromism of 4 by far.The dimethylamino group of derivative 3c causes an even higher sensitivity towards solvents, however, the fluorescence quantum yield strongly decreases in polar solvents.Multiple donor groups as in 3d also display a remarkably high solvatochromism with comparably high fluorescence quantum yields; even though weak fluorescence was still observed in polar DMF.A substitution with larger aryl groups like the 4-methoxynaphthyl moiety leads to 3g which displays a very distinct solvatochromism (a = À1.36) while still exhibiting high fluorescence quantum yields in polar solvents.Further extension of the conjugated framework to the methoxybiphenyl derivative 3l also induces such a pronounced fluorescent solvatochromism (a = À1.33)exceeding that of the anisyl-substituted species 3b.
Finally, the effect of the donor acceptor motif in 3b was further tested with 3j where the electron donating methoxy group was exchanged by an electron withdrawing cyano moiety.There is still a comparably high sensitivity to solvent polarity, but as expected, the effect of the donor-substituted derivatives was not reached (Table 1).The electronic properties of the 4-methoxynaphthyl derivative 3g are comparable to those of compound 3b.However, the slope parameter a is found to be nearly twice as much.This observation made us focussing more intensely on the geometrical arrangement of the chromophores and prompted us to investigate the influence of steric hindrance on mesomerism.In comparison, the optical properties of 3g (Fig. 3) and its methylated analogue 3h are only slightly different from each other (Fig. S35, ESI †).This indicates a similar intramolecular geometry.
A skew arrangement of the aromatic systems seems to be mainly influenced by peri hydrogen atoms of the naphthalene subunits.These findings were further confirmed by quantumchemical DFT calculations (B3LYP 6-311**G) as shown in Table 2, Fig. 4 and Table S10, S11 (ESI †).Hence, the steric influence of the methyl group in 3h is only subordinated (dihedral angle 77.381)   and does not affect the geometry significantly (70.601 for 3g).
The steric repulsion of the methyl groups in 3f arranges the two aromatic systems statically fixed.The nearly orthogonal geometry (87.421,Table 2 and Fig. 4) results in low fluorescence quantum yield of less than 0.05.A twisted geometry between the donor and the acceptor promotes charge transfer causing strong solvatochromism in fluorescence and large Stokes shifts.Moderate angles below 801 preserve high fluorescence quantum yields.A complete orthogonalisation (3f) quenches fluorescence where an obviously essential residual orbital overlap is lacking.We further confirmed this concept by heating a solution of 3f in diethylene glycol diethyl ether to 200 1C where the very weak fluorescence reversibly becomes intensified by a factor of 2 (Fig. S36, ESI †). 17  This is attributed to thermally induced vibronic perturbation of the nearly orthogonal arrangement enabling fluorescence.The proposed geometrical requirements for a distinct charge transfer are related to the TICT theory. 18However, our results imply that orthogonal arrangements between the donor and the acceptor completely quench the fluorescence.Significant fluorescence is attributed to skew conformations which tend to more planar arrangements in the excited state allowing significant orbital overlap.Thus, the optical properties, particularly the fluorescent solvatochromism is not the result of a twist-induced charge transfer.It is rather the result of an interplay of conformational change and electronic charge transfer depending on the dipole moment.By tuning the molecular geometry we could obtain a series of chromophores with adjustable fluorescence and quantum yields of up to more than 80%.

Conclusions
In summary, we have reported new, readily soluble and highly fluorescent derivatives of naphthalene-1,8-dicarboximides that have been obtained by Pd-catalyzed arylation in position 4.These compounds display a pronounced solvatochromic fluorescence.The sensitivity of the substituted naphthalimides towards solvent polarity was evaluated according to several theoretical approaches.A photo-induced charge transfer from the electron rich aryl moiety to the naphthalimide is enhanced in polar solvents and causes a bathochromic shift of the fluorescence.Furthermore, the electronic effects are accompanied by molecular dynamics.The intramolecular arrangement influences the intensity of the charge transfer.A skew geometry between the donor and the acceptor allows planarization in the first electronically excited state.This allows high fluorescent quantum yields and still favours a pronounced charge transfer resulting in both distinct solvatochromism and large Stokes shifts.In contrast to basic TICTtheory, no orthogonalization occurs and rectangular orientation leads to strongly quenched fluorescence.The presented synergy of electronic and geometric effects results in highly fluorescent compounds such as 3b and 3g with easily adjustable emission spectra controlled by medium effects.This provides very large Stokes shifts exceeding 200 nm (approx.1.6 eV) being of interest for various applications such as for frequency converters, fluorescence optical fibers and highly tunable light sources.

Table 1
Solvatochromism of the fluorescence of the carbox-imides 3, 4 and 5 a Fluorescence quantum yield F in chloroform.b Fluorescence lifetime t in ns in chloroform.c Slope a of the linear regression.d Coefficient r of correlation for applications of eqn (2).

Table 2
Optimized structures and calculated dipole moments of 3 (DFT B3LYP 6-311**G) Applied solvents: n-tetradecane, n-hexane, toluene, chloroform, dimethylformamide (DMF).a Calculated dihedral angle y in the ground state.b Calculated dihedral angle y in the first electronically excited state.c Dipole moment in the electronic ground state in Debye.d Dipole moment in the first electronically excited state in Debye.e Dihedral angle between phenyl moieties.