A controlled blue-shift in meso-nitrogen aryl fused DIPY and BODIPY skeletons

Marco Farinone , Joanna Cybińska and Miłosz Pawlicki *
Wydział Chemii, Uniwersytet Wrocławski, F. Joliot-Curie 14, 50-383 Wrocław, Poland. E-mail: milosz.pawlicki@chem.uni.wroc.pl; Web: http://mjplab.org/

Received 23rd February 2019 , Accepted 24th April 2019

First published on 25th April 2019


meso-Nitrogen substituted dipyrromethenes (DIPYs) and their boron(III) complexes (BODIPYs) were obtained by the method of nucleophilic aromatic substitution. The following intramolecular fusion with the formation of a 3H-[2,3-c]quinoline skeleton extends delocalisation, but shifts the absorption and emission hypsochromically. A further blue shift has been observed for a deprotonated form of the fused system. A nitrogen atom introduced into the system can be treated as a switching factor that controls the delocalisation significantly influencing the emission by shifting between the available amine and imine tautomeric forms.


Introduction

Strongly conjugated and extensively π-delocalized systems are constantly gaining a significant amount of attention because of their wide applicability assigned to their modifiable optical properties based on subtle initiators. The extended π-clouds of strongly conjugated heterocyclic derivatives (cyclic (i.e. porphyrinoids)1 or linear (i.e. dipyrromethene, DIPY))2 have been widely explored and they show strong potential for significant modifications of the optical response derived from skeletal changes (Fig. 1). The edge-fusion of arenes with the parent chromophore has been widely used as a powerful tool for modification of the optical properties of the main motif (macrocyclic or condensed carbon units) shifting the absorption and emission strongly bathochromically.3
image file: c9qo00294d-f1.tif
Fig. 1 Possible modifications of tetraphyrin(1.1.1.1) and the BODIPY core.

Nevertheless in light of the structural modifications applied for dipyrromethene (DIPY) skeletons and their boron(III) complexes (BODIPY) mostly focused on the meso-position (Fig. 1) a significant influence on the optical properties can be recorded in addition to allowing the formation of more complex systems.4 The further possibilities of skeletal modifications have been applied for the extension of the π-system and realized by means of the involvement of α[thin space (1/6-em)]5 and/or β[thin space (1/6-em)]6 positions (Fig. 1). Such modifications resulted in a spectacular red-shift of absorption and emission.6 The linear structures of BODIPY systems have been extensively used in chromophores where the extended delocalization and efficient emission were crucial for utilization.

A separate possibility of structural adjustments of pyrrole based chromophores can be realized by way of heteroatom involvement in the meso-position (Fig. 1, X), e.g. by adding an oxygen or nitrogen atom in the meso-position eventually forming structures with slightly disturbed electronic properties. Macrocyclic motifs, i.e. tetraphyrin(1.1.1.1), were extensively explored toward formation of differently substituted meso-positions which strongly changed the observed behaviour.7 An involvement of nitrogen in the meso-position in the tetraphyrin(1.1.1.1) skeleton influences the optical response because of an efficient orbital overlap achieved after the edge fusion of arenes.8 The extended delocalisation observed for such structures can be additionally modified using simple initiators (e.g. deprotonation) which introduce a drastic change to overall behaviour.9 From that perspective the scope of modifications at the meso-position of DIPY/BODIPY structures is rather limited10 and to the best of our knowledge the β/meso fusion of an arene with the DIPY/BODIPY chromophore has not been extensively explored11 and the structures containing nitrogen attached to the meso-position and extending the delocalisation eventually forming an additional heterocyclic subunit have not been reported to date.

Following these observations here we report on an efficient method for introducing arylamines to the meso-position of BODIPY and the following, post-synthetic fusion that changes delocalization within the molecule eventually creating a novel chromophore with a modified optical response.

Results and discussion

Synthesis and structural analysis of DIPY/BODIPY derivatives

Our major aim while focusing on the meso-modified BODIPYs/DIPYs was the formation of a fused structure, and also an analysis of potential tautomeric amine/imine equilibria in the final molecules. To solve this problem we prepared meso-arylamine BODIPY derivatives following a previously reported approach via a nucleophilic aromatic substitution (SNAr) starting from the derivatives containing good leaving groups 1 and 2 (Scheme 1).12 The internal fusion can be easily reached for highly activated arenes as documented for different porphyrinoids.8a,9a From these reasons we have obtained a series of BODIPY derivatives with differently activated arylamines – dimethoxyaniline 3a (yield 81%) and anisidine 3b (yield 70%) starting from 2 and 1, respectively. As a reference compound we have also prepared 3c by reacting 1 with aniline.12b
image file: c9qo00294d-s1.tif
Scheme 1 Two alternatives for the formation of meso-arylamine derivatives (conditions: (a) arylamine 2.0 eq., DCM, RT, Ar, overnight; (b) arylamine 2.0 eq., DCM, RT, Ar, overnight; (c) ZrCl4 5.0 eq., acetonitrile, reflux, N2, 2 h).

1H NMR spectroscopy has been reported as a sensitive tool for analysing tautomeric equilibria but it can also be easily applicable for identification of the degree of delocalisation and global aromaticity.13 While looking at the skeleton of 3a one can distinguish two types of heterocyclic nitrogen atoms – amine (C–N, pyrrole-like) and imine (C[double bond, length as m-dash]N, pyridine-like) ones in the BODIPY fragment (Scheme 1, blue) – which drastically modify proton chemical shifts as the increased conjugation accompanied by the noticeable contribution of pyridine-like nitrogen in the heterocyclic subunits strongly shifts the resonances assigned to the alpha position down-field. The 1H NMR spectrum recorded for 3a showed the resonance line of position 1 (Scheme 1) recorded at δ = 7.59 ppm (Fig. 2A) consistent with a significant contribution of pyridine like (C–N[double bond, length as m-dash]C) nitrogen in the equilibrium. In addition, both pyrrolic subunits of 3a present a single set of resonances. It suggests significant dynamics of the meso-arylamine substituent resulting in an effective two-fold symmetry giving equal positions to both pyrrolic subunits. In addition, the broad line at δ = 7.73 ppm, eventually assigned to the NH group located at the meso-nitrogen as documented in 2D experiments including the network of intramolecular NOE contacts (see ESI, Fig. S15–18) confirming the presence of the meso-amino form as a dominant tautomer with a delocalisation within two pyrroles forced by a coordination mode of boron(III).


image file: c9qo00294d-f2.tif
Fig. 2 1H NMR spectra of 3a (A) and 4a (B) (CDCl3, 600 MHz, 300 K). Assignment follows the numbering system presented in Scheme 1.

As postulated it was expected that the removal of boron can potentially shift the equilibrium towards the meso-imine form which in consequence would break the delocalization within the DIPY fragment and both pyrrolic subunits will behave as isolated heterocycles with drastically different spectroscopic properties when compared to the boron(III) complex.

In addition, the imine-bond has been reported as a rigid motif introducing an asymmetry to the final structure.9a To test this aspect we treated all boron(III) complexes with zirconium chloride18 (Scheme 1 path c) obtaining free bases 4a–c quantitatively. The 1H NMR spectrum showed a different picture of proton lines observed for 4a (Fig. 2B) with both alpha positions recorded at δ = 6.98 ppm and δ = 6.71 ppm up-field relocated by Δδ = 0.5–0.8 ppm when compared to 3a. The observed changes in the chemical shift range recorded mostly for alpha-positions are consistent with the removal of the possibility of observing a C[double bond, length as m-dash]N tautomeric form within the heterocyclic subunits from an equilibrium in 4a. The two broad lines observed at δ = 9.48 ppm and δ = 8.20 ppm were assigned to NH groups of both pyrrolic cycles. Thus all these observations were crucial for concluding that after removing boron from 3a, a significant change in tautomerism occurred and has been eventually assigned to meso-C–N with a significant contribution of an imine tautomer (Scheme 1) and pyrrole resonance forms in the meso-aminodipyrromethene in solution. A similar behaviour documented from spectroscopy has been observed for 4b and 4c (see the ESI).

A fusion of arene-amine derivatives and structural analysis

The edge fusion of substituents with the main chromophore has been widely used for modification of properties of chromophores including BODIPYs.11 Focusing on the task of constituting a new aromatic heterocyclic unit that would extend the delocalisation within the skeleton we have tested different conditions. The best results were observed for oxidation with DDQ under acidic conditions (Scheme 2, path a). All tests performed for the 3a–c series have shown different behaviours depending on the level of arene activation. We have not observed any products of internal fusion for 3b and 3c and the only isolated products were 4b and 4c, respectively, showing a necessity of significantly higher activation of the arene. It also shows a different behaviour of linear structures when compared with tetraphyrin(1.1.1.1) derivatives where the presence of a single alkoxy substituent was sufficient to observe very efficient formation of new C–C bonds.8,9a Nevertheless the dimethoxyaniline derivative 3a applied for these conditions gave the fused system 5 in 25% yield. The acidic conditions applied for the fusion reaction cause the removal of boron(III) from the starting material and formation of 3H-[2,3-c]quinoline substituted by a pyrrole subunit. 5 was also formed when the same conditions were applied for 4a (yield 27%). Both experiments suggested a need for higher activation of the arene to make the Scholl type reaction possible.14 The new six-membered heterocyclic ring (Scheme 2, blue) allows an extension of delocalization and also introduces a permanent asymmetry to the molecule which opens a possibility for different tautomeric forms in equilibrium. The reinsertion of boron(III) (Scheme 2, path c) gave a product where the dipy sub-section of 5 binds a cation eventually forming molecule 6 in 36% yield.
image file: c9qo00294d-s2.tif
Scheme 2 Fusion reaction (conditions: (a) DDQ 1.0 eq., methanesulfonic acid, 10 eq., DCM, RT, Ar, 1 h, 25%; (b) DDQ 1.0 eq., methanesulfonic acid 10 eq., DCM, RT, Ar, 1 h, 27%; (c) TEA, BF3OEt2 10 eq., DCM, 0° → RT, Ar, 3 h, 36%; and (d) CH(D)Cl3, TBAF, RT).

The 1H NMR analysis has proven a fully asymmetric shape of both fused compounds 5 and 6 (Fig. 3). Compound 5 showed both fragments with the NH group with different chemical shift ranges (Fig. 3A). The fused fragment was identified at δ = 7.45 ppm (1, α) and δ = 7.33 ppm (2, β) and the unfused one at δ = 7.04 ppm (9, α), δ = 6.89 ppm (7, β) and δ = 6.41 ppm (8, β). Based on the previous observations for the 3a/4a couple, it could be assumed that 5 stabilizes the imine form, in addition to being influenced by the extended aromaticity of the newly obtained heterocyclic unit. Thus the tautomeric form eventually assigned to 5 contains two protonated pyrrolic nitrogen atoms and extended delocalisation over the heterocyclic fragment with a significant contribution of the imine tautomer (C[double bond, length as m-dash]N). The 1H NMR analysis based on the 2D NOESY map showed a specific set of contacts (see ESI, Fig. S58 and S59) eventually confirming the presence of a tautomeric form where the two exchangeable hydrogen atoms are located at two nitrogen atoms of pyrroles. The expected free rotation around the C5–C6 bond allows a stabilization of the tautomeric form where the unfused pyrrole (Scheme 2, green) shows a hydrogen N–H–N interaction with the meso-nitrogen (Scheme 2, blue).


image file: c9qo00294d-f3.tif
Fig. 3 1H NMR spectra of 5 (A) and 6 (B) (CDCl3, 600 MHz, 300 K). Assignment follows the numbering system presented in Scheme 2.

As observed for the 3a/4a couple the amine contribution in the equilibrium is significantly populated for the complex while the imine form dominates in a free base. A similar behaviour has been documented for the fused systems as the formation of boron(III) complex 6 changes the spectroscopic behaviour and the 1H NMR spectrum showed significantly down-field shifted lines for both alpha positions δ = 7.96 ppm (1, α) and δ = 7.50 (9, α) (Fig. 3B) consistent with the postulated BODIPY type coordination and the changes in potential delocalisation within the dipyrromethene fragment competing with the quinoline one observed for 5. The presence of hydrogen at the meso-nitrogen (δ = 9.68 ppm) has been confirmed by the analysis of through space interactions observed on the NOESY map (Fig. S68 and S69) proving a BODIPY type of binding observed in 6.

13C NMR analysis

The carbon 13C NMR chemical shifts have been previously reported as a sensitive probe allowing us to assess the contribution of the C[double bond, length as m-dash]N–C subunit (pyridine like) vs. C–N–C (pyrrole like) in the pyrrole equilibrium as the contribution of carbon nitrogen multiple bonds strongly shifts the carbon resonance down-field.15

Such an analysis performed for the 3a (1 δ = 135.8 ppm, 3 δ = 120.7 ppm, 2 δ = 114.9 ppm)/4a (1/1′ δ = 121.6/122.0 ppm, 2/2′ δ = 109.0/110.2 ppm, 3/3′ δ = 114.9/116.3 ppm)/5 (1 δ = 126.1 ppm, 2 δ = 106.0 ppm, 9 δ = 120.5 ppm, 8 δ = 110.6 ppm, 7 δ = 108.8 ppm)/6 (1 δ = 138.6 ppm, 2 δ = 108.4 ppm, 9 δ = 132.2 ppm, 8 δ = 113.7 ppm, 7 δ = 114.7 ppm) series (Fig. 4) showed a significant change in the chemical shifts assigned to carbon atoms of both pyrrolic subunits involved in these structures. Referring to the pyrrole (δ = 108.2 ppm (β), δ = 118.5 ppm (α))17 or fully oxidized dipyrromethene (δ = 128.8 (β), δ = 117.2 ppm (β), δ = 143.1 ppm (α))15 we can conclude that both structures without boron(III) entrapped present a noticeable contribution of the pyrrole like tautomeric structure with a C[double bond, length as m-dash]N imine bond between meso-carbon and nitrogen. In contrast to this, the coordination of boron(III) with formation of 3a and 6 changes the spectroscopic parameters and forces a different tautomeric form with the C–N amine bond.


image file: c9qo00294d-f4.tif
Fig. 4 Schematic representation of recorded 13C chemical shifts for 3a (green), 4a (navy), 5 (red) and 6 (orange).

X-Ray analysis

Structural analyses performed by means of X-ray experiments showed a picture consistent with the spectroscopic conclusions for all considered derivatives. The crystal structures for 3a and 4a (Fig. 5A and B) confirmed a different character of the C(meso)–N interaction in both compounds. The C(meso)–N bond lengths (1.343(2) Å (3a) and 1.302(2) Å (4a)) consistently show the presence of an imine tautomeric form in 4a and a significant contribution of an amine tautomer in 3a. As documented in the crystals structures 3a presents a BODIPY like structure with the meso-NH amine functionality while 4a is build as two pyrroles with NH groups flanking a central imine tautomer. The diverse character of the central bond has also been supported by a different nature of C(meso)–C(alpha) interactions significantly shorter for 3a (1.424(3) and 1.427(3) Å) when compared with 4a (1.465(2) and 1.463(2) Å) which shows a different behaviour of the main BODIPY motif when compared to DIPY consistent with spectroscopic conclusions. In addition both pyrroles present in 4a have the bond lengths typical of isolated heterocycles with noticeably shorter α–β bonds (1.388(2) Å) and significantly longer β–β bonds (1.410(3) Å). The X-ray analysis performed for both fused derivatives 5 and 6 (Fig. 5C and D) is consistent with the spectroscopic conclusions and shows the presence of a form similar to the solution analysis also in a solid state. Thus the fused structure 5 entraps the 3H-pyrrolo[2,3-c]quinoline structural motif substituted by a pyrrole unit. A strong N–H–N hydrogen bond between the pyrrole NH functionality and the quinoline nitrogen can be expected to show an additional stabilizing control; nevertheless in the crystal structure an interaction with solvent molecules (acetonitrile) can be observed. Bond length analysis in the fused fragment is consistent with the aromatic character observed in the newly prepared fragment and two pyrrole like subunits observed within the main structure. A partially multiple character of C(meso)–N interactions is confirmed by the 1.326(6) Å bond length. Also a significant difference in the α–β and β–β bond lengths within pyrrolic fragments consistently supports the spectroscopic conclusions. The boron(III) complex 6 shows a different orientation as the coordination mode prefers the BODIPY fashion of binding and switching the tautomeric equilibrium towards the amine derivative consistent with the spectroscopic analysis. The C(meso)–N bond length recorded as 1.345(2) Å is similar to that observed for 3a suggesting an isolation of the fused fragment from the rest of the chromophore and the preferred tautomeric form of BODIPY consistent with the 1H NMR analysis.
image file: c9qo00294d-f5.tif
Fig. 5 X-Ray structures of 3a (A), 4a (B), 5 (C) and 6 (D) (colouring scheme: nitrogen – blue, oxygen – red, boron – yellow, and fluoride – green; thermal ellipsoids present 50% probability).

UV-Vis and fluorescence analysis

The BODIPYs have been widely explored as skeletons with extraordinary and tuneable absorption properties2 with a significant number of applied modifications mostly focused on the bathochromic shift of absorption. Nevertheless an introduction of nitrogen to the meso-position of BODIPY has been recorded as a hypsochromic shift factor.2 Considering this, the absorption properties observed for 3a are typical of such structural motifs and we did not observe any significant influence of alkoxy substituents on such behaviour as the absorption with the λmax observed at 420 nm (Fig. 6, red) is significantly blue shifted when compared to other BODIPY structures.2
image file: c9qo00294d-f6.tif
Fig. 6 Absorption (A) of 3a (red), 4a (black), 5 (blue) and 6 (green) (CH2Cl2, 295 K) and emission (C) of 5 (blue), 6 (green) and 6(–) (orange). Trace B presents a titration of 6 (λmax = 430 nm) with DBU and formation of 6(–) (λmax = 385 nm) (CH2Cl2, 295 K).

Removing boron(III) from 3a and shifting the tautomeric equilibrium to the dominant contribution of the imine form observed in 4a significantly change the absorption as the biggest chromophore with the structure is the 6π electron pyrrole/arene (Fig. 6, black). In contrast to a previously reported analysis of 3c,12b,16 where it has been reported as a non-fluorescent molecule, 3a has shown very weak fluorescence observed at 445 nm. Such behaviour is in contrast to those of other meso-derivatives of BODIPY which in their alkyl variants were observed to be strongly fluorescent.10 In contrast to this, the behaviour of 4a is absolutely mute in fluorescence spectroscopy. The behaviour of 4a can suggest that beside the amine–imine equilibrium another factor is responsible for the rather weak emission properties observed for meso-nitrogen derivatives. As documented in many examples including the BODIPY derivatives the dynamics of the meso substituent has a significant influence on the stability of the excited state and can very efficiently quench fluorescence. For these reasons it was crucial to explore the optical properties of both fused structures. The absorption spectrum recorded for 5 showed a red shifted transition when compared to 4a. It can be explained by the presence of an extended π-cloud of 3H-pyrrolo[2,3-c]quinoline that causes lower dynamics of the attached aryl but also stabilizes a specific tautomer with an extended aromatic character and formation of a quinoline fragment. Further changes in the absorption spectrum were observed for boron(III) complex 6 where the shape characteristic for the BODIPY chromophore was restored (Fig. 6, green).

The presence of boron(III) shifts the absorption to longer wavelengths and the λmax was recorded at 430 nm very subtly red-shifted when compared to 3a suggesting a significant contribution of the BODIPY structure and a noticeable isolation of the fused arene from global conjugation. As we have presented previously, a deprotonation of meso-nitrogen linked with tetraphyrin(1.1.1.1) resulted in a strong bathochromic shift of absorption and emission.9 Thus we decided to test such a possibility also for boron(III) complexes 3a and 6 where only one exchangeable hydrogen atom is present. The UV-Vis titration performed for 6 (Fig. 6B) showed a gradual disappearance of the starting material and, after addition of 1 eq. of DBU, the formation of a structure eventually assigned to 6(–) (Scheme 2, path d). To our surprise we did not observe a shift of absorbance to longer wavelengths but a hypsochromic shift of transitions to λmax = 385 nm has been recorded showing a blue shift by 45 nm when compared to 6. The deprotonation is reversible as after controlled addition of TFA, the starting material was recovered. The deprotonation does not modify the coordination mode as documented in 1H NMR titration (Fig. S74/75). A titration of 3a under the same conditions also showed a hypsochromic shift of absorbance (Fig. S96); nevertheless the emission of the final molecule was not improved. As expected, the fusion of the meso-aryl substituent rigidifies the molecule causing an appearance of fluorescence eventually recorded at λ = 410 nm (5) and λ = 450 nm (6) with a Stokes shift of ∼20 nm. The emission observed for 5 reflects a significant influence of the dynamics of the unfused pyrrole substituent attached to the meso-position as the fluorescence has rather low efficiency (Φ = 0.04, τ1 = 4.18 ns, kr = 9.6 × 106 s−1, knr = 2.3 × 108 s−1) even if significantly blue shifted. The boron(III) complex 6 presents a red (bathochromically) shifted emission (Φ = 0.10, τ1 = 4.71 ns, kr = 2.1 × 107 s−1, knr = 1.9 × 108 s−1) at the wavelength approximately similar to 3a but it is noticeably more efficient suggesting a significant influence of the meso-attached substituent dynamics on the electronic behaviour. Nevertheless deprotonation does not modify drastically the fluorescence properties as 6(–) emits at λmax = 405 nm (Φ = 0.32, τ1 = 3.12 ns, kr = 1.0 × 108 s−1, knr = 2.2 × 108 s−1) with a 25 nm blue shift when compared to 6. A similar spectral range of absorption and emission recorded for 3a and 6 consistently supports the presence of the BODIPY tautomeric form in both molecules and a smaller influence of the imine tautomeric form.

Theoretical studies

As documented spectroscopically both DIPY structures – unfused 4a and fused 5 – are observed in solution as tautomers where a multiple C–N bond is dominating; nevertheless we identified several possible tautomeric forms (Scheme 2) where two factors (the NH tautomerism within three available nitrogen atoms and the free rotation around C–C bonds) are in operation and influence the observed response. To gain insight into the observed dynamics we employed a theoretical analysis of both observed tautomeric equilibria.

The DFT optimized conformations of 4a showed the lowest energy of the spatial arrangement entrapped also in a crystal (Scheme 3A, 4-1) with two NH pyrrolic groups pointing toward the meso-imine nitrogen. The calculations extended to the higher basis sets (6-311++G(d,p); see the ESI) have confirmed the lowest energy of 4-1. The further imine tautomers distinguished within the equilibrium (4-2 and 4-3) show slightly higher energy consistently reproduced also for higher basis sets. A noticeably higher energy has been observed for structure 4-3 which can suggest a significant influence of an intramolecular N–H–N interaction that stabilizes both (4-1 and 4-2) tautomeric forms. In addition, a small energy difference observed for 4-1 and 4-2 where a rotation around C–C is a changed factor is consistent with such behaviour. The meso-amino tautomers (4-4, 4-5 and 4-6) are noticeably less favoured as the global energy predicted is about 6–7 kcal mol−1 higher when compared to 4-1. It shows a consistent picture with the spectroscopic observations that the imine forms are more preferred even if the two rotamers are so close in energy and cannot be distinguished by spectroscopy.


image file: c9qo00294d-s3.tif
Scheme 3 Tautomeric and rotation equilibria of 4a (A) and 5 (B) (B3LYP/6-311++G(d,p)).

A similar analysis performed for 5 (Scheme 3B) showed the lowest energy for the tautomer 5-1 (regardless of the basis set applied) where the N–H–N interaction is observed and in addition the maximum delocalisation of the fused 3H-[2,3-c]quinoline fragment is sustained. A rotation around a single C–C bond with formation of 5-2 increases the energy by ∼4 kcal mol−1 showing an influence of NH repulsion similar to 4-3. Slightly less favoured is a BODIPY-like form 5-3 where the meso-nitrogen is protonated and the stabilisation is forced by the resonance between two pyrrolic subunits. The probable reason for such behaviour is distracted delocalisation in the 3H-[2,3-c]quinoline fragment. The differences in energy exceeding 10 kcal mol−1 as observed for 5-6 significantly reduce the potential contribution of such a tautomeric form as documented spectroscopically.

The chemical shifts simulated for all analysed tautomeric forms showed good correlation consistently supporting the behaviour observed by spectroscopy with a dominant contribution of tautomers assigned to the lowest energy (4-1/4-2 and 5-1) (Tables S2 and S3). A similar analysis performed for boron(III) complexes 3a, 6 and 6(–) (Table S4) showed very good correlation of experimental proton chemical shifts with theoretical ones.

The TD-DFT calculations with the B3LYP functional and the basis set of 6-311++G(d,p) performed for all analysed tautomers of 4 and 5 consistently support a conclusion of the predominant contribution of the imine isomers with significantly smaller participation of amine ones characteristic of the BODIPY structures (see the ESI). The TD-DFT calculations applied for boron(III) derivatives 3a, 6 and 6(–) supported the behaviour observed by spectroscopy. The theoretically predicted spectra reproduced the blue shift for all analysed derivatives with a ∼45 nm hypsochromic shift of the most red-shifted transition similar to experimental values recorded for deprotonation and formation of 6(–).

The frontier orbital analysis showed a change in the HOMO–LUMO gap responsible for hypsochromically shifted transitions and also presented a noticeable modification of MO coefficients distinctively extended on arenes in fused systems (Fig. 7). The HOMO–LUMO gaps are comparable for both boron(III) complexes and nicely correlate with the experimentally observed UV transitions. The fusion and conversion from 3a to 6 destabilize both the HOMO and LUMO orbitals by 0.36 eV and 0.29 eV, respectively (Fig. 7). The biggest change has been recorded for the deprotonated structure where the HOMO and LUMO orbitals significantly increase the energy by 0.9 eV and 1.39 eV, respectively. The deprotonation modifies both the HOMO and LUMO orbitals eventually increasing the gap by ∼0.5 eV consistent with the experimentally recorded blue shift.


image file: c9qo00294d-f7.tif
Fig. 7 Energies of the frontier orbitals for 3a (A), 6 (B) and 6(–) (C).

Conclusions

The precisely planned derivatisation at the meso-position of DIPY/BODIPY skeletons with arylamine(s) influences the observed behaviour and changes the spectroscopic properties shifting the absorbance significantly hypsochromically and simultaneously reducing the emission. The tautomeric equilibrium analyses for DIPY and BODIPY showed a preference in the formation of the C(meso)–N imine tautomeric form for DIPYs and the C(meso)–N amine form for boron complexes as documented experimentally and theoretically. The intramolecular fusion performed for highly activated arenes introduces a 3H-[2,3-c]quinoline structural motif to the final molecule extending delocalisation and rigidifying the skeleton eventually limiting the dynamics. Such modification shifts the absorbance bathochromically when compared with the unfused systems but more importantly switches on the fluorescence. The meso-NH group can be treated as a switching factor that after deprotonation, in contrast to macrocyclic motifs, shifts the absorption and emission strongly hypsochromically showing potential for utilizing such motifs as chromophores/fluorophores responding to fundamental initiators.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Science Centre, Poland (2016/23/B/ST5/01186). We thank Dr Andrzej Bil for fruitful discussions. M. P. thanks the Wrocław Super-computer Centre (KDM WCSS) for sharing computation resources necessary for DFT calculations.

Notes and references

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Footnotes

Electronic supplementary information (ESI) available. CCDC 1896805–1896809. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c9qo00294d
Crystal Data: 3a. C17H16BF2N3O2 (M = 343.29 g mol−1): monoclinic, space group P21/c, a = 11.1306(3) Å, b = 11.7622(3) Å, c = 24.0194(7) Å, β = 91.217(2)°, V = 3143.92(15) Å3, Z = 8, T = 100(2) K, μ(MoKα) = 0.112 mm−1, Dcalc = 1.451 g cm−3, 20[thin space (1/6-em)]535 reflections measured (6.032° ≤ 2Θ ≤ 55.756°), 6900 unique (Rint = 0.0480, Rsigma = 0.0468) which were used in all calculations. The final R1 was 0.0599 (I > 2σ(I)) and wR2 was 0.1730 (all data); 4a. C17H17N3O2 (M = 295.33 g mol−1): monoclinic, space group C2/c, a = 15.1050(5) Å, b = 12.8664(4) Å, c = 15.5822(5) Å, β = 91.952(3)°, V = 3026.60(17) Å3, Z = 8, T = 100(2) K, μ(MoKα) = 0.087 mm−1, Dcalc = 1.296 g cm−3, 12[thin space (1/6-em)]949 reflections measured (6.334° ≤ 2Θ ≤ 73.754°), 4655 unique (Rint = 0.0614, Rsigma = 0.0786) which were used in all calculations. The final R1 was 0.0658 (I > 2σ(I)) and wR2 was 0.1757 (all data) 5. C17H16N3O2.5 (M = 302.33 g mol−1): monoclinic, space group P21/c, a = 10.6680(14) Å, b = 24.498(4) Å, c = 11.286(3) Å, β = 93.034(12)°, V = 2945.4(9) Å3, Z = 8, T = 100(2) K, μ(MoKα) = 0.094 mm−1, Dcalc = 1.364 g cm−3, 10[thin space (1/6-em)]639 reflections measured (3.824° ≤ 2Θ ≤ 49.998°), 5183 unique (Rint = 0.1393, Rsigma = 0.3222) which were used in all calculations. The final R1 was 0.0753 (I > 2σ(I)) and wR2 was 0.1374 (all data); 6. C19H17BF2N4O2 (M = 382.17 g mol−1): triclinic, space group P[1 with combining macron], a = 7.1859(4) Å, b = 12.0427(7) Å, c = 12.0895(9) Å, α = 93.791(5)°, β = 100.927(5)°, γ = 98.538(5)°, V = 1011.05(11) Å3, Z = 2, T = 100.00(10) K, μ(MoKα) = 0.095 mm−1, Dcalc = 1.255 g cm−3, 6891 reflections measured (3.436° ≤ 2Θ ≤ 57.524°), 4547 unique (Rint = 0.0210, Rsigma = 0.0511) which were used in all calculations. The final R1 was 0.0479 (I > 2σ(I)) and wR2 was 0.1299 (all data).

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