Synthesis and electronic properties of π-extended fl avins †

Flavin derivatives with an extended π-conjugation were synthesized in moderate to good yields from aryl bromides via a Buchwald–Hartwig palladium catalyzed amination protocol, followed by condensation of the corresponding aromatic amines with violuric acid. The electronic properties of the new compounds were investigated by absorption and emission spectroscopy, cyclic voltammetry, density functional theory (DFT) and time dependent density functional theory (TDDFT). The compounds absorb up to 550 nm and show strong luminescence. The photoluminescence quantum yields φPL measured in dichloromethane reach 80% and in PMMA (poly(methyl methacrylate)) 77%, respectively, at ambient temperature. The electrochemical redox behaviour of π-extended flavins follows the mechanism previously described for the parent flavin.

2][3] They play an important role in light, oxygen, and voltage-sensitive (LOV) domains sensitive for blue light excitation. 4However, flavins have also found many applications in photocatalysis [5][6][7][8][9][10][11] or as photosensitizers [12][13][14] in singlet oxygen generation for synthesis or in photodynamic disinfection. 13,15,16The local environment, 17 hydrogen bonding, 18,19 π-π stacking 20 or metal coordination 21,22 to the chromophore and the type and position of substituents 23 determine the redox potentials and the photophysical properties of flavin derivatives.Donor-acceptor systems, in which flavins are covalently linked to other chromophores showing different absorptions have been synthesized 24,25 and used in photo-voltaic devices, 26 molecular switching or molecular logic gates. 26,27Oxidized forms of flavin derivatives act as electron acceptors in photoinduced electron transfer (PET) 28,29 and are known to bind different metal ions inducing a positive shift in their one-electron reduction potentials 29 and long-lived charge-separation. 29 less explored option to modify the optical and electronic properties of flavins is the extension of their aromatic π-system.An extended conjugation length is expected to shift the absorption maxima to a longer wavelength and affect the extinction coefficient and redox properties of the chromophore. 30,31We report here the synthesis and electronic properties of three new flavins with extended aromatic π-systems.

Synthesis
The synthesis of new π-system extended flavins 3a, 3b and 3c 32 was accomplished by the condensation 33 of violuric acid 1 with the corresponding amines 2a-c as starting materials.The amines were obtained via a Buchwald-Hartwig amination 34 in good to very good yields.The subsequent condensation of amines 2a-2c with violuric acid 1 in boiling acetic acid easily furnished multi-gram quantities of flavins 3a-c in moderate to good yields (Scheme 1).
The structures of flavins 3a-3c were determined by NMR spectroscopy and confirmed by high resolution mass spectrometry and elemental analysis.The electronic properties of flavin derivatives 3a-3c were investigated by absorption and emission spectroscopy (Table 1), cyclic voltammetry, DFT and TDDFT calculations (Fig. 1).Furthermore, the fluorescence quantum yields of the chromophores were determined with an integrating sphere accessory.

Theoretical calculations
Computational investigations are particularly useful for understanding trends observed in the electrochemical and photophysical properties of molecular materials.Thus, density functional theory (DFT) and time dependent density func-tional (TDDFT) calculations were performed to gain additional understanding of the electronic structures of the π-expanded flavins (3a, 3b, and 3c).
Interestingly, the contour curves of the lowest unoccupied molecular orbitals (LUMOs) look similar for all three investigated compounds being largely unaffected by the π-extension (Fig. 1).Also, the LUMO energies are similar ranging from −2.88 eV to −2.96 eV.In contrast to this, the highest occupied molecular orbitals (HOMOs) are delocalized over large parts of the molecule for all investigated compounds (Fig. 1).Consequently, the HOMO energies vary in a wider energy range from −5.70 eV to −6.11 eV.
TDDFT calculations revealed that transitions between these frontier orbitals determine the lowest excited singlet state S 1 .The corresponding transition energies amount to 2.47 eV (3a), 2.44 eV (3b), and 2.85 eV (3c).This is in agreement with the experimental results (see below) which show that the emission energies of compounds 3a and 3b are similar, whereas the emission of compound 3c is clearly blue-shifted.This trend is also seen in the absorption spectra.

Electronic properties
The UV-visible absorption and emission spectral data of the flavins in two different organic solvents and in a polymer matrix (PMMA = poly(methyl methacrylate)) are summarized in Table 1.The absorption and emission spectra of flavin derivatives 3a, 3b, 3c, and butyl flavin derivative 4 recorded in DMSO are displayed in Fig. 2a and b.All new derivatives show a complex absorption profile characteristic for the presence of multiple chromophores.The flavins exhibit intense absorption bands in the range of 306-543 nm.The S 0 → S 1 band around 440 nm, characteristic for the parent flavin 4, is red shifted by 15 nm for compound 3c, 52 nm for 3b, and 57 nm for 3a, respectively, in DMSO solution, indicating a decreased HOMO-LUMO gap.Furthermore, the absorption bands in the range 306-400 nm show a vibrational fine structure characteristic for the isolated naphthalene, anthracene and pyrene chromophore. 35However, the molar extinction coefficient of anthracene derivative 3b is significantly higher than those observed for pyrene 3a and naphthalene 3c.A likely reason for this is the extended linear conjugated system.
Compounds 3a and 3b show an intense orange emission under daylight irradiation, while 3c emits in the green.
The emission spectrum of 3c shows a vibrational structure, similar to that of butyl flavin (4), with a spacing of the vibrational satellites amounting to about 1000 cm −1 .This energy spacing corresponds to the characteristic stretching modes of isoalloxazines.Furthermore, the structured emission indicates that the emission is originating from a localized π-π* transition.In contrast, the emissions of 3a and 3b are broad and do not exhibit any structure indicating that the emission originates from a state with some charge transfer character.
Table 1 Selected photophysical properties of flavins 3a-3c and 4 measured at ambient and liquid nitrogen temperature, respectively.λ abs.max is the peak wavelength of the absorption spectrum, λ em.max is the peak wavelength of the emission spectrum, τ the emission decay time, and Φ PL the photoluminescence quantum yield.PMMA: poly(methyl methacrylate)

Compound
Solvent polymer This assignment is also supported by the TDDFT calculation (Fig. 1), which predicts that the S 1 state is of partial charge transfer character for 3a and 3b due to HOMO orbitals delocalized over the entire molecule and LUMO orbitals localized on the isoalloxazine part.In addition, the emission maxima show a clear dependence on the polarity of the solvent, which is typical for charge transfer transitions.In the case of 3a, in DCM (ET(30) = 41.1) 36the emission peaks at 610 nm whereas it is red shifted by 18 nm in DMSO (ET(30) = 45.0) 36to 628 nm.Compound 3b exhibits a similar red shift of 20 nm from 585 nm to 605 nm when we go from DCM to DMSO.For flavin 3c a corresponding shift from 511 nm to 516 nm is observed.The emission quantum yields of the new compounds 3a-3c vary with solvent and temperature.In solution at T = 300 K the emission quantum yields are between 24 and 80% in DCM, 13 and 53% in DMSO and 13 and 77% in PMMA.The corresponding Φ PL values at nitrogen temperature increase, which may be explained by a decrease of non-radiative deactivations on cooling.The life times at 300 K of the excited states of the three compounds are in the range of 2 to 9 ns in DMSO and 6-11 ns in PMMA.The values increase only slightly when decreasing the temperature from 300 K to 77 K; in PMMA the same values are observed for both temperatures.We assume that the radiative emission at 300 K and 77 K originates from the singlet excited state being involved without the triplet state involvement.

Redox properties
][39] The variable scan rate cyclic voltammograms of compound 3a shown in Fig. 3 appear most similar to those obtained for riboflavin and can be interpreted based on the mechanism in Scheme 2. 39 Compound 3a (Fl ox ) is initially reduced at E 0 f(1) by one-electron to form the radical anion (Fl rad •− ).The radical anion reacts quickly with another flavin by proton transfer to form the neutral radical (Fl rad H • ) via eqn (1) plus the deprotonated flavin (Fl ox − ).Because Fl rad H • is easier to reduce than Fl ox , it is immediately further reduced at the electrode surface by one-electron to form Fl red H − at E 0 f(3) .Therefore, the first voltammetric process (wave 1) observed at ∼−1.0 V vs. Fc/Fc + actually involves two one-electron transfers interspaced with a proton transfer reaction.When the scan direction is reversed at approximately −1.5 V vs. Fc/Fc + , two oxidative peaks are observed.The first oxidative peak at ∼−1.0 V vs. Fc/Fc + (wave 2) is due to the one-electron oxidation of Fl rad •− back to the starting material, Fl ox , while the second electron transfer at −0.7 V vs. Fc/Fc + is due to the one-electron oxidation of Fl red H − to Fl rad H • (wave 3).If the equilibrium in eqn (1) in Scheme 2 favours the back reaction (as it does for riboflavin), 39 then any Fl rad H • that deprotonates on the voltammetric timescale will also undergo further one-electron oxidation in wave 3 to regenerate the starting material.As the scan rate is increased up to 20 V s −1 , the initial reduction process at ∼−1.0 V vs. Fc/Fc + becomes more chemically reversible shown by how the i ox p /i red p ratio approaches unity, due to the proton transfer step between Fl rad •− and Fl ox being outrun; hence the reduction process changes to a chemically reversible oneelectron transfer.
When the voltammetric scan is extended to more negative potentials, additional reduction processes are detected at ∼−1.7 V vs. Fc/Fc + (wave 4) and ∼−2.0 V vs. Fc/Fc + (wave 5) for compound 3a.Wave 4 is associated with the one-electron reduction of Fl ox − that is formed from the Fl ox reacting with Fl rad •− (E 0 f(4) ), and wave 5 is associated with the further oneelectron reduction of Fl rad •− to form Fl red 2− (E 0 f (5) ).When the forward potential scan is extended all the way past wave 5, on the reverse scan it can be observed that the oxidative wave 3 is larger than when the forward scan is only extended just past wave 1.The reason for wave 3 appearing larger is because Fl red H − (which undergoes oxidation in wave 3) can also be formed via Fl red 2− reacting with Fl ox to form Fl red H − (plus Fl ox − ) (eqn ( 2)).The voltammetric responses observed for compounds 3b and 3c appear somewhat different from 3a, but they can be interpreted based on exactly the same mechanism as for compound 3a; the subtle differences in the voltammetric waves can be accounted for by varying equilibrium constants for the homogeneous reactions given in Scheme 2. For example, at relatively slow scan rates the first voltammetric reduction process of 3b and 3c appears to be fully chemically reversible implying a simple chemically reversible electron transfer reaction (only waves 1 and 2 are observed), and the process remains chemically reversible as the scan rate is increased up to 20 V s −1 .The reason for the high chemical reversibility likely relates to the radical anion (Fl rad •− ) formed in the initial electron transfer step at E 0 f(1) undergoing a slow homogeneous reaction with Fl ox (eqn (1)), so on the short voltammetric timescale Fl rad •− survives fully at the electrode surface and is able to be converted back to Fl ox when the scan direction is reversed (Fig. 4 and 5).
When the scan is extended to more negative potentials, compounds 3b and 3c show an additional process at ∼−1.8 V vs. Fc/Fc + (wave 5) which is associated with the further oneelectron reduction of Fl rad •− to form Fl red 2− (E 0 f( 5) ).Wave 5 appears more clearly than was observed during the reduction of compound 3a because more Fl rad •− exists at the electrode surface in higher amounts (due to the slower proton transfer reaction with Fl ox ).Similarly, the reduction process associated with wave 4 (E 0 f(4) ) is not observed during the reduction of 3b and 3c because Fl ox − does not have time to form via the proton transfer reaction between Fl rad •− and Fl ox .The peak shape of the wave 5 reduction process is "sharp" and this is likely due to some adsorption of the dianion onto the electrode surface.The reason that wave 4 (reduction of Fl red H − to Fl rad 2•− ) is not observed for compounds 3b and 3c may arise either due to lower acidity of the N-H proton of the starting materials or due to lower basicity of their corresponding anion radicals (Fl rad •− ).
When then voltammetric scanning direction was switched at −2.2 V vs. Fc/Fc + after first reducing Fl rad •− to Fl red 2− , no reverse peak was detected associated with the oxidation of Fl red 2− back to Fl rad •− regardless of the scan rate, indicating that the dianion is only very short-lived for compounds 3b and 3c.Similarly, when the potential was first scanned to −2.2 V vs. Fc/Fc + , on the reverse scan only wave 2 was observed at slow scan rates.However, as the scan rate is increased, wave 3 becomes more pronounced and wave 2 diminishes in size.A reason for this apparently anomalous behaviour can be based on the equilibrium reactions that exist between the different species in Scheme 2 favouring the reformation of Fl rad •− .For example, when the dianion is formed, it immediately undergoes a proton transfer reaction with the starting material to form Fl red H − plus Fl ox − (Scheme 2, eqn (2)).Fl red H − then undergoes another reaction with Fl ox − to form two molecules of Fl rad •− (Scheme 2, eqn (3)).Therefore, at slow scan rates, Fl red 2− has time to convert all the way back to Fl rad •− so only wave 2 is observed, while at faster scan rates, there is only time for Fl red 2− to convert to Fl red H − and so wave 3 is mainly observed.

Experimental part
Computational procedures DFT and TDDFT calculations were performed using Gaussian 09. 40For all calculations (geometry optimizations and TDDFT energy calculations), B3LYP 41,42 was used as the functional in combination with the 6-31G** basis set. 43,44As the starting geometry, manually drawn structures were used.For all compounds, the structure was optimized prior to TDDFT energy calculations.
General procedure for the syntheses of compounds 3a-c (GP1) A mixture of amines 2a-c (1 eq.), violuric acid 1 (1.2 eq.) and 100 mL of glacial acetic acid was added to a round bottom flask and refluxed for 24 h.Acetic acid was removed under reduced pressure, giving a purple solid residue.The residue was purified by flash chromatography on silica gel (chloroform/methanol 50 : 1) to yield flavins 3a-c as yellow or red solids.

Conclusions
Flavin derivatives 3a-c were obtained from the condensation of naphthyl-, anthranyl-or pyrenyl-amines 2a-c with violuric acid.Extending the π-system of the parent flavin by annulation of benzene, the naphthalene or pyrene unit changes the electronic and redox properties of the chromophore significantly.The chromophore absorption shifts bathochromic and all three compounds show intensive emission with quantum yields of up to 80%.The reduction mechanism of the expanded flavins in DMSO as observed in cyclic voltammetry experiments can be interpreted analogously to the previously investigated parent flavin, with the subtle differences in the voltammetric behaviour due to varying equilibrium constants for the homogeneous reactions following electron transfer.

Fig. 1
Fig. 1 HOMOs and LUMOs of compounds 3a, 3b, and 3c.The plots result from DFT calculations performed on the B3LYP/6-31G** level of theory.The iso-contour value was set to 0.03.Further molecular orbitals and the corresponding energies are displayed in the ESI.†

Fig. 3
Fig. 3 Variable scan rate CVs of 2 mM 3a in DMSO with 0.2 M n-Bu 4 NPF 6 , recorded at a 1 mm Pt electrode at 22(±2) °C.The current data were scaled by multiplying by ν −0.5 .