Substitution Effects on the Photophysical and Photoredox Properties of Tetraaza[7]helicenes

: A series of substituted derivatives of tetraaza[7]helicenes were synthesized and the influence of the substitution on their photophysical and photoredox-catalytic properties was studied. The combination of their high fluorescence quantum yields of up to 0.65 and their circularly polarized luminescence (CPL) activity results in CPL brightness values ( B CPL ) that are among the highest recorded for [7]helicenes so far. A sulfonylation/hetarylation reaction using cyanopyridines as substrates for photoinduced electron transfer (PET) from the excited helicenes was conducted to test for viability in photoredox catalysis. DFT calculations predict the introduction of electron withdrawing substituents to yield more oxidizing catalysts.


Introduction
The research interest in helicenes has grown in recent years, driven by the increasing number of synthetic approaches [1][2][3][4][5][6][7][8] and the desire to exploit the unique properties of this class of compounds, which originate from their helical structure in combination with the extended aromatic system.Helicenes find a wide variety of applications, such as synthetic use in the preparation of chiral catalysts, often in the form of ligands, or more exotic applications including the construction of molecular machines. Since the emission is not linked to a specific functional group or moiety (ketone, BODIPY), but originates from the helically bent acene backbone, substitution in and decoration of the core structure allows for a variety of modifications to tune the electronic and optical properties. [17]In order to compare the CPL activity of different compounds, the dissymmetry factor of emission (g lum ), defined as 2(I L À I R )/(I L + I R ), is commonly used, with I L and I R being the intensity of light with left-handed and right-handed circular polarization, respectively.However, currently available CPL dyes show low g lum values, and efforts are being made to increase them to ranges that make the dyes suitable for applications such as CPL microscopy, 3D displays, and CP-OLEDs. [9,18]Another field of application of light-absorbing organic molecules utilizes the absorbed photon energy to enable chemical transformations via photocatalysis.[30] However, helicenes have not been broadly considered as potential photoredox catalysts, with the only reports to the best of our knowledge being the use of a helical carbenium ion by Gianetti et al. [31] and later Cozzi et al. [32] We recently investigated the use of a polyaza [7]helicene as a reductive photocatalyst [33] and here report a chromatographyfree gram-scale synthesis of the catalyst and provide further insight into its optical properties.In addition, five substituted azahelicenes were synthesized and characterized with respect to their (chir)optical properties, and the influence of the substitution pattern on the catalytic performance was tested.DFT calculations of three additional model compounds draw a path for further optimization by introducing electron-withdrawing substituents.

Synthesis
The 2-chloroquinolines 3 c-f (Scheme 1) with substituents in 6-and 8-position can be prepared in three steps from cinnamoyl chloride and the corresponding anilines.Aminolysis of the acid chloride yields anilides which were cyclized to the quinoline-2(1H)-ones in a Friedel-Crafts reaction with an excess of aluminum chloride either in chlorobenzene [35][36] or solvent-free in a melt, [37] leading to the elimination of benzene.Deoxychlorination with phosphoryl chloride gives the 2-chloroquinolines 3. 2-Chloro-4phenylquinoline 3 c was prepared in three steps from ethyl benzoylacetate.
Single crystals suitable for X-ray diffractometry were grown by evaporation of solutions of 7 c in CH 2 Cl 2 /pentane.The structure (Figure 1) shows the helically bent shape of the molecule, in which the torsional angles of the inner atoms add up to 71.4°(9.6°,26.9°, 4.2°, 24.4°, 6.3°), with most of the twisting on the two imidazole subunits.

Characterization of photophysical properties
Absorption and emission spectra were recorded in acetonitrile solution (Figure 2).The lowest energy absorption bands of 7 a-f tail into the blue spectral region and the broad fluorescence bands peak between 457 and 484 nm.Compared to the Scheme 1. Synthesis of the chloroquinolines 3 c-f.Scheme 2. Synthesis of the azahelicenes 7 a-f in two steps from chloroquinolines.unsubstituted helicene 7 a, the absorption and emission spectra of 7 b-f are red-shifted which is most pronounced for 7 c.This is due to their extended conjugated system (7 c) and the fluorination (7 d-f), respectively.The presence of excimers as a source of the shoulder at longer wavelengths was ruled out by measuring the fluorescence of 7 c at varying concentration, but no change in the spectrum was observed.Luminescence spectroscopy at 77 K reveals a fine structure of the fluorescence with three maxima in all cases (Table 1, Supporting Information Figure S7).Additionally, a weak, delayed emission between 525-650 nm was observed at low temperatures, which we assign to phosphorescence from excited triplet states.The corresponding triplet energies of 2.22-2.32eV were determined from the lowest energy phosphorescence band maxima λ max(ph) (Table 3).The assignment as triplet states was confirmed by quenching experiments with O 2 forming 1 O 2 (see below), which was detected via its emission signature at 1274 nm. [41]For 7 b-d and 7 f excitation spectra were recorded and closely follow the absorption spectra of the helicenes, confirming them as the source of the 1 O 2 production (Figure S8).
High fluorescence quantum yields (Φ fl ) of 0.20-0.65 with nanosecond emission lifetimes τ were measured for 7 a-f in deaerated acetonitrile and dichloromethane (Table 1).Under deaerated conditions, fluorescence quantum yields for the fluorinated helicenes 7 d (29 %) and 7 f (25 %) are lower compared to the other azahelicenes including 7 e, whose Φ fl remains above 40 %.46] For the azahelicenes studied here, a fluorination in 1,16-position (7 d) reduces Φ fl to a greater extent than fluorination in 3,14position (7 e and 7 f).Φ fl in dichloromethane are higher than in acetonitrile for 7 a-d, as is expected for a lower polarity solvent (ɛ MeCN = 36.64vs. ɛ CH2Cl2 = 8.93) [47] resulting in a decreased nonradiative decay rate. [48]nder air, quenching by oxygen lowered Φ fl , with quenching efficiencies in MeCN ranging from 20 % for the tetrafluorinated 7 f to 35 % for less-substituted 7 a and 7 b (SI, Table S1).The fluorescence lifetimes of 7 a-f also shorten in the presence of oxygen (Table S1) with quenching efficiencies matching those obtained from fluorescence quantum yields.
The (P)-and (M)-enantiomers of 7 a-f were resolved using semi-preparative HPLC with chiral stationary phases.Depending on the retention time differences and the solubility of the compounds, 3-10 mg could be purified in a single injection with stacked injections speeding up the separation of larger quantities.The purity of the separated enantiomers was assessed via analytical HPLC (SI, Figures S12-17).Optical rotations of the enantiomers were measured in chloroform and corrected for the determined enantiomeric excess (Table 2).The configurational stability and the activation parameters of enantiomerization of 7 a-f were determined by examining the Table 1.Absorption maxima λ max(abs) and the corresponding molar extinction coefficients ɛ, fluorescence maxima λ max(fl) , fluorescence quantum yields Φ fl , fluorescence lifetimes τ fl and phosphorescence maxima λ max(ph) for azahelicenes 7 a-f.Absorption (20 μM), fluorescence (20 μM) at room temperature and fluorescence lifetimes (100 μM) were measured in MeCN.Low temperature measurements were performed in EtOH/MeOH (4 : 1) glass at 100 μM concentration.
Compound λ max(abs) [nm]  (ɛ [ (r.t.) [b] (77 K) [c] MeCN [d]  racemization (k rac = 2k e ) via HPLC at constant temperatures over time (see Supporting Information for further information).Specific optical rotatory powers of the helicenes range from + 2300°cm 3 g À 1 dm À 1 ((P)-7 c) to + 3555°cm 3 g À 1 dm À 1 ((P)-7 f).The assignment of absolute configuration is based on the general relationship that (P)-helicenes are dextrorotatory and (M)-helicenes are levorotatory, [9][10] and was confirmed exemplarily by DFT calculation of the ECD spectrum of 7 a (Figure S18).The activation parameters of enantiomerization of the aza [7]helicenes (Table 2) show the expected effect of increased conformational stability for C-1 substituted derivatives (7 c, 7 f).The influence of temperature on ΔG � is small as was reported for carbon[n]helicenes. [49]The values of ΔG � calculated for 7 a-f lie between those of carbo [5]helicene (24.1 kcal/mol (298 K)) [49] and carbo [6]helicene (35.4 kcal/mol (298 K)) [49] and are lower than for [7]helicenes consisting only of six-membered rings, since the presence of the two five-membered rings drastically lowers the enantiomerization barrier. [9,49]The influence of the substituents in 3,14-and 5,12-position is much smaller, but the phenyl substituents in 7 c slightly lower the barrier of enantiomerization compared to the unsubstituted structure.It should be noted that the phenyl rings in 7 c are not fully conjugated to the π-system of the helix, as the planar configuration of the phenyl substituents is expected to be hindered to a similar extent as in 1-phenylnaphthalene. [50][51] An equilibrium torsion angle of 50-70°, depending on the solvent, has been reported for 1-phenylnaphthalene, [51] and an appropriate angle is observed in the crystal structure of rac-7 c (Figure 1, additionally see Supporting Information, CCDC deposit number 2241858).
ECD-spectra of compounds 7 a-f were recorded in dichloromethane solution and are shown in Figure 3.The (P)-enantiomers of the azahelicenes show positive Cotton bands at longer wavelengths.The 1,16-substituted 7 d and 7 f have less structured spectra compared to the compounds without substituents at this position, resulting in differences in the region around 330 nm.The spectra of the (M)-enantiomers appear as mirror images.A plot of the dissymmetry factor of absorption (g abs ) is given in the Supporting Information (Figure S19).
Circulary polarized luminescence (CPL) spectra were recorded (Figure 3) and the dissymmetry values g lum at λ max,em are given in Table 2.The absolute g lum values for the (P)-and (M)enantiomers match for each compound, except for 7 d, showing a lower value for the (M)-enantiomer.A difference was also observed in the optical rotation in this case.The differences for the enantiomers of 7 d may result from a luminescent impurity which cannot be removed in this particular case via the chromatography methods applied, which can be detected in the absorption and emission spectra of the two enantiomers (see Supporting Information, Figures S20, S21).Substitution lowers the CPL intensity and the unsubstituted 7 a remains the strongest CPL emitter in this series of helicenes with j g lum j = À 8.1 • 10 À 3 , which is consistent with the value determined by Otani et al. [34] To allow for a direct comparison to other CPLemitters, the CPL brightness (B CPL ), which was introduced by Arrico et al., was calculated using the molar extinction coefficient at excitation wavelength and the fluorescence quantum Enantiomerization barrier [ [34,52] 22.6 � 1.8 À 16.6 � 5.  yield of each helicene using Equation (1). [52]For the calculation, j g lum j was defined as the mean of the g lum absolute values for the (P)-and (M)-enantiomers.
The azahelicenes 7 a-f show exceptionally high B CPL values of 11.4-45.0M À 1 cm À 1 at the excitation wavelength used for CPL measurements (see Table 2, Footnote [b]).For 7 a the value we obtained is higher than in earlier reports (B CPL = 37.6) [52] due to the higher fluorescence quantum yield determined in our measurement.The increased extinction coefficient of 7 b yields a higher CPL brightness compared to 7 a, despite its lower fluorescence quantum yield and dissymmetry factor g lum .The CPL brightness of B CPL = 45 M À 1 cm À 1 for 7 b is among the highest of [7]helicenes compiled by Arrico et al., with only few, mostly dimeric, helicenes showing higher values. [52]

Photocatalytic properties
To explore the electronic structure of the azahelicenes and the effect of substitution DFT calculations were performed on the structures 7 a-f and additionally on model compounds 7 g-i (Figure 4) to investigate the influence of stronger acceptor substituents.
Calculated HOMO-and LUMO energies are included in Table 3 and the frontier orbitals are shown in the Supporting Information.Attachment of a methyl or phenyl group in 5,12position destabilizes the HOMO by + 0.14 eV and + 0.12 eV, respectively.Placing a CF 3 -group or a C 6 F 5 -group in the same position (model substrates 7 h, 7 i) results in a strong stabilization compared to the non-fluorinated substituents (À 0.71 eV CH 3 !CF 3 , À 0.47 eV Ph!C 6 F 5 ).The LUMO energy of the methylated 7 b is raised to a similar extent (+ 0.11 eV) as the HOMO, resulting in comparable absorption and emission bands of 7 a and 7 b but an easier oxidation of 7 b.Fluorination on the inner ring positions (1,16) does not influence the HOMO level but fluorination in the 3,14-position stabilizes it by À 0.23 eV.LUMOs of the fluorinated derivatives 7 d and 7 e are shifted similarly to the HOMOs, with the 1,16-F-substitution having a lesser stabilizing effect compared to the 3,14-substitution (À 0.06 eV and À 0.27 eV).Orbitals of the tetrafluorinated helicene 7 f show the combined effect of both substitutions.In model substrate 7 g the cyano group in 1,16-position, unlike a fluoro-substitution, does stabilize (À 0.17 eV) the HOMO but is also predicted to stabilize the LUMO to an even larger extent (À 0.51 eV) leading to a reduced energy gap of 3.23 eV.
The HOMO is predominantly localized on the central A-, Band C-rings, while the LUMO mainly extends to the outer (Cand D-) rings.The substituents only have minor effects on the orbital coefficients of the HOMOs.The LUMO of 7 c and 7 i is extended to the phenyl rings to a small extent while the orbital coefficients in the D-rings are smaller, in contrast to the LUMO  All potentials are given vs. SCE.b] From lowest λ max(ph) .
coefficients of the cyano-substituted 7 g, which are larger on the outer D-rings compared to any other derivative studied here.The calculations allow prediction of the (photo)redox and optical properties of the model substrates, and the estimated values are given in Table 3.Further information on the calculations of the excited state energies and redox potentials is given in the Supporting Information.
To estimate the photoredox potentials of the catalysts 7 a-f cyclic voltammograms were recorded (SI, Figure S9).Using the equation for the Gibbs energy of photoinduced electron transfer (Eq.( 2)), omitting the electrostatic work term, the excited state potentials of the helicenes were calculated.The excited state energy E 0,0 is approximated from the absorption and fluorescence spectra using the midpoint between the lowest energy absorption maximum and the fluorescence maximum. [26]The potentials are compiled in Table 3.
The oxidation potentials of the fluorinated helicenes 7 d-f are increased compared to the donor-substituted or unsubstituted helicenes 7 a-c, as expected for electronegative substituents.The effect is not very pronounced due to the strong mesomeric properties of the fluorine substituents which attenuate the inductive effect (resonance parameter R = À 0.39, field effect parameter F = 0.45). [53]The methyl-(7 b, R = À 0.18, F = 0.01) [53] and phenyl-substituted (7 c, R = À 0.13, F = 0.12) [53] derivatives show a comparable oxidation potential, while the excited state energy of 7 c is decreased as the extended πsystem reduces the HOMO-LUMO gap.7 c shows a second oxidation wave (E1 = 2 ,ox,2 = + 1.37 V) with both oxidations being reversible.Apart from this, reversibility of the first oxidation on the CV time scale was observed only for 7 a and 7 f.No peaks in the reductive region were observed except for 7 f (À 1.58 V), which corresponds to an electrochemical gap of 2.91 eV that compares well to the estimated optical gap E 0,0 = 2.88 eV.The expected position of the reductive peaks for 7 a-e is beyond the potential window of dichloromethane and sweeps to À 2 V failed to give peaks in these cases.7 h and 7 i are expected to show increased oxidation potentials compared to their nonfluorinated counterparts 7 b and 7 i.This effect is especially pronounced for 7 h, which should be a photoredox catalyst with a much lower reductive power in the excited state.
The photoredox-catalytic properties of 7 a-f were compared in a sulfonylation/hetarylation of styrenes earlier developed in our lab [54] using four different cyanopyridine substrates 8 a-d with varying reduction potentials.The results are compiled in Scheme 3. The stronger reducing derivatives 7 a-c give higher yields in this reaction compared to the fluorinated compounds 7 d-f, even if the catalyst's reduction potential matches or surpasses the substrates electron affinity by calculation as is the case for the combinations 7 d-f and 8 a.Another factor coming into play might be the singlet lifetime (Table 1) which is decreased for 7 d and 7 f.Certain combinations (7 d and 8 b, 7 e and 8 a) give high yields of the product even if the catalyst's reduction potential should not allow for efficient electron transfer to the substrate.
It was anticipated that the improved performance in the reaction with 8 d of 7 b compared to 7 a may also stem from a decreased decomposition rate trough styrene addition due blocking of the 6,11-positions by methyl groups.A comparison of the decay in acetonitrile solution containing 40 equiv.αmethylstyrene reveals the opposite to be true as the decomposition rate constant, assuming first order kinetics, for 7 b (k decomp = 6.1 � 0.3 • 10 À 4 s À 1 ) is twice the rate constant of 7 a (k decomp = 2.7 � 0.1 • 10 À 4 s À 1 ).While the spectral shift of 2 nm (Table 1) between 7 a und 7 b does increase the spectral overlap with the used lamp for 7 b (Supporting Information, Figure S23) it does so only to a small extent and cannot be the sole cause of the accelerated decomposition observed.The lower fluorescence quantum yield of 7 b (41 % vs. 48 % for 7 a) may be an indication of a higher intersystem crossing rate to the triplet state, from which the reaction of the catalyst with the styrene probably proceeds to decomposition.

Conclusions
The synthesis of substituted aza [7]helicenes from 2-chloroquinolines in a two-step sequence was demonstrated for six derivatives and the oxidative cyclization with PIFA proved a reliable method for each of the substrates used.The photophysical properties of the azahelicenes were extensively studied, a weak phosphorescence and the sensitization of 1 O 2 show the formation of a triplet state.The chiroptical properties of the helicenes were studied after chromatographic separation of the enantiomers.Electronic circular dichroism and circularly polarized luminescence spectra were recorded, giving g lum values in the 10 À 3 range for each of the derivatives.
The suitability of the helicenes as photoredox catalysts was evaluated in view of their reductive power in a sulfonylation/ hetarylation reaction with four benchmark cyanopyridine substrates.The strongly reductive variants 7 a and 7 b performed best in this reaction.Subtle differences in the catalyst structure result in significant effects on substrate preference.While the use of chiral catalysts of this type in enantioselective photocatalysis will be investigated, structural modifications may be required to ensure a sufficient enantiodiscrimination.The redox potentials of the helicenes were determined by cyclic voltammetry and the excited state potentials estimated.DFT calculations were performed to visualize the frontier orbitals and to evaluate three additional model compounds and calculate their expected redox potentials.The influence of the substitution on the localization of the HOMO and LUMO was small in the synthesized cases.

Experimental Section
Deposition Number 2241858 (for 7 c) contains the supplementary crystallographic data for this paper.These data are provided free of charge by the joint Cambridge Crystallographic Data Centre and Fachinformationszentrum Karlsruhe Access Structures service.
General procedure for the coupling of 2-chloroquinolines with pphenylenediamine: p-Phenylenediamine (1.0 eq), the corresponding 2-chloroquinoline (2.0 eq) and tributylamine (4.0 eq) were placed in round bottom reaction tube with a magnetic stir bar and sealed with a septum.After exchanging the atmosphere with N 2 the tube was closed with a screw cap and heated to 200 °C over night.The mixture was diluted with EtOAc and washed with NaCO 3 soln.(1 M) and brine.After drying with Na 2 SO 4 and evaporation the residue was recrystallized from toluene.
General procedure for the oxidation to azahelicenes: The helicene precursor was dissolved in HFIP and PIFA was added in portions (0.4 eq) every 30 min.After the last addition (2.0-2.4 equiv.are required) the mixture is stirred over night and diluted with CH 2 Cl 2 , then washed with NaHCO 3 soln.and Na 2 S 2 O 3 soln.and dried with Na 2 SO 4 .The crude product is evaporated onto silica and purified via flash chromatography (gradient CH 2 Cl 2 to CH 2 Cl 2 /MeOH = 97.5 : 2.5, Isolera system).

Figure 2 .
Figure 2. Absorption and emission spectra of 7 a-f in MeCN (20 μM) at room temperature.
c = 0.05 in CHCl 3 , a relative error of 5 % is assumed.[b] in CH 2 Cl 2 at λ max(fl) , λ exc: 7 a 337 nm; 7 b 335 nm; 7 c 345 nm; 7 d 345 nm; 7 e 343 nm; 7 f 350 nm.[c] in n AmylOH [d] errors were in the range of 9-13 cal/mol.[e] ɛ at the excitation wavelength of the CPL measurement and Φ fl in CH 2 Cl 2 are used for calculation.The mean value of g lum was used to calculated the CPL brightness; error is calculated by propagation of error assuming 1 = 2 Δg lum as the error of g lum and a relative error of 5 % for Φ fl .

Figure 4 .
Figure 4. Structures of model compounds 7 g-7 i and exemplary annotation of the ring system.

Table 3 .
Excited state energies, redox potentials and HOMO-LUMO energies of azahelicenes 7 a-f.Estimated values from HOMO-LUMO energies for 7 g-i are given in italics.