Carbonyl mediated fluorescence in aceno[n]helicenones and fluoreno[n]helicenes

Helicenes are very attractive chiral non-planar polycyclic aromatic hydrocarbons possessing strong chiroptical properties. However, most of the helicenes absorb light mainly in the ultraviolet region, with only a small segment in the blue part of the visible spectrum. Furthermore, carbo[n]helicenes exhibit only weak luminescence that limits their utilization. Herein, we demonstrate that peripheral decoration of the helicene backbone with an aryl-carbonyl group shifts the absorption to the visible region and simultaneously improves their fluorescence quantum yields. We thus show that the carbonyl group, commonly considered as detrimental to emission, has the capability of improving optical and photophysical properties. Two different families, aceno[n]helicenones and fluoreno[n]helicenes, are presented with comprehensive spectrochemical characterization. TD-DFT calculations were implemented to clarify their electronic profiles. We show that increasing the helical length in aceno[n]helicenes increases absorption onset, gabs and glum. Extension of the peripheral aromatic part in fluoreno[n]helicenes leads to a blue shift in both absorption and emission.


Introduction
The introduction of a carbonyl group into the aromatic skeleton signicantly reduces the HOMO-LUMO gap which is an essential parameter in organic electronics.This effect is oen evident by a shi of the absorption and the emission into the visible region.Even relatively short conjugated carbonyl systems are capable of absorption of long-wavelength parts of the visible spectrum.However, carbonyl groups are generally considered as detrimental to the uorescence quantum yield. 1 This is due to efficient intersystem crossing (ISC) between singlet and triplet excited manifolds of np* and pp* character.The ISC yield of aromatic ketones is related simultaneously to the very small energetic singlet-triplet gap (<0.2 eV) due to the orthogonal orientation between non-bonding n and anti-bonding p* orbitals and to the strong spin-orbit coupling following El-Sayed's rule. 2 Therefore, most aromatic ketones undergo extremely fast ISC within picoseconds, e.g.benzophenone: 5-10 ps, 3 xanthone: 2 ps, 4 anthrone: 70 ps. 5This results from the proviso that systems which possess low lying singlet 1 np* or 1 pp* (S1 and potentially S2) states are coupled with triplet states of similar energy and, intrinsically, different symmetry ( 3 pp* or 1 np*, respectively). 2 For this reason, the integration of a carbonyl group into the aromatic system would seem counter-intuitive for the purpose of improving the photophysical properties.Thus, the carbonyl group is rarely considered benecial for the luminescence.Herein we show that the installation of an aryl-carbonyl group at the periphery of the carbo[n]helicene helix not only shis the absorption by more than 100 nm (>4270 cm −1 ) but also signicantly increases the uorescence quantum yield (F F ) when compared to pristine carbo[n]helicenes.Although helicenes are known to exhibit very remarkable chiroptical properties such as high optical rotations (>±1000), circular dichroism, circularly polarized luminescence and nonlinear optical activity, 6 they show only weak luminescence: the uorescence quantum yield (F F ) of [6]helicene is F F = 0.04, [7]helicene F F = 0.02 and [n]helicene (where n $ 8) F F # 0.01. 7This is mainly due to the small oscillator strength and to signicant intersystem crossing (ISC). 8Furthermore, the absorption onset wavelength (l onset ) of helicenes increases initially with increasing length but saturates around 490 nm ( [11]helicene).Thus, pristine carbo [n]helicenes are colorless (n a Institute of Organic Chemistry and Biochemistry of the Czech Academy of Sciences, 166 10 Prague 6, Czech Republic.E-mail: samal@uochb.cas.cz# 6) or yellow (n $ 7) substances.In order to qualify helicenes for use in various luminescence-based (chiropto)electronic applications such as uorescent probes, signaling systems, OLEDs, bioimaging etc., it is necessary to improve their luminescence quantum yields and modulate their absorption/ emission (HOMO-LUMO gap) on demand.To improve photochemical parameters with emphasis on F F , a number of different approaches have been employed, 9 such as substitution, hetero-doping, lateral extension, or metal complexation.
Crassous, Favereau, Autschbach and coworkers have shown that the functionalization of [6]helicene allows the modulation of the circularly polarized luminescence, the absorption onset and the uorescence quantum yield (Fig. 1B). 10,11Matsuda, Hirose et al. have developed [7]helicene derivatives with enhanced circularly polarized luminescence and improved uorescence quantum yields (Fig. 1C). 12In the case of helicenes longer than [7]helicene, only theoretical studies have been conducted to investigate their chiroptical properties.It was shown that the dissymmetry factor of carbo[n]helicenes increases with the helix length n. 14 Recently, Narita, Müllen, Pieters and coworkers have demonstrated that a small increase (from n = 7 to 9) of the helix length in laterally extended [n]  helicenes can lead to a 10-fold increase of the dissymmetry factor (Fig. 1D). 13Herein, we show that the carbonyl group can do the job of increasing F F while simultaneously shiing the absorption onset to longer wavelengths.6][17][18][19] The non-emissivity was always attributed to the carbonyl group.Thus uorenone-fused helicenes have been extensively used only as precursors to synthesize a variety of highly luminescent helicene-like materials such as uorene-fused helicenes, 15,16 azahelicenes 20 or indeno-uorenes. 21,22In this study we show that installation of uorenone or acenone units at the periphery of the helicene (Fig. 2) has a very positive effect on their photophysical properties.The absorptions and emissions are shied into the visible region and the uorescence quantum yields are greatly enhanced.In addition, our study demonstrates that the established rule that the carbonyl group is detrimental to such properties has important limitations.We show that the carbonyl group can indeed improve the optical and photophysical properties.Furthermore, we show that the absorption and luminescence dissymmetry factors (g abs and g lum ) in the obtained systems strongly depend on the helix length n, in contrast to pristine helicenes.

Results and discussion
For this study we have designed and synthesized two families of carbo[n]helicenes.The rst family, aceno[n]helicenones 1A-8A (Fig. 2 and 3), are helicenes that are peripherally annulated via an aryl-carbonyl group that formally creates an acene unit.The second family, uoreno[n]helicenes 1F-8F, is annulated such that a uorenone unit is inserted into the helicene backbone.Installing an aromatic carbonyl group in this way at the periphery of the helicene scaffold has a signicant effect on the  optical properties of the studied helicenes.For example, the absorption is shied to the visible region by more than 50 nm (2160-3370 cm −1 ) in the aceno[n]helicenone family and more than 100 nm (4270-6000 cm −1 ) in the uoreno-derivatives, and the uorescence quantum yields are increased up to six times when compared to the pristine helicenes.We have deployed two versatile approaches providing aceno-/uoreno[n]helicenone series 1A-8A/1F-8F (Fig. 2).
The rst approach is based on the Mallory photo-cyclization of suitable bromo-stilbene precursors ST (prepared by Wittig reaction) providing directly bromo-helicenes Hel-Br.The bromo atom fulls two crucial functions that are (1) deactivation of the ortho-position against photocyclization and thus directing the cyclization toward the helicene and (2) being a coupling partner in a subsequent Suzuki reaction.The second approach is based on [2 + 2 + 2] cycloisomerization of suitable tri-ynes 3YN, developed by I. Stará and I. Starý et al. 23 Intramolecular cyclization (catalyzed by a Co I catalyst) followed by elimination/ aromatization provides directly arylated helicenes Hel-Ar bearing carboxylic ester group.Basic hydrolysis of the ester group leads to the acids.The helicenic esters Hel-Ar and their corresponding acids consist of two unseparable atropodiastereomers (for details, see ESI † page S158).The nal acid-catalyzed cyclization afforded aceno[n]helicenones and uoreno[n]helicenes 1A-8A/1F-8F simultaneously in ratios varying between 1 : 1 and 1 : 7. The uoreno[n]helicenes were always formed preferentially.The density functional theory (DFT) calculations at the level of the uB97X-D/Def2SVP on the formation process of 1A and 1F suggested that one atropodiastereomer preferentially produces 1F, while the other atropodiastereomer affords a mixture of 1A and 1F (see ESI † page S189).This result of the calculation showed a good agreement with the experimental outcomes and corroborates that uoreno[n]helicenones are formed preferentially.The signicant difference in polarity of aceno[n]helicenones and uoreno[n]helicenes allows us to very effectively separate the two helicenes by traditional column chromatography.All the nal structures are well soluble in common organic solvents, and thus they were characterized by proton and carbon nuclear magnetic resonance spectroscopy ( 1 H and 13 C NMR) and high-resolution mass spectrometry (HR-MS).In addition, the structures of the uoreno[n]helicenes 5F, 6F and 8F were unambiguously conrmed by single crystal X-ray diffraction (for details, see ESI † page S165).

Photophysical and electrochemical properties of aceno[n] helicenones 1A-8A
The structures of the series have been designed in order to systematically study how the length of the helicene and acene parts will affect their absorption/emission and their chiroptical properties such as electronic circular dichroism (ECD), circularly polarized luminescence (CPL) and optical rotatory power.Isoelectronic aceno[n]helicenones 4A and 5A are unique systems which differ in the position of Clar sextets and in the position of a double bond relative to the carbonyl group (in blue color, Fig. 5a and b).This reveals the relation of the geometry with the physicochemical properties.First, aceno[n]helicenones 3A, 5A, 7A and 8A have been investigated to probe the effect of the helicene length on absorption and emission (Table 1).The ultraviolet-visible (UV-Vis) absorption and photoluminescence (PL) spectra were recorded rst in nonpolar hexane (10 −5 M) (as a representative example of 5A, Fig. 4).The electronic absorption spectra of 3A, 5A, 7A and 8A cover the entire ultraviolet region and a major part of the blue region with a maximum located at 460 nm, 480 nm, 489 nm and 504 nm, respectively.
These absorption maxima located in the visible light region render these compounds orange.The absorption maximum is red-shiing with each further benzene annulation.A major increase is observed when going from aceno [6]helicenone 3A to aceno [7]helicenone 5A, which is most likely related to the fact that the two terminal rings of the [7]helicene backbone of 5A overlap.This overlap brings additional through-space conjugation and a perceptibly red-shied absorption.This aceno[n] helicenone series follows a regular trend of decrease of the gap (E 0,0 ) with increase of the length of the p-system (for details, see ESI † Table S1, page S147 and S158).The lowest absorption bands arise from the allowed p / p* transitions with high oscillator strengths (f = 0.43 for 3A; f = 0.19 for 5A; f = 0.16 for 7A and f = 0.16 for 8A) and correspond mainly to S 0 / S 1 transitions.In nonpolar hexane, the uorescence spectra of the 1A-8A series show a well-structured vibrational emission band which can be ascribed to a locally excited (LE) state character.Very small Stokes shis (#530 cm −1 , #11 nm) are pointing to small changes of the electron distributions in the ground and excited states.In all cases of aceno-derivatives 1A-8A, solvato-uorochromism was observed.The polarity of the environment has only a limited effect on the absorption, causing a small red-shi of the long-wavelength absorption bands.However, passing from low-polarity hexane to polar dichloromethane (DCM) or acetonitrile (ACN) has a pronounced effect on the emission spectra.There is a notable disappearance of the vibrational features of the emission band, accompanied by a strong red-shi.Such large Stokes shis (1220-2420 cm −1 , 30-60 nm) indicate that the dipole moment of low-lying excited states (S 1 ) is larger than in the ground state (S 0 ).According to the calculation results, the dipole moments of aceno-helicenes 1A-8A in the ground state are of the order of 4-5 D, and in the excited state 8-12 D, which corroborate to the experimentally observed Stokes shis with increasing of solvent polarity (for details, see ESI † page S163).This difference of dipole moments characterizes an electron-donor-acceptor (EDA) system.In our case, the carbonyl group behaves as an acceptor unit owing to its strong electron-withdrawing effect, and the electron rich helicene wing functions as a donor.This electron ow is visible in the electron density distribution of HOMO and LUMO, obtained by density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations based on B3LYP functional and 6-31g(d) basis set.Although the HOMO and LUMO are extended over the whole molecule, the main density of the HOMO is located on the helicene wing and the main density of the LUMO is distributed over the carbonyl-acene part (Fig. 5a and b).Moreover, the symmetry of these orbitals, compared to the orbitals of the parent helicene, is disturbed and causes a clear    increase in oscillator strengths for the transitions between the S 0 and S 1 states (for details see ESI † page S162).This is accompanied by an increase in the uorescence yield of acenohelicenes 1A-8A.However, the uorescence yield is limited by non-radiative intersystem crossing processes taking place by different channels, i.e. to the numerous triplet states with energies close to that of the S 1 state.However, the effect is less prominent as the length of the helix increases.For instance, there is a six-fold F F increase in a case of aceno [6]helicenone 3A in polar DCM (F F = 0.085) or ACN (F F = 0.078), when compared to the value in nonpolar hexane (F F = 0.013), while for aceno [9]  helicenone the F F are comparable in nonpolar or polar environment (for details see the Table 1 and S147 †).The emission is characterized by short uorescence lifetimes (2-4 ns) of all the aceno-derivatives 1A-8A, much shorter than the lifetimes of the corresponding [n]helicenes (10-15 ns). 25 In the next step, we have investigated effect of the acene length on the photophysical properties.For this purpose, the aceno-derivatives 1A, 2A, 5A and 6A were synthesized.In the acene series, the absorption maximum systematically increases by ca. 100 nm with each additional benzene ring. 26,27In contrast, increasing the length of the acene part in the aceno[n]helicenones by one benzene ring leads to a smaller bathochromic shi than the one caused by an increase in the length of the helicene part.A linear one-benzene ring annulation of the acene part red-shis the absorption maximum by only 4 nm (1A / 2A, 459 nm / 463 nm or 5A / 6A, 480 nm / 484 nm), whereas elongation of the helicene part by one benzene ring red-shis the absorption maximum by 9 nm (1A / 4A, 459 nm / 468 nm or 5A / 7A, 480 nm / 489 nm).We assume that this is due to the extra through-space conjugation caused by the overlap of the terminal helicene rings.Linear benzannullation of the acene part increases the F F of 2A and 6A in nonpolar hexane (F F of 1A / 2A: 0.036 / 0.050 and 5A / 6A: 0.033 / 0.079).The uorescence quantum yield of 2A decreases but that of 6A increases in polar DCM (F F of 1A / 2A, 0.125 / 0.083 and 5A / 6A, 0.066 / 0.112).Last but not least, the isoelectronic derivatives 4A and 5A allow us to study the relation between the geometry and the physicochemical properties.Both structures are composed of tetracene and [7]helicene units.They differ in the position of the ring with double bond character (in blue color, Fig. 5a and b) relative to the carbonyl group.This position can be controlled by the arrangement of the tetracene and [7]  helicene units and thus by localization of the Clar p-sextets in the helicene part (green color, Fig. 5a and b).The position and thus the conjugation of the double bond to the carbonyl group may affect the physicochemical properties of the whole aromatic system.It can also have consequences for the chemical reactivity, for instance 4A and 5A may behave differently upon nucleophilic attack (1,2-vs.1,4-addition). 28,29We found that most of the photophysical and chiroptical properties of 4A and 5A are comparable and only the absorbance maxima of 5A (480 nm, double bond in para-position to the carbonyl) is red-shied by 12 nm when compared to 4A (468 nm, double bond in ortho-position to the carbonyl).However, the geometry strongly affects the redox behavior, as evidenced by cyclic voltammetry (CV).
All the aceno-derivatives 1A-8A exhibit weak phosphorescence in hexane at 5 K, but this emission is absent in DCM.The phosphorescence emission band has two maxima in the 600-800 nm range.The singlet-triplet gap (DE S0-T1 ) was determined from the uorescence and phosphorescence onset wavelengths.Plotting the singlet-triplet gaps (DE T1-S0 ) of the aceno[n]helicenones 3A, 5A, 7A and 8A we observed a systematic decrease with increasing length of the helicene part, which conrms that the energy of the triplet state is associated with conjugation length (see ESI † page S158).Likewise, the energy gap DE S1-T1 between the lowest excited singlet (S 1 ) and triplet (T 1 ) states decreases with increasing length of the helicene part.Surprisingly, elongation of the acene part (1A / 2A or 5A / 6A) leads to an increase of the gap DE S0-T1 , in stark contrast to the pristine acenes. 30The energy-state level diagrams of 1A-8A were obtained by TD-DFT calculations.All energy diagrams exhibit that higher triplet states (T 3 , T 4 and potentially T 5 ) are energetically close (DE S1-T4/3 # 0.1 eV) to the S 1 state and may constitute a main channel for non-radiative intersystem crossing (ISC, S 1 / T n ).The resulting population of the T 1 state (by nonradiative T n / T 1 ) is manifested by the emission of phosphorescence.The efficient ISC (together with the nonradiative internal conversion, S 1 / S 0 ) compete with the radiative depopulation of the S 1 state, and the result is reduced quantum yield of the uorescence emission.
Cyclic voltammetry (CV) measurements were performed in acetonitrile to determine the redox properties of 1A-8A.All exhibit two reduction processes, except for 4A and 8A, which exhibit three reversible reductions.The rst reductions show sharp reversible waves with half-wave potentials around E red 1/2 z −1.5 V (for details, see ESI †).The second reduction is irreversible, except for 4A and 8A.1A-8A all do not show any oxidation wave below +1.0 V (vs.Fc/Fc + ), suggesting a poor electron donating ability of the helicene wing.As already mentioned, isoelectronic 4A and 5A differ only in the position of the double bond (blue color, Fig. 5a and b) relative to the carbonyl group (para-/ortho-, determined by Clar's aromatic sextets).Interestingly, the systems with a double bond adjacent to the carbonyl group (4A and 8A) exhibit reversible three electron reductions with E red 1/2 = −1.66V, −2.20 V and −2.50 V for 4A and −1.62 V, −2.16 V and −2.53 V for 8A (vs.Fc/Fc + ).This particular geometry renders the doubly reduced species stable at the time scale of the CV.We suppose that the stability of the species aer two single electron reductions of 4A and 8A implies that they contain same number of Clar p-sextets (for details, see ESI † page S164).From the onset of the reduction wave, the LUMO energy levels of 1A-8A were calculated.The HOMO energy levels were calculated from the wavelength of the absorption onset.

Photophysical and electrochemical properties of uoreno[n] helicenes 1F-8F
The absorption onset of the uoreno[n]helicenes 1F-8F is located at longer wavelengths than 500 nm, which makes them red in the solid state (Fig. 7).The absorption onset is systematically red-shied with each benzannulation of the helix.There is increase an of 54 nm when going form [6]helicene 3F to [9]  helicene 8F (3F l onset = 534 nm, 5F l onset = 549 nm, 7F l onset = 560 nm, 8F l onset = 588 nm).Interestingly, the biggest increment (28 nm) is observed when going from [8]helicene 7F to [9]  helicene 8F and not when going from [6]helicene 3F to [7]helicene 5F, where the terminal rings start to overlap and extra through space-conjugation appears.Unfortunately, we do not have a satisfying explanation for that observation at this moment.The longest absorption band (assigned to p / p*) of the uoreno-derivatives is broad and featureless, except for 2F, 3F and 6F, whose spectra are more structured.
While the absorption spectra of 2F, 3F and 6F red-shi and broaden with increasing polarity, those of the other uoreno homologs are not modied by changing the polarity of the environment, suggesting that there are no signicant changes of the electron distributions in the ground state.The emission spectra of 1F-8F are broad (spanning over 200 nm) with two weakly visible maxima in nonpolar hexane.The emission spectra in polar DCM are broad and featureless with very large Stokes shis (2730-5350 cm −1 ).This suggests that the excited state S 1 has partial LE character in a low-polar environment and CT character in a polar media.In this system the uorenone substructure behaves as an acceptor group owing to the strong electron-withdrawing effect of the carbonyl, and the electron rich helicene wings behave as a donor.1F-8F all exhibit moderate uorescence in nonpolar hexane and very weak emission (except 2F and 6F, see below) in a polar environment.This difference is governed mainly by the higher non-radiative decay rate constant k nr in polar media, whereas the radiative rate constant k r is independent of the polarity.The k nr increases as the length of the helicene increases, whereas k r remains almost unchanged, and thus the F F decreases from [6]helicene 3F (F F = 0.115) to [9]helicene 8F (F F = 0.029).The uorescence lifetimes (5-10 ns) are higher than in the aceno[n]helicenones 1 A-8A (2-4 ns) but lower than for the parent helicenes (10-15 ns). 25 Annulation of one benzene ring in the linear direction of the uorenone affects considerably the photophysical  properties.Primarily, extension of 1F and 5F by one benzene ring leads to a blue-shi of the absorption by 20 nm (1F / 2F, 535 nm / 515 nm; 5F / 6F, 549 nm / 528 nm, Table 2).This hypsochromic shi is due to both electronic and geometric effects.The electronic effect includes an increase of the LUMO energy caused by a supplementary electron donation from the new annulated ring to the carbonyl group.This extra electron density lowers the carbonyl (C]O) bond strength, as conrmed by infrared spectroscopy.The stretching frequencies of 1F (n CO = 1700 cm −1 ) and 5F (n CO = 1700 cm −1 ) are higher than those of 2F (n CO = 1692 cm −1 ) and 6F (n CO = 1692 cm −1 ), respectively.The length and thus the bond strength of the carbonyl group of 5F and 6F (5F = 1.220Å vs. 6F = 1.226Å) was probed by single crystal XRD which conrmed the donating effect of the extra benzene ring.Interestingly, the one benzene ring annulation (5F / 6F) signicantly increases the distance of the two terminal rings in solid state (distance between the centroids of the overlapping benzene rings A-C, 5F A-C = 3.814 Å, 6F A-C = 4.047 Å, for details, see ESI † page S165) of the helicene backbone, with a disruptive effect on the through-space conjugation.However theoretical calculations do not show such a large distance between the terminal rings (5F A-C = 3.71 Å; 6F A-C = 3.70 Å, calculated at the level wB97X-D/Def2SVP using Gaussian 16 Rev C.02).The extra benzene ring has a strong effect on the F F both in nonpolar (F F in hexane of 1F / 2F, 0.055 / 0.22 and 5F / 6F, 0.042 / 0.12) and in polar environment (F F in DCM of 1F / 2F, 0.006 / 0.139 and 5F / 6F, 0.006 / 0.10).The oscillator strength was only slightly improved, but the nonradiative decay rate constant k nr was drastically reduced in both nonpolar and polar media, leading to an increased F F .These uoreno[n]helicenes show a regular trend of decreasing gap (E 0,0 ) with increasing length of the helicene.The decrease of the gap, when going from uoreno [6]helicene 3F to uoreno [6]  helicene 8F is less prominent (E 0,0 3F-8F = 0.19 eV) than for the homologous aceno[n]helicenones 3A and 8A (E 0,0 3A-8A = 0.25 eV).1F-8F do not exhibit phosphorescence even at 5K.This can be explained by their energy-level diagram proles.All the diagrams of 1F-8F have similar proles and show that there is only one triplet state below the lowest excited state S 1 .All the higher triplet states are well above the S 1 .Thus, the ISC from the S 1 to T 2 and to higher triplet states is endothermic and suppressed at low temperatures.T 1 is the only triplet state which could participate in ISC from S 1 .However, the relatively high energy difference (E S1-T1 > 0.4 eV) and similar nature of S 1 and T 1 states (spin-orbit-coupling, SOC, in case of 6F = 0.143 and 7F = 0.124) renders ISC inefficient.Therefore, the internal conversion from S 1 to S 0 becomes the only nonradiative deactivation channel.Based on our experiments and theoretical calculations, the DE S0-T1 gap systematically decreases as the length of the helicene part increases, as the energy of the triplet state is associated with conjugation length (see ESI † page S158).The energy of T 1 is decreasing to the same extent as the energy of S 1 , thus the energy gap DE S1-T1 remains constant as the length of the helix increases.Benzannulation of the uorene part lis the S 1 but has only a minuscule effect on the T 1 , thus there is an increase of the DE S1-T1 gap.CV measurements were performed to determine the electrochemical properties of 1F-8F.All exhibit two reversible reduction waves.From the onset of the reduction wave, the LUMO energy levels of 1F-8F were calculated.The HOMO energy levels were calculated from the absorption onset wavelength, i.e. from the optical gap (for details, see ESI † Table S2 page S157).

Conclusions
In summary, we have made a comprehensive investigation about the structure-property relationship of carbonyl-aceno[n] helicenes 1A-8A and carbonyl-uoreno[n]helicenes 1F-8F.We analyzed their structural, photophysical and chiroptical properties using NMR, CV, ECD, CPL and single crystal XRD, and the experimental results were conrmed by TD-DFT calculations.
We have shown that the installation of a carbonyl group at the periphery of the carbo[n]helicene backbone improves greatly the photophysical properties.The aceno[n]helicenones 1A-8A and uoreno[n]helicenes 1F-8F are much more emissive than the helicenes.This highlights the limitations of the generally accepted rule of nonemissivity of aromatic carbonyl compounds.Elongation of the helix leads to convergent evolution of the HOMO-LUMO energy gap for both families.Elongation of the acene part leads to an expected red-shi, but benzene ring annulation on the uorenone part causes a counterintuitive blue shi.This divergent evolution of the energy gaps was corroborated by TD-DFT calculations.CD and CPL have shown large absorption and luminescence dissymmetry factors g abs and g lum , which are dependent on the length of the acene/uorenone and helicene part.g abs and g lum substantially increase with the helical length (n) in the family of aceno[n] helicenones 1A-8A.In contrast, our theoretical calculations show that the g lum of uoreno[n]helicenes 1F-8F is decreasing as the length of the helix increases.

Fig. 5
Fig. 5 (a) Electronic configuration of 4A and its cyclic voltammogram (b) electronic configuration of 5A and its cyclic voltammogram.

Table 1
The evolution of the photophysical properties upon elongation of the helix