Tuning Electron-Withdrawing Strength on Phenothiazine Derivatives: Achieving 100 % Photoluminescence Quantum Yield by NO2 Substitution

The weak fluorescence (quantum yield <1 % in cyclohexane) of phenothiazine ( PTZ ) impedes its further application. In addition, the nitro group (NO₂) is a well-known fluorescence quencher. Interestingly, we obtained a highly fluorescent chromophore by combining these two moieties, forming 3-nitrophenothiazine ( PTZ-NO₂ ). For comparison, a series of PTZ derivatives bearing electron-withdrawing groups (EWGs; CN and CHO) or electron-donating groups (EDGs; OMe) at the 3-position have been designed and synthesized. The phenothiazines bearing EWGs exhibited enhanced emission compared with the parent PTZ or EDG derivatives. Computational approaches unveiled that for PTZ and PTZ-OMe , the transitions are from HOMOs dominated by π orbitals to LUMOs of mixed sulfur nonbonding–π* orbitals, and hence are partially forbidden. In contrast, the EWGs lower the energy level of the lone-pair electrons on the sulfur atom, thereby suppressing the mixing of the nonbonding orbital with the π* orbital in the LUMO, such that the allowed ππ* transition becomes dominant. This work thus demonstrates a judicious chemical design to fine-tune the transition character in PTZ analogues, with PTZ-NO₂ attaining 100 % emission quantum yields in nonpolar solvent.     

Ab initio calculations on urea

Multiconfiguration wave functions constructed from contracted Gaussian-lobe functions have been found for the ground and valence-excited states of urea. ICSCF molecular orbitals of the excited states were used as the parent configurations for the CI calculations except for the ¹A₁(π → π*) state. The ¹A₁(π → π*) state used as its parent configuration an orthogonal linear combination of natural orbitals obtained from the second root of a three-configuration SCF calculation. The lowest excited states are predicted to be the n π → π* and π → π* triplet states. The lowest singlet state is predicted to be the n π → π* state with an energy in good agreement with the one known UV band at 7.2 eV. The π → π* singlet state is predicted to be about 1.9 eV higher, contrary to several previous assignments which assumed the lowest band was a π → π* amide resonance band. The predicted ionization energy of 9.0 eV makes this and higher states autoionizing.     

White‐Fluorescent Dual‐Emission Mechanosensitive Membrane Probes that Function by Bending Rather than Twisting

Bent N,N′‐diphenyl‐dihydrodibenzo[a,c]phenazine amphiphiles are introduced as mechanosensitive membrane probes that operate by an unprecedented mechanism, namely, unbending in the excited state as opposed to the previously reported untwisting in the ground and twisting in the excited state. Their dual emission from bent or “closed” and planarized or “open” excited states is shown to discriminate between micelles in water and monomers in solid‐ordered (S_o), liquid‐disordered (L_d) and bulk membranes. The dual‐emission spectra cover enough of the visible range to produce vesicles that emit white light with ratiometrically encoded information. Strategies to improve the bent mechanophores with expanded π systems and auxochromes are reported, and compatibility with imaging of membrane domains in giant unilamellar vesicles by two‐photon excitation fluorescence (TPEF) microscopy is demonstrated.     

Phenothiazine Scope: Steric Strain Induced Planarization and Excimer Formation

Phenothiazine derivatives based on the 10‐phenyl‐10H‐phenothiazine ( NAS ) chromophore, namely 7‐phenyl‐7H‐benzo[c]phenothiazine ( NAS‐1 ) and 12‐phenyl‐12H‐benzo[a]phenothiazine ( NAS‐2 ), were designed and synthesized. NAS‐1 and NAS‐2 are constitutional isomers with different steric strains imposed on the phenothiazine core moiety. In solution, the more‐strained NAS‐2 possesses a bent structure and undergoes photoinduced structural planarization (PISP). In the crystal, despite the absence of PISP, bent NAS‐2 exhibits prominent excimer emission as well as emission mechanochromism, which is not observed in the planar‐like NAS and NAS‐1 . This unconventional observation results from the bent core structure facilitating π–π stacking of the peripheral naphthalene moieties. Two‐photon‐coupled depth‐dependent emission shows spectral differences between the surface and kernel of the NAS‐2 crystal, and is believed to be a general phenomenon, at least in part, for materials exhibiting emission mechanochromism.     

Exciplex Emission from a Boron Dipyrromethene (Bodipy) Dye Equipped with a Dicyanovinyl Appendage

The photophysical properties of a prototypic donor–acceptor dyad, featuring a conventional boron dipyrromethene (Bodipy) dye linked to a dicyanovinyl unit through a meso‐phenylene ring, have been recorded in weakly polar solvents. The absorption spectrum remains unperturbed relative to that of the parent Bodipy dye but the fluorescence is extensively quenched. At room temperature, the emission spectrum comprises roughly equal contributions from the regular π, π* excited‐singlet state and from an exciplex formed by partial charge transfer from Bodipy to the dicyanovinyl residue. This mixture moves progressively in favor of the locally excited π, π* state on cooling and the exciplex is no longer seen in frozen media; the overall emission quantum yield changes dramatically near the freezing point of the solvent. The exciplex, which has a lifetime of approximately 1 ns at room temperature, can also be seen by transient absorption spectroscopy, in which it decays to form the locally excited triplet state. Under applied pressure (P<170 MPa), formation of the exciplex is somewhat hindered by restricted rotation around the semirigid linkage and again the emission profile shifts in favor of the π, π* excited state. At higher pressure (170<P<550 MPa), the molecule undergoes reversible distortion that has a small effect on the yield of π, π* emission but severely quenches exciplex fluorescence. In the limiting case, this high‐pressure effect decreases the molar volume of the solute by approximately 25 cm³ and opens a new channel for nonradiative deactivation of the excited‐state manifold.     

Photoinduced Excited State Intramolecular Proton Transfer of New Schiff Base Derivatives with Extended Conjugated Chromophores: A Comprehensive Theoretical Survey

This paper presented comprehensive theoretical investigation of excited state intramolecular proton transfer (ESIPT) of four new large Schiff base derivatives with extended conjugated chromophores. The properties of the ground state and the excited state of phototautomers of C1 to C4 [ C1 : 2‐(4′‐nitro‐stilbene‐4‐ylimino)methylphenol; C2 : 2‐(4′‐cyano‐stilbene‐4‐ylimino)methylphenol; C3 : 2‐(4′‐methoxyl‐stilbene‐4‐ylimino)methylphenol; C4 : 2‐(4′‐N,N‐diethylamino‐stilbene‐4‐ylimino)methylphenol], which included geometrical parameter, energy, rate constant, frontier orbit, Mulliken charge, dipole moment change, were studied by DFT (density functional theory), CIS (configuration interaction singles‐excitation), TDDFT (time‐dependent DFT) methods to analyze the effects of chromophore part on the occurrence of ESIPT and the role of substituent groups. The structural parameter calculation showed that the shorter R_{H? N} and larger R_{O? H} from enol to enol* form, and less twisted configuration in the excited state implied that these molecules could undergo ESIPT as excitation. Stable transition states and a low energy barrier were observed for C1 to C4 . This suggested that chromophore part increased some difficulty to undergo ESIPT for these molecules, while the possibility of occurrence of ESIPT was quite high. The negative ΔE^* (?9.808 and ?9.163 kJ/mol) of C1 and C2 and positive ΔE^* (0.599 and 1.029 kJ/mol) of C3 and C4 indicated that withdrawing substituent groups were favorable for the occurrence of ESIPT. The reaction rate constants of proton transfer of these compounds were calculated in the S₀ and S₁ states respectively, and the high rate constants of these compounds were observed at S₁ state. C1 even reached at 1.45×10¹⁵ s^?1 in the excited state, which is much closed to 2.05×10¹⁵ s^?1 of the parent moiety (salicylidene methylamine). Electron‐donating and electron‐withdrawing substituent groups had different effects on the electron density distribution of frontier orbits and Mulliken charges of the atoms, resulting in different dipole moment changes in enol*→keto* process. These differences in turn suggested that C1 and C2 had more ability to undergo ESIPT than C3 and C4 . The ultraviolet/visible absorption spectra, normal fluorescence emission spectra and ESIPT fluorescence emission spectra of these compounds were predicted in theory.     

Cu_2O(111)催化水煤气变换反应机理的理论研究

采用密度泛函理论结合周期平板模型方法系统地研究了水煤气变换反应在Cu_2O(111)表面上的反应机理,包括氧化还原机理、羧基机理和甲酸根机理.结果表明,在Cu_2O(111)表面,羧基机理和甲酸根机理均可行,且甲酸根机理更为有利,其最佳反应途径为H_2O~*→H~*+OH~*;CO(g)+H~*+OH~*→trans-HCOOH~*(1)→cis-HCOOH~*→CO_2~*+H_2(g).其中trans-HCOOH~*(1)→cis-HCOOH~*为其决速步,该基元反应的能垒仅为59 kJ·mol~(-1).羧基机理的最优反应路径同样是以H_2O的解离反应开始,随后CO(g)+OH~*→cis-COOH~*→trans-COOH~*→CO_2(g)+H~*,最后产生的两个吸附的H原子先迁移再结合生成H_2,整个反应的控速步骤为H原子的迁移,迁移能垒为96 kJ·mol~(-1).氧化还原机理则由于OH解离需要越过一个很高的能垒(254 vs.187 kJ·mol~(-1))而不可行.     

Temperature‐Dependent Behavior of the Dual Fluorescence of 2‐(3‐Fluorophenyl)‐2,3‐dihydro‐1H‐benzo[f]isoindole‐1,3‐dione

The fluorescence behavior of 2‐(3‐fluorophenyl)‐2,3‐dihydro‐1H‐benzo[f]isoindole‐1,3‐dione ( 1 ) was studied in solvents of different polarity and viscosity. Dual luminescence is observed and the short‐wavelength emission is found to increase considerably with the solvent polarity. The ratio of the fluorescence quantum yield of the two states emitting, the one (SW*) at short wavelength and the other (LW*) at long wavelength, shows a bell‐shaped dependence on the reciprocal of the temperature in diethyl ether, butyronitrile, and propane‐1,2,3‐triol triacetate (glycerol triacetate; GTA). This has been interpreted as the result of a reversible interconversion between the two states. The enthalpy difference between the SW* and LW* excited states, as deduced from the slope of the ln (Φ/Φ) vs. 1/T curves in the high temperature range, is found to be solvent polarity and solvent viscosity independent as the same value (−7.3 kJ/mol) is obtained in the three above‐mentioned solvents. The independence from polarity is the consequence of a similar difference in dipole moment between the ground‐state and the SW* and LW* excited states (4.5 and 4.9 D, respectively, derived from solvatochromy). The activation energy of the SW*→LW* step deduced from the low temperature measurements in the nonviscous solvents, increases with solvent polarity (11.6 and 17.5 kJ/mol for diethyl ether and butyronitrile, respectively); they are greater than the viscous‐flow activation energy of the solvents indicating that the resolvation of the excited dipole controls the kinetics. In the nonviscous solvents, the LW* state originates from the SW* state, while in the viscous GTA solution, both states are formed simultaneously within the 1‐ps laser pulse.     

Tuning the Conformation and Color of Conjugated Polyheterocyclic Skeletons by Installing ortho‐Methyl Groups

ortho‐Methyl effects are exploited to tune steric hindrance between side‐chain N,N′‐diaryls and polycyclic dihydrodibenzo[a,c]phenazine, and in turn control the conformations of N,N′‐diphenyl‐dihydrodibenzo[a,c]phenazine (DPAC) and its ortho‐methyl derivatives M x ‐M y (x=0, 1 or 2, y=1 or 2, x and y correlate with the number of methyl groups in the ortho‐positiond of N,N′‐diphenyl). The magnitude of steric hindrance increases as x and y increase, and the V‐shaped dihydrodibenzo[a,c]phenazine skeleton is gradually tuned from a bent (DPAC) to planar ( M2‐M2 ) structure in the ground state. As a result, the relaxation of the excited‐state structure of DPAC and its numerous analogues could be mimicked by model structures M x ‐M y, demonstrating for the first time the the conformation change from bent‐to‐planar and hence a large range of energy‐gap tuning of polycyclic conjugated structures controlled by the steric hindrance.     

Electronic structure of benzaldehyde: A comparative study of the lowest excited singlet π* ← π and π* ← n states

VE-PPP, CNDO/2, and CNDO/s-CI methods have been used to investigate the electronic spectrum and structure of benzaldehyde. Electronic charge distributions and bond orders in the ground and lowest excited singlet π* ← π and π* ← n states of the molecule have been studied. The molecule has been shown to be nonplanar in the lowest π* ← n excited singlet state, in agreement with the conclusions drawn from the study of vibrational spectra. Dipole moments in both excited states have been shown to be larger than the ground-state value. Thus, the ambiguity in the experimental result for the π* ← π n excited singlet state dipole moment has been resolved. It has been shown that the n orbital is mainly localized on the CHO group. Furthermore, charge distributions, dipole moments, and molecular geometries are shown to be very different in the excited singlet π* ← π and π* ← n states.     

Synthesis,crystal structure,and luminescence of a new zinc(II) coordination polymer based on 2-naphthol-5-carboxylate and 4,4′-bipyridine

To further explore the coordination possibilities of naphthalene-based carboxylic acids, a Zn^II coordination polymer, [Zn₃(L)₆(bipy)₂] _n (1), with bulky 2-naphthol-5-carboxylate (L) and bridging 4,4′-bipyridine (bipy), was synthesized and characterized. Structural analysis reveals that 1 is a 1-D polymeric chain with trinuclear units as nodes, which are further extended via interchain secondary interactions, such as O–H ··· O hydrogen-bonding and aromatic π ··· π stacking interactions, to form an overall 3-D framework. Complex 1 exhibits strong solid-state luminescence emission at room temperature, mainly originating from intraligand π→π* transition of L.     

Theoretical studies on the diazacyclopentadienylidenes: An analysis of aromatic versus nonaromatic systems and singlet versus triplet reactivity

Ab initio molecular orbital calculations have been carried out on the various electronic states of 2,3- ( 6 ), 2,4- ( 7 ), 2,5- ( 8 ), and 3,4-diazacyclopentadienylidene ( 9 ) at the fully geometry optimized 6–31G* level, with single point calculations being carried out at the MP2/6–31G* level. The calculated geometries are interpreted in terms of the degree of occupancy and the nature of the π and σ-nonbonded MO's. At the 6–31G* level the five π-electron, π,σ-triplet states were calculated to be considerably lower in energy. At the MP2/6–31G* level, however, with 7 the six π-electron singlet state is calculated to lie only slightly above the five π-electron triplet (0.4 kcal mole⁻¹), whereas with 8 and 9 the aromatic six π-electron singlet states are calculated to be lower in energy (9.0 and 8.1 kcal mole⁻¹). With 3 and 9 the aromatic six π-electron σ-triplet states lie only 3.6 and 5.5 kcal mole⁻¹ above the lowest energy states. It is concluded that, in general, the energy gained by having an electron in a lower energy σ-type MO instead of a higher energy π MO effectively offsets the energy gained by having an aromatic π system. The results are discussed in terms of the observed chemistry of 6–9 and their substituted systems.     

Photochemische Reaktionen. 68. Mitteilung. Die Photoisomerisierung von α,β-ungesättigten γ,δ-Epoxyketonen: 9α, 10α- und 9β, 10β-Oxido-3-oxo-17β-acetoxy-Δ4-östren

Irradiation in the n→π* absorption band of the α,β-unsaturated γ,δ-epoxyketone 5 in ethanol at ?65° exclusively afforded the rearranged ene-dione 13 , whereas at + 24° under otherwise unchanged reaction conditions or upon triplet sensitization with Michler's ketone and with acetophenone at + 24° essentially identical mixtures of 13 (major product), 14 , and 15 were obtained. Selective π→π* excitation of 5 at ?78° and + 24° led to similar product patterns. The 9β,10β-epimeric epoxyketone 7 selectively isomerized to 14 and 15 at + 24° and n → π* or π → π* excitation. Neither the epoxyketones 5 and 7 nor the photoproducts 13–15 were photochemically interconverted. In separate photolyses each of the latter gave the double bond isomers 16 , 18 , and 19 , respectively. Cleavage of 13 to the dienone aldehyde 17 competed with the double bond shift ( → 16 ) when photolyzed in alcoholic solvents instead of benzene. The selective transformations 5 → 13 (at ?65° and n → π* excitation) and 7 → 14 + 15 are attributed to stereoelectronic factors facilitating the skeletal rearrangements of the diradicals 53 and 55 , the likely primary photoproducts resulting from epoxide cleavage in the triplet-excited compounds 5 and 7 , via the transition states 54 , 56 , and 57 . The loss of selectivity in product formation from 5 at higher temperature and n → π* excitation or triplet sensitization is explicable in terms of radical dissociation into 58 and 59 increasingly participating at the secondary thermal transformations of 53 . The similar effect of π → π* excitation even at ?78° indicates that some of the π,π* singlet energy may become available as thermal activation energy. It is further suggested that the considerably lesser ring strain in 14 and 15 , as compared with 13 , is responsible that selectivity in product formation from 7 is maintained also at +24° and at π → π* excitation.     

N−H‐Type Excited‐State Proton Transfer in Compounds Possessing a Seven‐Membered‐Ring Intramolecular Hydrogen Bond

A series of compounds containing 5‐(2‐aminobenzylidene)‐2,3‐dimethyl‐3,5‐dihydro‐4H‐imidazol‐4‐one ( o ‐ABDI ) as the core chromophore with a seven‐membered‐ring N?H‐type intramolecular hydrogen bond have been synthesized and characterized. The acidity of the N?H proton and thus the hydrogen‐bond strength can be fine‐tuned by replacing one of the amino hydrogen atoms by a substituent R, the acidity increasing with increasing electron‐withdrawing strength of R, that is, in the order H<COCH₃<COPh<Tosyl<COCF₃. The tosyl and trifluoroacetyl derivatives undergo ultrafast, irreversible excited‐state intramolecular proton transfer (ESIPT) that results in proton‐transfer emission solely in the red region. Reversible ESIPT, and hence dual emission, involving the normal and proton‐transfer tautomers was resolved for the acetyl‐ and benzyl‐substituted counterparts. For o ‐ABDI , which has the weakest acidity, ESIPT is prohibited due to its highly endergonic reaction. The results clearly demonstrate the harnessing of ESIPT by modifying the proton acidity and hydrogen‐bonding strength in a seven‐membered‐ring intramolecular hydrogen‐bonding system. For all the compounds studied, the emission quantum yields are weak (ca. 10^?3) in dichloromethane, but strong in the solid form, ranging from 3.2 to 47.4 %.     

The synthesis,X‐ray structure analysis,and photophysical characterization of 4‐[(E)‐2‐(4‐cyanophenyl)diazenyl]‐morpholine

A novel triazene, 4‐[(E)‐2‐(4‐cyanophenyl)diazenyl]‐morpholine ( 1 ) was prepared via a diazonium ion coupling reaction between 4‐aminobenzonitrile and morpholine. The x‐ray structure of 1 was determined and evidenced π delocalization in the triazene subunit. The room temperature absorption spectrum of 1 in acetonitrile was dominated by an intense triazene‐centered π→π* transition at 325 nm. Compound 1 was observed to be luminescent, with an emission maximum at 434 nm in room temperature acetonitrile solution. The emission spectrum of 1 in propionitrile glass at 77K exhibited a narrowed emission band with a maximum at 449 nm. Broad emission from 400–700 nm with poorly resolved vibrational structure was observed from solid 1 at room temperature. J. Heterocyclic Chem., 2011.     

Single Benzene Green Fluorophore: Solid‐State Emissive,Water‐Soluble,and Solvent‐ and pH‐Independent Fluorescence with Large Stokes Shifts

Benzene is the simplest aromatic hydrocarbon with a six‐membered ring. It is one of the most basic structural units for the construction of π conjugated systems, which are widely used as fluorescent dyes and other luminescent materials for imaging applications and displays because of their enhanced spectroscopic signal. Presented herein is 2,5‐bis(methylsulfonyl)‐1,4‐diaminobenzene as a novel architecture for green fluorophores, established based on an effective push–pull system supported by intramolecular hydrogen bonding. This compound demonstrates high fluorescence emission and photostability and is solid‐state emissive, water‐soluble, and solvent‐ and pH‐independent with quantum yields of Φ=0.67 and Stokes shift of 140 nm (in water). This architecture is a significant departure from conventional extended π‐conjugated systems based on a flat and rigid molecular design and provides a minimum requirement for green fluorophores comprising a single benzene ring.     

Dérives de l'imidazo[2,1-b]thiazole. V. Transposition allylique observée au cours de la synthése d'(imino-2 alkyl-3 thiazolinyl-4) acetates d'ethyle et de (dihydro-5,6 imidazo[2,1-b]thiazolyl-3) acetates d'éthyle

Ethyl (6-phenyl-5,6-dihydroimidazo[2,1-b]thiazol-3-yl) acetate hydrobromide shows an allylic transposition in alkaline medium and gives ethyl (6-phenyl-2,3,5,6-tetrahydroimidazo[2,1-b]thiazol-3-ylidene)acetate. This compound possesses an exocyclic double bond which is stable upon acidification. The intermediate ethyl (2-imino-3-alkylthiazolin-4-yl)-acetates undergo an analogous transposition upon treatment with base, which is reversible upon acidification. These transpositions were observed in the pmr data. The steric constraint and π p π conjugation are discussed in this work.     

Single Benzene Green Fluorophore: Solid‐State Emissive,Water‐Soluble,and Solvent‐ and pH‐Independent Fluorescence with Large Stokes Shifts

Benzene is the simplest aromatic hydrocarbon with a six‐membered ring. It is one of the most basic structural units for the construction of π conjugated systems, which are widely used as fluorescent dyes and other luminescent materials for imaging applications and displays because of their enhanced spectroscopic signal. Presented herein is 2,5‐bis(methylsulfonyl)‐1,4‐diaminobenzene as a novel architecture for green fluorophores, established based on an effective push–pull system supported by intramolecular hydrogen bonding. This compound demonstrates high fluorescence emission and photostability and is solid‐state emissive, water‐soluble, and solvent‐ and pH‐independent with quantum yields of Φ=0.67 and Stokes shift of 140 nm (in water). This architecture is a significant departure from conventional extended π‐conjugated systems based on a flat and rigid molecular design and provides a minimum requirement for green fluorophores comprising a single benzene ring.     

The 3[ndσ*(n+1)pσ] Emissions of Linear Silver(I) and Gold(I) Chains with Bridging Phosphine Ligands

The complexes [Au₃(dcmp)₂][X]₃ {dcmp=bis(dicyclohexylphosphinomethyl)cyclohexylphosphine; X=Cl^? ( 1 ), ClO₄^? ( 2 ), OTf^? ( 3 ), PF₆^? ( 4 ), SCN^?( 5 )}, [Ag₃(dcmp)₂][ClO₄]₃ ( 6 ), and [Ag₃(dcmp)₂Cl₂][ClO₄] ( 7 ) were prepared and their structures were determined by X‐ray crystallography. Complexes 2 – 4 display a high‐energy emission band with λ_max at 442–452 nm, whereas 1 and 5 display a low‐energy emission with λ_max at 558–634 nm in both solid state and in dichloromethane at 298 K. The former is assigned to the ³[5dσ*6pσ] excited state of [Au₃(dcmp)₂]³⁺, whereas the latter is attributed to an exciplex formed between the ³[5dσ*6pσ] excited state of [Au₃(dcmp)₂]³⁺ and the counterions. In solid state, complex [Ag₃(dcmp)₂][ClO₄]₃ ( 6 ) displays an intense emission band at 375 nm with a Stokes shift of ≈7200 cm^?1 from the ¹[4dσ*→5pσ] absorption band at 295 nm. The 375 nm emission band is assigned to the emission directly from the ³[4dσ^*5pσ] excited state of 6 . Density functional theory (DFT) calculations revealed that the absorption and emission energies are inversely proportional to the number of metal ions (n) in polynuclear Au^I and Ag^I linear chain complexes without close metal???anion contacts. The emission energies are extrapolated to be 715 and 446 nm for the infinite linear Au^I and Ag^I chains, respectively, at metal???metal distances of about 2.93–3.02 Å. A QM/MM calculation on the model [Au₃(dcmp)₂Cl₂]⁺ system, with Au???Cl contacts of 2.90–3.10 Å, gave optimized Au???Au distances of 2.99–3.11 Å in its lowest triplet excited state and the emission energies were calculated to be at approximately 600–690 nm, which are assigned to a three‐coordinate Au^I site with its spectroscopic properties affected by Au^I???Au^I interactions.     

Circular dichroism spectra of 1,2,3,4,5,6-Hexahydro-2,6-methano-3-benzazocines

Circular dichroism curves of levorotatory 1,2,3,4,5,6-hexahydro-2,6-methano-3-benzazocin-8-ol derivatives have been determined and correlated with stereochemical features. Compounds with the rectus configuration at C-6 were found to exhibit negative extrema for the long wavelength phenolic π → π* transition. The remaining Cotton effects have been ascribed to specific aromatic π → π* transitions.