The photochemical characteristics of aromatic enediyne compounds substituted with electron donating and electron withdrawing groups

trans- and cis-1-(4-Dimethylaminophenyl)-6-(4-nitrophenyl)hex-3-ene-1,5-diynes (trans- and cis-DANE) were synthesized and their photochemical properties were studied. The absorption spectra of trans-DANE red-shifted compared with the parent compound bisphenylethynylethene (BEE) due to intramolecular charge transfer. The fluorescence spectra, Stokes shift, fluorescence lifetime, fluorescence quantum yield, and quantum yield of trans-to-cis photoisomerization of trans-DANE showed strong dependence upon the solvent polarity in the less-polar region. No fluorescence emission from trans-DANE was observed in medium-polar and polar solvents. The quantum yield of cis-to-trans isomerization was almost solvent independent. The donor-acceptor substituents shifted the equilibrium between the trans perpendicular triplet state and the trans planar triplet state to the trans triplet state, and resulted in an increase in the triplet lifetime. Comparison of the photochemical properties of trans-DANE with trans-4-dimethylamino-4'-nitrostilbene (DANS) suggests that trans-DANE is a possible fluorescent probe in the non-polar region.     

The triplet state of 4-nitro-N,N-dimethynaphthylamine

Nanosecond laser photolytic studies of 4-nitro-N,N-dimethylnaphthylamine (4-NDMNA) in nonpolar and polar solvents at room temperature show a transient species with an absorption maximum in the 500-510-nm range. This species is assigned to the lowest triplet excited state of 4-NDMNA. The absorption maximum of this state is independent of solvent polarity, and its lifetime is a function of the hydrogen donor efficiency of the solvent. In n-hexane the lifetime 1/k of the triplet state is 9.1 × 10^?6 sec, while in acetonitrile 1/k is 2.0 × 10^?7 sec. The hydrogen abstraction rate constant k_H of the triplet state with tributyl tin hydride (Bu₃SnH) in n-hexane is 1.7 × 10⁷M^?1·sec^?1, while in the case of isopropyl alcohol as hydrogen donor, k_H is 4.0 × 10⁷M^?1·sec^?1. The activation energy for the hydrogen abstraction by the triplet state from Bu₃SnH in deaerated n-hexane is 0.6 kcal/mol. The lack of spectral shift with increasing solvent polarity, and the appreciable hydrogen abstraction reactivity of the triplet state, also independent of solvent polarity, seem to indicate that this excited state is an n-π* state which retains its n-π* character even in polar media.     

Long‐Lived Room‐Temperature Deep‐Red‐Emissive Intraligand Triplet Excited State of Naphthalimide in Cyclometalated IrIII Complexes and its Application in Triplet‐Triplet Annihilation‐Based Upconversion

Cyclometalated Ir^III complexes with acetylide ppy and bpy ligands were prepared (ppy=2‐phenylpyridine, bpy=2,2′‐bipyridine) in which naphthal ( Ir‐2 ) and naphthalimide (NI) were attached onto the ppy ( Ir‐3 ) and bpy ligands ( Ir‐4 ) through acetylide bonds. [Ir(ppy)₃] ( Ir‐1 ) was also prepared as a model complex. Room‐temperature phosphorescence was observed for the complexes; both neutral and cationic complexes Ir‐3 and Ir‐4 showed strong absorption in the visible range (ε=39600 M ^?1 cm^?1 at 402 nm and ε=25100 M ^?1 cm^?1 at 404 nm, respectively), long‐lived triplet excited states (τ_T=9.30 μs and 16.45 μs) and room‐temperature red emission (λ_em=640 nm, Φ_p=1.4 % and λ_em=627 nm, Φ_p=0.3 %; cf. Ir‐1 : ε=16600 M ^?1 cm^?1 at 382 nm, τ_em=1.16 μs, Φ_p=72.6 %). Ir‐3 was strongly phosphorescent in non‐polar solvent (i.e., toluene), but the emission was completely quenched in polar solvents (MeCN). Ir‐4 gave an opposite response to the solvent polarity, that is, stronger phosphorescence in polar solvents than in non‐polar solvents. Emission of Ir‐1 and Ir‐2 was not solvent‐polarity‐dependent. The T₁ excited states of Ir‐2 , Ir‐3 , and Ir‐4 were identified as mainly intraligand triplet excited states (³IL) by their small thermally induced Stokes shifts (ΔE_s), nanosecond time‐resolved transient difference absorption spectroscopy, and spin‐density analysis. The complexes were used as triplet photosensitizers for triplet‐triplet annihilation (TTA) upconversion and quantum yields of 7.1 % and 14.4 % were observed for Ir‐2 and Ir‐3 , respectively, whereas the upconversion was negligible for Ir‐1 and Ir‐4 . These results will be useful for designing visible‐light‐harvesting transition‐metal complexes and for their applications as triplet photosensitizers for photocatalysis, photovoltaics, TTA upconversion, etc.     

THE EFFECT OF MEDIUM POLARITY ON THE HEMATOPORPHYRIN-SENSITIZED PHOTOOXIDATION OF l-TRYPTOPHAN

Abstract The 83 μM hematoporphyrin (HP)-sensitized photooxidation of 0.1 mM tryptophan in aqueous solution buffered at pH 7.4 or in binary mixtures of phosphate buffer and organic solvents of higher (formamide) or lower (N,N-dimethylformamide, methanol, ethanol, tetrahydrofuran) polarity proceeds by a pure singlet oxygen (¹O₂) mechanism as suggested by azide quenching experiments, the rate-enhancing action of deuterated solvents, and the lack of any significant reaction between triplet HP and tryptophan. Both the first-order rate constant of the photoprocess and the photooxidation quantum yield (φ= 0.011 in phosphate buffer at pH 7.4) increase when the medium polarity is increased (e.g. φ= 0.024 in 90% formamide); this results mainly from the greater quantum yield of ¹O₂ generation and the longer lifetime of ¹O₂. The intrinsic reactivity of ¹O₂ with tryptophan is independent of formamide concentration. A moderate decrease in the medium polarity (e.g. in the range 0-30% methanol) enhances the efficiency of tryptophan photooxidation (φ= 0.014 in 30% methanol) as a result of the enhanced quantum yields of triplet HP and ¹O₂ formation. In contrast, the overall photooxidation rate is depressed at high concentrations of low-polarity organic solvents (e.g. φ= 0.0039 in 90% methanol) due to a 5.5-fold drop of the rate constant for the ¹O₂-tryptophan reaction which counteracts the enhancement of the lifetime and quantum yield of triplet HP and ¹O₂. The solvent composition also affects the equilibria between monomeric and multimeric forms of HP. However, under our experimental conditions, the aggregation state of HP appears to exert only a minor influence on the efficiency of tryptophan photooxidation.     

Mécanisme de solvolyse des cis- et trans-(p-toluènesulfonates) d'aryl-2-cyclopentyle I. Etude de la première étape de la solvolyse: effets isotopiques de l' atone de deutérium en position 2, effets de sel basique et effet spécial de sel

Solvolysis Mechanism of cis - and trans-2-Arylcylopentyl p-Toluenesulfonates. The Step: 1-Deuterium Isotope Effects, Basic Salt Effects, and Special Salt Effect We have studied the first step of the solvolysis of cis and trans-2-arylcyclopentyl p-toluenesulfonates in HCOOH, AcOH, and EtOh. All substrates show a high kinetic 1-deuterium isotope effect (k_H/k_D(1) >1.15). This fact indicates that first step leads to classical intimate ion-pair Which dissociates to a solvet-separated ion-pair, without participation either of solvent, the 2-aryl group, or a H-atom at C(2). The slight influence of added basic ions on reaction rate allows us to exclude any direct solvent attack on the covalent substrate even in the most favorable case, i.e. ethanolysis of 2-(p-nitrophenyl)cylopentyl-p-toluenesulfonates. Furthermore, solvent-separated ion pair formation is indicated by the special salt effect induced by LiClO₄.     

Energy Dissipation Processes of singlet‐excited 1‐Hydroxyfluorenone and its Hydrogen‐bonded Complex with N‐methylimidazole¶

Effects of solvent, pH and hydrogen bonding with N‐methylimidazole (MIm) on the photophysical properties of 1‐hydroxyfluorenone (1HOF) have been studied. Fluorescence lifetime, fluorescence quantum yield and triplet yield measurements demonstrated that intersystem crossing was the dominant process in apolar media and its rate constant significantly diminished with increasing solvent polarity. The acceleration of internal conversion in alcohols paralleled the strength of intermolecular hydrogen bonding. The faster energy dissipation from the singlet‐excited state in cyclohexane was attributed to intramolecular hydrogen bonding. The pK_a of 1HOF decreased from 10.06 to 5.0 on light absorption, and H₃O⁺ quenched the singletexcited molecules in a practically diffusion‐controlled reaction. On addition of MIm in toluene, dual fluorescence was observed, which was attributed to reversible formation of excited hydrogen‐bonded ion pair. Rate constants for the various deactivation pathways were derived from the combined analysis of the steady‐state and the time‐resolved fluorescence results.     

Effects of temperature on the triplet behavior of the styrylphenanthrene isomers

The properties of the triplet state of five styrylphenanthrene (StPh) trans isomers were studied in 2-methyltetrahydrofuran (MTHF) as a function of temperature. At room temperature the T-T absorption was observed only for 4- and 9-StPh, while under these conditions 1-, 2-, and 3-StPh have too low a quantum yield of triplet formation (Φ_T <0.02); their T-T absorption spectra were obtained at low temperature. Φ_T of 1- and 2-StPh increases more than tenfold on going from 293 to 77 K, and the triplet lifetime (τ_T) increases by four orders of magnitude and approaches values of 5–40 ms at 77 K. The change in τ_T is explained in terms of an equilibrium between trans and perpendicular (perp) conformations of the lowest triplet state in fluid solution and temperature and viscosity effects on the trans → perp rotation. Evidence is presented for the existence of two conformeric trans triplet states of 3-StPh at 77 K. Semi-empirical calculations were performed to obtain the energy of the triplet state, the wavelengths of several T-T absorption maxima (λ_T), and the oscillator strength. The calculated λ_T values coincide with those measured in n-hexane.     

Nitrogen photochemistry. Cocaine and its model compounds

The ultraviolet irradiation of the π → π* transition of cocaine, p-anisoyl-l-ecognine methyl ester, p-toluoyl-l-ecognine methyl ester benzoyl tropine and benzoylpseudotropine in methanol using a Corex filter produces the corresponding N-demethylated products. Formaldehyde is quantitatively produced. 1-Methyl-3-piperidyl benzoate and 1-methyl-4-piperidyl benzoate yield benzoic acid under the same conditions but no demethylated products. Phenylacetyl-l-ecognine methyl ester gives no demethylation during irradiaiton. The phosphorescence bands of cocaine and its model compound, methyl benzoate, have been shown to be strongly dependent upon solvent polarity suggesting charge transfer in the triplet state.     

Quantitative estimation of trans-1,4 and cis-1,4 isomers in polychloroprene by high-resolution NMR

In the ¹H-NMR spectrum of polychloroprene dissolved in C₆D₆, the ?CH proton signal was separated into two triplet peaks. These triplet signals were assigned to the ?CH proton in the trans-1,4 and cis-1,4 isomers by measurement of ¹H-NMR spectra of 3-chloro-1-butene and a mixture of trans- and cis-2-chloro-2-butene as model compounds for the 1,2, trans-1,4 and cis-1,4 isomers. In ¹H-NMR spectra (220 Mcps) of polychloroprene dissolved in C₆D₆, two triplet signals were separated completely from which the relative concentrations of trans-1,4 and cis-1,4 isomers could be obtained quantitatively.     

Cationic oligomerization of anethole: Selective synthesis of dimers and trimers

A linear unsaturated dimer of anethole [II, (E)-1,3-bis(p-methoxyphenyl)-2-methylpentene-1], was prepared in 98% yield with an acetyl perchlorate (AcClO₄) catalyst in a nonpolar solvent (C₆H₆) at a high temperature (70°C). At 0°C a linear unsaturated trimer was formed in high yield with dimer II. The geometric structure of the linear unsaturated dimers was determined by analysis of the nuclear Overhauser effect (NOE) on their ¹H-NMR spectra. NOE analysis showed that at 0°C with AcClO₄, trans-anethole gives the (E)-form (II), whereas cis-anethole yields the (Z)-form. On the other hand, with a metal-halide catalyst (BF₃OEt₂) a cyclic dimer and a cyclic trimer were produced in high yields in a polar solvent [(CH₂Cl₂)] at 70°C; higher oligomers (≥ tetramer) were scarcely formed. The effect of catalysts on the structure of oligomers was explained in terms of the interaction between a growing carbocation and a counterion.     

MATRIX EFFECTS ON THE RADIATIONLESS TRANSITION' S1→T1 AND T1→S0 FOR BENZENE AND N, N, N', N'-TETRAMETHYLPARAPHENYLENE-DIAMINE MOLECULES

Abstract— The present study attempts to correlate the phosphorescence life time τ_p at 77°K of a definite solute: tetramethylparaphenylenediamine (TMPD) with various solvents viscosity and polarity. A few experiments with benzene in the same solvents are also reported. The following results have been obtained:

• 1 The measured τ_p vary regularly with the sample immersion time in liquid N₂, reaching a constant value after a few hours. This effect is related to the glass matrix relaxation. The rate constant K_isc (S, ₁→T₁) is also found to vary during relaxation of the solvent.
• 2 In the expression giving the nonradiative rate constant K_nr (T₁→S₀), the bimolecular quenching term appears negligible for high viscosity matrices i.e. for η= 10⁹ poises for benzene and for TMPD. K_nr is found to vary linearly with log η, as well as the intersystem crossing S₁→T₁ rate constant K^isc.
• 3 Both K_nr (T₁→S₀) and K^isc (S₁→T₁), increase with decreasing polarity of the solvent.
• 4 From our own observations and literature data[6] for C₆H₆ it appears that solvent viscosity does not contribute appreciably to the observed temperature effect on the solute τ_p when only a monomolecular triplet deactivation is operative.

     

Konkurrenz von Endoperoxid- und Hydroperoxid-Bildung bei der Umsetzung von Singulett-Sauerstoff mit cyclischen,konjugierten Dienen. Teil I

Competition of Endoperoxide and Hydroperoxide Formation in the Reaction of Singlet Oxygen with Cyclic, Conjugated Dienes Rose-bengal-sensitized photooxygenation of (?)-(R)-α-phellandrene ( 1 ) in MeOH at room temperature yielded a complex mixture of products, contrary to previous reports describing cis-(3S, 6R)-epidioxy-p-menthene ( 2 ) and trans-(3R, 6S)-epidioxy-p-menthene ( 3 ) as the only products. The mixture was separated by prep. HPLC (silica gel, pentane/Et₂O 9:1). Besides the known endoperoxides 2 (yield 39%) and 3 (26%), all those hydroper-oxides, which can be deduced from an ene reaction of ¹O₂ with 1 , were isolated, i.e. 4β-p-mentha-2,5-dien-1β-yl hydroperoxide ( 4 ) (14%), 4β-p-mentha-2,5-dien-1α-yl hydroperoxide ( 5 ) (9%), (2R, 4R)-p-mentha-1(7), 5-dien-2-yl hydroperoxide ( 6 ) (2,1%), (2S, 4R)-p-mentha-1(7),5-dien-2-yl hydroperoxide ( 7 ) (1,5%) and (1R)-p-mentha-3,6-dien-yl hydroperoxide ( 8 ; 1,5%; Scheme 1). Furthermore, the constant cis/trans ratio for all diastereoisomeric pairs ( 2 / 2 , 4 / 2 , 6 / 2 ) was striking. With the help of the two possible conformers 1a and 1b of the starting material a model of a common first step for endoperoxide as well as for hydroperoxide formation is developed. A photooxygenation at ?50° supports this model. The absolute value of the cis/trans ratio changes in the same way for the endoperoxides and the hydroperoxides.     

Synthesis and characterization of trans -Mo(CO)4( p -C5NH4SO3Na)2

Reaction between Mo(CO)₆ and p-C₅NH₄SO₃Na (1:2 (Mo: p-C₅NH₄SO₃Na) stoichiometric ratio) gave the trans-Mo(CO)₄(p-C₅NH₄SO₃Na)₂ complex, (1), in 80% yield. Complex (1) has been characterized by FTIR, ¹H and ¹³C NMR spectroscopy. Complex (1) has most likely an idealized D_4h geometry with trans N-bound p-C₅NH₄SO₃Na ligands.     

Laser photolysis study of trans→cis photoisomerization of trans-1-phenyl-2-(2-naphthyl) ethylene

The direct trans→cis photoisomerization of trans-1-phenyl-2-(2-naphthyl) ethylene (trans-PNE) in liquid solution at room temperature was studied by the nanosecond laser photolysis technique. The time-resolved S_n←S₁ and T_n←T₁ absorption spectra were observed with trans-PNE at 300 K and 77 K. The lifetime of the triplet state of trans-PNE was found to be much shorter in liquid solution at room temperature than in rigid solution at 77 K. This fact and the effect of a triplet quencher shows that the photoisomerization of trans-PNE occurs mainly via the triplet state.     

Effect of Substituents,Solvent, and Temperature on the Reactivity of the Zwitterionic Peroxides Arising from the Photo-Oxygenation of 2-(Methoxymethylidene)- and 2-(Phenoxymethylidene)adamantane

The photo-oxygenation of 2-(methoxymethylidene)adamantane ( 3 ) creates a zwitterionic peroxide which may be captured by acetaldehyde to give the corresponding pair of diastereoisomeric tricyclo[3.3.1.1^3,7]decane-2-spiro-6′ -(3′ -methyl-5′ -methoxyl′, 2′, 4′ -trioxanes) ( 4 ). Ease of capture depends strongly on solvent polarity and temperature. When these are low, yields of trioxine are high (～ 80%). Conversely, 1,2-dioxetane formation is favoured at high temperature and solvent polarity. 2-(Phenoxymethylidene)adamantane ( 5 ), on photo-oxygenation, only gives the corresponding 1,2-dioxetene, even in the presence of acetaldehyde. From a Hammett, study of the-oxygenation of the enol ether 5 and p-methoxy, p-methyl, p-chloro and m-chloro derivatives, 9, 11, 13 , and ( 15 ), a good linear relation was found between substituent constants and oxygenation rates which yielded reaction constants (ρ) of 2.59, ?2.40, ?1.09, and ?0.90 in benzene, AcOET, CH₂Cl₂, and MeOH respectively. This data to the formation of a zwitterionic peroxide which enjoys stabilization from its won substituents and by competing solvation and further explains the predominance of dioxetane to the detriment of trioxane formation.     

Photophysical Properties of Gilvocarcins V and M and Their Binding Constant to Calf Thymus DNA

Abstract— Absorption and emission techniques were used to characterize the ground (S₀), singlet (S|) and triplet states (T₁) of gilvocarcin V (GV) and gilvocarcin M (GM) in different solvents. Aggregation of GV with dimerization constant equal to 7800 M^?1is observed in 10% dimethyl-sulfoxide (DMSO)/water. The photophysical properties of the S, state of these molecules are more sensitive to changes in solvent characteristics than the corresponding ground states. The absorption of visible light by GV and GM results in a higher dipole moment of the excited state causing a red shift in the fluorescence spectra with increasing solvent polarity. The fluorescence quantum yield remains practically unchanged with changes in solvent properties unless water is present as a co-solvent. Both φ_f and φ_f values corresponding to GV in DMSO are larger than those of GM, whereas in 10% DMSO/H₂O the opposite is observed. Thus, GV is more susceptible to other deactivation pathways besides emission in the presence of water than GM. The relative phosphorescence quantum yield (φ_p= 0.03) and the triplet energy (E_T= 52 kcal/mol) of GV and GM are similar. The S₀-S₁ energy difference is 63 kcal/mol for GV, whereas for GM it is 67. Thus, the singlet-triplet energy difference is 11 and 15 kcal/mol, respectively. The PM3/CI calculated electronic structures of these compounds are consistent with the observed photophysical properties. The dark binding constants of GV to calf thymus DNA ([1.1–0.08] × 10⁶M^?1) are about an order of magnitude larger than those of GM ([0.24–0.018] × 10⁶M^?1) at different ionic strengths (0–2.00 M NaCl). Also, the number of gilvocarcin molecules bound per base pair is smaller for GM than for GV. These differences in dark DNA binding parameters between GV and GM could have implications in the large photocytotoxic ability of GV as compared to GM.     

Synthesis and absorption/emission properties of novel poly(silylenearylenevinylene)s

Polymerization of p-(dimethylsilyl)phenylacetylene in toluene at 25 and 80 °C with RhI(PPh₃)₃ catalyst afforded highly regio- and stereoregular poly(dimethylsilylene-1,4-phenylenevinylene)s [cis- and trans-poly( 1a )s] containing 98% cis- and 99% trans-vinylene moieties, respectively. The trans-type polymers exhibited redshifts and hyperchromic effects in the ultraviolet–visible spectrum as compared with the cis-type counterparts. Photoirradiation of cis- and trans-poly( 1a )s gave cis-rich mixtures at equilibrium states. The trans and cis polymers exhibited different emission properties, for example—trans polymer, emissn λ_max = 400 nm, quantum yield: 3.4 × 10⁻³ and cis polymer, emissn λ_max = 380 nm, quantum yield: 1.5 × 10⁻³. Besides poly( 1a ), poly(dimethylsilylenearylenevinylene)s containing biphenylene and phenylenesilylenephenylene units [poly( 3 )] were prepared. The extent of conjugation in these polymers decreased in the orders of biphenylene > phenylene > phenylenesilylenephenylene as well as trans-vinylene > cis-vinylene. The quantum yield of the trans-rich polymer with biphenylene moiety was fairly large and 0.15. Polyaddition of 1,4-bis(dimethylsilyl)benzene and three types of diethynylarenes (4,4′-diethynylbiphenyl, 2,7-diethynylfluorene, and 2,6-diethynylnaphthalene) catalyzed by RhI(PPh₃)₃ provided novel regio- and stereoregular polymers [poly( 6 )]. These polymers displayed blue light emission with high quantum yields (4–81%). © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 3615–3624, 2003     

Solvent and chelation effects on the photoreduction of cobalt(III)–amine complexes in aqueous–organic solvent media

The photoreduction of trans-[Co(NH₃)₄Cl₂]⁺, trans-[Co(en)₂Cl₂]⁺, [Co(dien)Cl₃], [Co(trien)Cl₂]⁺, and [Co(tetren)Cl]²⁺, ions has been studied using a low pressure Hg vapour lamp as light source (254 nm) in aqueous–organic solvents [0–30% (v/v) MeOH or 1,4-dioxane]. Quantum yields for Co^II production by redox decomposition have been determined in all the cases, and increase considerably with the increase in concentration of MeOH or 1,4-dioxane in the binary solvent mixtures under investigation. A plot of log(quantum yield) versus the Grunwald–Winstein Parameter, Y, which is a measure of solvent ionizing power, shows that a different blend of general and specific solvent interacts with the solute. This kind of specific solvent interaction on the reactant/excited state has been analysed using multiple regression: viz. Krygowski–Fawcett and Kamlet–Taft equations. Reasons for the difference in reactivity with chelation are also discussed.     

Photophysical Properties of Coumarin‐30 Dye in Aprotic and Protic Solvents of Varying Polarities¶

Experimental results on various photophysical properties of coumarin‐30 (C30) dye, namely, Stokes' shift (Δv), fluorescence quantum yield (τ_f), fluorescence lifetime (τ_f), radiative rate constant (k_f) and nonradiative rate constant (k_nr), as obtained using absorption and fluorescence measurements have been reported. Though in most of the solvents the properties of C30 show more or less linear correlation with the solvent polarity function, Δf= [(ε ‐ 1)/(2ε+ 1) ‐ (n² ‐ 1)/ (2n²+ l)], they show unusual deviations in nonpolar solvents at one end and in high‐polarity protic solvents at the other end. From the solvent polarity and temperature effect on the photophysical properties of the dye, following inferences have been drawn: ( 1 ) in nonpolar solvents, the dye exists in a nonpolar structure, where its 7‐NEt₂ substituent adopts a pyramidal configuration and the amino lone pair is out of resonance with the benzopyrone π cloud; ( 2 ) in medium to higher polarity solvents, the dye exists in a polar intra‐molecular charge transfer structure, where the 7‐NEt₂ group and the 1,2‐benzopyrone moiety are in the same plane and the amino lone pair is in resonance with the benzopyrone π cloud; ( 3 ) in protic solvents, the dye‐solvent intermolecular hydrogen bonding influences the photophysical properties of the dye; and ( 4 ) in high‐polarity protic solvents, the excited C30 undergoes a new activation‐controlled nonradiative deexcitation process because of the involvement of a twisted intra‐molecular charge transfer (TICT) state. Contrary to most other TICT molecules, the activation barrier for this deexcitation process in C30 is observed to increase with solvent polarity. A rational for this unusual behavior has been given on the basis of the solvent polarity‐dependent stabilization and crossing of relevant electronic states and the relative propensity of interconversion among these states.     

Effect of the Media on the Quantum Yield of Singlet Oxygen (O2(1Δg)) Production by 9H‐Fluoren‐9‐one: Solvents and Solvent Mixtures

We have investigated the effect of a series of 18 solvents and mixtures of solvents on the production of singlet molecular oxygen (O₂(¹Δ_g), denoted as ¹O₂) by 9H‐fluoren‐9‐one (FLU). The normalized empirical parameter E derived from E_T(30) has been chosen as a measure of solvent polarity using Reichardt's betaine dyes. Quantum yields of ¹O₂ production (Φ_Δ) decrease with increasing solvent polarity and protic character as a consequence of the decrease of the quantum yield of intersystem crossing (Φ_ISC). Values of Φ_Δ of unity have been found in alkanes. In nonprotic solvents of increasing polarity, Φ_ISC and, therefore, Φ_Δ decrease due to solvent‐induced changes in the energy levels of singlet and triplet excited states of FLU. This compound is a poor ¹O₂ sensitizer in protic solvents, because hydrogen bonding considerably increases the rate of internal conversion from the singlet excited state, thus diminishing Φ_Δ to values much lower than those in nonprotic solvents of similar polarity. In mixtures of cyclohexane and alcohols, preferential solvation of FLU by the protic solvent leads to a fast decrease of Φ_Δ upon addition of increasing amounts of the latter.