Effect of substituents on spectral,luminescent, and time-resolved spectral properties of 2,6-diarylidene derivatives of cyclohexanone

Spectral, luminescent, and time-resolved properties of 2,6-dibenzylidene cyclohexanone and its symmetric derivatives in the para-position of benzene ring (dimethoxy, dimethylthio, and bis-N,N-diethylamino groups) have been studied in media of different polarity (cyclohexane, acetonitrile, dimethylsulfoxide, and methanol). Introduction of electro-donating substituents to dibenzylidene cyclohexanone results in the red shift of the absorption maximum by 81–138 nm upon passing from the nonpolar to polar medium. Bis(N,N-diethylamino)dibenzylidene cyclohexanone fluoresces in polar solvents, acetonitrile, dimethylsulfoxide and methanol. The bathofluoric shift on passing from acetonitrile to methanol is 39 nm. The relative quantum yield of fluorescence in acetonitrile is seven times that in methanol. An intermediate species absorbing in the range of 400–560 nm with a maximum at 430–460 nm has been observed under the action of the laser pulse on dibenzylidene cyclohexanone solutions in acetonitrile. The intermediate species has a lifetime of 40, 20, and 7 μs for dibenzylidene cyclohexanone and its dimethylthio and dimethoxy derivatives, respectively.     

Exciplex mediated photoinduced electron transfer reactions of phthalocyanine-fullerene dyads

Evidences of an intramolecular exciplex intermediate in a photoinduced electron transfer (ET) reaction of double-linked free-base and zinc phthalocyanine-C60 dyads were found. This was the first time for a dyad with phthalocyanine donor. Excitation of the phthalocyanine moiety of the dyads results in rapid ET from phthalocyanine to fullerene via an exciplex state in both polar and nonpolar solvents. Relaxation of the charge-separated (CS) state Pc(*+)-C60(*-) in a polar solvent occurs directly to the ground state in 30-70 ps. In a nonpolar solvent, roughly 20% of the molecules undergo transition from the CS state to phthalocyanine triplet state (3)Pc*-C60 before relaxation to the ground state. Formation of the CS state was confirmed with electron spin resonance measurements at low temperature in both polar and nonpolar solvent. Reaction schemes for the photoinduced ET reactions of the dyads were completed with rate constants obtained from the time-resolved absorption and emission measurements and with state energies obtained from the fluorescence, phosphorescence, and voltammetric measurements.     

Activation Enthalpy and Entropy for the Decay of 9-Cyanophenanthrene Exciplexes

Activation parameters were studied for the decay of 9-cyanophenanthrene exciplexes with some weak electron donors (the Gibbs energy of electron transfer G ^* _et ranging from –0.02 to –0.09 eV), which displayed fairly high emission in both nonpolar and polar aprotic solvents. It was shown that the activation enthalpy of decay for the exciplexes is low, while the activation entropy reaches –(100–150) J mol^–1 K^–1, which is consistent with the two possible decay mechanisms: by dissociation into free radical ions or by intersystem crossing into the triplet state.     

Photoinduced Charge Transfer in a Conformational Switching Chlorin Dimer–Azafulleroid in Polar and Nonpolar Media

In the present study, a biomimetic reaction center model, that is, a molecular triad consisting of a chlorin dimer and an azafulleroid, is synthesized and its photophysical properties are studied in comparison with the corresponding molecular dyad, which consists only of a chlorin monomer and an azafulleroid. As evidenced by ¹H NMR, UV/Vis, and fluorescence spectroscopy, the chlorin dimer–azafulleroid folds in nonpolar media into a C₂‐symmetric geometry through hydrogen bonding, resulting in appreciable electronic interactions between the chlorins, whereas in polar media the two chlorins diverge from contact. Femtosecond transient absorption spectroscopy studies reveal longer charge‐separated states for the chlorin dimer–azafulleroid; ≈1.6 ns in toluene, compared with the lifetime of ≈0.9 ns for the corresponding chlorin monomer–azafulleroid in toluene. In polar media, for example, benzonitrile, similar charge‐separated states are observed, but the lifetimes are inevitably shorter: 65 and 73 ps for the dimeric and monomeric chlorin–azafulleroids, respectively. Nanosecond transient absorption and singlet oxygen phosphorescence studies corroborate that in toluene, the charge‐separated state decays indirectly via the triplet excited state to the ground state, whereas in benzonitrile, direct recombination to the ground state is observed. Complementary DFT studies suggest two energy‐minima conformations, that is, a folded chlorin dimer–azafulleroid, which is present in nonpolar media, and another conformation in polar media, in which the two hydrophobic chlorins wrap the azafulleroid. Inspection of the frontier molecular orbitals shows that in the folded conformation, the HOMO on each chlorin is equivalent and is shared owing to partial π–π overlap, resulting in delocalization of the conjugated π electrons, whereas the wrapped conformation lacks this stabilization. As such, the longer charge‐separated lifetime for the dimer is rationalized by both the electron donor–acceptor separation distance and the stabilization of the radical cation through delocalization. The chlorin folding seems to change the photophysical properties in a manner similar to that observed in the chlorophyll dimer in natural photosynthetic reaction centers.     

Environmental effects on the absorption and luminescence properties of aromatic amines: Aniline

Emission and absorption spectra of aniline have been studied in different solvents at 300 K and 77 K. This molecule exhibits broad and structureless fluorescence and poorly resolved phosphorescence in glassy polar ethanol and non-polar methylcyclohexane and n-hexane matrices at 77 K. Unlike the behaviour in a polar medium, fluorescence is found to be more intense than phosphorescence in non-polar media. In contrast to the case of glassy matrices, aniline shows highly stuctured emission and excitation spectra in crystalline cyclohexane and benzene matrices at 77 K. Excited state dipole moment calculation shows that both the singlet states contain charge transfer character, which is responsible for the strong phosphorescence in polar glassy medium. Spectral analysis indicates that the aniline molecule is slightly distorted in both the singlet and triplet states. The marked reduction in phosphorescence intensity of this molecule observed in non-polar matrices, particularly in a benzene crystalline matrix, relative to that in an ethanol matrix, is attributable to the decreased intersystem crossing rate and increased non-radiative rate from the emitting triplet state.     

The reactive state in the photo-rearrangement of o-nitrobenzaldehyde

The primayy step of the o-nitrobenzaldehyde-o-nitrosobenzoic acid photorearrangement in solution has been studied by flash absorption with 35 ps 355 nm light pulses. Flash photolysis of o-nitrobenzaldehyde in acetonitrile or THF solutions produces a transient absorption with a maximum at ca. 440 nm. Formation of the transient was < 35 ps, the laser pulse width, and within experimental error, no furthrr buildup was observed. The transient which decayed at nanosccond times is attributed to a remarkably reactive ketene intermediate formed by H abstraction of the aldehydic hydrogen by the excited state of the nitro group. Decay of the ketene was more rapid in water-acetonitrile, methanol-acetonitrile, tert-butyl alcohol and in THF than in acetonitrile solution. It is suggested that the intramolecular reaction of the ketene intermediate is enhanced in THF relative to acetonitrile because of the ability of THF to faciliaate proton transfer associated with the reaction. The addition of the triplet quencher cis-piperylene to a solution of o-nitrobenzaldehyde in THF did not accelerate decay of the transient nor reduce its yield. The n,π* triplet excited state band observed in the 625–650 nm region for a number of the nitroaromatic compounds was not observed in the case of o-nitrobenzaldehyde. The results provide evidence that in the direct irradiation on o-nitrobenzaldehyde in THF or acetonitrile solutions, the intramolecular reaction occurs from the singlet rather than the triplet excited state.     

Isomerization of ion pairs of symmetrical indopolycarbocyanine dyes

It was shown that from a polar solvent to a nonpolar solvent the rate constant of cis-trans isomerization of the dyes studied decreases sharply (by a factor of 10) on account of the formation of ion pairs (between the dye cation and anion in a nonpolar solvent). This is explained by an increase in the order of the bond of the polymethine chain of the dye, around which isomerization occurs. The formation of ion pairs was found to have virtually no effect on the lifetime of the triplet state of dyes; the sharp increase in the decay rate of the triplet state for dye 6 is due to the heavy-atom effect (the I^– anion).N. N. Semenov Institute of Chemical Physics, Russian Academy of Sciences, 117977 Moscow. Translated from Izvestiya Akademii Nauk, Seriya Khimicheskaya, No. 1, pp. 107–113, January, 1992.     

Photoinduced processes in bis(diethylaminobenzylidene)cyclohexanone and its bis(aza-18-crown-6)-containing analogue in acetonitrile

Photoinduced processes in bis(diethylaminobenzylidene)cyclohexanone (CH1) and its bis(aza-18-crown-6) derivative (CH2) in acetonitrile at ambient temperature and 77 K have been studied. The absorption, fluorescence, and phosphorescence spectra of CH1 and CH2 are similar. The probability of the formation of the triplet state is higher for CH2 molecules (λ_T-T^max = 660 nm, lifetime τ_T ~ 20 μs). The lifetime of the CH1 molecule in the triplet state is estimated at τ_T ~ 2–3 μs. Photoisomers of CH1 and CH2 are formed along with the triplet state. According to DFT calculation results, the formation of trans–cis photoisomers of CH1 and CH2 is the most energetically favorable.     

Chlorin–Bacteriochlorin Energy‐transfer Dyads as Prototypes for Near‐infrared Molecular Imaging Probes: Controlling Charge‐transfer and Fluorescence Properties in Polar Media

The photophysical properties of two energy‐transfer dyads that are potential candidates for near‐infrared (NIR) imaging probes are investigated as a function of solvent polarity. The dyads ( FbC‐FbB and ZnC‐FbB ) contain either a free base (Fb) or zinc (Zn) chlorin (C) as the energy donor and a free base bacteriochlorin (B) as the energy acceptor. The dyads were studied in toluene, chlorobenzene, 1,2‐dichlorobenzene, acetone, acetonitrile and dimethylsulfoxide (DMSO). In both dyads, energy transfer from the chlorin to bacteriochlorin occurs with a rate constant of ～(5–10 ps)^?1 and a yield of >99% in nonpolar and polar media. In toluene, the fluorescence yields (Φ _f = 0.19) and singlet excited‐state lifetimes (τ～5.5 ns) are comparable to those of the benchmark bacteriochlorin. The fluorescence yield and excited‐state lifetime decrease as the solvent polarity increases, with quenching by intramolecular electron (or hole) transfer being greater for FbC‐FbB than for ZnC‐FbB in a given solvent. For example, the Φ _f and τ values for FbC‐FbB in acetone are 0.055 and 1.5 ns and in DMSO are 0.019 and 0.28 ns, whereas those for ZnC‐FbB in acetone are 0.12 and 4.5 ns and in DMSO are 0.072 and 2.4 ns. The difference in fluorescence properties of the two dyads in a given polar solvent is due to the relative energies of the lowest energy charge‐transfer states, as assessed by ground‐state redox potentials and supported by molecular‐orbital energies derived from density functional theory calculations. Controlling the extent of excited‐state quenching in polar media will allow the favorable photophysical properties of the chlorin–bacteriochlorin dyads to be exploited in vivo. These properties include very large Stokes shifts (85 nm for FbC‐FbB , 110 nm for ZnC‐FbB ) between the red‐region absorption of the chlorin and the NIR fluorescence of the bacteriochlorin (λ _f = 760 nm), long bacteriochlorin excited‐state lifetime (～5.5 ns), and narrow (≤20 nm) absorption and fluorescence bands. The latter will facilitate selective excitation/detection and multiprobe applications using both intensity‐ and lifetime‐imaging techniques.     

Photochemistry of naphthalimide photoacid generators

The photophysical properties of a series of 1,8-naphthalimide photoacid generators were studied by steady state fluorescence and phosphorescence spectroscopy. Emission and excitation anisotropies, triplet quantum yields in polar and nonpolar solvent and photoacid generation were evaluated. The singlet excited state exhibits a low polarity and is strongly deactivated by an efficient intersystem crossing process. In protic solvent, a homolytic singlet cleavage of the N-O bond occurs and leads to the acid production. The existence of a triplet state close to the singlet state was clearly evidenced. The presence of close singlet excited states is supported by fluorescence anisotropy and picosecond laser spectroscopy experiments. Results of DFT calculations well confirm the experimental contentions and yield important information about the cleavage process involved in such compounds.     

Coupled electron and proton transfer processes in 4-dimethylamino-2-hydroxy-benzaldehyde

TDDFT calculations, picosecond transient absorption, and time-resolved fluorescence studies of 4-dimethylamino-2-hydroxy-benzaldehyde (DMAHBA) have been carried out to study the electron and proton transfer processes in polar (acetonitrile) and nonpolar (n-hexane) solvents. In n-hexane, the transient absorption (TA) as well as the fluorescence originate from the ππ* state of the keto form (with the carbonyl group in the benzaldehyde ring), which is produced by an intramolecular proton transfer from the initially excited ππ* state of the enol form (OH group in the ring). The decay rate of TA and fluorescence are essentially identical in n-hexane. In acetonitrile, on the other hand, the TA exhibits features that can be assigned to the highly polar twisted intramolecular charge transfer (TICT) states of enol forms, as evidenced by the similarity of the absorption to the TICT-state absorption spectra of the closely related 4-dimethylaminobenzaldehyde (DMABA). As expected, the decay rate of the TICT-state of DMAHBA is different from the fluorescence lifetime of the ππ* state of the keto form. The occurrence of the proton and electron transfers in acetonitrile is in good agreement with the predictions of the TDDFT calculations. The very short-lived (～1 ps) fluorescence from the ππ* state of the enol form has been observed at about 380 nm in n-hexane and at about 400 nm in acetonitrile.     

Triplet excited states of nitronaphthalenes. II. b-Nitronaphthalene

Nanosecond flash photolysis of b-nitronaphthalene (b-NO₂C₁₀H₇) in nonpolar and polar solvents shows a transient species with maximum absorption and lifetime dependent on solvent polarity. In deaerated n-hexane the absorption maximum and lifetime (1/k) are 425 nm and 530 nsec, while in deaerated ethanol the corresponding values are 470 nm and 1.7 ·sec. This transient absorption is attributed to the triplet excited state of b-NO₂C₁₀H₇, and the observed red shift as well as its longer lifetime in polar solvents are indicative of the intramolecular charge transfer character of this state. The change of dipole moment accompanying the transition T₁ → T_n, as well as rate constants for electron and proton transfer reactions involving the T₁ state of b-NO₂C₁₀H₇, were determined. The spectroscopic and kinetic data obtained in this work indicate that the triplet state of b-NO₂C₁₀H₇ behaves like a n-π* state in nonpolar media, while in polar solvents the n-π* character of the state is reduced with a simultaneous increase in the charge transfer character.     

SINGLET AND TRIPLET ENERGY TRANSFER IN α-DIKETONE DERIVATIVES OF URACIL and THYMINE

Irradiation of 1-(3,4-dioxopentyl)uracil (UPD) and 1-(3.4-dioxopentyl)thymine (TPD) in acetonitrile solution at 25°C, at the wavelength (280 nm) where only the pyrimidine absorbs the light, sensitizes both fluorescence and phosphorescence of the diketone chromophore in the sidechain. From comparison of the intensity in the corrected excitation spectra with the absorption spectra in acetonitrile solution, it was estimated that the yield of singlet energy transfer in UPD was 0.17 and in TPD was 0.44. It was also observed that the ratio of phosphorescence to fluorescence was greater in the sensitized emission than in that from direct excitation of the diketone chromophore. The yield of triplet energy transfer thus measured corresponds to minimum values for the yields of intersystem crossing from singlet excited state to triplet excited state of 0.075 in the uracil chromophore of UPD and of 0.14 in the thymine chromophore of TPD. These are in agreement with other recent values for these quantities. The value of this type of system as an intramolecular triplet counter is discussed.     

Static luminescence quenching by oxygen and nitric oxide for phenanthrene in glassy matrices

Steady-state and pulsed methods have been applied to static phosphorescence and fluorescence quenching for phenanthrene caused by oxygen and nitric oxide. The characteristic fluorescence quenching distances have been determined for oxygen (8.7±0.5 Å) and nitric oxide (8.9±0,5 Å); the phosphorescence quenching involves an exchange mechanism, and measurements have been made on the characteristic quenching distances R₀ and exchange-interaction attenuation parameters L. Estimates have been made on the rate constants for dynamic quenching of the triplet excited states of phenanthrene in low-viscosity liquids, which agree well with the measured values for hexane.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 24, No. 6, pp. 667–672, November–December, 1988.     

Triplet–Triplet Energy Transfer from a UV‐A Absorber Butylmethoxydibenzoylmethane to UV‐B Absorbers

The phosphorescence decay of a UV‐A absorber, 4‐tert‐butyl‐4′‐methoxydibenzolymethane (BMDBM) has been observed following a 355 nm laser excitation in the absence and presence of UV‐B absorbers, 2‐ethylhexyl 4‐methoxycinnamate (octyl methoxycinnamate, OMC) and octocrylene (OCR) in ethanol at 77 K. The lifetime of the lowest excited triplet (T₁) state of BMDBM is significantly reduced in the presence of OMC and OCR. The observed quenching of BMDBM triplet by OMC and OCR suggests that the intermolecular triplet–triplet energy transfer occurs from BMDBM to OMC and OCR. The T₁ state of OCR is nonphosphorescent or very weakly phosphorescent. However, we have shown that the energy level of the T₁ state of OCR is lower than that of the enol form of BMDBM. Our methodology of energy‐donor phosphorescence decay measurements can be applied to the study of the triplet–triplet energy transfer between UV absorbers even if the energy acceptor is nonphosphorescent. In addition, the delayed fluorescence of BMDBM due to triplet–triplet annihilation was observed in the BMDBM–OMC and BMDBM–OCR mixtures in ethanol at 77 K. Delayed fluorescence is one of the deactivation processes of the excited states of BMDBM under our experimental conditions.     

Influence of the solvent on the quenching of the fluorescence of anthracene by aromatic amines

The influence of the solvent on the quenching of the fluorescence of anthracene by diethylaniline (DEA) and aniline has been investigated. Fluorescence quenching in these systems obeys the equation of Stern and Volmer. A transition from kinetics-controlled to diffusion-controlled processes takes place with the increase in viscosity () in nonpolar solvents. In polar solvents the quenching rate is limited by diffusion over the whole viscosity range investigated. These peculiar effects of the solvent can be explained by the traditional three-stage quenching scheme of the singlet-excited anthracene by electron donors. It is also possible that the increase in the quenching constant () in polar as compared to nonpolar solvents with the same is related to an increase in the rate constant of non-emitting deactivation in polar media, induced by the quenching agent. In the case of the quenching of fluorescence of anthracene by aniline the process is diffusion-controlled, regardless of the nature of the solvent.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 21, No. 3, pp. 370–374, May–June, 1985.     

The first observation of the effect of dendritic structure to produce the triplet excited state of the core stilbene by dendron excitation

We report that both singlet and triplet energy transfers in stilbene-cored benzophenone dendrimers (trans-BPST) took place quite efficiently. On excitation (290 nm) of stilbene group, the intramolecular singlet energy transfer from the excited core stilbene to the benzophenone part (99.7%) was confirmed by quenching of the fluorescence from the core stilbene. The benzophenone in the excited singlet state is known to undergo intersystem crossing to give its excited triplet state quantitatively. However, the very weak phosphorescence from benzophenone part in trans-BPST was observed even at 77 K. The phosphorescence intensity of trans-BPST is only 1% of that of model compound (4-methylbenzophenone) at 77 K. During the irradiation, the absorption spectra also changed due to the trans-cis isomerization. This is probably due to the ultrafast triplet energy transfer from the benzophenone to produce the triplet state stilbene.     

Electron transfer in porphyrin—quinone cyclophanes studied on the pico- and femto-second time scale

Electron transfer in porphyrin—quinone cyclophanes is investigated by fluorescence and absorption spectroscopy with pico- and femto-second pulses. In nonpolar solvents, the S₁ state of the porphyrin shows a lifetime from 300 ps up to several nanoseconds, depending upon the number of quinones and upon their electron affinity. Comparative measurements in polar solvents demonstrate very fast electron transfer on a time scale between 1 and 10 ps. The results are analyzed with the aid of quantum-chemical calculations which give the energy of the charge transfer states and the relevant coupling strengths. For nonpolar solvents, theory suggests fluctuation-induced charge separation and/or direct radiationless internal conversion from the porphyrin S₁ to the ground state. In polar solution, the molecules exist in a tilted configuration with strong electronic coupling and charge transfer states well below the S₁ level, resulting in fast electron transfer and subsequent charge recombination within 10–40 ps.     

Selectivity of liquid-adsorption chromatography on hydroxylated silica gel with nonpolar and polar eluents

Summary The effect of nonpolar (hexane, decane, benzene) and polar (water, isopropanol, water+isopropanol) eluents on separation of aromatic hydrocarbons and their derivatives in liquid-adsorption chromatography on hydroxylated silica gel columns has been studied. The work has revealed that the traditional application areas of hydroxylated silica gel in liquid chromatography may be extended. In particular it has been shown that, first, polar compounds may be separated on hydroxylated silica gel with nonpolar eluents at high temperatures (100–150°C) and that polar and nonpolar compounds may be separated using pure water as eluent. The effect of adsorbed moisture on retention behaviour of silica gel with nonpolar eluents has also been investigated.Presented at the 14th International Symposium on Chromatography London, September, 1982     

One‐Component Thioxanthone Acetic Acid Derivative Photoinitiator for Free Radical Polymerization

Acetic acid–based thioxanthone (TXCH₂COOH) was synthesized and characterized and used as a photoinitiator for free radical photopolymerization of methyl methacrylate (MMA) in the absence and presence of a tertiary amine (MDEA) in different solvents. Different absorption properties were observed depending on the solvent. Fluorescence and phosphorescence experiments were also carried out successfully. The fluorescence quantum yield was found to be 0.09 and the phosphorescence lifetime was calculated as 138 ms at 77 K. The photoinitiator undergoes efficient intersystem crossing into the triplet state and the lowest triplet state possesses π–π* configuration. Laser flash photolysis experiments show that transient absorption of TXCH₂COOH is similar to the parent thioxanthone and the triplet lifetime was calculated as 2.3 μs at 630 nm.