Weak Phosphorescence Research Articles

Excited states and emission properties of 2-naphthaldehyde

Emission, excitation and absorption spectra of 2-naphthaldehyde (2-NA) have been measured in the vapor phase and in solution at room temperature and 77 K. In an isopentane solution the apparent origins of S 1 (n, π ∗) and T 2 (n,π ∗) are observed at 381 and 404 nm, respectively, while in the vapor phase those of S 2 (π,π ∗) and T 1 (π,π ∗) are observed at 335 and 500 nm, respectively. 2-NA vapor exhibits weak phosphorescence from the lowest 3(π,π ∗) state, the quantum yield of which decreases significantly upon the excitation into the S 3(π,π ∗) state. Excitation into the S 3(π,π ∗) state is found to induce the decomposition of 2-NA vapor to yield naphthalene.

Excited State Dynamics of Rh(II) Tetramesityl Porphyrin Monomer from Nanosecond Transient Absorption and Emission Spectroscopy

The excited states of [tetrakis(2,4,6-trimethylphenyl)porphyrinato]rhodium(II), Rh(II)TMP, have been studied using nanosecond transient absorption spectroscopy, emission spectroscopy, and electrochemistry. Rh(II)TMP has a long-lived excited state, with a 460 nm absorption band, that decays with a time constant of 180 ± 22 ns, in benzene and 205 ± 28 ns in 1,3-difluorobenzene. No transient absorption features are seen between 600 and 700 nm, but weak phosphorescence is detected at 743 nm at 77 K. Thus, the excited state has the characteristics of a triplet (π,π*) state, although a low lying, charge transfer state had been expected from iterative extended Huckel calculations. The phosphorescence is biphasic, and highly temperature dependent, consistent with a trip-quartet [4T] lowest excited state, lying not far below a trip-doublet [2T] state. Electrochemistry establishes that the Rh(II)/(I) reduction potential is more negative than −1.5 V versus SSCE while the porphyrin cation reduction potential is +0.55 V. These potentials place the lowest LMCT state at >2.05 V, well above the triplet states. In addition to elucidating the photophysics of a novel type of metalloporphyrin, these results have significant bearing on the feasibility of using Rh(II) porphyrins in photoinduced catalytic systems.

Phosphorescence of a Stiff cis-Stilbene Analogue, 5H-Dibenzo[a,d]cycloheptene

Weak phosphorescence of a stiff cis-stilbene analogue, 5H-dibenzo[a,d]cycloheptene single crystal, has been observed at 77 K. The energy of the T1−S0 transition is 222 kJ mol-1, and the lifetime of the lowest excited triplet state is 14 ms. For interpreting the results, the ground state geometry of the molecule has been determined by the X-ray crystallography. The torsion angles around C(ethylene)C(ethylene) and C(ethylene)−C(phenyl) are 0° and 32.6°, respectively. The results are discussed and compared with those of cis-stilbene.

Phosphorescence of β-carotene

The lowest triplet energy level of all-trans β-carotene has been directly established as 88 ± 3 kJ mol–1 from the weak phosphorescence observed using a sensitive Fourier transform based interferometer.

Fluorescence emission from mechanical pulp sheets II. Estimation of quantum yields

The luminescence of dry paper sheets made from lignin-containing peroxide-bleached wood pulps was investigated by steady state spectrofluorometry. The emission was predominantly fluorescence, although a very weak phosphorescence was also detected. Fluorescence spectra, corrected for incident light intensity and detector response and recorded in emission and excitation modes, revealed the existence of more than one emitting excited state for excitation wavelengths between 300 and 470 nm. A relative method for determining the fluorescence quantum yield using rhodamine-6G (R6G) as standard was developed, and the data thus obtained indicate that fluorescence is a very inefficient deactivation pathway for the excited states.

Excited state properties of 2-acrylated anthraquinones

Abstract The absorption, fluorescence and phosphorescence properties of 2-acrylamido and 2-acryloxyanthraquinones are examined in different solvents. The latter has an unusual marked blue-shifted absorption maximum compared with that of the acrylamido derivative. Also, unlike most 2-substituted anthraquinones these molecules exhibit negligible fluorescence. In the former case, weak phosphorescence in observed which is associated with rapid deactivation of the lowest excited 1 ππ* state by intersystem crossing to the second excited 3 nπ* state followed by internal conversion to the lowest excited 3 ππ* state and hence radiative emission to the ground-state. In the latter case, very strong phosphorescence is observed which appears to occur from the second excited 3 nπ* state and resembles that of anthraquinone. No evidence is found for a low lying excited 3 ππ* state. This effect is associated with vibronic coupling of the excited 1 nπ* and 1 π* states resulting in rapid intersystem crossing to the 3 nπ* state followed by emission to the ground-state. The data are discussed in the context of their activities for photocrosslinking polymers.

Orientational effects in intramolecular electronic energy transfer in bichromophoric molecules II: Triplet-triplet transfer

Intramolecular triplet energy transfer has been investigated in a series of bichromophoric molecules incorporating a cyclic α-diketone and substituted aromatic ring. The distance as well as the relative orientation between the two chromophores varies as a function of the number of methylene groups in the chains joining them. Previous work with these compounds has shown that intramolecular singlet energy transfer between the aromatic and diketone moieties shows a distance dependence which fits with the exchange mechanism. Phosphorescence of the diketone moiety (including temperature dependence) was used to evaluate the efficiency of triplet energy transfer. Direct excitation of the diketone resulted in weak phosphorescence, indicating that intersystem crossing is inefficient. However, excitation of the aromatic moiety resulted in much more intense phosphorescence as a result of the formation of triplet diketone via triplet energy transfer. Complementariness between singlet and triplet energy transfer was evidenced by the fact that some compounds in which singlet energy transfer was more efficient exhibited weaker phosphorescence and vice versa. Quantitative correlations between the triplet energy transfer efficiency and interchromophore distance were not possible.

Modification of photochemical reactivity by zeolites: Location of guests within faujasites by heavy-atom-induced phosphorescence

Heavy-atom-induced phosphorescence of naphthalene included in faujasites has been observed and evidence obtained for the existence of direct interaction between the heavy metal cations present within the zeolite supercage and the included aromatic guest. Naphthalene when included in faujasites exchanged with rubidium and cesium as cations exhibits intense phosphorescence and possesses short singlet and triplet lifetimes. When adsorbed in faujasites exchanged with lithium, sodium and potassium as cations, naphthalene gives intense fluorescence and weak phosphorescence with long singlet and triplet lifetimes.

Luminescence and photochemistry of 9-nitroanthracene at 4.2 K

The luminescence resulting from photoexcitation of 9-nitroanthracene in degassed ethyl alcohol and in a polyvinylbutyral film at 4.2 K indicates that it occurs completely from photochemical products. In contrast to previous studies of this molecule the interpretation of data is facilitated at this low temperature, which prevents diffusion, thermal activation and bimolecular events. The primary photoproduct is 9-anthrol, and an additional emission with a maximum at about 520 nm is tentatively assigned as the 9-anthrolate anion. Although an extremely weak phosphorescence emission is observed at 685 nm, it still appears that 9-nitroanthracene is non-phosphorescent, and that this emission originates from substituted anthracene photoproducts.

Photophysics of pseudoazulenes: 7-azaindole derivatives

Three pseudoazulenes derived from 7-azaindole reveal photophysical properties characteristic of this class of compounds: strongly temperature-dependent S 1 → S 0 fluorescence, weak phosphorescence, very small S 1—T 1 splitting, decrease in dipole moment on excitation to S 1 (ππ*) and S 2(ππ*) states and dimerization at low temperatures in non-polar solvents. Photoacoustic studies show that internal conversion is the dominant channel for the deactivation of the lowest excited singlet state.

Photoisomerization of 1,2-diarylethylenes: Photophysical properties of 5( E)-styryl-1,3-dimethyluracil

The photophysical properties of 5( E)-styryl-1,3-dimethyluracil (5( E)-SDU) were investigated. The fluorescence quantum yield and the lifetime are low and short relative to those of the parent aromatic hydrocarbon; this is probably because of the contribution of the n,π * state to the fast radiationless decay. However, the fluorescence of 5( E)-SDU at room temperature is not significantly affected by the solvent polarity, the presence of salts or the pH. This observation indicates that the 1(n,π *) state lies above the lowest 1(π,π *) state and that the vibronic mixing between the two states is not extensive. The highly positive fluorescence and the fluorescence excitation polarization confirm that the lowest excited singlet state has the π,π * configuration. The strong temperature dependence of the fluorescence intensity may be due to the operation of the activated process of twisting in the first excited singlet state competing with flurescence, and the rotation barrier is estimated to be 3.0 kcal mol −1. The strong quenching of fluorescence intensity by ethyl iodide at room temperature may be indicative of inefficient intersystem crossing. Moreover, only very weak phosphorescence is detected at 77 K and its quantum yield is too low to estimate in either isopentane-ethyl ether (1:1, v/v) or ethanol, supporting the idea of inefficient intersystem crossing suggested by laser flash photolysis results. Therefore, the major radiationless process appears to be internal conversion including rotatory radiationless decay. From the relatively long phosphorescence lifetime and its highly negative polarization the lowest triplet state is considered to be the 3(π,π *) state.

Luminescence emission of isotactic polypropylene

AbstractIn order to elucidate the origin of luminescent species in unstabilized commercial isotactic polypropylene (IPP), effects of hexane extraction and exposure of the extracted IPP film to the atmosphere on both fluorescence and phosphorescence emissions were examined. Fluorescence emission from the unextracted IPP film disappeared by the extraction, and only a very weak phosphorescence emission was found. During the exposure of the extracted IPP film to the atmosphere, both fluorescence and phosphorescence emissions appeared again. The intensity of the spectra increased with the exposure time and finally almost reached that from the unextracted IPP film. The results indicated that main fluorescent species in IPP are foreign impurities originating from an ambient atmosphere, and that contribution of a very minor amount of phosphorescence species such as carbonyl groups incorporated with the polymer was scarce.

Optical multichannel detection system suitable for a Fabry–Perot interferometer

In order to detect the circular fringes of a Fabry–Perot interferometer with high sensitivity the electronic scanning of an optical multichannel analyzer (OMA) has been modified. Compared to the common scanning Fabry–Perot photomultiplier combination this modification yields a gain in sensitivity of more than a factor of 10. Furthermore, single pulses or strongly fluctuating sources may be measured without the noise problems of a scanning interferometer. The modified OMA is capable of resolving about 90 circles. The system is applied to the detection of the fine structure of weak laser-excited phosphorescence spectra.

Absorption and emission characteristics of disubstituted 3-phenylmethylene-1(3B)-isobenzofuranones

The absorption, fluorescence and phosphorescence characteristics of a series of disubstituted 3-phenylmethylene-1(3B)-isobenzofuranones - (X,Y)BPH's - organic luminophores with a stilbene skeleton, have been studied. Depending on the substituents, (X,Y)BPH's fluorescence in the range 400 - 700 nm with a maximal fluorescence quantum yield of 0.65; for some of them a weak phosphorescence in EPA, 77° K is also observed. The changes in the dipole moments upon excitation were measured.

On the luminescence of 1,4-dinitronaphthalene

1,4-dinitronaphthalene in hydrocarbon media is non-fluorescent and shows a weak phosphorescence. In protic solvents a weak fluorescence, with intensification of phosphorescence, has been observed at 77 K. In hydrocarbon media, another energy-transfer route has been observed which quenches fluorescence and results in excimer emission.

ChemInform Abstract: CALCULATED SPECTRA OF BENZALDEHYDE AND BENZOIC ACID

Abstract A modified Intermediate Neglect of Differential Overlap technique is used to examine the absorption and emission spectra of benzaldehyde and the benzoic acid monomer and cyclic dimer. The lowest energy singlet and triplet of benzaldehyde are found to be of n-π ∗ type. The strong spin-orbit interaction 3 (n-π ∗ ) ⇝ 1 (π-π ∗ ) is held responsible for the observed weak fluorescence-strong phosphorescence. A lowest 1 (n-π ∗ ) has been calculated for the monomer of benzoic acid. The presence of this transition accounts for the non-fluorescent-strongly phosphorescent nature of the monomer via the coupling 3 (π-π ∗ ) ⇝ 1 (n-π ∗ ) . The calculations suggest that the 1 ( n -π ∗ ) will lie above the 1 (π-π ∗ ) in the dimer and thus provide an explanation for the observed dimer fluorescence.

Calculated spectra of benzaldehyde and benzoic acid

A modified Intermediate Neglect of Differential Overlap technique is used to examine the absorption and emission spectra of benzaldehyde and the benzoic acid monomer and cyclic dimer. The lowest energy singlet and triplet of benzaldehyde are found to be of n-π ∗ type. The strong spin-orbit interaction 3(n-π ∗) ⇝ 1(π-π ∗) is held responsible for the observed weak fluorescence-strong phosphorescence. A lowest 1(n-π ∗) has been calculated for the monomer of benzoic acid. The presence of this transition accounts for the non-fluorescent-strongly phosphorescent nature of the monomer via the coupling 3(π-π ∗) ⇝ 1(n-π ∗) . The calculations suggest that the 1( n-π ∗) will lie above the 1(π-π ∗) in the dimer and thus provide an explanation for the observed dimer fluorescence.

Reversible photo-oxidation of zinc tetraphenylporphine by benzo-1,4-quinone

Zinc tetraphenylporphine shows fairly weak fluorescence and phosphorescence but intense, long-lived triplet absorption. Benzo-1,4-quinone quenches the singlet and triplet excited states of ZnTPP and the quenching probability increases with increased polarity of the solvent. Quenching leads to formation of a complex which, for the triplet reaction in toluene, is sufficiently long-lived to be observed by flash spectroscopy. In polar solvents, quenching leads to production of separated ions, the yield being a function of the solvent dielectric constant.

Porphyrins. XXXVII. Absorption and emission of weak complexes with acids, bases, and salts

Absorption and emission data are reported for species resulting from addition of acids, alkali, and metal salts to H 2(Etio) (etioporphyrin) and H 2(TPP) (tetraphenylporphyrin) in various organic solvents. With acid addition, dications of both TPP and Etio are observed in equilibrium with free base, and a monocation of Etio is also observed in acetone, but not in benzene. The fluorescence yield of H 4(TPP)Cl 2 was approximately 0.1, and that of H 4(TPP)Br 2, about 0.01 in acetone. The fluorescence yield of H 4(Etio)Cl 2 was approximately twice that of H 4(Etio)Br 2 in THF. With alkali addition, absorption spectra show a predicted variation with metal, and all species show strong fluorescence and weak phosphorescence, which is consistent with only a small heavy atom effect. In addition, three distinct species were observed in the fluorescence of Na 2(TPP) complexes in THF at 77 K. With metal salt addition, spectra closely resembling monocation and dication spectra are produced in acetone for Etio and TPP, respectively. For all cases studied, except perhaps uranyl salts with TPP, fluorescence yields agreed to within about 25% of that of H 3(Etio) acetate for Etio complexes and about 25% of that of H 4(TPP)Cl 2 for TPP complexes, indicating shielding of the metal salt from the porphyrin π cloud by solvent molecules.

The polarized electronic spectra and electric field spectra of benzo-diazoles. II. 2,1,3-benzothiadiazole

The S 1 ← S 0 absorption spectra of 2,1,3-benzothiadiazole (BTD) have been measured at 4.2 K in four different host crystals: naphthalene, durene, p-dichlorobenzene (DCB) and p-dibromobenzene. Detailed vibrational analyses are given for BTD imbedded in napthalene and DCB. The polarization measurements show that the S 1 state has B 2 symmetry, like its selenium analogue (BSD). The transition is dominated by a single totally symmetric mode - 484 cm −1. The Herzberg-Teller coupling contributes only a very small fraction of the total intensity. The Stark measurements of a DCB sample containing both BTD and BSD enabled us to compare the charge distribution of BTD and BSD in the state S 1. The Stark splittings of BTD are 17% greater than the splittings of BSD. Reorganization of the σ-core during the excitation is used to explain the difference. The drastic change in dipole moment upon excitation implies that the S 1←S 0 transitions of BTD and BSD are not localized in the six-membered ring as suggested by previous workers. Weak phosphorescence of BTD in napthalene and DCB and singlet-triplet absorption spectrum of neat BTD have been observed. The heavy atom effect of spin-orbit coupling is to explain the ST absorption intensity of BTD and BSD.