Lowest Energy Excited Singlet State Research Articles

High-Pressure Optical Properties and Chemical Stability of Picene

Picene is a polycyclic aromatic hydrocarbon belonging to the class of phenacenes which have been recently found to behave as high-temperature superconductors upon alkali metal doping. The electronic properties of organic crystals can be finely and largely modified by the density changes obtained by the application of an external pressure. In this work, the role of pressure in tuning the optical properties of crystalline picene has been investigated from room conditions up to 15 GPa through the measurement of UV–visible absorption spectra, two-photon excitation profiles, and one- and two-photon excited fluorescence spectra in a diamond anvil cell. The pressure dependence of the optical band gap was determined, and the frequencies of several vibronic bands belonging to electronic transitions from the ground state (S0) to the four lowest-energy excited singlet states (S1 to S4) were determined as a function of pressure. We evidence a very different density dependence of the transition energy of S0 → S1, which undergoes a remarkable red shift of ∼400 cm–1/GPa, and of the transitions from S0 to the higher excited states, which remain constant in the whole investigated range. This is consistent with a S1 state of 1La character in solid picene. The high-pressure chemical stability of solid picene was investigated through visible absorption and Fourier transform infrared spectroscopy (FTIR). A chemical transformation involving the bulk picene crystal occurs above ∼23 GPa, giving rise to a disordered material similar to the amorphous hydrogenated carbon obtained in the pressure-induced reactivity of benzene. The combination of electronic and vibrational data allows us to identify the presence of reaction intermediates at ∼10 GPa, preferentially forming at crystal defects.

HOMO-LUMO transitions in solvated and crystalline picene

The optical properties of picene at ambient conditions have been investigated through the measurement of UV/Vis absorption and fluorescence spectra and of excitation profiles, using one- and two-photon excitation, in solution and in the crystal phase. For solvated picene an assignment of the vibronic structure of the transitions to the four lowest-energy excited singlet states (S(1)-S(4)) has been obtained from the absorption data, and the vibronic structure of the fluorescence spectra has been assigned. The absorption and fluorescence spectra of the solid phase can be interpreted according to the single molecule analysis. Nevertheless, the strong increase of the optical density in the spectral region of the lowest HOMO-LUMO transitions and the frequency shift of absorption and fluorescence bands may be explained by a mixing of the states of adjacent molecules in the crystal. Moreover, peculiar emission features depending on the crystal dimensions (10(-1) to 10(2) μm) are observed.

On the Photophysical Properties of New Luminol Derivatives and their Synthetic Phthalimide Precursors

The photophysical properties of a series of structurally related 4-aminophthalimides and the corresponding 5-aminophthalic hydrazides (luminols) are reported. Absorption, steady-state, and time-resolved fluorescence spectra of luminols exhibited substitution, solvent, and pH dependence. Singlet lifetimes have been determined by time-resolved laser flash spectroscopy. UV spectra in gas phase and DMSO solution were calculated by TD-DFT which revealed the existence of two low-energy excited singlet states with strong pH-sensitivity.

Application of the Perimeter Model to the Assignment of the Electronic Absorption Spectra of Gold(III) Hexaphyrins with [4n+2] and [4n] π‐Electron Systems

The electronic excited states of two forms of meso-hexakis(pentafluorophenyl)-substituted gold(III) hexaphyrin(1.1.1.1.1.1) have been investigated by density functional calculations and magnetic circular dichroism (MCD) spectroscopy, in order to assign their low-energy excited singlet states. We found that the perimeter model can be successfully applied to the interpretation of the electronic states. In the case of the neutral forms (Au(2)-N, Au-N), the absorption bands observed in the NIR and visible region can be assigned to pi-pi* transitions referred to as the L and B bands, respectively, analogous to the Q and Soret bands of regular porphyrins. In marked contrast with the neutral forms, the absorption bands of the reduced forms (Au(2)-R and Au-R) are attributed to pi-pi* transitions involving six frontier molecular pi orbitals. By applying the 4N-electron perimeter model, the six orbitals are labeled as h(-), h(+), s(-), s(+), l(-), and l(+), while the observed absorption bands can be assigned to the S, N(1), N(2), P(1), and P(2) transitions, in order of increasing energy.

DFT and TDDFT study of carbon trioxide

Carbon trioxide has been studied using DFT and TDDFT methods with different combinations of GGA functionals and correlation consistent basis sets. The C 2 v isomer is calculated to be 2.5 kcal mol −1 and 12.5 kcal mol −1 lower in energy than D 3 h with a conversion energy barrier of 5.0 kcal mol −1 and 12.9 kcal mol −1 by PW91PW91 and B3LYP, respectively. All of the above include zero-point energy corrections. The geometry changes almost linearly along the reaction coordinate and the TS structure is approximately an average of D 3 h and C 2 v with an imaginary O–C–O bending mode of 421 i cm −1. However, D 3 h is more favored in low energy excited singlet states. These states also promote the C 2 v → D 3 h conversion.

The effect of solvent polarity on the photophysical properties of 4-cyano-(4′-methylthio)diphenylacetylene: A prototypic donor–acceptor system

The photophysical properties of the target compound are extremely sensitive to changes in solvent polarity since the lowest-energy excited states possess considerable charge-transfer character. Excitation results in a greatly increased dipole moment, with the resultant excited singlet state retaining a lifetime of ca. 1 ns in all solvents. Radiative decay involves coupling between the lowest-energy excited singlet state and both the ground state and an upper excited singlet state. The level of coupling to the upper singlet decreases in non-polar solvents, presumably due to symmetry factors. The radiative rate constant decreases smoothly with increasing solvent polarity function as the molecule acquires an ever increasing dipolar character. Non-radiative decay includes both intersystem crossing and internal conversion, but the former process dominates in polar solvents. The excited singlet state lifetime is very weakly dependent upon temperature in the solid state. However, in polar solutions where the Stokes' shift decreases with decreasing temperature, there is clear evidence for an activated process. This is believed to involve coupling to the upper-lying singlet excited state.

Photophysical properties of pyrene-(2,2′-bipyridine) dyads

Two photoactive dyads bearing pyrene and 2,2′-bipyridine terminals bridged by an ethynylene or a Pt(II) bis(σ-acetylide) moiety have been studied by fluorescence and transient absorption spectroscopy following excitation into the pyrene unit. Pyrene-like fluorescence and phosphorescence are observed for the Pt(II) bis(σ-acetylide)-bridged dyad but emission lifetimes are shortened by the internal heavy-atom effect. There is a modest thermodynamic driving force for electron transfer from the pyrene-like excited singlet state to the appended 2,2′-bipyridine but this process does not compete effectively with intersystem crossing to the triplet manifold. Connecting the terminals by a single ethynylene group promotes intramolecular charge transfer from pyrene to 2,2′-bipyridine and perturbs both absorption and fluorescence spectra. In polar solvents pyrene-like fluorescence is not observed and the lowest-energy excited singlet state contains appreciable charge-transfer character, whereas in nonpolar solvents pyrene-like π,π* fluorescence is seen. The intramolecular charge-transfer state is relatively long-lived in acetonitrile and decays to form the pyrene-like π,π* triplet state in quantitative yield. In fact, direct charge recombination to the ground state is extremely slow in this system, despite the close proximity of the reactants and the fact that they are connected by a highly-conductive “spacer’' group, because of a negligible Franck–Condon factor. The results are discussed in terms of the ability of the Pt(II) bis(σ-acetylide) to function as an insulator. Coordination of a metal centre to the vacant 2,2′-bipyridine opens up a new route for deactivation of the pyrene-like excited singlet state in both dyads.

Dual Fluorescence of Novel Modified Carotenoids

Despite only a small energy difference between the lowest energy excited singlet states S1 and S2, both the carotenoids 1 and 2 exhibit dual fluorescence. For the first time the energetic position of the S1 state, which is not accessible by UV/VIS excitation, could be estimated. In 1 it lies 3000–3500 cm−l lower than the S2 state; in 2 this gap is 3400–4000 cm−1, which is also presumed to be the S2–S1 difference in β-carotene.

ChemInform Abstract: Lowest Energy Excited Singlet State of Isolated cis‐Hexatriene

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Lowest energy excited singlet states of isomers of alkyl substituted hexatrienes

Vibrationally resolved S0→S1 excitation spectra for the alkyl substituted linear polyenes heptatriene, octatriene, and decatriene seeded into a supersonic He expansion have been measured by resonance enhanced multiphoton ionization spectroscopy. As is the case for the parent compound hexatriene, the lowest energy excited singlet state in all of these molecules is the 2 1Ag state. The measurement of S0→S1 excitation spectra of three of the four double bond isomers of heptatriene gives a detailed picture of the dependence of the electronic structure of the 2 1Ag state on molecular conformation. The three isomers for which spectra are presented have the cis configuration at either the central or the alkyl substituted double bond, or both. For the case of the mono cis species with the cis configuration at the alkyl substituted double bond the spectra show the presence of two single bond conformers. Because of the increased number of distinguishable isomers and conformers for octatriene and decatriene we were unable to unambiguously separate the observed spectra of these molecules into contributions from single specific molecular conformations. However, the increased excitation intensity in the low frequency region relative to that in the C–C and C=C stretching region for octatriene and decatriene as compared to heptatriene suggests that vibrational relaxation is enhanced in the more complex molecules. In the case of unsubstituted hexatriene, previously reported spectra show that the 2 1Ag state has lower symmetry than does the ground state (most likely due to nonplanarity at the terminal carbon atoms). There is no evidence for an analogous distortion in the excitation spectra measured for the alkyl substituted hexatrienes.

ChemInform Abstract: Lowest Energy Excited Singlet State of 2,2′:5′,2′′‐Terthiophene, an Oligomer of Polythiophene.

Fluorescence excitation and emission spectra have been measured for solid solutions of 2,2’:5’,2‘‐terthiophene in n‐decane at 77, 10, and 4.2 K. At 4.2 K the spectra exhibit full vibrational resolution (origin inhomogeneous FWHM approximately 5 cm−1). At 4.2 K narrow band excitation and detection establish that there are four independent but nearly identical excitation/emission pairs with origins at 24 806, 24 827, 24 835, and 24 879 cm−1. Whether this multiplicity is caused by the presence of different isomers or comes from a single isomer that can occupy in the n‐decane lattice in four different ways is not known with certainty. However, the similarity of the vibronic development and energetic considerations suggest that the spectra arise from a single isomer present in four rather similar n‐decane sites or from four slightly different conformations of a single isomer. The overlap of excitation and emission origins and the vibrational development of the spectra establish that the S0 to S1 transition is ...

Lowest energy excited singlet state of 2,2′:5′,2′′-terthiophene, an oligomer of polythiophene

Fluorescence excitation and emission spectra have been measured for solid solutions of 2,2′:5′,2″-terthiophene in n-decane at 77, 10, and 4.2 K. At 4.2 K the spectra exhibit full vibrational resolution (origin inhomogeneous FWHM approximately 5 cm−1). At 4.2 K narrow band excitation and detection establish that there are four independent but nearly identical excitation/emission pairs with origins at 24 806, 24 827, 24 835, and 24 879 cm−1. Whether this multiplicity is caused by the presence of different isomers or comes from a single isomer that can occupy in the n-decane lattice in four different ways is not known with certainty. However, the similarity of the vibronic development and energetic considerations suggest that the spectra arise from a single isomer present in four rather similar n-decane sites or from four slightly different conformations of a single isomer. The overlap of excitation and emission origins and the vibrational development of the spectra establish that the S0 to S1 transition is symmetry allowed (probably reasonably described as the 1 1A1 to 1 1B2 transition where the excited state is derived from the ground state by the promotion of one electron from the HOMO to LUMO).

Photochemical Behavior in Copolymers of 2(2-Hydroxy-5-vinylphenyl)2H-benzotriazole and Methyl Methacrylate: Photochemical Processes in Polymeric Systems. 6

Abstract The photochemical behavior of poly[2(2-hydroxy-5-vinylphenyl)2H-benzotriazole)]-co-methyl methacrylate was studied by spectroscopic and analytical techniques; no degradation of this copolymer occurred when the bulk polymer was exposed for several thousand hours of accelerated aging by ultraviolet irradiation. However, by the end of the period, surface deterioration became noticeable. Picosecond flash photolysis on copolymer solutions demonstrated that the lowest energy excited singlet state undergoes rapid internal conversion with little or no triplet yield. The observed short singlet lifetime (∼25 ps) is undoubtedly responsible for the photostability of this copolymer system. Analysis of the surface exposed to ultraviolet irradiation indicates some photooxidative degradation of the methyl methacrylate portion of the copolymer, but the surface 2(2-hydroxy-5-vinylphenyl)2H-benzotriazoles of the macromolecules remain unchanged.

Excited state spectroscopy of 1,5-naphthyridine: Identification of the lowest energy excited singlet state as 1B g(1nπ*)

The electronic absorption spectra of excited-state 1,5-naphthyridine from 400 to 650 nm are reported in several solvents. The time evolution of these spectra indicate that they arise from one or more Sn←S1 transitions. Comparison of these observed spectra with calculated excited state absorption spectra obtained using the INDO molecular orbital formalism demonstrates excellent agreement of the transition energies and oscillator strengths with Sn(1Au)←S1(1Bg,nπ*) transitions. These results conclusively identify the lowest excited singlet state of 1,5-naphthyridine as 1Bg(1nπ*).

Low energy excited singlet states of some monophenyl substituted 5-membered heterocycles and their photoisomerization

The two low-lying singlet excited states of 14 mono-phenylated 5-membered heterocycles have been studied with respect to their energy level ordering, the oscillator strengths of the pertinent transitions, the state wave functions and the one-particle density matrices. Two types of lowest energy singlet states could be revealed by means of the configuration analysis (CA) and the similarity analysis (SA). One state (labelled “state A”) is characterized by a delocalization over the whole π system, whereas the other (labelled with “state B”) is locally excited at the phenyl moiety. An attempt is made to link this classification with the different photoisomerization pathways of the title compounds.

Electronic absorption spectrum of methyl allenyl sulfide

The ultraviolet absorption spectrum of methyl allenyl sulfide CH 3SCHCCH 2 was recorded in solution from 30000–50000 cm −1 in order to inquire further into the role of sulfur 3d valence-shell and 4s Rydberg-type atomic orbitals for the low-lying excited singlet states in unsaturated organic sulfides. It is shown that sulfur 3d atomic orbitals cannot be neglected for the interpretation of the low-energy excited singlet states of unsaturated organic sulfides.

ChemInform Abstract: THE LOWEST ENERGY EXCITED SINGLET STATES AND THE CIS‐TRANS PHOTOISOMERIZATION OF STYRENE

Chemischer InformationsdienstVolume 9, Issue 18 Physical Organic Chemistry ChemInform Abstract: THE LOWEST ENERGY EXCITED SINGLET STATES AND THE CIS-TRANS PHOTOISOMERIZATION OF STYRENE G. L. BENDAZZOLI, G. L. BENDAZZOLISearch for more papers by this authorG. ORLANDI, G. ORLANDISearch for more papers by this authorP. PALMIERI, P. PALMIERISearch for more papers by this authorG. POGGI, G. POGGISearch for more papers by this author G. L. BENDAZZOLI, G. L. BENDAZZOLISearch for more papers by this authorG. ORLANDI, G. ORLANDISearch for more papers by this authorP. PALMIERI, P. PALMIERISearch for more papers by this authorG. POGGI, G. POGGISearch for more papers by this author First published: May 2, 1978 https://doi.org/10.1002/chin.197818044AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume9, Issue18May 2, 1978 RelatedInformation

The lowest energy excited singlet states and the cis-trans photoisomerization of styrene

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTThe lowest energy excited singlet states and the cis-trans photoisomerization of styreneG. L. Bendazzoli, G. Orlandi, P. Palmieri, and G. PoggiCite this: J. Am. Chem. Soc. 1978, 100, 2, 392–395Publication Date (Print):January 1, 1978Publication History Published online1 May 2002Published inissue 1 January 1978https://doi.org/10.1021/ja00470a007RIGHTS & PERMISSIONSArticle Views85Altmetric-Citations35LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (484 KB) Get e-Alerts Get e-Alerts

Energy dependence of radiationless relaxation in methylglyoxal

Measurements are reported for the biexponential decay parameters as a function of excitation energy for luminescence from the lowest energy excited singlet state (S 1) of methylglyoxal. The results are interpreted in terms of a kinetic scheme incorporating reversible decay between S 1 and a set of isoenergetic accepting levels. Correspondence with a quantum mechanical model yields coupling matrix elements and densities of accepting levels. Some fundamental decay rate constants for S 1 in glyoxal, methylglyoxal, and in biacetyl are found to be strikingly similar. This similarity may be explained with a simple theoretical scheme.

Polyene spectroscopy: The lowest energy excited singlet state of diphenyloctatetraene and other linear polyenes

Optical absorption and emission spectra at liquid helium temperatures of solutions of all-trans-1,8-diphenyloctatetraene in single crystals of bibenzyl and polycrystalline n-paraffin matrixes are presented which show an excited singlet state at a lower energy than the 1Bu state responsible for the strong absorption of all linear polyenes. The transition from the ground state to this level has an oscillator strength of about 0.05. The location of this low energy weak transition removes a number of discrepancies between the expected and observed fluorescence properties of diphenyloctatetraene. The observed vibronic pattern is consistent with the Raman spectrum which is also reported. Substantial indirect evidence is presented which indicates that many, and perhaps all, linear polyenes have a similar ordering of excited states. A review of the available information on diphenylpolyenes is presented. A correlation of the oscillator strength of the fluorescence transition as deduced from the emission lifetime with the separation of the low energy excited states for a number of polyenes is noted. The implications for the photochemistry of linear polyenes including the visual chromophores are pointed out.