Pronounced Vibrational Structure Research Articles

Vibrationally Resolved Inner-Shell Photoexcitation of the Molecular Anion C2.

Carbon 1s core-hole excitation of the molecular anion C2 - has been experimentally studied at high resolution by employing the photon-ion merged-beams technique at a synchrotron light source. The experimental cross section for photo-double-detachment shows a pronounced vibrational structure associated with and core excitations of the C2 - ground level and first excited level, respectively. A detailed Franck-Condon analysis reveals a strong contraction of the C2 - molecular anion by 0.2 Å upon this core photoexcitation. The associated change of the molecule's moment of inertia leads to a noticeable rotational broadening of the observed vibrational spectral features. This broadening is accounted for in the present analysis which provides the spectroscopic parameters of the C2 - and core-excited levels.

Circularly Polarized Light Probes Excited-State Delocalization in Rectangular Ladder-type Pentaphenyl Helices.

Ladder-type pentaphenyl chromophores have a rigid, planar π-system and show bright fluorescence featuring pronounced vibrational structure. Such moieties are ideal for studying interchromophoric interactions and delocalization of electronic excitations. We report the synthesis of helical polymers with a rigid square structure based on spiro-linked ladder-type pentaphenyl units. The variation of circular dichroism with increasing chain length provides direct evidence for delocalization of electronic excitations over at least 10 monomeric units. The change in the degree of circular polarization of the fluorescence across the vibronic side bands shows that vibrational motion can localize the excitation dynamically to almost one single unit through breakdown of the Born-Oppenheimer approximation. The dynamic conversion between delocalized and localized excited states provides a new paradigm for interpreting circular dichroism in helical polymers such as proteins and polynucleic acids.

Circularly Polarized Light Probes Excited‐State Delocalization in Rectangular Ladder‐type Pentaphenyl Helices

AbstractLadder‐type pentaphenyl chromophores have a rigid, planar π‐system and show bright fluorescence featuring pronounced vibrational structure. Such moieties are ideal for studying interchromophoric interactions and delocalization of electronic excitations. We report the synthesis of helical polymers with a rigid square structure based on spiro‐linked ladder‐type pentaphenyl units. The variation of circular dichroism with increasing chain length provides direct evidence for delocalization of electronic excitations over at least 10 monomeric units. The change in the degree of circular polarization of the fluorescence across the vibronic side bands shows that vibrational motion can localize the excitation dynamically to almost one single unit through breakdown of the Born‐Oppenheimer approximation. The dynamic conversion between delocalized and localized excited states provides a new paradigm for interpreting circular dichroism in helical polymers such as proteins and polynucleic acids.

Photophysical Study on the Rigid Pt(II) Complex [Pt(naphen)(Cl)] (Hnaphen = Naphtho[1,2-b][1,10]Phenanthroline and Derivatives.

The electrochemistry and photophysics of the Pt(II) complexes [Pt(naphen)(X)] (Hnaphen = naphtho[1,2-b][1,10]phenanthroline, X = Cl or C≡CPh) containing the rigid tridentate C^N^N-coordinating pericyclic naphen ligand was studied alongside the complexes of the tetrahydro-derivative [Pt(thnaphen)(X)] (Hthnaphen = 5,6,8,9-tetrahydro-naphtho[1,2-b][1,10]phenanthroline) and the N^C^N-coordinated complex [Pt(bdq)(Cl)] (Hbdq = benzo[1,2-h:5,4-h’]diquinoline. The cyclic voltammetry showed reversible reductions for the C^N^N complexes, with markedly fewer negative potentials (around −1.6 V vs. ferrocene) for the complexes containing the naphen ligand compared with the thnaphen derivatives (around −1.9 V). With irreversible oxidations at around +0.3 V for all of the complexes, the naphen made a difference in the electrochemical gap of about 0.3 eV (1.9 vs. 2.2 eV) compared with thnaphen. The bdq complex was completely different, with an irreversible reduction at around −2 V caused by the N^C^N coordination pattern, which lacked a good electron acceptor such as the phenanthroline unit in the C^N^N ligand naphen. Long-wavelength UV-Vis absorption bands were found around 520 to 530 nm for the C^N^N complexes with the C≡CPh coligand and were red-shifted when compared with the Cl derivatives. The N^C^N-coordinated bdq complex was markedly blue-shifted (493 nm). The steady-state photoluminescence spectra showed poorly structured emission bands peaking at around 630 nm for the two naphen complexes and 570 nm for the thnaphen derivatives. The bdq complex showed a pronounced vibrational structure and an emission maximum at 586 nm. Assuming mixed 3LC/3MLCT excited states, the vibronic progression for the N^C^N bdq complex indicated a higher LC character than assumed for the C^N^N-coordinated naphen and thnaphen complexes. The blue-shift was a result of the different N^C^N vs. C^N^N coordination. The photoluminescence lifetimes and quantum yields ΦL massively increased from solutions at 298 K (0.06 to 0.24) to glassy frozen matrices at 77 K (0.80 to 0.95). The nanosecond time-resolved study on [Pt(naphen)(Cl)] showed a phosphorescence emission signal originating from the mixed 3LC/3MLCT with an emission lifetime of around 3 µs.

Assignment of Core and Surface States in Multicolor-Emissive Carbon Dots.

It is important to reveal the luminescence mechanisms of carbon dots (CDs). Herein, CDs with two types of optical centers are synthesized from citric acid in formamide by a solvothermal method, and show high photoluminescence quantum yield reaching 42%. Their green/yellow emission exhibits pronounced vibrational structure and high resistance toward photobleaching, while broad red photoluminescence is sensitive to solvents, temperature, and UV-IR. Under UV-IR, the red emission is gradually bleached due to the photoinduced dehydration of the deprotonated surface of CDs in dimethyl sulfoxide, while this process is hindered in water. From the analysis of steady-state and time-resolved photoluminescence and transient absorption data together with density functional theory calculations, the green/ yellow emission is assigned to conjugated sp2 -domains (core state) similar to organic dye derivatives stacked within disk-shaped CDs; and the broad red emission-to oxygen-containing groups bound to sp2 -domains (surface state), whereas energy transfer from the core to the surface state can happen.

UV transition moments of tyrosine.

To assist polarized-light spectroscopy for protein-structure analysis, the UV spectrum of p-cresol, the chromophore of tyrosine, was studied with respect to transition moment directions and perturbation by solvent environment. From linear dichroism (LD) spectra of p-cresol aligned in stretched matrices of poly(vinyl alcohol) and polyethylene, the lowest π-π* transition (Lb) is found to have pure polarization over its entire absorption (250-300 nm) with a transition moment perpendicular to the symmetry axis (C1-C4), both in polar and nonpolar environments. For the second transition (La), polarized parallel with the symmetry axis, a certain admixture of intensity with orthogonal polarization is noticed, depending on the environment. While the Lb spectrum in cyclohexane shows a pronounced vibrational structure, it is blurred in methanol, which can be modeled as due to many microscopic polar environments. With the use of quantum mechanical (QM) calculations, the transition moments and solvent effects were analyzed with the B3LYP and ωB97X-D functionals in cyclohexane, water, and methanol using a combination of implicit and explicit solvent models. The blurred Lb band is explained by solvent hydrogen bonds, where both accepting and donating a hydrogen causes energy shifts. The inhomogeneous solvent-shift sensitivity in combination with robust polarization can be exploited for analyzing tyrosine orientation distributions in protein complexes using LD spectroscopy.

Azacyanines of the pyrrolopyrrole series.

The reaction of POCl3-activated, readily soluble diketopyrrolopyrrole (DPP) with 2-aminoheteroaromatics to yield 1:1 and 1:2 hydrogen chelates is described. Complexation of these hydrogen chelates with boron reagents results in thermally and photochemically stable fluorescent dyes (PP-azacyanines). The 1:2 complexes in particular absorb at long wavelengths and are brightly fluorescing. The rich photophysics of the new compounds are presented. Both the pronounced vibrational fine structure of the S0 → S1 transitions and the observed fluorescence phenomena allow detailed conclusions to be made on the correlation between molecular structure and optical properties.

Size and shape dependent photoluminescence and excited state decay rates of diamondoids

We present photoluminescence spectra and excited state decay rates of a series of diamondoids, which represent molecular structural analogues to hydrogen-passivated bulk diamond. Specific isomers of the five smallest diamondoids (adamantane-pentamantane) have been brought into the gas phase and irradiated with synchrotron radiation. All investigated compounds show intrinsic photoluminescence in the ultraviolet spectral region. The emission spectra exhibit pronounced vibrational fine structure which is analyzed using quantum chemical calculations. We show that the geometrical relaxation of the first excited state of adamantane, exhibiting Rydberg character, leads to the loss of Td symmetry. The luminescence of adamantane is attributed to a transition from the delocalized first excited state into different vibrational modes of the electronic ground state. Similar geometrical changes of the excited state structure have also been identified in the other investigated diamondoids. The excited state decay rates show a clear dependence on the size of the diamondoid, but are independent of the particle geometry, further indicating a loss of particle symmetry upon electronic excitation.

Intermolecular exciton coupling and vibronic effects in solid-state circular dichroism: a case study

The electronic circular dichroism (CD) spectrum of the sesquiterpenoid 1β,10β-epoxydesacetoxymatricarin (1) measured in the microcrystalline solid state differs from the solution one in the appearance of a pronounced vibrational fine structure in the long-wavelength region (n-π* enone transition) and of a new moderately intense band in the π-π* region. TDDFT CD calculations were run on input structures derived from the X-ray geometry of 1 to reproduce the impact of exciton-type couplings between proximate molecules in the crystals. The vibrational structure of the CD spectrum was also reproduced for the isolated molecule by modelling the potential energy surfaces at the harmonic level and taking into account Duschinsky and Herzberg-Teller effects.

Disentanglement of triplet and singlet states of azobenzene: direct EELS detection and QMC modeling

Singlet and triplet excited states of trans-azobenzene have been measured in the gas phase by electron energy loss spectroscopy (EELS). In order to interpret the strongly overlapping singlet and triplet bands in the spectra a set of large-scale correlated quantum Monte-Carlo (QMC) simulations was performed. The EELS/QMC combination of methods yields an excellent agreement between theory and experiment and for the two low-lying excited singlet and two low-lying triplet states permitted their unambiguous assignment. In addition, EELS revealed two overlapping electronic states in the band commonly assigned as S(2), the lower one with a pronounced vibrational structure, the upper one structureless. Finally, the agreement between theory and experiment was shown to further increase by taking computationally into account the finite temperature effects.

Boron 1s photoelectron spectrum of B11F3: Vibrational structure and linewidth

The boron 1s photoelectron spectrum of (11)BF(3) has been measured at a photon energy of 400 eV and a resolution of about 55 meV. The pronounced vibrational structure seen in the spectrum has been analyzed to give the harmonic and anharmonic vibrational frequencies of the symmetric stretching mode, 128.1 and 0.15 meV, as well as the change in equilibrium BF bond length upon ionization, -5.83 pm. A similar change in bond length has been observed for PF(3) and SiF(4), but a much smaller change for CF(4). Theoretical calculations for BF(3) that include the effects of electron correlation give results that are in reasonable accord with the experimental values. The Lorentzian (lifetime) width of the boron 1s core hole in BF(3) is found to be 72 meV, comparable to the value of 77 meV that has been reported for CF(4).

Photoelectron Spectroscopy on Small Anionic Copper-Carbonyl Clusters

Anion photoelectron spectroscopy in combination with density functional theory (DFT) calculations has been used to study mono- and dinuclear copper-carbonyl clusters Cun(CO)m- (n = 1, 2; m = 2-5). The adiabatic detachment energies of the anions have been measured which correspond to the electron affinities of the respective neutral species. The corresponding values are 0.95 eV for Cu(CO)2, 1.02 eV for Cu(CO)3, 1.04 eV for Cu(CO)4, 1.43 eV for Cu2(CO)4, and 1.19 eV for Cu2(CO)5. All spectra exhibit a pronounced vibrational fine structure on the adiabatic photodetachment peak. The energy splitting is close to the energy of the C-O stretching vibration of the neutral cluster (final state). The DFT calculations clearly indicate that in all clusters the highest occupied molecular orbital (HOMO) is a CO-derived π* orbital. Furthermore the calculations are used to give insight into geometry, frontier orbitals, vibrational frequencies and spin multiplicity of the neutral and anionic clusters.

Study of Vibronic Interactions in Impurity Centers by Conjugate Fluorescence and Absorption Spectra with a Poorly Resolved Vibrational Structure

The conjugate fluorescence and fluorescence excitation spectra of recently synthesized substituted arylpolyene (C6H5-[CH=CH]2-C6H4-NH2) are studied in solid n-octane at a temperature of 4.2 K. The spectra exhibit a weakly pronounced vibrational structure. A method of determination of the vibronic interaction parameters responsible for the shape of the spectra is developed. The method is based on the modeling of the spectra by series of vibronic bands, each of which consists of a narrow zero-phonon line and a broad phonon wing (phonon sideband). This makes it possible to calculate the fluorescence and fluorescence excitation spectra with the weakly pronounced vibrational structure and compare them with the measured spectra. The deviations from the mirror symmetry between the measured fluorescence and fluorescence excitation conjugate spectra are explained by the combined effect of the Franck-Condon and Herzberg-Teller interactions. The parameters of these interactions are determined.

Photodissociation spectroscopy and dynamics of Mg+-formaldehyde

We have carried out photodissociation spectroscopy studies of the bimolecular complex Mg+(H2CO) in the visible and near-uv regions. The work is supported by electronic structure calculations of the ground and low-lying excited states of the complex. Mg+-formaldehyde is bound in a C2v Mg+–O=CH2 geometry with a theoretical bond energy of De″(Mg-OCH2)=1.35 eV. The complex shows absorption bands that correlate with Mg+-based and formaldehyde-based radiative transitions. The lowest-energy band is assigned as à 2A′(2B1)←X̃ 2A1, to an excited state of mixed Mg+(3pπ) and H2CO(π*) orbital character. The band exhibits complex vibrational structure with considerable excitation of the CH2 out-of-plane wag and C=O stretch modes; the vibrational frequencies are shifted dramatically from their values in the ground state, showing the effect of a significant weakening of the C=O bond and out-of-plane distortion of the complex. Excitation in the Mg+-based B̃ 2A′(2B2)←X̃ 2A1 band shows predominantly low-frequency vibrational motions assigned to the intermolecular in-plane wag and Mg-O stretch modes. Birge–Sponer analysis gives the Mg–O bond energy in the ground state as De″=1.29 eV. Partially resolved rotational substructure clearly demonstrates a change in geometry from a linear or near linear Mg-O-C (C2v) ground state to a bent (Cs) excited state [θ(Mg-O-C)=139°±3°]. Spectroscopic rotational constants are in very good agreement with ab initio predictions for this band. The Mg+-based D̃ 2A1←X̃ 2A1 band also exhibits pronounced vibrational structure including strong Mg–O and C=O stretch signals, consistent with formation of a partial Mg–O σ bond in this state. Mg+ is the major dissociation product through the uv–visible region. However, in the B̃←X̃, C̃←X̃, and D̃←X̃ absorption bands, we also observe a substantial branching to the reactive dissociation product MgH+. The reactive branching ratio increases with photon energy through the absorption bands, reaching a reactive quantum yield of ∼13 in the D̃←X̃ band. Our results suggest that there is no significant activation barrier to reaction above the endothermicity.

Tetraaminoperylenes: their efficient synthesis and physical properties.

Trimethylsilylation of 1,8-diaminonaphthalene gave 1,8-bis(trimethylsilylamino)naphthalene (1 a), which was in turn lithiated with two molar equivalents of n-butyllithium to give the tris(thf)-solvated dilithium diamide [1,8-[(Me(3)SiN)Li(thf)](2)C(10)H(6)](thf) (2 a). Metal exchange of 2 a with TlCl was carried out in two steps, via the previously characterized mixed-metal amide [1-[(Me(3)SiN)Li(thf)(2)]-8-[(Me(3)SiN)Tl]C(10)H(6)], to give the dithallium diamide [1,8-[(Me(3)SiN)Tl](2)C(10)H(6)] (3 a). Thermolysis of 3 a cleanly gave a 1:1 mixture of the 4,9-bis(trimethylsilylamino)perylenequinone-3,10-bis(trimethylsilylimine) (4 a) and 1 a. By this route, a whole series of silylated homologues of 4 a was obtained in good yields, while the same method proved to be inefficient for the synthesis of the alkyl-substituted analogues. Compound 4 a and its tert-butyldimethylsilyl derivative 4 d were reduced with sodium amalgam to give, after protonation, the corresponding 3,4,9,10-tetraaminoperylenes 7 a and 7 d. Cyclic voltammetry showed two reversible, closely spaced reduction waves (E(red 1)=-1.39, E(red 2)=-1.59 V versus SCE) corresponding to this conversion. The perylenes 7 a and 7 d are thought to be the primary products in the reaction cascade leading to the perylene derivatives, involving the thermal demetalation of the thallium amides, possibly via Tl(II)bond;Tl(II) intermediates, first to give 7 a and its analogues. The final oxidation of the tetraaminoperylenes by one molar equivalent of 3 a and analogous thallium amides gave the quinoidal derivatives such as 4 a and 4 d, a step that could be studied by direct reaction of the isolated species. The UV/Vis absorption spectra of the 4,9-bis(silylamino)perylenequinone-3,10-bis(silylimines) are characterized by a long-wavelength absorption band with a pronounced vibrational structure (lambda(max)=639 nm, lg epsilon =4.53) attributed to a pi*<--pi and a pi*<--n absorption band at 454 nm (lg epsilon 4.83), along with intense absorption in the UV region. A weak red emission with a rather low quantum yield (Phi(fl)=0.001, lambda(max)=660 nm) is observed upon irradiation of a sample; the lifetime of the emission is only 66 ps. The low emission quantum yield is attributed to the *pi<--n transition of the amino perylene, which induces strong spin-orbit coupling, leading to a large triplet yield. The triplet state was probed by transient absorption spectroscopy and found to have a lifetime of 200 ns in air, and 1100 ns in argon-flushed solution. Treatment of 4 a with a stoichiometric amount of KF in methanol/water under phase-transfer conditions (with the cryptand [C 222]) gave an almost quantitative yield of the parent compound 4,9-diaminoperylenequinone-3,10-diimine (8). Treatment of 8 with two molar equivalents of the ruthenium complex [Ru(bpy)(2)(acetone)(2)](PF(6))(2), generated in situ, yielded the blue dinuclear ruthenium complex [(bpy)(4)Ru(2)[mu(2)-N,N':N",N"'-[[4,9-(NH(2))(2)-3,10-(NH)(2)]C(20)H(8)]]](PF(6))(4) (9), the redox properties of which were studied by cyclic voltammetry. The difference in the potentials of the two one-electron redox steps (225 mV) indicates strong coupling of the metal centers through the 4,9-diaminoperylenquinone-3,10-dimine bridging ligand and corresponds to a comproportionation constant K(c) of 6.3 x 10(3). The UV/Vis absorption spectrum of the mixed valent form, which is stable in air, has a characteristic intervalence charge-transfer (IVCT) band in the near infrared at 930 nm (lg epsilon =3.95), from which an electronic coupling parameter J of 760 cm(-1) could be estimated, placing compound 9 at the borderline between the class II and class III cases in the Robin-Day classification.

Absorption and luminescence spectroscopy of the MnO4 centers in alkali halide crystals

The low-temperature (8 K) optical absorption, near-infrared photoluminescence (NIRPL) and photoluminescence excitation spectra (PLE) of MnO4− and MnO42− in the KI and RbI crystals are reported. The most intense low-energy absorption bands in visible region are assigned to ligand-to-metal charge transfer (LMCT) bands due to 1A1→1T2 (1t1–2e) (MnO4−) and 2E→2T2 (1t1–2e) (MnO42−) intramolecular transitions, respectively. In both crystals, LMCT bands exhibit a fairly well-resolved vibrational structure due to the Mn–O stretching mode (∼750 cm−1). The broad NIRPL bands, observed for both systems in the similar (850–1200 nm) spectral region, display pronounced vibrational structure caused by electron coupling mostly with the bending O–Mn–O mode (∼340 cm−1). The model is proposed that explains the NIRPL spectra in MnO4−- and MnO42−-doped crystals as activated by two types of the MnO42− centers with Td and Cs symmetries. The NIRPL bands are attributed to d–d (2T2→2E) ligand–field transition of the single 3d-electron of manganese in MnO42−.

Depletion spectrum of ozone in a molecular beam. Evidence for interference effects in the Hartley band photodissociation

The absorption spectrum of the Hartley band of ozone in a cold molecular beam has been investigated using a hole-burning depletion technique over the wavelength range 273.2–284 nm. This spectrum shows a more pronounced vibrational structure than the corresponding absorption spectrum measured at ∼200 K which is often used as a reference for the quality of ab initio calculations. Comparison of the depletion spectrum with the wavelength-dependent fluctuation of the vibrational state distribution of the photofragment O 2( 1Δ g ) indicates that this fluctuation is to a significant extent due to an interference effect.

Spectroscopic characterization of the ZnNe van der Waals molecule in theX0+(41S0)andD1(41P1)energy states

A molecular jet-expansion beam of ZnNe seeded in Ne was crossed with a pulsed-laser beam produced by a Nd:YAG (yttrium aluminum garnet) laser-pumped dye laser. The dye laser was frequency doubled with a BBO-C second-harmonic-generation (SHG) crystal. Both the dye laser and the SHG were synchronously scanned over a range of $\ensuremath{\lambda}=2130--2170\AA{}$ in the vicinity of the Zn atomic $4{}^{1}{P}_{1}--4{}^{1}{S}_{0}$ resonance line (2138.65 \AA{}), producing an excitation spectrum related to the $D1(4{}^{1}{P}_{1})\ensuremath{\leftarrow}{X0}^{+}(4{}^{1}{S}_{0})$ transition in the ZnNe molecule. The spectral traces, which show a pronounced vibrational structure, were subjected to LeRoy's near-dissociation expansion (NDE) analysis, yielding ${\ensuremath{\omega}}_{e},$ ${\ensuremath{\omega}}_{e}{x}_{e},$ ${D}_{e},$ and long-range molecular potential parameters. Computer simulation of the excitation spectrum, based on a calculation of the Franck-Condon factors, yielded additional information on the equilibrium internuclear separation for the $D1$ and ${X0}^{+}$ states. The NDE analysis provided long-range characteristics for the ${X0}^{+}$ state of the CdNe and HgNe molecules as well. A simple dispersive model, and a combination rule for van der Waals interactions in the ground state of ZnNe, CdNe, and HgNe molecules, are discussed.

Photodissociation spectroscopy and dynamics of MgC2H4+

The weakly bound ion–molecule complex MgC2H4+ has been studied by photodissociation spectroscopy in a reflectron time-of-flight mass spectrometer over the spectral range 218–510 nm. Mg+ is the major photofragment throughout this range, although for λ&amp;lt;270 nm, charge-transfer dissociation to C2H4+ is observed as a minor channel. We have identified five absorption bands of MgC2H4+. The spectral assignment is facilitated by results from ab initio calculations for the ground and low-lying excited states of MgC2H4+. Three of the bands, 1 2B2←1 2A1, 1 2B1←1 2A1, and 2 2A1←1 2A1, are based primarily in the metal-centered Mg+(3p 2P←3s 2S) atomic transition. One of the remaining bands is assigned as 2 2B2←1 2A1, a transition correlating with the a 3B1u←X 1Ag forbidden band of C2H4, with mixed charge-transfer character. The final band, 3 2A1←1 2A1, is assigned to a metal-to-ligand charge-transfer transition, enhanced by coupling with the nearby 2 2A1 state. The 1 2B2←1 2A1 band is a broad continuum, indicative of fast predissociation in the upper state. A nonadiabatic dissociation mechanism involving C=C π-bond activation by Mg+(3p) is suggested by ab initio calculations. The 1 2B1←1 2A1 band shows pronounced vibrational structure with a strong progression in the Mg+–CH4 intermolecular stretch (ν2), and weaker progressions assigned to combination bands built on the intermolecular out-of-plane wag (ν3), and a CH2–CH2 wag (ν7). The observed vibrational constants are ω2=329, x22=−2.3, ω3=439, and ω7=1024 cm−1. Measurement of the photofragment kinetic energy release determines the bond dissociation energies for the ground state [D0″(Mg+–C2H4)=0.7±0.2 eV], and for the 1 2B1 excited state, [D0′(Mg+–C2H4)=1.8±0.2 eV]. Spectroscopic constants are in good agreement with ab initio predictions.

Unprecedented observation of pronounced vibrational structure in charge‐transfer absorption and emission spectra

AbstractTwo rigidly bridged electron donor‐acceptor systems are reported that display discrete, intramolecular charge‐transfer (CT) absorption and emission spectra in solution at room temperature. For both the CT‐absorption is highly structured irrespective of solvent, for one also the CT‐fluorescence in a nonpolar solvent is structured.