Crystallization‐Induced Reversal from Dark to Bright Excited States for Construction of Solid‐Emission‐Tunable Squaraines

Squaraines (SQs) with tunable emission in the solid state is of great importance for various demands; however a remaining challenge is emission quenching upon aggregation. Herein, a unique SQ, named as CIEE‐SQ, is designed to exhibit strong emission in crystal, undergoing crystallization‐induced reverse from dark ¹(n+σ,π*) to bright ¹(π,π*) excited states. Such an excited state of CIEE‐SQ can be subtly tuned by molecular conformation changes during the unexpected temperature‐triggered single‐crystal to single‐crystal (SCSC) reversible transformation. Furthermore, co‐crystallization between CIEE‐SQ and chloroform largely stabilize the ¹(π,π*) state, enhancing the transition dipole moment and decreasing the reorganization energy to boost the fluorescence, which is promising in data encryption and decryption.     

Spektroskopische Untersuchung einiger α,β-ungesättigter cyclischer Ketone

The results of the spectroscopic investigation of the steroidal enones 1–6 can be summarized as follows:

• 1. Direct absorption and phosphorescence excitation techniques have been used to locate the ³(n,π*) states, and in each case it has been found to be the second triplet state.
• 2. The lowest excited state in each case is assigned as ³(π,π*) state.
• 3. The diffuseness in the phosphorescence emission from the ³(π,π*) states is attributed to a large change in the molecular geometry upon excitation (probably to a non-planar configuration).
• 4. The diffuseness in the S→ T_n,π* absorption is correspondingly attributed to interaction between the ³(n,π*) and ³(π,π*) states. A summary of the energy levels for these compounds is given in Fig. 4.

     

Restriction of Access to the Dark State: A New Mechanistic Model for Heteroatom‐Containing AIE Systems

Restriction of intramolecular motion (RIM), as the working mechanism of aggregation‐induced emission (AIE), cannot fully explain some heteroatom‐containing systems. Now, two excited states are taken into account and a mechanism, restriction of access to dark state (RADS), is specified to elaborate RIM and complete the picture of AIE mechanism. A nitrogen‐containing molecule named APA is chosen as a model compound; its weak fluorescence in solution is ascribed to the easy access from the bright (π,π*) state to the close‐lying dark (n,π*) state. By either metal complexation or aggregation, the dark state is less accessible due to restriction of the molecular motion leading to the dark state and elevation of the dark state energy, thus the bright state emission is restored. RADS is powerful in elucidating the AIE effect of molecules with excited states favoring non‐radiative decay, including overlap‐forbidden states such as (n,π*) and CT states, spin‐forbidden triplet states, and so on.     

Photophysical Properties of Pyrazinopsoralen,A New Monofunctional Psoralen*

Abstract— Pyrazinopsoralen (PzPs), a new monofunctional psoralen, has a UV absorption spectrum similar to other psoralens except that it absorbs more strongly in the long-UVA than 8-methoxypsoralen. The solvent effects on the UV absorption and fluorescence emission spectra indicate that the lowest excited singlet state is the π,π* state like other psoralen derivatives. It shows a much lower fluorescence quantum yield (0.0008 in ethanol at room temperature) than the other psoralens as expected by the increased proximity effect (vibronic perturbation) due to close ¹(n,π*) to ¹(π,π*) states. The fluorescence lifetime was 1.05 ns in methylcyclohexane with a single exponential decay, while more than two components were observed in other solvents with the short-lived component being the major (>95%). The triplet state of PzPs could not be detected by phosphorescence, laser flash excitation (T-T absorption) and singlet oxygen formation probably due to very low φ_isc, or short lifetime of the triplet state (τ_T) caused by the fast T₁→ S₀ intersystem crossing.     

High-resolution absorption spectra of diphenylnmethylenes

High-resolution absorption spectra of the following diphenylmethylenes (DPM_s) dispersed in benzophenone crystals at liquid-helium temperatures are presented: DPM-h₁₀, DPM-d₁₀, 4-chloro-DPM, and 4-bromo-DPM. The substituent effects concerning the electronic structure, transition energy and intensity are discussed. From polarization measurements, the electronic configurations of the ground and the first excited triplet states of these DPM_s are assigned as (pπ)¹(pσ)¹ and (pσ)¹(π*)¹, respectively. Further studies reveal a second excited triplet state, designated as (pπ)¹(π*)¹, which lies less than 1000 cm^-1 above the first excited triplet state of DPM. Diffuse broad bands appear as common features in all the spectra. Such diffuseness is discussed in terms of electron-phonon coupling of the low-lying excited states.     

Acid/base sensitive platinum terpyridyl complex: Switching between metal-to-ligand charge transfer (MLCT), ligand-to-ligand charge transfer (LLCT), and intraligand charge transfer (ILCT) states

A platinum(II) 2,2′:6′,2″-terpyridyl complex (2) with a hydroxylphenyl substituent on the terpyridyl ligand and a dimethylamino substituent on the phenylacetylide ligand was synthesized and characterized. Complex 2 exhibits a metal-to-ligand charge transfer (¹MLCT) absorption band at ca. 410 nm and a ligand-to-ligand charge transfer (¹LLCT) band at ca. 536 nm. It exhibits dual emission at ca. 450 nm and ca. 560 nm at room temperature when excited at 334 nm, which originates from the ¹π,π* state and the ³MLCT/³π,π* state, respectively. Dramatic color change was observed for 2 with addition of acid and base. Its emission at 560 nm was enhanced in acidic solution and quenched in basic solution. The changes in absorption and emission could be attributed to the variation of the nature of the lowest excited state from LLCT to MLCT in acidic solution and to LLCT/ILCT at basic solution. The drastic color and emission intensity changes in acidic and basic solutions suggest that 2 could potentially be a colorimetric and luminescent acid/base sensor.     

Dipole moments and electronic charge distributions of the 1, 3(nπ*) excited states of formaldehyde,benzaldehyde and benzophenone. A theoretical study at the INDO level including the doubly- and triply-excited configurations

The dipole moments and charge distributions of the ^1,3(nπ*) excited states of formaldehyde, benzaldehyde and benzophenone have been investigated theoretically. This study was performed within the framework of the INDO approximations by introducing into the electronic wavefunctions doubly- and triply-excited configurations selected with respect to the (nπ*) configuration of suitable spin multiplicity. One basis set of MOs was used in the calculations on formaldehyde and on benzophenone, while two basis sets were used in the calculations on benzaldehyde. The results show, in particular, that the ^1,3(nπ*) excited states of benzaldehyde have a dipole moment lower than that of the corresponding ground state.     

Photoluminescence of Acenaphthenone in Organic Solvents and Aqueoug Media

The configuration of the lowest excited state of acenaphthenone, S₁(π, π^*) or T₁(π, π^*), depending on the solvent, dominates photoluminescence. The T₁(n, π^*) state in aprotic organic solvents is responsible for the phosphorescence of acenaphthenone. The wavelengths of the phosphorescence measured in benzene are 576 nm and 635 nm (vibronic) with 3.3 × 10^?4 quantum efficiency. However, the S₁(π, π^*) state in protic solution which dominates the fluorescence emission depending upon acidity is the most distinctive feature of acenaphthenone. The wavelengths of the emissions are 446 nm under water solvation with 0.185 quantum efficiency and 538 nm with 0.097 quantum efficiency under high acidity. The emission at 446 nm is assigned from a H-bonded keto-form excited state, whereas the emission at 538 nm is probably due to the excited state of protonated keto-form. The pK_a value in aqueous solution measured by diminution of fluorescence in basic solutions is 12.5 ± 0.4.     

Photochemistry of β,γ-enones-VIII: On the remarkable photostability of some β,γ.β',γ'-dienones and the 1,3-acyl shift photoreactivity of two β,γ.γ',δ'-dienones

The direct irradiation of the β,γ.β',γ'-dienones 1–5 and the β,γ.γ',δ'-dienones (E)-6a, (E)-7a and 8a at λ 300 nm has been studied. The β,γ.β,γ'-dienones 1–5 are remarkable photostable for λ ? 300 nm, even upon prolonged irradiation, in contrast to simple β,γ-enones which upon irradiation exhibit α-cleavage, γ-hydrogen abstraction, (E)-(Z) isomerization and oxetane formation. The observed photostability of the β,γ.β',γ'-dienones is rationalized in terms of a rapid radiationless decay of the excited singlet state, enhanced by CT-interaction between the carbonyl ¹(n-π*) state and the homoconjugated 1,4-diene moiety, which precludes fluorescence, photochemical reactions and intersystem crossing (ISC).The β,γ.γ',δ'-dienones (E)-(6a), (E)-7a and 8a exhibit only a 1,3-acyl shift (1,3-AS) without (E)-(Z) isomerization of the alkenyl moiety, to yield (E)-6b, (E)-7b and 8b. It is concluded that the 1,3-AS proceeds from the ¹(n-π*) state with a rate which is very large relative to the rate of ISC to the ³(n-π*) state, thus precluding any internal triplet energy transfer (1TET) from the ³(n-π*) to the ³(π-π*) state which would manifest itself by (E)-(Z) isomerization.     

Electronically excited states of membrane fluorescent probe 4-dimethylaminochalcone. Results of quantum chemical calculations

Quantum-chemical calculations of ground and excited states for membrane fluorescent probe 4-dimethylaminochalcone (DMAC) in vacuum were performed. Optimized geometries and dipole moments for lowest-lying singlet and triplet states were obtained. The nature of these electronic transitions and the relaxation path in the excited states were determined; changes in geometry and charge distribution were assessed. It was shown that in vacuum the lowest existed level is of (n, π*) nature, and the closest to it is the level of (π, π*) nature; the energy gap between them is narrow. This led to an effective (1)(π, π*) →(1)(n, π*) relaxation. After photoexcitation the molecule undergoes significant transformations, including changes in bond orders, pyramidalization angle of the dimethylamino group, and planarity of the molecule. Its dipole moment rises from 5.5 Debye in the ground state to 17.1 Debye in the (1)(π, π*) state, and then falls to 2 Debye in the (1)(n, π*) state. The excited (1)(n, π*) state is a short living state; it has a high probability of intersystem crossing into the (3)(π, π*) triplet state. This relaxation path explains the low quantum yield of DMAC fluorescence in non-polar media. It is possible that (3)(π, π*) is responsible for observed DMAC phosphorescence.     

Static and dynamical potential surface distortions in aromatic aldehyde 3A″ (nπ*) states

Two quanta of the aldehyde H-wag vibrational mode are observed with relatively constant activity in the vibrational structure of ³A″ (nπ*) → ¹A′ phosphorescence spectra of benzaldehyde, deuterated benzaldehydes, pyridine aldehydes, p-methylbenzaldehyde, p-fluorobenzaldehyde, and p-chlorobenzaldehyde at 4.2°K in a methylcyclohexane polycrystalline environment. The fundamental is either missing or very weak. Contrasted to the aldehyde H-wag mode, higher quanta of the CHO torsional mode are observed in the phosphorescence of the same molecules with variable intensity and progression length. These results are interpreted in terms of a static (i.e., valence) distortion of the ³nπ potential surface along the aldehyde H-wagging coordinate, and pseudo-Jahn-Teller distortion along the CHO torsional coordinate. The valence distortion is analogous to a distortion found for the ³nπ* state in propynal and its origin probably lies in excess electron density at the carbonyl carbon in the excited state. The dynamical Jahn-Teller distortion is ascribed to vibronic interaction between ³A″ (nπ) and the nearest ³A′(nπ*) states.     

Aromatic Amides: A Smart Backbone toward Isolated Ultralong Bright Blue-Phosphorescence in Confined Polymeric Films

We describe an aromatic amide skeleton for manipulation of triplet excited states toward bright long-lived blue phosphorescence. Spectroscopic studies and theoretical calculations demonstrated that the aromatic amides can promote strong spin-orbit coupling between (π,π*) and the bridged (n,π*) states, and enable multiple channels to populate the emissive ³(π,π*), as well as facilitate robust hydrogen bonding with polyvinyl alcohol to suppress non-radiative relaxations. Isolated inherent deep-blue (0.155, 0.056) to sky-blue (0.175, 0.232) phosphorescence with high quantum yields (up to 34.7 %) in confined films are achieved. The blue afterglow of the films can last for several seconds and are showcased in information display, anti-counterfeiting, and white light afterglow. Owing to the high population of ³(π,π*) states, the smart aromatic amide skeleton provides an important molecular design prototype to manipulate triplet excited states for ultralong phosphorescence with various colors.     

Mechanism of the photochemical process of singlet oxygen production by phenalenone

Phenalenone (PN) is a very efficient singlet oxygen sensitiser in a wide range of solvents. This work uses ab initio quantum chemical calculations (CASSCF/CASPT2 protocol) to study the mechanism for populating the triplet state of PN responsible for this reaction, the (3)(π-π*) state. To describe in detail this reaction path, the singlet and triplet low-lying excited states of PN have been studied, the critical points of the potential energy surfaces corresponding to these states located and the vertical and adiabatic energies calculated. Our results show that, after the initial population of the S(2) excited state of (π-π*) character, the system undergoes an internal conversion to the (1)(n-π*) state. After populating the dark S(1) state, the system relaxes to the (1)(n-π*) minimum, but rapidly populates the triplet manifold through a very efficient intersystem crossing to the (3)(π-π*) state. Although the population of the minimum of this triplet state is strongly favoured, a conical intersection with the (3)(n-π*) surface opens an internal conversion channel to this state, a path accessible only at high temperatures. Radiationless deactivation processes are ruled out on the basis of the high-energy barriers found for the crossings between the excited states and the ground state. Our computational results satisfactorily explain the experimental findings and are in very good agreement with the experimental data available. In the case of the frequency of fluorescence, this is the first time that these data have been theoretically predicted in good agreement with the experimental results.     

Spezifisch π→π*-induzierte Reaktionen von γ-Dimethoxymethylcyclohexen-2-onen: 1,3-Umlagerung und Wasserstoffabstraktion durch das α-Kohlenstoffatom

When α,β-unsaturated γ-dimethoxymethyl cyclohexenones are excited to the S₂(π,π*) state, certain unimolecular reactions can be observed to compete with S₂ → S₁ internal conversion. These reactions do not occur from the S₁(n,π*) or the lowest T(π,π* and n,π*) states. They comprise the radical elimination of the formylacetal substituent (cf. 8 , 9 → 32 + 33 ), γ → α formylacetal migration (cf. 6 → 27 , 8 → 30 , 9 → 34 , 12 → 37 ), and a cyclization process involving the transfer of a methoxyl hydrogen to the α carbon and ring closure at the β position (cf. 6 → 28 , 8 → 31 , 12 → 38 , 20 → 40 + 41 ). The quantum yield of the ring closure 20a → 40a + 41a is 0.016 at ≤ 0.05M concentration. It is independent of the excitation wavelength within the π→π* absorption band (238–254 nm), but Φ ( 40a + 41a ) decreases at higher concentrations. According to the experimental data the reactive species of these specifically π→π*-induced transformations is placed energetically higher than the S₁(n,π*) state, and it is either identical with the thermally equilibrated S₂(n,π*) state, or reached via this latter state. The linear dienone 14 undergoes a similar π→π*-induced cyclization (→ 42 ) whereas the benzohomologue 26 proved unreactive, and the dienone 22 at both n → π and π→π* excitation only gives rise to rearrangements generally characteristic of cross-conjugated cyclohexadienones.     

Triplet excited states of cyclic disulfides and related compounds: electronic structures, geometries, energies, and decay

We have performed a computational study on the properties of a series of heterocycles bearing two adjacent heteroatoms, focusing on the structures and electronic properties of their first excited triplet states. If the heteroatoms are both heavy chalcogens (S, Se, or Te) or isoelectronic species, then the lowest excited triplet state usually has (π*, σ*) character. The triplet energies are fairly low (30-50 kcal mol(-1)). The (π*, σ*) triplet states are characterized by a significantly lengthened bond between the two heteroatoms. Thus, in 1,2-dithiolane (1b), the S-S bond length is calculated to be 2.088 ? in the singlet ground state and 2.568 ? in the first triplet excited state. The spin density is predicted to be localized almost exclusively on the sulfur atoms. Replacing one heavy chalcogen atom by an oxygen atom or an NR group results in a significant destabilization of the (π*, σ*) triplet excited state, which then no longer is lower in energy than an open-chain biradical. The size of the heterocyclic ring also contributes to the stability of the (π*, σ*) triplet state, with five-membered rings being more favorable than six-membered rings. Benzoannulation, finally, usually lowers the energy of the (π*, σ*) triplet excited states. If one of the heteroatoms is an oxygen or nitrogen atom, however, the corresponding lowest triplet states are better described as σ,π-biradicals.     

Penning ionization optical spectroscopy: Metastable helium (He23S) with nitric oxide

The interaction between NO(X²π) and metastable He(2³S) atoms has been investigated by emission spectrometry. Several reaction channels have been identified, leading to NO⁺(A¹π), or to electronically excited N or O atoms. The NO⁺(A⁺π-X¹Σ⁺) banded emission spectrum was observed in the range 123-190 nm, and it was analyzed for vibrational and rotational populations. The NO⁺(A) state vibrational distribution, determined with a new set of Franck—Condon factors for NO⁺(A–X), is in approximate accord with the calculated NO⁺ (A) - NO(X) Franck—Condon (FC) factors; however the υ' = 2 – 5 levels are overpopulated relative to the FC values. The NO⁺(A) state rotational populations are shifted to higher J-values than the precursor, NO(X). Emission was observed from several excited states of O and N in both the vacuum ultraviolet and red regions of the spectrum. Comparison of total rates from excited atomic fragments with emissions from NO⁺(A) showed that the cross-section for dissociative excitation was similar to that for Penning ionization giving NO⁺(A).     

Structure and dynamics of the lowest triplet state in p-benzoquinone: I. An isotope effect study on the optical absorption,emission and ODMR spectra

The results of detailed spectroscopic experiments on the lowest nπ* triplet state of p-benzoquinone-h₄, -dh₃, 2,6-d₂h₂, -d₄ and -CH₃ in mixed and isotopic mixed crystals are presented and analyzed. The origin of the lowest B_1g (nπ*) singlet-triplet transition in p-benzoquinone-h₄ (PBQ-h₄) is shown to be induced by asymmetric isotopic substitution and the oscillator strength of this origin is seen to be accounted for by a corresponding decrease in intensity of a level 16.9 cm^?1 higher in energy in the pure PBQ-h₄ crystal. The combined oscillator strength of these close lying levels is measured and found to be almost independent of deuteration.These results are discussed in reference to the previously proposed double minimum potential model for the lowest nπ* triplet state in PBQ-h₄ and the applicability for this model is critically examined.Optical absorption experiments on heavily doped isotopic mixed crystals of PBQ-h₄ in PBQ-d₄ show hydrogen (deuterium) bounding effects between translational inequivalent molecules to be primarily responsible for the observed cluster states. These hydrogen bounding effects also induce the electronic origin of the B_1g (nπ*) triplet state in case of a translational inequivalent dimer.A detailed vibrational analysis of the phosphorescence spectrum of PBQ-h₄ in a PBQ-d₄ host crystal at 1.8 K is presented and it is shown that the unobserved origin of the B_1g (nπ*) triplet state of PBQ-h₄ is located at 18609 ± 1 cm^?1 and that the inversion splitting in this lowest excited state amounts to 21 ± 1 cm^?1 in this mixed crystal system. An isotope effect is study on the vibronic structure in the emission spectrum further indicates that the excited state structure of PBQ is isotope dependent.The observed large isotope effect on the ZFS parameters of the lowest triplet state of PBQ-h₄ is demonstrated to be an intramolecular phenomenon and explained as an isotope dependent spin-orbit contribution to the ZI-S parameters, induced by localization of the nπ* excitation on oxygen.Finally the dynamics of energy migration in the dilute PBQ-h₄ in PBQ-d₄ isotopic mixed crystal is probed by concentration and temperature dependent phosphorescence intensity measurements and it is suggested that trap-exciton band communication effects are of importance in this system.     

Photochemical Reduction of 2-Bromo-4,4-dimethyl-2-cyclohexenone

The photochemical behaviour of 2-Bromo-4,4-dimethyl-2-cyclohexenone (1) was studied in 2-propanol and cyclohexane. In both solvents (n-π*)-excitation followed by intersystem crossing leads to population of a low-lying triplet (T₁) state, which can be quenched by 1,3-cyclohexadiene but does not undergo chemical transformation efficiently. (π-π*)-Excitation affords 4,4-dimethyl-2-cyclohexenone ( 2 ) as the only product. While in 2-propanol 2 is formed in 60% from the S₂-state and in 40% from the T₂-state, in cyclohexane reduction occurs exclusively from this upper triplet state. The T₂-state can also be populated via energy transfer using acetone or benzene as sensitizer. The mechanistic dissimilarities for the reduction of excited 1 in either 2-propanol or cyclohexane are discussed.     

Time-resolved EPR and laser photolysis investigations of photoinduced ω-bond dissociation in an aromatic carbonyl compound having triplet π,π* character

Photochemical properties of photoinduced ω-bond dissociation in p-phenylbenzoylbenzyl phenyl sulfide (PPS) having the lowest triplet state (T₁) of π,π* character in solution were investigated by time-resolved EPR and laser flash photolysis techniques. PPS was found to undergo photoinduced ω-bond cleavage in the excited lowest singlet state (S₁(n,π*)) with a quantum yield (Φ_rad) of 0.15 for the radical formation, which was independent of excitation wavelengths. Based on the facts of the observation of the absorption spectrum of triplet PPS upon triplet sensitization of xanthone, and absence of CIDEP signal, ω-cleavage was shown to be absent in the T₁(π,π*) state of PPS. Considering the electronic character of the excited and dissociative states of PPS, a schematic energy diagram for the ω-bond dissociation of PPS was shown.     

A theoretical study of the excited‐state decay of acylhydrazones

Acylhydrazones is a novel yet underexploited class of molecular switches. In the present paper, we investigated the excited‐state decay of three model systems of acylhydrazones in the gas phase by a combination of electronic structure calculations and Tully's surface hopping dynamic simulations. Our computational results demonstrated that the S₂(n_Nπ*) state decay of the three model systems leads to both the imine‐like photo‐isomerization through the S₁(n_Nπ*)/S₀ intersection and population of the S₁(n_Oπ*) state that will cross to the triplet manifold. The position of phenyl substituent was found to have an effect on the ratio of the two S₁ states. The present theoretical work provides some understandings of the intramolecular mechanism for de‐population of the excited electronic states of acylhydrazones.