Vibronic effects accelerate the intersystem crossing processes of the through-space charge transfer states in the triptycene bridged acridine–triazine donor–acceptor molecule TpAT-tFFO

Quantum chemical studies employing combined density functional and multireference configuration interaction methods suggest five excited electronic states to be involved in the prompt and delayed fluorescence emission of TpAT-tFFO. Three of them, a pair of singlet and triplet charge transfer (CT) states (S₁ and T₁) and a locally excited (LE) triplet state (T₃), can be associated with the (Me → N) conformer, the other two CT-type states (S₂ and T₂) form the lowest excited singlet and triplet states of the (Me → Ph) conformer. The two conformers, which differ in essence by the shearing angle of the face-to-face aligned donor and acceptor moieties, are easily interconverted in the electronic ground state whereas the reorganization energy is substantial in the excited singlet state, thus explaining the two experimentally observed time constants of prompt fluorescence emission. Forward and reverse intersystem crossing between the singlet and triplet CT states is mediated by vibronic spin–orbit interactions involving the LE T₃ state. Low-frequency vibrational modes altering the distance and alignment of the donor and acceptor π-systems tune the S₁ and T₃ states (likewise S₂ and T₃) into and out of resonance. The enhancement of intersystem crossing due to the interplay of vibronic and spin–orbit coupling is considered a general feature of organic through-space charge-transfer thermally activated delayed fluorescence emitters.

DFT/MRCI quantum chemical studies suggest five excited electronic states to be involved in the prompt and delayed fluorescence emission of TpAT-tFFO.     

High Stokes shift long-wavelength energy gap regulated fluorescence in the series of nitro/dimethylamino-substituted ortho-analogs of POPOP

A series of novel nitro-substituted ortho-analogs of POPOP was synthesized. Like the most of the other known compounds of this class, the synthesized molecules demonstrate high Stokes shift fluorescence emission owing to the planarization of their molecules at electronic excitation. Significant fluorescence quenching in polar solvents was described as the “energy gap law” action rather than the specific effect of the dialkylamino group excited state twisting.      

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.     

Excited state intramolecular proton transfer in a new o-hydroxy Schiff base in non polar solvents at room temperature and 77 K

A new orthohydroxy Schiff base, 7-ethylsalicylidenebenzylamine (ESBA) has been synthesised. The excited state intramolecular proton transfer (ESIPT) processes have been investigated by means of absorption, emission and nanosecond spectroscopy at room temperature and at 77 K in non polar solvents. The ESIPT is evidenced by a large Stokes shifted emission (11 000 cm⁻¹) only at 77 K. From fluorescence and excitation spectra it is suggested that at least three different species are present in the excited state at room temperature. Our theoretical calculation at AM1 level confirm the cis-isomer to be the only viable form in the ground state.     

Modeling the structure,absorption spectra,and cis-trans isomerization of thiacarbocyanine dyes

The relative stability of the trans-and cis-isomers of 3,3′-diethylthiacarbocyanine (Dye1) and 3,3′-diethyl-9-methylthiacarbocyanine (Dye2)¹, as well as sections of the potential energy surfaces along the internal coordinate of the isomerization reaction, were studied using the density functional theory. Calculation of the minimum energy pathway for the isomerization reaction showed that the barrier for rotation about the C₈–C₉ bond is higher for Dye1 than for Dye2. Local minimums were found for the singlet excited state of the 8,9-cis-and trans-isomers of the dyes. In the case of the trans-isomers, substantial changes in the dye structure do not occur and the local minimum of the excited state corresponds to the geometry of the starting trans-isomers, which favors efficient fluorescence. A search for the nearest local minimum of the singlet excited state of the 8,9-cis-isomers leads to structures, which differ significantly from the starting structures, and the intensity of the S₁ → S₀ transition in those structures appears to be practically zero. The results are in agreement with experimental data on the absorption, fluorescence, and fluorescence excitation spectra of the dyes.     

Fluorescence of phthalocyanines: Emission from an upper excited state

The weak (φ_f < 10³) fluorescence at around 430 nm of the S² upper excited singlet state of metal-free phthalocyanine and metallophthalocyanines is presented. Polarization measurements indicate that the emission is short-lived (< 800 ps) contrary to the intense (φ_f > 0.3) normal emission at around 700 nm originating from S¹ and having a lifetime in the 4.1 to 10.6 ns range, depending on the solvent. The short wavelength emitting S² excited state has been populated by a two photon absorption process using the excitation light at 695 nm of a pulsed ruby laser. This process is shown not to involve the triplet state but the following stepwise two photon absorption process: .     

Excited state dipole moments of fluoroanilines from solvatochromic shift measurements

The dipole moments of fluorinated anilines in the first excited singlet state (¹L_b) have been determined from the solvent shifts of absorption and fluorescence spectra. It is concluded that in the monofluoro isomers as well as in aniline itself this dipole moment must be of the order of 5 debye, whereas the gas phase dipole moment is estimated to be some 2 debye only from Stark effect measurements. Ortho-substituted difluoro- and trifluoroanilines show anomalous Stokes shifts of the absorption and fluorescence spectra which are indicative of substantial reorganization of their nuclear framework in the excited state; in these cases no excited state dipole moment could be determined.     

Emission energies and photophysical properties of ladder oligo(p-aniline)s

Emission properties and the photophysics of three ladder oligo(p-aniline)s; namely 5,11-diethyl-6,12-dimethylindolo[3,2-b]carbazole (DIMER 2P), 14-ethyl-5,8-dihydro-diindolo[3,2-b:2′,3′-h]carbazole (TRIMER 2P), and 5,8,14-triethyl-diindolo[3,2-b:2′,3′-h]carbazole (TRIMER 3P) are presented. The optimization (relaxation) of the first singlet excited electronic state (S₁) has been done using the restricted configuration interaction (singles) (RCIS/6-31G*) approach. The excitation to the S₁ state does not cause important changes in the geometrical parameters of the compounds, as is experimentally corroborated by the small Stokes shifts. Emission energies from the relaxed excited states have been obtained from TDDFT calculations performed on the S₁ optimized geometries and have been correlated with the corresponding fluorescence spectra of the derivatives dissolved in dichloromethane. A good agreement has been found between TDDFT emission energies and the (0,0) fluorescence bands. As predicted from theoretical calculations, all compounds exhibit small Stokes shift, which testify the rigidity of these ladder compounds. Moreover, this theoretical approach provides a good evaluation of the bathochromic shifts caused by the increase in the conjugation length or by the presence of alkyl chains on the nitrogen atoms. Finally, the fluorescence quantum yield and lifetime of the compounds in dichloromethane have been obtained. From these data, the radiative and nonradiative rate constants of the deactivation of the S₁ state have been determined.     

Effect of N-pyridyl substitution and hydrogen bonding on the deactivation of singlet excited 1,2-naphthalimide

Photophysical properties of 1,2-naphthalimide (1) and N-(4-pyridyl)-1,2-naphthalimide (2) as well as the effect of their hydrogen bonding with phenols have been studied in toluene. Fluorescence emission is the dominant energy dissipation pathway of the singlet excited 1. Introduction of the 4-pyridyl substituent into the imide moiety significantly accelerates the internal conversion due to the efficient vibronic coupling between close-lying S1 and S2 excited states, however, the rate of triplet formation exhibits negligible change. In contrast with the behavior of the corresponding substituted phenyl derivatives, 2 does not emit dual fluorescence because of the less extensive conjugation within the molecule. Fluorescence quenching with phenols takes place both in dynamic and static processes. Electron transfer is slower in the hydrogen bonded complex where phenols are linked to the pyridyl moiety due to the larger distance between the electron donor and acceptor components.     

Molecular design of efficient yellow- to red-emissive alkynylgold(iii) complexes for the realization of thermally activated delayed fluorescence (TADF) and their applications in solution-processed organic light-emitting devices

Here, we report the design and synthesis of a new class of fused heterocyclic alkynyl ligand-containing gold(iii) complexes, which show tunable emission colors spanning from the yellow to red region in the solid state and exhibit thermally activated delayed fluorescence (TADF) properties. These complexes display high photoluminescence quantum yields of up to 0.87 and short excited-state lifetimes in sub-microsecond timescales, yielding high radiative decay rate constants on the order of up to 10⁶ s⁻¹. The observation of the drastic enhancement in the emission intensity of the complexes with insignificant change in the excited-state lifetime upon increasing the temperature from 200 to 360 K indicates an increasing radiative decay rate. The experimentally estimated energy splitting between the lowest-lying singlet excited state (S₁) and the lowest-lying triplet excited state (T₁), ΔE_S1–T1, is found to be as small as ∼0.03 eV (250 cm⁻¹), comparable to the value of ∼0.05 eV (435 cm⁻¹) obtained from computational studies. The delicate choice of the cyclometalating ligand and the fused heterocyclic ligand is deemed the key to induce TADF through the control of the energy levels of the intraligand and the ligand-to-ligand charge transfer excited states. This work represents the realization of highly emissive yellow- to red-emitting gold(iii) TADF complexes incorporated with fused heterocyclic alkynyl ligands and their applications in organic light-emitting devices.

We report the design of a new class of fused heterocyclic alkynyl ligand-containing gold(iii) complexes, which shows tunable emission colors spanning yellow to red region and exhibits thermally activated delayed fluorescence (TADF) properties.     

Further studies of the continuous UV emission produced by electron impact on CF4

The intense continuous UV emission which extends from 200 nm to beyond 500 nm produced by electron impact on carbontetrafluoride, CF₄, has been investigated in a crossed electron-beam — gas-beam apparatus as well as in a gas-target apparatus under single collision conditions with special emphasis on the regime of impact energies below 50 eV. Systematic measurements of excitation functions (relative emission cross sections) at various wavelengths between 225 nm and 420 nm revealed the same onset of 23.5±2 eV and the same energy dependence for all excitation functions independent of the selected wavelength indicating that the continuous emission consists of a single fluorescence contribution. Evidence of a second onset around 40 eV, roughly 16 eV above the first onset which is an energy close to the ionization energy of atomic fluorine (17.4 eV), was found — more or less prominent — in all excitation functions. The presence of the second onset lends strong support to the assignment of the excited (CF ₃ ⁺ )* fragment ion as the emitting source of the emission continuum. We identify the lower onset at 23.5 eV with the electron-impact induced breakup of the parent CF₄ molecule into an excited (CF ₃ ⁺ )* ion and a ground state fluorine atom (neutral channel), whereas the second onset around 40 eV indicates the presence of an ionic fragmentation channel in which an excited (CF ₃ ⁺ )* fragment ion is produced together with a fluorine ion, F⁺. Wavelength scans taken at impact energies above and below the second onset revealed no significant difference in the envelope of the observed emission continuum which further supports the notion that the emission continuum consists of a single fluorescence contribution.     

Excitation energy dependence of the quantum yields of fluorescence and electron formation from aqueous phenol by means of the heavy atom effect

The formation of hydrated electrons and H atoms from phenol in neutral aqueous solutions has been studied by excitation in its first and second excited singlet state. The quantum yields of both H and

show a strong increase at higher excitation energies. However, this increase cannot fully account for the corresponding decrease of the fluorescence quantum yield. Using Cs⁺ as a heavy atom quencher, it could be shown that there are at least two pathways for electron formation, one in competition to internal conversion to the fluorescing state and another occurring after population of the fluorescing state, but most likely not from the triplet state.     

Planarized Intramolecular Charge Transfer: A Concept for Fluorophores with both Large Stokes Shifts and High Fluorescence Quantum Yields

Fluorophores were successfully used in several areas of chemistry and biochemistry. For many purposes, however, it is necessary that the fluorescence compound features a high fluorescence quantum yield as well as a large Stokes shift. The latter is, for example, achieved by the use of a twisted intramolecular charge‐transfer (TICT) compound, which shows a twisted geometry in the excited state. However, the higher the twisting is, the lower becomes in general the fluorescence quantum yield as the resulting emission from the twisted state is forbidden. In order to escape this dilemma, we propose the model of planarized intramolecular charge‐transfer (PLICT) states. These compounds are completely twisted in the ground states and planar in the excited states. By means of quantum chemical calculations (time‐dependent (TD)‐B3LYP and CC2) and experimental studies, we could demonstrate that 1‐aminoindole and its derivatives form photoinduced PLICT states. They show both very large Stokes shifts ( =9000–13 500 cm^?1, i.e., λ=100–150 nm) and high fluorescence quantum yields. These characteristics and their easy availability starting from the corresponding indoles, make them very attractive for the use as optical switches in various fields of chemistry as well as biological probes.     

Absorption band shape and emission in rhodopsin

Abstract— –The Planck law relationship between absorption and emission spectra is applied to the spectral sensitivity curve of human rod vision, assumed to be equivalent to the absorption spectrum of the visual pigment rhodopsin, to compute a hypothetical emission spectrum for rhodopsin. When 17, 100 cm^-1 is used as the reflection axis, the mirror image of this hypothetical emission substantially matches the activation spectrum over 2500 cm^-1. The predicted emission increases exponentially at long wavelengths, in contrast to published observations of fluorescence at 580 nm; this discrepancy, and an estimate of excited state lifetime based on the Planck law theory, suggest that emission occurs from a meta-stable vibrationally-excited state. The exceptionally slow falloff in absorption at long wavelengths is explained as being due to the smaller dependence of potential energy on angle of twist about bonds in the polyene chain in the lowest π, π* singlet state of rhodopsin than in its ground state; a model assuming six or seven essentially flat vibronic modes in the excited state accurately fits the observed action spectrum.     

Excited state proton transfer reaction of two new intramolecularly hydrogen bonded Schiff bases at room temperature and 77K.

Two new orthohydroxy Schiff bases, 7-phenylsalicylidene benzylamine (PSBA) and 7-ethylsalicylideneaniline (ESA) have been synthesized. The excited state intramolecular proton transfer (ESIPT) and the structure of PSBA and ESA in its crystalline form and in the solvents n-hexane, n-heptane and 1,4-dioxane have been investigated by means of absorption, emission and nanosecond spectroscopy at room temperature and 77K. One ground state species has been detected both in neutral and basic solutions of both PSBA and ESA: the cis-enol form with an intramolecular hydrogen bond. The ESIPT and formation of keto tautomer are evidenced by a large Stokes shifted emission (approximately 12000 cm(-1)) at room temperature only in the case of ESA. On the other hand the keto tautomer is the predominant species at 77K in a solid matrix and as a solid sample at room temperature both in the case of ESA and PSBA. In the case of both ESA and PSBA the more intense, higher energy emission is due to the species which has not undergone ESIPT and attributed mainly due to cis-enol form. The trans-enol form is also observed by changing the excitation wavelength. Both the compounds are found to undergo a structural change to a zwitterionic and intermolecular hydrogen bonded form in the presence of a strong base like triethylamine. From the nanosecond measurements and quantum yield of fluorescence we have estimated the decay rates of proton transfer reaction in the case of PSBA. Our theoretical calculation at the AM1 level of approximation shows that the ground singlet state has a rather large activation barrier both in the case of PSBA and ESA. The barrier height is much lower on the corresponding excited singlet surface only in the case of ESA. The process is predicted to be endothermic in the ground state and exotherrmic in the excited singlet state.     

Bond length alternation in ground and HOMO→LUMO excited states in polyenes. Dynamic Stokes shift?

Complete-active-space self-consistent-field calculation of the reorganisation energy, , corresponding to the strongly allowed HOMOLUMO transition in planar polyenes in the trans form (C _{2 h} symmetry), gives >0.5 eV. This large depends on the fact that the short and long bond lengths of the excited ¹B _u (or ³B _u ) state compared to the ¹A _g ground state are almost cancelled. The emission redshift (Stokes shift) in molecules with the same type of system is quite small, however, which suggests that the Stokes shift may be dynamic, owing to the presence of another excited state at lower or about the same energy. Acknowledgement.We congratulate Björn on his birthday and at the same time thank him for the CASSCF method and for many years of collaboration and help from him and his collaborators to make this wonderful method work in our laboratory.Contribution to the Björn Roos Honorary Issue     

Solvent-dependent radiationless transitions in fluorenone: A probe for hydrogen bonding interactions in the cyclodextrin cavity

Fluorescence lifetimes fluorescence quantum yields and triplet yields were measured for fluorenone in various hydroxylic and non-hydroxylic solvents, and in-cyclodextrin complexes. The rate of singlet-triplet intersystem crossing, which decreases with increasing polarity, was found to be a good indicator of nonspecific solvent-solute interactions, while the rate of direct internal conversion from the singlet excited state was correlated with hydrogen bonding. The fast internal conversion of singlet excited fluorenone/-cyclodextrin complexes shows that the probe molecule, while embedded within the cyclodextrin cavity, still remains hydrogen bonded.Dedicated to Professor Szejtli.     

Fluorescence from the second excited singlet state of [18] annulenes

The emission spectra of the second excited singlet state of [18] annulene and of monofluoro [18] annulene (in a 3-methylpentane glass at 4 K) are reported. The large energy gap between the first and second excited singlet states inhibits fast internal conversion and favours the appearance of S₂ → S₀ emission. In addition, fluorescence from the S₁ state can be observed in monofluoro [13] annulene by exciting into the S₂ or directly into the S₁ absorption.     

Triplet Energy of 2, 2-Dimethylisoindene from Electron-Energy-Loss Spectroscopy and Photoinduced Triplet Energy Transfer

The excited electronic states of 2, 2-dimethylisoindene ( 1 ) have been studied by electron-energy-loss spectroscopy. Its vertical gas-phase triplet (1³B₂), and singlet (1¹B₂) excitation energies are 1.61 and 3.19 eV, respectively. The excited states are thus lowered by 0.49 eV and 1.21 eV, respectively, when compared to the corresponding states of (all-E)-octatetraene, which serves as a reference compound. These shifts are partially reproduced by ZINDO calculations. The spectra give no evidence for a 2¹A_g state below the 1¹B₂ state, but this lack of observation does not exclude its existence. The lowest triplet state T₁( 1 ) was further characterized by flash photolysis. T₁( 1 ) was observed as a transient intermediate, λ ≤ 350 nm, with a lifetime of 8 m?s in degassed hexane. The adiabatic excitation energy of T₁( 1 ) was bracketed to the range of 1.1 ± 0.1 eV by energy-transfer experiments. Relationships between the energies of the lowest excited singlet and triplet states of 1 and the lowest excited doublet state of its radical cation ${1}^{+\kern0pt {.}}$ – essentially a non-Koopmans' state – are discussed.     

Non-radiative decay processes in isolated SO2 molecules excited within the first allowed absorption band (2500–3200 Å)

New and previously published SO₂ fluorescence emission data related to non-radiative decay processes are considered in light of the recent observations of Brus and McDonald. All the present data are consistent with the previous conclusion of Mettee that non-radiative processes in SO₂ singlet photochemistry are unimportant. It appears that any small inefficiency in the emission of light quanta for SO₂ excited at short wavelengths (2650 A) is largely due to the population of a second very short-lived state which is quenched effectively even at pressures down to 1 μ. The very low efficiency of quanta production which we observed at long wavelengths (3020 Å) appears to have its origin not only in the second easily quenched state, but more importantly, in the diffusional loss of the long-lived singlet which for these conditions has a 20-fold greated lifetime than was expected previously.