EFFECT OF THE BROMINE ATOM UPON THE DISSOCIATION CONSTANTS OF BROMOPYRIDINES,BROMOQUINOLINES AND 4-BROMOISOQUINOLINE IN THEIR LOWEST EXCITED SINGLET AND TRIPLET STATES*

Abstract— The lowest excited singlet-state dissociation constants (pK^S_a) of bromosubstituted pyridines, quinolines, and isoquinolines were determined from the pH-dependent shifts in their electronic absorption spectra. The lowest excited triplet-state dissociation constants (pK^T_a) of bromosubstituted quinolines and 4-bromoisoquinoline were obtained from the shifts of the 0–0 phosphorescence bands measured in rigid aqueous solution at 77 K. The pK^S_a values indicate that the basicity of these brominated nitrogen heterocycles is increased in the lowest excited singlet state by 2 to 10 orders of magnitude as compared with the ground state. The pK^T_a values are found to be significantly different from the corresponding ground-state pK_a values, indicating that the basicity of bromoquinolines is increased in the lowest excited triplet state by 1.7 to 3.0 pK units. The enhancement of the excited singlet-and triplet-state basicity of brominated nitrogen heterocycle derivatives as compared with the unsuhstituted parent compounds is attributed to the increased electron-donor conjugative interactions of the bromine atom pπ orbitals with π orbitals in the lowest excited singlet and triplet state.     

The acid—base equilibria of dimethylphenols in the ground and the first excited singlet states

Measurements of the acidity constants in the ground (pK_a(S₀)) and the lowest excited singlet (pK_a(S₁)) states for 2,3-, 2,4-, 2,5-, 2,6-, 3,4- and 3,5-dimethylphenols in aqueous solution have been carried out spectrophotometrically at 25°C. The pK_a values in S₀ have been derived from the absorption spectra and the pK_a values in S₁ were estimated by means of the Forster cycle. It is found that the hydroxy group is more acidic in the first excited singlet than in the ground state.     

Influence of medium acidity upon the luminescence properties of 2,5-diphenyl-1,3,4-oxadiazole and 2,5-diphenyl-1,3-oxazole

The luminescence properties of 2,5-diphenyl-1,3,4-oxadiazole and 2,5-diphenyl-1,3-oxazole in aqueous solutions of sulfuric acid (pH 7 to H_o − 10) were studied. The spectral parameters are essentially acidity dependent, which is due to the acid-base equilibria of these heterocycles both in the ground and in the first excited singlet states. The difference between these two compounds is governed by their dissimilar solvation. The basicity constants of 2,5-diphenyl-1,3,4-oxadiazole in the S₀ state (pK_BH+ = − 2.49) and 2,5-diphenyl-1,3-oxazole in the S₀ and S₁ states (pK_BH+ = −1.93, pK_BH+^* = 1.95) were experimentally obtained. The enthalpies of formation, electron charge density, and geometry of the bases and corresponding conjugate acids in the S₀ and S₁ states were calculated by the MNDO method.     

Time-Dependent Density Functional Theory Study on the Electronic Excited State of Hydrogen-Bonded Clusters Formed by 2-Hydroxybenzonitrile (<Emphasis Type="Italic">o</Emphasis>-Cyanophenol) and Carbon Monoxide

Time-dependent density functional theory (TDDFT) method has been carried out to investigate excited-state hydrogen-bonding dynamics between 2-hydroxybenzonitrile (o-cyanophenol) and carbon monoxide. We have demonstrated that intermolecular hydrogen bond between 2-hydroxybenzonitrile (o-cyanophenol) and C=O group are significantly strengthened in the electronically excited state by theoretically monitoring the changes of the bond lengths of hydrogen bonds and hydrogen-bonding groups in different electronic states. In this study, we firstly analyze frontier molecular orbitals (MOs). Our results are consistent with the intermolecular hydrogen bond strengthening in the electronically excited state of Coumarin 102 in alcoholic solvents, which has been demonstrated for the first time by Zhao and Han. Moreover, the calculated electronic excitation energies of the hydrogen bonding C=O and O–H groups are markedly red-shifted upon photoexcitation, which illustrates the hydrogen bonds strengthen in the electronically excited state again. And the geometric structures in both ground state and the S₁ state of this hydrogen-bonded complex are calculated using the density functional theory (DFT) and TDDFT methods, respectively.     

Photoisomerization of Arylazopyrazole Photoswitches: Stereospecific Excited‐State Relaxation

Electronic structure calculations and nonadiabatic dynamics simulations (more than 2000 trajectories) are used to explore the Z–E photoisomerization mechanism and excited‐state decay dynamics of two arylazopyrazole photoswitches. Two chiral S₁/S₀ conical intersections with associated enantiomeric S₁ relaxation paths that are barrierless and efficient (timescale of ca. 50 fs) were found. For the parent arylazopyrazole (Z8) both paths contribute evenly to the S₁ excited‐state decay, whereas for the dimethyl derivative (Z11) each of the two chiral cis minima decays almost exclusively through one specific enantiomeric S₁ relaxation path. To our knowledge, the Z11 arylazopyrazole is thus the first example for nearly stereospecific unidirectional excited‐state relaxation.     

Fluorescence of 5-fluorouracil upon the transition from the second singlet excited state <Emphasis Type="Italic">S</Emphasis><Subscript>2</Subscript> to the ground state

Effect of the wavelength of excitation light (λ_ex) on the fluorescence excitation and emission spectra of 5-fluorouracil in acidic solution (pH 2.5) was studied upon excitation at the S ₂ ← S ₀-transition absorption band. It has been found that direct excitation at the second or the shorter wavelength absorption band results in 5-fluorouracil fluorescence that originates not only from the first excited state S ₁ but is also due the transition from the second excited state S ₂ to the ground state.     

A DFT/TDDFT Investigation of Excited State Dynamical Mechanism of (E)‐1‐((2,2‐Diphenylhydrazono)methyl)naphthalen‐2‐ol

The excited‐state intramolecular proton transfer (ESIPT) mechanism of a new compound (E )‐1‐((2,2‐diphenylhydrazono)methyl)naphthalen‐2‐ol ( EDMN ) sensor, reported and synthesized by Mukherjee et al . [Sensors Actuat. B‐Chem . 2014, 202 , 1190], is investigated in detail theoretically. The calculations on primary bond lengths, bond angles, and the corresponding infrared (IR) vibrational spectra and hydrogen‐bond energy involved in intramolecular hydrogen bond between the S₀ and S₁ states confirm that the intramolecular hydrogen bond is strengthened in the S₁ state, which reveals the tendency of ESIPT reaction. The fact that the experimental absorption and emission spectra are well reproduced demonstrates the rationality and effectiveness of the time‐dependent density functional theory (TDDFT) level of theory we adopt here. Furthermore, intramolecular charge transfer based on the frontier molecular orbitals (MOs) gives indication of the ESIPT reaction. The constructed potential energy curves of both the S₀ and S₁ states while keeping the O─H distance of EDMN fixed at a series of values are used to illustrate the ESIPT process. The lower barrier of ~3.934 kcal/mol in the S₁ state potential energy curve (lower than the 8.254 kcal/mol in the S₀ state) provides the transfer mechanism.     

Structure of Pyridazine in the S1 State: Experiment and Theory

The molecular structure of pyridazine in the first electronically excited state (S₁) is deduced from the combined use of resonance‐enhanced two‐photon ionization and mass‐analyzed threshold ionization spectroscopic methods. The equation‐of‐motion coupled‐cluster single and double (EOM–CCSD) calculation gives the distorted planar geometry for the most stable structure of the S₁ pyridazine. The symmetry constraint of C_2v is relaxed to that of C_s, and consequently many in‐plane vibrational modes are found to be optically active in both S₁–S₀ and D₀–S₁ excitation spectra, being appropriately assigned from the comparison of their frequencies with ab initio values. This indicates that the S₁–S₀ excitation is partially localized, and provides an alternative explanation for the long‐standing spectroscopic puzzle in S₁ pyridazine.     

Acid-base properties of N-methyl-substituted 1,2-dihydroquinolines in the ground and excited states

Changes in the absorption and fluorescence spectra of 1,2,2,3-tetramethyl-(1), 1,2,2,4-tetramethyl-(2), 6-ethoxy-1,2,2,4-tetramethyl-(3), and 1,2,6-trimethyl-1,2-dihydroquinolines (4) were studied in aqueous solution over a wide pH range from 1.0 to 12.0. The quantum yields of fluorescence and the values of pK _a of dihydroquinolines (DHQs) under study in the ground and excited states were determined, pK _a = 4.5, 3.8, 4.5, and 4.2 for the ground state of compounds 1–4, respectively, and pK _a ∼ 1.7 for the S ₁^* state for all DHQs.     

Low-temperature thermodynamic properties of <Emphasis Type="Italic">L</Emphasis>-cysteine

Heat capacity C _p(T) of the orthorhombic polymorph of L-cysteine was measured in the temperature range 6–300 K by adiabatic calorimetry; thermodynamic functions were calculated based on these measurements. At 298.15 K the values of heat capacity, C _p; entropy, S _m⁰(T)-S _m⁰(0); difference in the enthalpy, H _m⁰(T)-H _m⁰(0), are equal, respectively, to 144.6±0.3 J K⁻¹ mol⁻¹, 169.0±0.4 J K⁻¹ mol⁻¹ and 24960±50 J mol⁻¹. An anomaly of heat capacity near 70 K was registered as a small, 3–5% height, diffuse ‘jump’ accompanied by the substantial increase in the thermal relaxation time. The shape of the anomaly is sensitive to thermal pre-history of the sample.     

Molecular Structure, Vibrational Spectrum and Ionization Energy of m-Dimethoxybenzene

The optimized molecular geometries of the three rotamers of m-dimethoxybenzene in the ground So and electronically excited Sl states were predicted by ab initio and density functional theory （DFF） calculations. Their vibrational spectra in the St state were studied by one color resonant two photon ionization （1C-R2PI） method, and their ionization energies were measured by two color resonant two photon ionization （2C-R2PI） experiment. The optimized molecular geometries showed that the total energy of conformer a was the lowest in the So state. Most of the active vibrations assigned from the 1C-R2PI spectrum were found to be of the in-plane ring modes. The ionization energies （IE） of conformers a, b and c were determined to be 63521, 64487 and 63755 cm^-1, respectively.     

Comparative analysis of the vibrational structure of the absorption spectra of acrolein in the excited (<Emphasis Type="Italic">S</Emphasis><Subscript>1</Subscript>) electronic state

The assignments of absorption bands of the vibrational structure of the UV spectrum are compared with the assignments of bands obtained by the CRDS method in a supersonic jet from the time of laser radiation damping for the trans isomer of acrolein in the excited (S ₁) electronic state. The ν_00trans = 25861 cm⁻¹ values and fundamental frequencies, including torsional vibration frequency, obtained by the two methods were found to coincide in the excited electronic state (S ₁) for this isomer. The assignments of several absorption bands of the vibrational structure of the spectrum obtained by the CRDS method were changed. Changes in the assignment of (0-v′) transition bands of the torsional vibration of the trans isomer in the Deslandres table from the ν_00trans trans origin allowed the table to be extended to high quantum numbers v′. The torsional vibration frequencies up to v′ = 5 were found to be close to the frequencies found by analyzing the vibrational structure of the UV spectrum and calculated quantum-mechanically. The coincidence of the barrier to internal rotation (the cis-trans transition) in the one-dimensional model with that calculated quantum-mechanically using the two-dimensional model corresponds to a planar structure of the acrolein molecule in the excited (S ₁) electronic state.     

A CASSCF study on the lowest π→π* excitation of urocanic acid

The photochemical cis/trans isomerization of urocanic acid (UCA, (E)‐3‐(1′H‐imidazol‐4′‐yl)propenoic acid) was investigated using complete active space SCF (CASSCF) ab initio calculations. The singlet ground state and the triplet and the singlet manifolds of the lowest‐lying π→π* (HOMO→LUMO) excitation of the neutral and the anionic UCA were calculated using the 6‐31G* and the 6‐31+G* basis sets, respectively. The torsional barrier of the double bond of the propenoic acid moiety in UCA is observed to be considerably lower in the T₁ and S₁ excited states of the neutral UCA and in the T₁ but not in the S₁ excited state of the anionic UCA, as compared to the S₀ state of the respective protonation form. The cis‐isomer of both the neutral and the anionic UCA is lower in energy than the trans‐isomer in the S₀, T₁, and S₁ states. This energy difference is larger in the excited states than in the ground state, probably due to strengthening of the intramolecular hydrogen bond of cis‐UCA as the molecule is excited. The results of the calculations, interpreted in terms of the idea that UCA is deprotonated upon electronic excitation, led to construction of a new model for the photoisomerization mechanisms of UCA. According to this model, the trans‐to‐cis isomerization proceeds via both the triplet and the singlet manifolds in the deprotonated form of UCA. This isomerization may occur in the S₀ state of the neutral UCA as well. The cis‐to‐trans isomerization is suggested to proceed only in the S₀ state of the neutral UCA. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 72: 25–37, 1999     

Theoretical study on S<Subscript>1</Subscript>(<Superscript>1</Superscript>B<Subscript>3u</Subscript>) state electronic structure and absorption spectrum of pyrazine

Making use of a set of quantum chemistry methods, the harmonic potential surfaces of the ground state (S₀(¹ A _g)) and the first (S₁(¹ B _3u)) excited state of pyrazine are investigated, and the electronic structures of the two states are characterized. In the present study, the conventional quantum mechanical method, taking account of the Born-Oppenheimer adiabatic approximation, is adopted to simulate the absorption spectrum of S₁(¹ B _3u) state of pyrazine. The assignment of main vibronic transitions is made for S₁(¹ B _3u) state. It is found that the spectral profile is mainly described by the Franck-Condon progression of totally symmetric mode ν_6a. For the five totally symmetric modes, the present calculations show that the frequency differences between the ground and the S₁(¹ B _3u) state are small. Therefore the displaced harmonic oscillator approximation along with Franck-Condon transition is used to simulate S₁(¹ B _3u) absorption spectra. The distortion effect due to the so-called quadratic coupling is demonstrated to be unimportant for the absorption spectrum, except the coupling mode ν_10a. The calculated S₁(¹ B _3u) absorption spectrum is in reasonable agreement with the experimental spectra. Supported by Taiwan National Science Council (Grant Nos. NSC 96-2113-M-009-021 and NSC 96-2811-M-009-023)     

Accessing the Triplet State in Heavy‐Atom‐Free Perylene Diimides

Previous studies of perylenediimides (PDIs) mostly utilized the lowest singlet excited state S₁. Generation of a triplet excited state (T₁) in PDIs is important for applications ranging from photodynamic therapy to photovoltaics; however, it remains a formidable task. Herein, we developed a heavy‐atom‐free strategy to prompt the T₁←S₁ intersystem crossing (ISC) by introducing electron‐donating aryl (Ar) groups at the head positions of an electron‐deficient perylenediimide (PDI) core. We found that the ISC efficiency increases from 8 to 54 % and then to 86 % by increasing the electron‐donating ability of head‐substituted aryl groups from phenyl (p‐PDI) to methoxyphenyl (MeO‐PDI) and then to methylthioxyphenyl (MeS‐PDI). By enhancing the intramolecular charge‐transfer (ICT) interaction from p‐PDI to MeO‐PDI, and then to MeS‐PDI, singlet oxygen generation via energy‐transfer reactions from T₁ of PDIs to ³O₂ was demonstrated with the highest yield of up to 80 %. These results provide guidelines for developing new triplet‐generating PDIs and related rylene diimides for optoelectronic applications.     

MATRIX EFFECTS ON THE RADIATIONLESS TRANSITION' S1→T1 AND T1→S0 FOR BENZENE AND N, N, N', N'-TETRAMETHYLPARAPHENYLENE-DIAMINE MOLECULES

Abstract— The present study attempts to correlate the phosphorescence life time τ_p at 77°K of a definite solute: tetramethylparaphenylenediamine (TMPD) with various solvents viscosity and polarity. A few experiments with benzene in the same solvents are also reported. The following results have been obtained:

• 1 The measured τ_p vary regularly with the sample immersion time in liquid N₂, reaching a constant value after a few hours. This effect is related to the glass matrix relaxation. The rate constant K_isc (S, ₁→T₁) is also found to vary during relaxation of the solvent.
• 2 In the expression giving the nonradiative rate constant K_nr (T₁→S₀), the bimolecular quenching term appears negligible for high viscosity matrices i.e. for η= 10⁹ poises for benzene and for TMPD. K_nr is found to vary linearly with log η, as well as the intersystem crossing S₁→T₁ rate constant K^isc.
• 3 Both K_nr (T₁→S₀) and K^isc (S₁→T₁), increase with decreasing polarity of the solvent.
• 4 From our own observations and literature data[6] for C₆H₆ it appears that solvent viscosity does not contribute appreciably to the observed temperature effect on the solute τ_p when only a monomolecular triplet deactivation is operative.

     

Ab initio configuration interaction study on electronically excited 4-dimethylamino-4′-cyanostilbene

Ab initio configuration interaction calculations have been performed to examine the electronic structures of both trans-4-dimethylamino-4′-cyanostilbene (DCS) and four types of perpendicularly twisted DCSs, trans-DCS is predominantly excited into the S₁ state out of low-lying excited states. The S₁ state is an intramolecular charge-transfer (ICT) state in which the dipole moment is about twice as large as that in S₀. The excited DCS at the 4-dimethylanilino twisted conformation, which becomes S₁ in polar solvents, has a very much larger dipole moment than that in S₁ to trans-DCS. This means that the geometrical structure of the twisted ICT (TICT) is the 4-dimethylanilino twisted form, not the dimethylamino twisted one which is well know from the TICT structure of 4-dimethylaminobenzonitrile. Received: 16 December 1998 / Accepted: 19 March 1999 / Published online: 9 September 1999     

Radiationless decay of excited states of tetrahydrocannabinol through the S 1–S 0 (conical) intersection

The ground and electronically excited singlet states of tetrahydrocannabinol have been studied theoretically using density functional and time-dependent density functional methods. The vertical excitation energies, the equilibrium geometries as well as the adiabatic excitation energies have been determined. Opening of the six-membered ring between the oxygen and carbon atoms has been considered as photochemical reaction path. This mechanism leads to a typical excited-state intramolecular hydrogen-transfer process and produces low-lying S ₀–S ₁ intersection (possible conical intersection, CI) which provides a channel for effective radiationless deactivation of the electronically excited state.     

Spectroscopic Properties of 4‐Pyridoxic Acid as a Function of pH and Solvent

The photophysical properties and acid/base equilibria of 4‐pyridoxic acid (=3‐hydroxy‐5‐(hydroxymethyl)‐2‐methylpyridine‐4‐carboxylic acid), the final product of the catabolism of vitamin B₆, have been studied in aqueous solutions. The ground state of 4‐pyridoxic acid exhibits the different protonated forms A – D in the range of H₀=?6 to pH 11.5. HMQC‐ and HMBC‐NMR Studies allowed the pH‐dependent assignment of the different C‐atoms, and the evaluation of the deprotonation sequence. The 3‐OH group in the ground state has a ‘pK_a’ of H₀=?0.64, which is much lower than that found for other vitamin B₆ related compounds. The pK_a value of the 4‐COOH group is 5.4. Fluorescence studies showed that the same species exist at the lowest excited singlet state, but in different pH ranges. The 3‐OH group is four pH units more acidic in the lowest excited singlet state than in the ground state. Excitation spectra and emission decays in the pH range of 8 to 11.5 indicate that the pyridine N‐atom is more basic in the excited singlet state than in the ground state. The emission spectra are red‐shifted in protic solvents, in agreement with an intramolecular H‐bond between the ionized 3‐OH group and the nonionized 4‐COOH group.     

Ultrafast Relaxation Dynamics of the Excited States of 1‐Amino‐ and 1‐(N,N‐Dimethylamino)‐fluoren‐9‐ones

The dynamics of the excited states of 1‐aminofluoren‐9‐one (1AF) and 1‐(N,N‐dimethylamino)‐fluoren‐9‐one (1DMAF) are investigated by using steady‐state absorption and fluorescence as well as subpicosecond time‐resolved absorption spectroscopic techniques. Following photoexcitation of 1AF, which exists in the intramolecular hydrogen‐bonded form in aprotic solvents, the excited‐state intramolecular proton‐transfer reaction is the only relaxation process observed in the excited singlet (S₁) state. However, in protic solvents, the intramolecular hydrogen bond is disrupted in the excited state and an intermolecular hydrogen bond is formed with the solvent leading to reorganization of the hydrogen‐bond network structure of the solvent. The latter takes place in the timescale of the process of solvation dynamics. In the case of 1DMAF, the main relaxation pathway for the locally excited singlet, S₁(LE), or S₁(ICT) state is the configurational relaxation, via nearly barrierless twisting of the dimethylamino group to form the twisted intramolecular charge‐transfer, S₁(TICT), state. A crossing between the excited‐state and ground‐state potential energy curves is responsible for the fast, radiationless deactivation and nonemissive character of the S₁(TICT) state in polar solvents, both aprotic and protic. However, in viscous but strong hydrogen‐bond‐donating solvents, such as ethylene glycol and glycerol, crossing between the potential energy surfaces for the ground electronic state and the hydrogen‐bonded complex formed between the S₁(TICT) state and the solvent is possibly avoided and the hydrogen‐bonded complex is weakly emissive.