Prakriti Ranjan Bangal

Excited state photodynamics of 4-N,N-dimethylamino cinnamaldehyde: A solvent dependent competition of TICT and intermolecular hydrogen bonding

Abstract This paper describes the dual fluorescence of 4-N,N-dimethylamino cinnamaldehyde (DMACA) in various solvents. Based on major findings the first fluorescence band has been assigned to be arising out of delocalized excited state (DE) and the anomalous fluorescence band in polar aprotic solvents has been assigned to twisted intramolecular charge transfer (TICT) state whereas, in protic solvent it is arising out due to hydrogen bonding interaction between hydrogen donor part of the solvent and carbonyl group of the probe molecule. This hydrogen bonding being an efficient fluorescence quenching channel trammels the TICT formation and thus an apparent competition exists between TICT and hydrogen bonding formation in protic solvent. The calculated absorption spectrum generated from CNDO calculations follows nicely the experimental absorption band. The AM1 calculations corroborate the large dipole moment change and the charge transfer in orthogonal position as found from experimental observations.

Modulation of photophysics due to orientational selectivity of 4-N,N-dimethylamino cinnamaldehyde β-cyclodextrin inclusion complex in different solvents

Abstract This paper delineates some results on the twisted intramolecular charge transfer (TICT) dynamics of 4- N , N -dimethylamino cinnamaldehyde (DMACA) encapsulated in β-cyclodextrin in aqueous and non-aqueous solvents at room temperature. 4- N , N -Dimethylamino cinnamaldehyde is found to form a 1:1 inclusion complex with β-CD in both aqueous and non-aqueous solvents with a binding constant higher in aqueous solvent. The most important feature of the inclusion complexes of 4- N , N -dimethylamino cinnamaldehyde, as revealed from photophysics of normal and twisted intramolecular charge transfer bands, is that there is a preferential orientation of 4- N , N -dimethylamino cinnamaldehyde inside β-CD cavity in aqueous solvent with dimethyl group sticking outside and in non-aqueous solvents it is just the opposite.

Photophysics of 4-dimethylamino cinnamic acid in different environments

Abstract 4-dimethylamino cinnamic acid (DMACA) has been studied by steady state and time-resolved fluorescence spectroscopy in different environments in order to get the information about its photophysics and photochemistry. Dual fluorescence in polar medium has been assigned to be arising out of delocalized excited state (DE) and twisted intramolecular charge transfer (TICT) state from different experimental and theoretical considerations. The computed excited state dipole moments in different twisted geometries evince that a twist of 90° of N(CH3)2 produces minimum energy state and maximum dipole moment change. The experimentally measured value (9.23 D) for dipole moment change from ground to excited state tallies with the corresponding value computed theoretically (7.5 D) for twisted geometry. Polarization studies in polar, nonpolar solvents and in β-CD cavity suggest that the emission transition moment is perpendicular to the molecular axis confirming the presence of twisted conformer. Weak acid protonates the carbonyl group while strong acid favors protonation of amino nitrogen. The solvent dependent absorption and emission spectra were found to be due to intramolecular structure change rather than ‘solvent coordinate’ effect. TICT band intensity is enhanced in β-CD inclusion complex due to modulation of photophysics by it while the normal (DE) band failed to appear. The fluorescence dependence of DMACA inclusion complex on polarity and viscosity has been discussed in detail. Quantum chemical calculations with AM1 Hamiltonian have been performed to find the actual geometry producing twisted intramolecular charge transfer state by twisting N(CH3)2 group to different angles and to confirm experimental findings qualitatively.

Vibronic interaction and photophysics of chalcone derivatives

Abstract A comprehensive photophysical study of 1-(4-methoxy-1-phenyl)-3-(1-naphthyl) prop-2-en-1-one (MPNPO) and naptho (1, 2b) furan (NF) in different solvents at room temperature and at 77 K in ethanolic and methylcyclohexane glass shows a strong phosphorescence emission in NF. The phosphorescence emission and excitation spectra of NF along with their polarizations and phosphorescence lifetimes suggest that the perturbation of the zero point level of the emitting state ( 3 π, π∗) by a nearby ( 3 n,π∗) state leads to a number of spectral features, which are not expected from ( 3 π, π∗) phosphorescence. It is concluded that the strong solvent dependent triplet quantum yield and lifetime of NF at 77 K is due to the dependence of radiationless transitions on vibronic spin-orbit coupling between ( 1 n, π∗ ) and ( 3 π, π∗) states.

Excited state dynamics of 4(1H-pyrrole 1-yl) benzoic acid and different environmental effects

Abstract The absorption and fluorescence characteristics of 4(1 H -pyrrole 1-yl) benzoic acid (PBA) in solvents of different polarities, acidities, glass matrices and also in β-cyclodextrin cavity were studied with regard to the influence of flexibility of the molecule in both the ground and excited sates. The study of dynamics of excited state reveals the presence of two competitive fluorescence emission from a common excited state of PBA. Of these two fluorescence bands one originates from delocalized excited (DE) state [i.e., Franck-Condon (F-C) excited state] and the other from twisted intramolecular charge transfer (TICT) state. The excitation of ground state of PBA leads to the immediate formation of F-C excited state and the TICT state is formed within 220 ps from the parent F-C excited state. The 220 ps time has also been interpreted as the time required for the molecule to relax by rotational motion along the C-N bond of PBA to achieve the TICT state. The steady state photophysical measurements show that the molecule is very weakly fluorescent and nonradiative rate parameters get prominence compared to radiative rate parameters. The fluorescence and phosphorescence studies at 77 K have also been reported. Quantum chemical calculations of the energies and dipole moments were performed for planar and different twisted conformations to confirm experimental findings qualitatively.

Excited state dynamics of 4-Pyrrolidino pyridine in different solvents

Abstract Photophysical properties of 4-Pyrrolidino Pyridine (PP) have been delineated from the absorption, steady state and time-resolved spectroscopy in different solvents. Preferential hydrogen bonding of PP with water and also protonation with acid at intercyclic (pyrrolidino) nitrogen lone pair site could be observed. The intramolecular exciplex as predicted from semi-empirical calculations controls the photophysics in weakly polar and nonpolar solvents. In water, a ground-state pretwisted pyrrolidino group forms hydrogen-bonded complex that dominates over the intramolecular exciplex and this H-bonded exciplex acts as a quenching channel for the dual fluorescence. Intermolecular exciplexes of PP were found to form in binary and ternary solutions with polar solvent containing N-atom. Fluorescence in β-cyclodextrin cavity resembles intramolecular exciplex in ethanol with high quantum yield.

Role of hydrogen bonding in the photophysical properties of isomeric tetrapyridylporphyrins in aprotic solvent.

Extensive photophysical properties of isomeric tetra-2-pyridylporphyrin (TpyP(2)), tetra-3-pyridylporphyrin (TpyP(3)), and tetra-4-pyridylporphyrin (TpyP(4)) have been studied in the presence of a series of phenols of increasing hydrogen bonding power in dichloromethane solution by employing UV/vis spectroscopy; steady-state, time-resolved fluorescence spectroscopy; and transient absorption spectroscopic techniques. The change of absorption spectra of all three porphyrins as a function of different phenol concentrations established the preference of hydrogen bonded complex formation to the peripheral pyridyl nitrogen rather than the pyrrole nitrogen of the porphyrin macrocycle. The fluorescence behaviors of the porphyrins which were observed upon addition of different phenols point to a marked dependence on the nature of the added phenols. Phenols with an electron withdrawing group do not quench the fluorescence of porphyrins, whereas phenols with an electron donating group quench the singlet porphyrin both in static and dynamic pathways. A remarkable difference in quenching behaviors of singlet excited porphyrin by 4-methylphenol (4-MePhOH) and 4-MeOPhOH/4-EtOPhOH (4-EtOPhOH = 4-ethoxyphenol) are observed. The quenching of singlet excited porphyrins by 4-MePhOH is attributed to be purely static in nature, and the H-bond provides a strong nonradiative channel to singlet excited porphyrins. However, the quenching of singlet excited porphyrins by 4-MeOPhOH/4-EtOPhOH is mostly dynamic, and it is ascribed to be the reductive quenching of single excited porphyrins. Picosecond transient absorption study with TpyP(2) and 4-MeOPhOH provides the evidence of porphyrin radical anion and phenol radical cation of equal lifetime, which indicates the fact that electron transfer occurs from phenol to singlet excited porphyrin. The temperature effect on dynamic quenching by 4-MeOPhOH/4-EtOPhOH and kinetic deuterium isotope effect established the reaction to be a photoinduced concerted proton coupled electron transfer.

Ultrafast Time-Resolved Emission and Absorption Spectra of meso-Pyridyl Porphyrins upon Soret Band Excitation Studied by Fluorescence Up-Conversion and Transient Absorption Spectroscopy

A comprehensive study of ultrafast molecular relaxation processes of isomeric meso-(pyridyl) porphyrins (TpyPs) has been carried out by using femtosecond time-resolved emission and absorption spectroscopic techniques upon pumping at 400 nm, Soret band (B band or S2), in 4:1 dichloromethane (DCM) and tetrahydrofuran (THF) solvent mixture. By combined studies of fluorescence up-conversion, time-correlated single photon counting, and transient absorption spectroscopic techniques, a complete model with different microscopic rate constants associated with elementary processes involved in electronic manifolds has been reported. Besides, a distinct coherent nuclear wave packet motion in Qy state is observed at low-frequency mode, ca. 26 cm(-1) region. Fluorescence up-conversion studies constitute ultrafast time-resolved emission spectra (TRES) over the whole emission range (430-710 nm) starting from S2 state to Qx state via Qy state. Careful analysis of time profiles of up-converted signals at different emission wavelengths helps to reveal detail molecular dynamics. The observed lifetimes are as indicated: A very fast decay component with 80 ± 20 fs observed at ∼435 nm is assigned to the lifetime of S2 (B) state, whereas being a rise component in the region of between 550 and 710 nm emission wavelength pertaining to Qy and Qx states, it is attributed to very fast internal conversion (IC) occurring from B → Qy and B → Qx as well. Two distinct components of Qy emission decay with ∼200-300 fs and ∼1-1.5 ps time constants are due to intramolecular vibrational redistribution (IVR) induced by solute-solvent inelastic collisions and vibrational redistribution induced by solute-solvent elastic collision, respectively. The weighted average of these two decay components is assigned as the characteristic lifetime of Qy, and it ranges between 0.3 and 0.5 ps. An additional ∼20 ± 2 ps rise component is observed in Qx emission, and it is assigned to the formation time of thermally equilibrated Qx state by vibrational cooling/relaxations of excess energy within solvent. This relaxed Qx state decays to ground as well as triplet state by 7-8 ns time scale. The femtosecond transient absorption studies of TpyPs in three different excitations at S2 (400 nm), Qy (515 nm), and Qx (590 nm) along with extensive global and target model analysis of TA data exclusively generate the true spectra of each excited species/state with their respective lifetimes along with microscopic rate constants associated with each state. The following five exponential components with lifetime values of 65-70 fs, ∼0.3-0.5 ps, ∼20 ± 2 ps, ∼7 ± 1 ns, and 1-2 μs are observed which are associated with S2, Qy, hot Qx, thermally relaxed Qx, and lowest triplet (T1) states, respectively, when excited at S2, and four (Qy, hot Qx, thermally relaxed Qx, and lowest triplet (T1) states) and three (hot Qx, thermally relaxed Qx, and lowest triplet (T1) states) states are obtained when excited at 515 nm (Qy) and 590 nm (Qx), respectively, as expected. The TA results parallel the fluorescence up-conversion studies, and both the results not only compliment each other but also unveil the ultrafast internal conversion from S2 to Qy, S2 to Qx, and Qy to Qx for all three isomers in a similar fashion with nearly equal characteristic decay times.

Vibronic interaction and triplet state photophysics of phenylisatin and oxindole

Abstract Photophysical study of phenylisatin and oxindole triplet states have been made at room temperature and in different glasses at 77K. Qualitatively, in all respects the compounds have identical spectroscopic characteristics. Phosphorescence emission, excitation along with their polarization and lifetime suggest that a perturbation of the zero-point level of emitting state ( 3 ππ*) by a close-lying triplet state ( 3 nπ*) leads to a number of new spectral features. The experimental observations have been interpreted satisfactorily in terms of a switch ( 3 ππ* state to 3 nπ*) in the character of the lowest triplet states (T 1 and T 2 ) and also a similar switch in the character of the excited singlet states S 1 and S 2 for a change of glass matrix from MCH to ethanol. Invoking of first order and second order spin-orbit coupling explains the phosphorescence emission unambiguously.

INTERMOLECULAR INTERACTIONS OF 4-PYRROLIDINO PYRIDINE: A SIMULATION STUDY

Abstract Simulations of intermolecular interaction by the AM1 method have been performed. The hydrogen bonding complex between and 4-pyrrolidino pyridine (PP) and water molecules with 1:n complexes have been considered to investigate possible stable complex configurations and to calculate the stable interaction energy. These calculations confirm the influence of water molecules on twisting of the pyrrolidino group in ground state. The excited state simulation predicts that the energy minimized geometry of the PP molecule takes an almost sandwich like structure, confirming the formation of an intramolecular exciplex in the gas phase as well as in nonpolar or in weakly polar solvents in line with the experimental findings.