Solvent sensitive intramolecular charge transfer dynamics in the excited states of 4-N,N- dimethylamino-4 0 -nitrobiphenyl (original) (raw)
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Ultrafast Excited State Twisting Dynamics of Molecular Systems in Condensed Phase
This thesis reports the nature of potential energy surface (PES) along the geometrical coordinate(s) responsible for the excited state relaxation of molecular systems in condensed phase, using femtosecond laser spectroscopy and theoretical calculations. The excited state relaxation of several molecular systems involves torsional motion of molecular fragments, and such twisting dynamics acts as the major non-radiative decay channel. These torsional motions cause depletion of the excited state population following a certain specific path defined by the PES. The main objective was to understand the mechanistic pathways of electronic relaxation of the photo-excited molecules, and to reveal the non-radiative channels rendering the molecules highly non-fluorescent. An emphasis was given to understand the influence of molecular structure in promoting a particular coordinate as a non-radiative relaxation channel. In this regard, the effect of twisting dynamics of various molecular fragments was exclusively established for (S)-(−)-1-(4-Nitrophenyl)-2-pyrrolidinemethanol and trans-4-dimethylamino-4-nitrostilbene. Ultrafast twisting dynamics of auramine-O in compliance with temperature dependent measurements was studied to reveal the presence of an activation barrier in an otherwise thoroughly accepted barrierless excited state relaxation model of auramine-O. Further, the knowledge of excited state relaxation mechanism of model chromophore analogs was implemented to explain the highly fluorescent nature of naturally occurring Green Fluorescent Protein. The ultrafast excited state relaxation behavior of a triphenylmethane dyes was contemplated and subsequently used to estimate the viscosity of water in a nano-confined region.
Ultrafast Solvent-Assisted Electronic Level Crossing in 1-Naphthol
Angewandte Chemie International Edition, 2013
Nonadiabatic electronic transitions between potential energy surfaces play a dominant role in many photoinduced chemical reactions, such as charge transfer, [1a-c] branching pathways in dissociation, and cis/trans isomerizations. Furthermore, nonadiabatic internal conversion (IC) to the ground state (S 0 ) is believed to protect photoexcited natural chromophores (e.g. nucleic acid bases of DNA. ) against UV radiation damage. The theoretical description of nonadiabatic level crossing (LC) dynamics is a highly active field of research for both gas phase and condensed phase molecular systems. Importantly, in the condensed phase the solvent often provides more than just a heat bath for the reactive system, as it can strongly affect the outcome of different reaction products. Herein, we report the experimental observation of LC occurring in 60 fs between the two lowest electronic excited singlet states ( 1 L a and 1 L b ) of 1-naphthol (1N).
The Journal of Physical Chemistry A, 2005
Femtosecond fluorescence upconversion and transient absorption experiments have been performed to monitor the photoinduced electronic, geometry, and solvent relaxation dynamics of 1,8-bis(dimethylamino)naphthalene dissolved in methylcyclohexane or n-hexane, n-dodecane, dichloromethane, and acetonitrile. The data have been analyzed by using a sequential global analysis method that gives rise to species associated difference spectra. The spectral features in these spectra and their dynamic behavior enable us to associate them with specific processes occurring in the molecule. The experiments show that the internal charge-transfer lπ* state is populated after internal conversion from the 1 L a state. In the lπ* state the molecule is concluded to be subject to a large-amplitude motion, thereby confirming our previous predictions that internal charge transfer in this state is accompanied by the formation of a two-center three-electron bond between the two nitrogen atoms. Solvent relaxation and vibrational cooling in the lπ* state cannot be separated in polar solvents, but in apolar solvents a distinct vibrational cooling process in the lπ* state is discerned. The spectral and dynamic characteristics of the final species created in the experiments are shown to correspond well with what has been determined before for the relaxed emissive lπ* state.
Ultrafast Intramolecular Electronic Energy-Transfer Dynamics In a Bichromophoric Molecule
J. Phys. Chem. …, 2004
Intramolecular electronic energy-transfer (intra-EET) dynamics has been investigated in 2-(9-anthryl)-1Himidazo [4,5-f] [1,10]-phenanthroline (AIP), a newly synthesized bichromophoric molecule, using the steadystate and time-resolved absorption and fluorescence spectroscopic techniques. In AIP, anthracene (AN) and 1H-imidazo [4,5-f] [1,10]-phenanthroline (IP) molecules are directly linked to each other through a CC σ bond and without any intervening molecular bridge. Two constituent chromophoric moieties of this bichromophoric molecule interact relatively weakly in the ground state. In the excited singlet state, however, the AN moiety transfers its excitation energy quantitatively (the efficiency of energy transfer, φ EET , is near unity) and rapidly (the rate of energy transfer, k EET , is 1.8 × 10 11 s-1 in methanol) to the unexcited IP moiety. k EET decreases linearly with increase in viscosity of the solvents, and the process is significantly retarded in rigid glass matrixes. These observations suggest that, for an efficient EET process, the molecule needs to attain a conformational geometry, which is different from that of the ground state, by undergoing a conformational relaxation process following photoexcitation. The theoretically calculated energy-transfer rate (5.1 × 10 9 s-1) due to the Förster dipole-dipole-induced resonance-interaction mechanism is about 2 orders of magnitude smaller than the experimentally determined energy-transfer rate. Hence, the Dexter throughspace exchange-interaction mechanism, which becomes predominant at shorter interchromophoric separation (R ∼ 6.3 Å in AIP) and requires specific conformation for efficient orbital overlap, should have the major contribution to the intra-EET process in AIP. Viscosity dependence of k EET suggests that we possibly measure the rate of the conformational relaxation process using the intra-EET process as the probe.
A B S T R A C T Solvent polarity dependent relaxation pathways of the excited state of 4-dimethylamino-b-nitrostyrene (DANS) have been investigated using steady state and transient absorption spectroscopic techniques in femtosecond and nanosecond timescale. The molecule undergoes ultrafast twisted intramolecular charge transfer (TICT) relaxation involving the rotation of dimethylaniline group in polar solvents. In the medium and low polarity solvents, TICT relaxation rate is retarded due to larger barrier of twisting along TICT coordinate. In nonpolar solvents, the S 1 state population undergo efficient intersystem crossing (ISC) to triplet surface without any conformational change. However, trans ! cis isomerisation in nonpolar solvents takes place from the excited triplet manifold, as monitored by steady state photo-irradiation experiments. The ISC efficiency decreases with the increase in solvent polarity, which is explained by invoking solvent polarity dependent stabilization of charge transfer S 1 state relative to a receiver triplet state. Comparison of the excited state dynamics of DANS with less conjugated para-dialkylamino nitrobenzene derivatives, reported previously, suggests that additional p-conjugation between the donor and acceptor groups stabilizes planar ICT state resulting to slower TICT relaxation.
Ultrafast Dynamics of the Excited States of 1-(p-Nitrophenyl)-2-(hydroxymethyl)pyrrolidine
The Journal of Physical Chemistry A, 2012
Ultrafast Dynamics of the Excited States of 1-(p-Nitrophenyl)-2-hydroxymethylpyrrolidine 6.1. Introduction Understanding the structure and dynamics of the excited states of numerous donor (D)acceptor (A) substituted aromatic molecules, D-Ar-A (Ar is the aromatic spacer moiety connecting D and A) or simply D/A, has been the subject of extensive theoretical and experimental investigations. 57,58 The simplest and the most well studied molecule of this type is dimethylaminobenzonitrile (DMABN), which shows dual fluorescence because of the emissive properties of both the local excited (LE) and intramolecular charge transfer (ICT) states. Since the first report of this phenomenon in 1961, 211 dual fluorescence has been observed with a large number of D/A molecules in solutions, in crystals as well as in the gas phase. 58, 212-229 It is now well-established that this process is an intramolecular phenomenon and that the LE and ICT states are structural conformers. ICT state is characterized by twisted intramolecular charge transfer (TICT) state in which dimethylamino group is considered to be twisted to a configuration perpendicular to the plane of the phenyl ring. Variation of donor and acceptor strength in D-Ar-A molecules results to extend of charge transfer properties and dynamics of the excited states. Among a large number of disubstituted benzenes as the donoracceptor molecules, p-nitroaniline (PNA) has served as the simplest but the most important model molecule for both theoretical as well as experimental investigations of the ICT reactions
The Influence of Push–Pull States on the Ultrafast Intersystem Crossing in Nitroaromatics
The Journal of Physical Chemistry B, 2013
The photochemistry of nitro-substituted polyaromatic compounds is generally determined by the rapid decay of its S 1 state and the rapid population of its triplet manifold. Previous studies have shown that such an efficient channel is due to a strong coupling of the fluorescent state with specific upper receiver states in the triplet manifold. Here we examine variations in this mechanism through the comparison of the photophysics of 2-nitrofluorene with that of 2-diethylamino-7-nitrofluorene. The only difference between these two molecules is the presence of a diethylamino group in a push−pull configuration for the latter compound. The femtosecond-resolved experiments presented herein indicate that 2-nitrofluorene shows ultrafast intersystem crossing which depopulates the S 1 emissive state within less than a picosecond. On the other hand, the amino substituted nitrofluorene shows a marked shift in its S 1 energy redounding in the loss of coupling with the receiver triplet state, and therefore a much longer lifetime of 100 ps in cyclohexane. In polar solvents, the diethylamino substituted compound actually shows double peaked fluorescence due to the formation of charge transfer states. Evaluation of the Stokes shifts in different solvents indicates that both bands correspond to intramolecular charge transfer states in equilibrium which are formed in an ultrafast time scale from the original locally excited (LE) state. The present study addresses the interplay between electron-donating and nitro substituents, showing that the addition of the electron-donating amino group is able to change the coupling with the triplet states due to a stabilization of the first excited singlet state and the rapid formation of charge transfer states in polar solvents. We include calculations at the TD-DFT level of theory with the PBE0 and B3LYP functionals which nicely predict the observed difference between the two compounds, showing how the specific S(π−π*)−T(n−π*) coupling normally prevalent in nitroaromatics is lost in the push−pull compound.
The Journal of Physical Chemistry A, 1999
Nonradiative excited-state relaxation via charge transfer induced twisting of N,N-dimethylaminobenzylidene-1,3-indandione (DMABI) in solution was investigated by means of picosecond transient absorption and femtosecond fluorescence spectroscopy.The analysis of the experimental data allows us to distinguish between different reaction stages of the molecular twisting in excited and ground states, as well as to consider the molecular dynamics along reactive (twisting) and nonreactive (solvation) coordinates. Quantum chemical calculations of the electronic structure of DMABI and its dependence on the molecular conformation are discussed in order to identify the bond(s) involved in the twisting reaction. It is found that such a twisting bond is the CdC double bond, which is substantially weakened in the excited state.