Shahnawaz Rafiq

Excited State Relaxation Dynamics of Model Green Fluorescent Protein Chromophore Analogs: Evidence for Cis–Trans Isomerism

Two green fluorescent protein (GFP) chromophore analogs (4Z)-4-(N,N-dimethylaminobenzylidene)-1-methyl-2-phenyl-1,4-dihydro-5H-imidazolin-5-one (DMPI) and (4Z)-4-(N,N-diphenylaminobenzylidene)-1-methyl-2-phenyl-1,4-dihydro-5H-imidazolin-5-one (DPMPI) were investigated using femtosecond fluorescence up-conversion spectroscopy and quantum chemical calculations with the results being substantiated by HPLC and NMR measurements. The femtosecond fluorescence transients are found to be biexponential in nature and the time constants exhibit a significant dependence on solvent viscosity and polarity. A multicoordinate relaxation mechanism is proposed for the excited state relaxation behavior of the model GFP analogs. The first time component (τ(1)) was assigned to the formation of twisted intramolecular charge transfer (TICT) state along the rotational coordinate of N-substituted amine group. Time resolved intensity normalized and area normalized emission spectra (TRES and TRANES) were constructed to authenticate the occurrence of TICT state in subpicosecond time scale. Another picosecond time component (τ(2)) was attributed to internal conversion via large amplitude motion along the exomethylenic double bond which has been enunciated by quantum chemical calculations. Quantum chemical calculation also forbids the involvement of hula-twist because of high activation barrier of twisting. HPLC profiles and proton-NMR measurements of the irradiated analogs confirm the presence of Z and E isomers, whose possibility of formation can be accomplished only by the rotation along the exomethylenic double bond. The present observations can be extended to p-HBDI in order to understand the role of protein scaffold in reducing the nonradiative pathways, leading to highly luminescent nature of GFP.

Microviscosity inside a nanocavity: a femtosecond fluorescence up-conversion study of malachite green.

Femtosecond fluorescence up-conversion measurements of malachite green (MG) have been carried out to confirm the relaxation pathway and subsequently to probe the microviscosity of water trapped in a nanoconfined environment using an AOT (sodium dioctylsulfosuccinate, aerosol-OT) reverse micelle as a model system. The experimental results reveal a strong dependence of S(1) state relaxation dynamics of MG on solvent viscosity while a very weak dependence has been observed for the S(2) state relaxation. The time-dependent density functional theory (TD-DFT) calculations have been used to construct potential energy surfaces of MG by pursuing an intramolecular rotation along the torsional coordinate of the phenyl rings. On synchronization with the experimental observations, the computational results comprehend the existence of a conical intersection along the S(1) and S(0) potential energy surfaces, which leads to mixed vibrational levels of S(1) and S(0) characteristics. The results suggest that the conical intersection is along the torsional coordinate of N,N-dimethyl substituted phenyl ring. Correlating the observed dynamics of MG in a confined system with the relaxation time of MG in different glycerol-water mixtures, we assert the determination of the microviscosity of water inside the AOT reverse micelle. The data confer that the microviscosity of water in an AOT water pool of w(0) = 2 (9 cP) is almost 9 times higher than the bulk water. As we increase the w(0) from 2 to 40, the microviscosity decreases monotonically to 5.68 cP, and the decrease is observed to be exponential in nature.

Femtosecond excited-state dynamics of 4-nitrophenyl pyrrolidinemethanol: evidence of twisted intramolecular charge transfer and intersystem crossing involving the nitro group.

Ultrafast excited-state relaxation dynamics of a nonlinear optical (NLO) dye, (S)-(-)-1-(4-nitrophenyl)-2-pyrrolidinemethanol (NPP), was carried out under the regime of femtosecond fluorescence up-conversion measurements in augmentation with quantum chemical calculations. The primary concern was to trace the relaxation pathways which guide the depletion of the first singlet excited state upon photoexcitation, in such a way that it is virtually nonfluorescent. Ground- and excited-state (singlet and triplet) potential energy surfaces were calculated as a function of the -NO(2) torsional coordinate, which revealed the perpendicular orientation of -NO(2) in the excited state relative to the planar ground-state conformation. The fluorescence transients in the femtosecond regime show biexponential decay behavior. The first time component of a few hundred femtoseconds was ascribed to the ultrafast twisted intramolecular charge transfer (TICT). The occurrence of charge transfer (CT) is substantiated by the large dipole moment change during excitation. The construction of intensity- and area-normalized time-resolved emission spectra (TRES and TRANES) of NPP in acetonitrile exhibited a two-state emission on behalf of decay of the locally excited (LE) state and rise of the CT state with a Stokes shift of 2000 cm(-1) over a time scale of 1 ps. The second time component of a few picoseconds is attributed to the intersystem crossing (isc). In highly polar solvents both the processes occur on a much faster time scale compared to that in nonpolar solvents, credited to the differential stability of energy states in different polarity solvents. The shape of frontier molecular orbitals in the excited state dictates the shift of electron density from the phenyl ring to the -NO(2) group and is attributed to the charge-transfer process taking place in the molecule. The viscosity dependence of relaxation dynamics augments the proposition of considering the -NO(2) group torsional motion as the main excited-state relaxation coordinate.

Origin of strong synergism in weakly perturbed binary solvent system: a case study of primary alcohols and chlorinated methanes.

A strong synergistic solvation was observed for the mixtures of hydrogen bond donating and accepting solvent pairs. The nature of the interactions between two solvent pairs was investigated with different dye molecules viz. coumarin 480, coumarin 153, 4-aminophthalimide, and p-nitroaniline. Coumarin 480 in differenet alcohols-CHCl(3) (alcohols: MeOH, EtOH, BuOH) binary mixture shows a strong synergism, which is explained in the backdrop of solvent-solvent interactions. Fluorescence quenching of C480 by 1,2-phenylenediamine in the binary solvent mixture exhibited the maximum deviation in quenching constant corresponding to ~0.45 mol fraction of MeOH in MeOH-CHCl(3) binary mixture and hence suggested the maximum extent of hydrogen-bonding interactions prevailing at this proportion of mixture. The solvation behavior of MeOH-CHCl(3) mixture shows strong probe dependence with no synergism observed in p-nitroaniline, which is ascribed to its higher ground state dipole moment (8.8 D) relative to C480 (6.3 D). Interestingly, the strong synergistic signature observed through spectrophotometric measurement of C480 in alcohol-CHCl(3) binary mixture is absent when studied by fluorescence measurement. The higher excited state dipole moment of coumarin 480 (13.1 D) is considered to be the driving force for the absence of synergism in the excited state. In such strongly perturbed systems (due to high dipole moment values) the dominant phenomenon is preferential solvation. Analysis of proton NMR of MeOH-CHCl(3) binary solvent mixture indicates the existence of MeOH-CHCl(3) clusters in the stoichiometric ratio of 1:2.15. Refractive index measurement also infers the existence of hydrogen bonded network structure between MeOH and CHCl(3). A modified Bosch solvent exchange model has been used to determine the feasibility of synergistic behavior and polarity parameter of the mixed solvent structure of MeOH-CHCl(3) binary solvent mixture.

Dynamics of Solvent Response in Methanol–Chloroform Binary Solvent Mixture: A Case of Synergistic Solvation

Steady-state absorption, emission, and femtosecond transient absorption spectroscopies were used to ascertain the static and dynamic nature of the solvent response of methanol-chloroform binary solvent mixtures of different stoichiometric ratios using 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran (DCM) as the probe molecule. The appearance of synergistic solvation behavior in the steady-state absorption measurements can be explained in terms of solvent-solvent interactions through an extended hydrogen-bonding network. The disappearance of such synergistic behavior in the excited state of the DCM dye was recently proposed by us to be due to the weak nature of the intermolecular interactions present in binary solvent mixtures (J. Phys. Chem. B 2012, 116, 1345). It was anticipated and subsequently confirmed by the dynamics of the solvent response that the disruption of the weak interactive solvent network is the main reason for the absence of the synergism in the excited state. As expected, we observed the slowest dynamics for the mixture with X(MeOH) = 0.45, with an average solvation time of 12.03 ps, which is much higher than the values for the pure bulk counterparts (⟨τ(s)⟩(Methanol) = 4.32 ps and ⟨τ(s)⟩(Chloroform) = 1.32 ps). The unprecedented slowing of solvation for DCM is probably due to the rigid interactive methanol-chloroform solvent system in the first solvation shell, followed by solvent rearrangements around the solute dipole. Overall interactions present within the methanol-chloroform binary solvent mixture furnish clear evidence of solvent association through weak hydrogen bonding.

Slow Intramolecular Vibrational Relaxation Leads to Long-Lived Excited-State Wavepackets

Broadband optical pump and compressed white light continuum probe were used to measure the transient excited-state absorption, ground-state bleach, and stimulated emission signals of cresyl violet solution in methanol. Amplitude oscillations caused by wavepacket motion in the ground and excited electronic states were analyzed. It was found that vibrational coherences in the excited state persist for more than the experimental waiting time window of 6 ps, and the strongest mode had a dephasing time constant of 2.4 ps. We hypothesize the dephasing of the wavepacket in the excited state is predominantly caused by intramolecular vibrational relaxation (IVR). Slow IVR indicates weak mode-mode coupling and therefore weak anharmonicity of the potential of this vibration. Thus, the initially prepared vibrational wavepacket in the excited state is not significantly perturbed by nonadiabatic coupling to other electronic states, and hence the diabatic and adiabatic representations of the system are essentially identical within the Born-Oppenheimer approximation. The wavepacket therefore evolves with time in an almost harmonic potential, slowly dephased by IVR and the pure vibrational decoherence. The consistency in the position of node (phase change in the wavepacket) in the excited-state absorption and stimulated emission signals without undergoing any frequency shift until the wavepacket is completely dephased conforms to the absence of any reactive internal conversion.

Spectroscopic evidence of the presence of an activation barrier in the otherwise barrierless excited state potential energy surface of auramine-O: A femtosecond fluorescence up-conversion study

A widely acclaimed model for the excited state relaxation dynamics of auramine-O involves orientational relaxation of dimethylanilino moieties along the barrierless excited state potential energy surface (PES). Such a model would necessitate similar excited state dynamics in media offering similar viscous drag. However, we have noticed an interesting experimental observation showing auramine-O to have ~8 times larger fluorescence quantum yield in chloroform than in methanol, though both the solvents have the same viscosity. The femtosecond fluorescence transients of auramine-O in chloroform surprisingly depict a rise in fluorescence transients, which has not been observed before. This, along with the simultaneous observation of unexpectedly large fluorescence lifetime and multi-exponential transients (in chloroform) questions the thoroughly accepted barrierless model of auramine-O relaxation dynamics, as the barrierless model would demand a short lifetime and single-exponential decay. Temperature dependent quantum yield measurements along with solvent dependent excited state multi-coordinate time-dependent density functional theory calculations further unveil the exact nature of PES. All these results concomitantly conclude that, at-least in chloroform, upon photo-excitation auramine-O must pass over an activation barrier before damping the excited state population into ground state via a sink function through adiabatic coupling of the electronic states.

Ultrafast Photophysics of a Dinitrogen-Bridged Molybdenum Complex

Among the many metal-dinitrogen complexes synthesized, the end-on bridging (μ2, η1, η1-N2) coordination mode is notoriously unreactive for nitrogen fixation. This is principally due to the large activation energy for ground-state nitrogen-element bond formation and motivates exploration of the photoexcited reactivity of this coordination mode. To provide the foundation for this concept, the photophysics of a dinitrogen-bridged molybdenum complex was explored by ultrafast electronic spectroscopies. The complex absorbs light from the UV to near-IR, and the transitions are predominantly of metal-to-ligand charge transfer (MLCT) character. Five excitation wavelengths (440, 520, 610, 730, and 1150 nm) were employed to access MLCT bands, and the dynamics were probed between 430 and 1600 nm. Despite the large energy space occupied by electronic states (ca. 1.2 eV), the dynamics were independent of the excitation wavelength. In the proposed kinetic model, photoexcitation from a Mo-N═N-Mo centered ground state populates the π*-state delocalized over two terpyridine ligands. Due to a large terpyridine-terpyridine spatial separation, electronic localization occurs within 100 fs, augmented by symmetry breaking. The subsequent interplay of internal conversion and intersystem crossing (ISC) populates the lowest 3MLCT state in 2-3 ps. Decay to the ground state occurs either directly or via a thermally activated metal-centered (3MC) trap state having two time constants (10-15 ps, 23-26 ps [298 K]; 103 ps, 612 ps [77 K]). ISC between 1MLCT and 3MLCT involves migration of energized electron density from the terpyridine π* orbitals to the Mo-N═N-Mo core. Implication of the observed dynamics for the potential N-H bond forming reactivity are discussed.

From coherent to vibronic light harvesting in photosynthesis

Photosynthetic organisms are a remarkable example of nanoscale engineering and have mastered the process of solar energy harvesting over billions of years of evolution. Therefore, researchers seek insights from the light collection mechanisms of photosynthetic machinery. The initial energy transfer stage of photosynthesis, which begins with light absorption and leads to charge separation, is remarkably robust in conditions of strong energetic disorder, extreme physiological temperatures, and low light flux - very different from conventional solar conversion materials [1-3]. However, determining the key principles which are responsible for efficient conversion is a challenging task due to the complexity of the photosynthetic systems. The field encountered a fascinating lead in 2007 when oscillatory features were discovered in two-dimensional electronic spectroscopic data - the optical analogue of 2D NMR - and were assigned to quantum coherence between donor and acceptor electronic states [4••]. In this review, we describe the evolution in our understanding of quantum effects in photosynthetic energy transfer. A vibronic model is described to demonstrate the current opinion on how quantum effects can optimize energy transfer.

Nitrogen-Free Bifunctional Bianthryl Leads to Stable White-Light Emission in Bilayer and Multilayer OLED Devices

White organic light-emitting diodes (WOLEDs) are at the center stage of OLED research today because of their advantages in replacing the high energy-consuming lighting technologies in vogue for a long time. New materials that emit white light in simple devices are much sought after. We have developed two novel electroluminescent materials, referred to as BABZF and BATOMe, based on a twisted bianthryl core, which are brilliantly fluorescent, thermally highly stable with high Td and Tg, and exhibit reversible redox property. Although inherently blue emissive, BABZF leads to white-light emission (CIE ≈ 0.28, 0.33) with a moderate power efficiency of 2.24 lm/W and a very high luminance of 15 600 cd/m2 in the fabricated multilayer nondoped OLED device. This device exhibited excellent color stability over a range of applied potential. Remarkably, similar white-light emission was captured even from a double-layer device, attesting to the innate hole-transporting ability of BABZF despite it being non-nitrogenous, that is, lacking any traditional hole-transporting di-/triarylamino group(s). Similar studies with BATOMe led to inferior device performance results, thereby underscoring the importance of dibenzofuryl groups in BABZF. Experimental as well as theoretical studies suggest the possibility of emission from multiple species involving BABZF and its exciplex and electroplex in the devices. The serendipitously observed white-light emission from a double-layer device fabricated with an unconventional hole-transporting material (HTM) opens up new avenues to create new non-nitrogenous HTMs that may lead to more efficient white-light emission in simple double-layer devices.