Article pubs.acs.org/JPCB
Conformational Control of Ultrafast Molecular Rotor Property: Tuning Viscosity Sensing Efficiency by Twist Angle Variation Rajib Ghosh,*,† Archana Kushwaha,‡ and Dipanwita Das‡ †
Radiation and Photochemistry Division, Bhabha Atomic Research Centre, Mumbai 400085, India Department of Chemistry, Institute of Chemical Technology, Nathalal Parekh Marg, Matunga, Mumbai 400019, India
‡
S Supporting Information *
ABSTRACT: Fluorescent molecular rotors find widespread application in sensing and imaging of microscopic viscosity in complex chemical and biological media. Development of viscositysensitive ultrafast molecular rotor (UMR) relies upon the understanding of the excited-state dynamics and their implications for viscosity-dependent fluorescence signaling. Unraveling the structure−property relationship of UMR behavior is of significance toward development of an ultrasensitive fluorescence microviscosity sensor. Herein we show that the ground-state equilibrium conformation has an important role in the ultrafast twisting dynamics of UMRs and consequent viscosity sensing efficiency. Synthesis, photophysics, and ultrafast spectroscopic experiments in conjunction with quantum chemical calculation of a series of UMRs based on dimethylaniline donor and benzimidazolium acceptor with predefined ground-state torsion angle led us to unravel that the ultrafast torsional dynamics around the bond connecting donor and acceptor groups profoundly influences the molecular rotor efficiency. This is the first experimental demonstration of conformational control of small-molecule-based UMR efficiencies which can have wider implication toward development of fluorescence sensors based on the UMR principle. Conformationcontrolled UMR efficiency has been shown to exhibit commensurate fluorescence enhancement upon DNA binding. recent years, molecular-rotor-based fluorescence sensors and imaging agents gained widespread interest toward viscosity mapping of chemical systems such as sol−gel transition or elastic modulus of polymer nanocomposites.33,34 Prof. Theodorakis and co-workers have developed several molecular rotors based on DCVJ and explored microscopic fluidity of several complex chemical and biological materials.16−22 Employing fluorescence lifetime imaging microscopic (FLIM) technique, Prof. Kuimova and co-workers have extensively used BODIPY and porphyrin-based molecular rotors to map the viscosity of different compartments of cell, aerosol surface, lipids, and so on.23−30 Local viscosity measurements of heterogeneous chemical systems have been explored using different molecular rotors.31−45 Cationic molecular rotors such as thioflavin-T and auramine-O are being extensively used as excellent viscosity reporters, not only for biomolecules but also for complex chemical fluids.46−49 The excellent fluorescence sensitivity of the molecular rotor is due to occurrence of ultrafast nonradiative-torsion-induced emission quenching in nonviscous medium and different degree of inhibition of the nonradiative process depending on surrounding viscosity. To develop an efficient viscosity sensor, it is necessary to underpin
1. INTRODUCTION Viscosity is an important parameter which governs solutionphase reaction rate of virtually all chemical and biological reactions.1−10 Due to the heterogeneous nature of complex chemical and biological systems, the diffusion-controlled bimolecular reactions are mainly governed by the viscosity at microscopic and nanoscopic length scale.11−15 Design and development of an efficient viscosity sensor based on a fluoroscent molecular rotor is key to map the viscosity of the nanoscopic compartments of chemical and biological environments.16−34 Ultrafast molecular rotor (UMR) is a class of molecules which provides information about rigidity and microscopic viscosity by virtue of their viscosity-dependent fluorescence signaling. Ultrafast torsion around a single or double bond introduces efficient nonradiative deactivation in a nonviscous medium resulting in weak emission. Depending on the rigidity offered by the surrounding environment, nonradiative torsional motion gets proportionately restricted, rendering fluorescence intensity enhancement and lifetime lengthening. Thus, mapping fluorescence intensity and lifetime of a molecular rotor provides direct information about the rigidity of heterogeneous chemical, biological, and biomimetic environments. Excellent sensitivity along with the ease of fluorescence measurement technique has made UMR-based sensing modality an attractive tool for microrigidity mapping of chemical and biological systems. In © 2017 American Chemical Society
Received: June 17, 2017 Revised: August 27, 2017 Published: August 28, 2017 8786
DOI: 10.1021/acs.jpcb.7b05947 J. Phys. Chem. B 2017, 121, 8786−8794
Article
The Journal of Physical Chemistry B
Scheme 1. Molecular Structure of the Three Moleculesa
the influencing factors which govern torsional speed of a molecular rotor. The ability to control the nonradiative torsional relaxation rate in a molecular system in a desired manner is the key to optimize the sensor efficiency. A priori assessment of the factors governing the rate of torsional motion is a prerequisite, and tailoring the molecular rotor property is critical to develop efficient fluorescence sensor and imaging agents for complex chemical fluids, biomacromolecules, and cells.50−55 A molecular level understanding of how the torsional relaxation depends on the ground-state conformation and finding a structure− function relationship of excited-state torsional relaxation is desirable to develop better sensors based on the UMR principle. Herein we reveal that the ground-state conformation of molecular rotors needs to be optimized to place the molecule in the excited potential energy surface (PES) so that a barrierless torsional motion sets in. We show that the molecular rotor property can systematically be tuned by the conformation control of the ground-state geometry, and the torsional angle between the two rotating segments of the molecular rotor is shown to have a significant influence on the molecular rotor efficiency, as the shape of excited-state PES changes from barrier crossing to barrierless. As a test example, we have also shown that this conformational engineering to molecular rotor efficiency is commensurate with the extent of fluorescence enhancement upon DNA binding. We believe the information gained from the present study will be of immense help toward development of more efficient ultrafast-molecular-rotor-based ultrasensitive microviscosity senor.
a The dihedral angle (Φ) between dimethylaniline (DA) and benzimidazolium (BI) subunits is tuned by the nature of substituent R1 and R2.
measured in a pump−probe spectrometer (ExciPro from CDP corporation, Russia) coupled with a femtosecond amplified laser (Amplitude Technologies, France). The details of the experimental setup can be found elsewhere.58 Samples were excited by 100 fs laser pulse at 390 nm and probed in the visible region at the magic angle polarization of pump and probe pulse. The transient absorption spectra were corrected for group velocity dispersion. Transient kinetics at selected wavelengths were fitted with multiexponential function convoluted with the instrument response function. The instrument response function of our fluorescence up-conversion and transient absorption spectrometer is about 200 fs. Density functional theory (DFT) and time-dependent DFT (TDDFT) calculations were performed using GAMESS software packages.59 Structural optimizations were performed employing B3LYP functional and 6-G311(d,p) basis set.60,61 Solvent effect was incorporated by polarizable continuum model (PCM) as implemented in GAMESS.62,63
2. MATERIAL AND METHODS 4-(1-H-Benzo[d]imidazol-2-yl)-N,N-dimethylaniline was synthesized by condensation of 4-N,N-dimethylaminobenzaldehyde and o-phenylenediamine. 4-(1H-benzo[d]imidazol-2-yl)N,N-dimethylaniline was dimethylated by iodomethane to yield DABI-3. DABI-1 was prepared by protonation of benzimidazole nitrogen of 4-(1H-benzo[d]imidazol-2-yl)-N,N-dimethylaniline with dilute perchloric acid. N,N-Dimethyl-4-(1-methyl-1Hbenzo[d]imidazol-2-yl)aniline was prepared by condensation of 4-N,N-dimethylaminobenzaldehyde and N-methyl-O-phenylenediamine. DABI-2 was prepared by protonation of monomethyl derivative. Details of synthesis and characterization are given in the Supporting Information (Scheme S1 and Figure S1). In aqueous solution, colorimetric pH titration provides the pKa in the range of 5.2−5.5, confirming protonation of benzimidazole nitrogen (Figure S2, Supporting Information).56 All solvents were of spectroscopic grade (Spectrochem India) and used as received. Milli-Q water (Resistance >18 MΩ) was used for experiments in aqueous medium. Calf thymus DNA was obtained from Aldrich. All experiments were performed at room temperature (24 °C) unless otherwise specified (Scheme 1) Steady-state absorption and emission spectra were recorded in a JASCO spectrophotometer (Model: V670) and HoribaJobin-Yvon Spectrofluorimeter (model: Fluorolog 3), respectively. Fluorescence quantum yields were measured using quinine sulfate (
