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Conformational Control of Ultrafast Molecular Rotor Property: Tuning Viscosity Sensing Efficiency by Twist Angle Variation Rajib Ghosh, Archana Kushwaha, and Dipanwita Das J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.7b05947 • Publication Date (Web): 28 Aug 2017 Downloaded from http://pubs.acs.org on August 29, 2017
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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, *E-mail:
[email protected] ‡
Department of Chemistry, Institute of Chemical Technology, Nathalal Parekh Marg, Matunga,
Mumbai 400019, India,
Abstract Fluorescent molecular rotors find widespread application in sensing and imaging of microscopic viscosity in complex chemical and biological media. Development of viscosity sensitive ultrafast molecular rotor (UMR) relies upon the understanding of the excited state dynamics and their implications to viscosity dependent fluorescence signaling. Unraveling structure-property relationship of UMR behavior is of significance towards development of ultrasensitive fluorescence microviscosity sensor. Herein we show that the ground state equilibrium conformation has 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 towards development of fluorescence sensors based on UMR principle. Conformation-controlled UMR efficiency has been shown to exhibit commensurate fluorescence enhancement upon DNA binding.
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1. Introduction: Viscosity is an important parameter which governs solution phase 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 efficient viscosity sensor based on 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 dependence fluorescence signaling. Ultrafast torsion around a single or double bond introduces efficient nonradiative deactivation in nonviscous medium resulting 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 recent years, molecular rotor based fluorescence sensors and imaging agents gained widespread interest towards viscosity mapping of chemical systems such as sol-gel transition or elastic modulus of polymer nanocomposites.33,
34
Prof. Theodorakis and coworkers 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 coworkers 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 measurement of heterogeneous chemical systems have been explored using different other molecular rotors.31-45 Cationic molecular rotors such as thioflavin-T and auramine-O are being extensively used as excellent viscosity reporter, not only for bio-molecules 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 2 ACS Paragon Plus Environment
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process depending on surrounding viscosity. To develop an efficient viscosity sensor, it is necessary to underpin 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, bio-macromolecules 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 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 torsional angle between the two rotating segments of the molecular rotor is shown to have significant influence to the molecular rotor efficiency, as the shape of excited state PES change from barrier crossing to barrierless. As a test example, we have also shown that this conformational engineering to molecular rotor efficiency commensurate with the extent of fluorescence enhancement upon DNA binding. We believe the information gained from present study will be of immense help towards development of more efficient ultrafast molecular rotor based ultrasensitive microviscosity senor. R2 CH 3
N N
+ N
Φ
CH 3
R1
R 1 = R 2 =H : DABI-1 R 1 = H, R 2 =CH 3 : DABI-2 DABI-3 R 1 = R 2 =CH 3 : Scheme 1: Molecular structure of the three molecules. The dihedral angle (Φ) between dimethylaniline (DA) and benzimidazolium (BI) subunits is tuned by the nature of substituent R1 and R2. 3 ACS Paragon Plus Environment
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2. Material and Methods: 4-(1-H-benzo[d]imidazol-2-yl)-N,N-dimethylaniline was synthesized by condensation of 4-N, Ndimethylaminobenzaldehyde
and
o-phenylenediamine.
4-(1H-benzo[d]imidazol-2-yl)-N,N-
dimethylaniline was di-methylated 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-1H-benzo[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).54 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. Steady state absorption and emission spectra were recorded in a JASCO spectrophotometer (Model: V670) and Horiba-Jobin-Yvon Spectrofluorimeter (model: Fluorolog 3), respectively. Fluorescence quantum yields were measured using quinine sulfate (
