Planar Intramolecular Charge Transfer Research Articles

Twisted 8-Acyl-1-dialkyl-amino-naphthalenes Emit from a Planar Intramolecular Charge Transfer Excited State

Fluorescence from dialkylamino donor–acyl acceptor substituted 1,8-naphthalene derivatives can occur either from a planar (PICT) or a twisted (TICT) intramolecular charge transfer excited state. The photophysical properties of 8-acetyl-1-(dimethyl-amino)naphthalene (3) and 8-pivaloyl-1-(dimethyl-amino)naphthalene (4) are compared with 1-methyl-2,3-dihydronaphtho[1,8-bc]azepin-4(1H)-one (5). In 3 and 4, both the carbonyl and amino groups are forced to twist out of the plane of the naphthalene ring. In 5, these groups are nearly coplanar with the naphthalene. Neither 3 nor 4 fluoresce as strongly as 5, but all three show similar degrees of solvato-chromism and all are strongly quenched by alcohol solvents. Nitrile 6, 8-cyano-1-(dimethyl-amino)naphthalene, does not show the same degree of solvato-chromism as 3–5, nor is it as affected by alcohols. Calculations corroborate the experimental results, indicating that 3–5 emit from a PICT excited state.

Leveraging Coupled Solvatofluorochromism and Fluorescence Quenching in Nitrophenyl-Containing Thiazolothiazoles for Efficient Organic Vapor Sensing.

Solvatofluorochromic molecules provide strikingly high fluorescent outputs to monitor a wide range of biological, environmental, or materials-related sensing processes. Here, thiazolo[5,4-d]thiazole (TTz) fluorophores equipped with simple alkylamino and nitrophenyl substituents for solid-state, high-performance chemo-responsive sensing applications are reported. Nitroaromatic substituents are known to strongly quench dye fluorescence, however, the TTz core subtly modulates intramolecular charge transfer (ICT) enabling strong, locally excited-state fluorescence in non-polar conditions. In polar media, a planar ICT excited-state shows near complete quenching, enabling a twisted excited-state emission to be observed. These unique fluorescent properties (spectral shifts of 0.13 - 0.87eV and large transition dipole moments Δµ = 20.4 - 21.3 D) are leveraged to develop highly sought-after chemo-responsive, organic vapor optical sensors. The sensors are developed by embedding the TTz fluorophores within a poly(styrene-isoprene-styrene) block copolymer to form fluorescent dye/polymer composites (ΦF = 70 - 97%). The composites respond reversibly to a comprehensive list of organic solvents and show low vapor concentration sensing (e.g., 0.04% solvent saturation vapor pressure of THF - 66ppm). The composite films can distinguish between solvent vapors with near complete fluorescent quenching observed when exposed to their saturated solvent vapor pressures, making this an extremely promising material for optical chemo-responsive sensing.

Tuning the Photophysics of Two-Arm Bis[(dimethylamino)styryl]benzene Derivatives by Heterocyclic Substitution.

The identification of novel molecular systems with high fluorescence and significant non-linear optical (NLO) properties is a hot topic in the continuous search for new emissive probes. Here, the photobehavior of three two-arm bis[(dimethylamino)styryl]benzene derivatives, where the central benzene was replaced by pyridine, furan, or thiophene, was studied by stationary and time-resolved spectroscopic techniques with ns and fs resolution. The three molecules under investigation all showed positive fluorosolvatochromism, due to intramolecular charge-transfer (ICT) dynamics from the electron-donor dimethylamino groups, and significant fluorescence quantum yields, because of the population of a planar and emissive ICT state stabilized by intramolecular hydrogen-bond-like interactions. The NLO properties (hyperpolarizability coefficient and TPA cross-section) were also measured. The obtained results allowed the role of the central heteroaromatic ring to be disclosed. In particular, the introduction of the thiophene ring guarantees high fluorescent quantum yields irrespective of the polarity of the medium, and the largest hyperpolarizability coefficient because of the increased conjugation. An important and structure-dependent involvement of the triplet state was also highlighted, with the intersystem crossing being competitive with fluorescence, especially in the thiophene derivative, where the triplet was found to significantly sensitize molecular oxygen even in polar environment, leading to possible applications in photodynamic therapy.

Aggregation-induced emission and reversible high-contrast mechanofluorochromic behavior of dicyanoethylenes modified by xanthene and carbazole

Two novel donor-acceptor type molecules, XA-CE-CAR and XA-CE-BCAR, were created and synthesized using xanthene, carbazole and dicyanoethylene. The two luminophores possessed effective emissions in the solid-state with the respective fluorescence efficiencies of 0.256 and 0.238 and showed distinct aggregation-induced emissions. Particularly, XA-CE-CAR and XA-CE-BCAR exhibited high-contrast mechanofluorochromic (MFC) characteristics. When the as-prepared powders of the two compounds underwent grinding, the colors of emission shifted from yellow-green to yellow and then orange-yellow, respectively. Accordingly, the emission peaks of the two samples were red-shifted from 500 to 505 nm to 563 and 581 nm, respectively. Moreover, after dichloromethane vapor fuming, the emission colors and wavelengths of the ground powders were restored to conditions that were similar to those of the as-prepared powders. Powder X-ray diffraction revealed that the transformation between the crystalline and amorphous states following external stimuli was responsible for the MFC characteristics of XA-CE-CAR and XA-CE-BCAR. The reduced gap between bands resulting from the planar intramolecular charge transfer as well as an increase in π–π interactions, exciton coupling, and orbital overlap between neighboring molecules are the reasons for the redshifts in the photoluminescence spectra following grinding.

Highly Emissive Luminogens in Both Solution and Aggregate States

Highly Emissive Luminogens in Both Solution and Aggregate States

Design, synthesis, and mechanochromic luminescence of twisted donor–acceptor bisarylic methanone derivatives

Two bisarylic methanone derivatives, DBF-BZ-POZ and DBF-BZ-PTZ, which have phenoxazine or phenothiazine as electron donors and carbonyl group as electron acceptor are presented. Both molecules have highly distorted molecular conformation and exhibit obvious intramolecular charge transfer (ICT), effective solid-state emissions, and high-contrast mechanofluorochromic (MFC) properties. Both DBF-BZ-POZ and DBF-BZ-PTZ have good solid-state luminescence, efficiencies that reach 33.5% and 38.9%, respectively. The fluorescence colors of the as-prepared powders for both molecules are bright blue-green. The colors change to bright yellow with external mechanical stimulation and red shifts of 104 and 82 nm are observed in the photoluminescence (PL) spectra. Moreover, the MFC behavior of DBF-BZ-POZ and DBF-BZ-PTZ are reversible when their ground samples are subjected to fuming with the DCM vapor. Powder X-ray diffraction results indicate that the MFC phenomenon of DBF-BZ-POZ and DBF-BZ-PTZ may be attributed to crystalline-to-amorphous transitions in the material. The red shifts observed in the PL spectra after the material is ground are due to the smaller band gap induced by the planar intramolecular charge transfer (PICT) and the increased π-π interactions, the increase of exciton coupling, as well as the increased orbital overlap between neighboring molecules.

Mechano-responsive D–A luminogen based on bisarylic methanone derivative with brightly tricolored mechanochromic luminescence

A new luminogen PTZ-BZ-DBF bearing an electron donor phenothiazine unit and an electron acceptor carbonyl group is designed and synthesized. This molecule adopts highly twisted molecular conformation and exhibits intramolecular charge transfer (ICT), effective solid-state luminescence, and high contrast mechanofluorochromic (MFC) properties. The as-prepared powders show bright yellow-green fluorescence. Under conditions of external mechanical stimulation, the color changes to bright orange-red, and a red shift of 87 nm is observed in the emission spectral profile. Meanwhile, the solid-state luminescence efficiency increases from 20.5% to 28.5%. It is different from general MFC materials, when fumed using DCM vapor or annealed at 150 °C, the fluorescence peaks blue shift to approximately 560 nm, and the yellowish-emitting Y-powders are obtained. The ground solids and Y-powders can be converted to the as-prepared solid powders following the process of solvent recrystallization. Thus, tricolor switching is realized. The results obtained using the PXRD and DSC techniques indicate that the MFC phenomenon can be attributed to the process of crystalline-to-amorphous transition. The red shift of the PL spectra is attributed to the planar intramolecular charge transfer (PICT) process induced by conformational planarization in response to external forces.

Twisted phenothiazine-terminated bisarylic methanone π-architecture exhibiting high-contrast mechanofluorochromism

Bisarylic methanone π-architecture PTZ-BZ-DBT with carbonyl group as the acceptor and phenothiazine as the donor was synthesized. Theoretical calculations and photophysical properties in solution demonstrated that the obtained donor-acceptor (D-A) type molecule PTZ-BZ-DBT adopts a molecular conformation of high distortion and significant intramolecular charge transfer (ICT) behavior, which endows it not only with intense solid-state luminescence but also with a high-contrast mechanofluorochromic (MFC) feature. The original as-prepared samples of PTZ-BZ-DBT display bright yellow-green fluorescence. When the as-prepared samples are ground, the ground solid powders exhibit orange-red fluorescence are obtained. Accordingly, during the grinding process, the emission wavelength of the solid powders red shifts from 518 nm of the as-prepared samples to 617 nm of the ground samples, and thus a spectral shift of 99 nm is observed. Different from general MFC materials, if the ground solid powders are fumed by DCM vapor or annealed at 150 °C, the fluorescence peaks blue shift to about 564 nm, and the yellow-emitting Y-powders are gained. However, the ground samples can be converted into the as-prepared solid powders through solvent recrystallization; thus tri-color switching is obtained. The PXRD, fluorescence lifetimes, DSC and the spectral analysis demonstrate that the remarkable MFC nature of PTZ-BZ-DBT is attributed to crystalline-to-amorphous transition. Further, the red shift of the fluorescence band after grinding can be ascribed to the extension of conjugate length and subsequent planar intramolecular charge transfer (PICT) induced by conformational planarization under external forces.

Excellent and reversible mechanofluorochromism in donor–acceptor π-systems based on bisarylic methanone derivatives

Mechanofluorochromic (MFC) materials characterized by high-contrast fluorescent colors and/or high intensities have rarely been reported. This can be attributed to the complex mechanism and molecular design associated with MFC phenomenon. Herein, the route followed for the preparation of two D–A type bisarylic methanone fluorescent molecules CAR-BZ-POZ and CAR-BZ-PTZ, has been presented. Results obtained from systematic photophysical experiments reveal that CAR-BZ-POZ and CAR-BZ-PTZ are characterized by twisted intramolecular charge-transfer (TICT) states, highly distorted molecular conformations, good solid-state emission properties, and high-contrast reversible mechanofluorochromism. The as-prepared CAR-BZ-POZ and CAR-BZ-PTZ solids emitted intense yellow-green fluorescence (531 and 550 nm, respectively), and the solid-state luminescence efficiencies recorded were 32.54% and 24.92%, respectively. When the samples were ground, orange-red fluorescence was emitted (602 and 594 nm, for CAR-BZ-POZ and CAR-BZ-PTZ, respectively). Results obtained by conducting mechanistic studies suggest that the MFC phenomenon can be attributed to the phase transition (from crystalline to amorphous phase) of the molecules. The red-shift in the PL spectral profiles, achieved following the process of grinding, could be attributed to the extension of the molecular conjugation length and the planar intramolecular charge transfer (PICT) properties.

Photophysical Properties of BADAN Revealed in the Study of GGBP Structural Transitions.

The fluorescent dye BADAN (6-bromoacetyl-2-dimetylaminonaphtalene) is widely used in various fields of life sciences, however, the photophysical properties of BADAN are not fully understood. The study of the spectral properties of BADAN attached to a number of mutant forms of GGBP, as well as changes in its spectral characteristics during structural changes in proteins, allowed to shed light on the photophysical properties of BADAN. It was shown that spectral properties of BADAN are determined by at least one non-fluorescent and two fluorescent isomers with overlapping absorbing bands. It was found that BADAN fluorescence is determined by the unsolvated “PICT” (planar intramolecular charge transfer state) and solvated “TICT” (twisted intramolecular charge transfer state) excited states. While “TICT” state can be formed both as a result of the “PICT” state solvation and as a result of light absorption by the solvated ground state of the dye. BADAN fluorescence linked to GGBP/H152C apoform is quenched by Trp 183, but this effect is inhibited by glucose intercalation. New details of the changes in the spectral characteristics of BADAN during the unfolding of the protein apo and holoforms have been obtained.

Mechano-induced photoluminescence colour change in tetraphenylethylene -based aza-N,O-chelated boron difluoride complexes

Two new asymmetric aza-N,O-chelated organoboron complexes DMFPT ((E)-5-(4-(dimethylamino)phenyl)-2,2-difluoro-3-(4-(1,2,2-triphenylvinyl)benzylidene)-2,3-dihydro-1,3,4,2-oxadiazaborol-3-ium-2-uide) and DFPT ((E)-2,2-difluoro-5-phenyl-3-(4-(1,2,2-triphenylvinyl)benzylidene)-2,3-dihydro-1,3,4,2-oxadiazaborol-3-ium-2-uide) have been designed and synthesized. The incorporation of a dimethylamino donor unit into organoboron complexes resulted in a more pronounced AIE and mechanofluorochromic response. PXRD (powder X-ray diffraction), DSC (differential scanning calorimetry) and spectral data indicated that the reversible mechanofluorochromic behavior of the two asymmetric organoboron compounds was attributed to the crystalline-amorphous phase transition. The planarization of the molecular conformation and subsequent PICT (planar intramolecular charge transfer) resulted in the red-shift of fluorescence spectra upon grinding. The newly emerged organoborons may be the candidates for potential applications in sensing, and optical data storage fields, and so on. We expect the obtained results would be helpful to promote the future design and applications of more advanced aza-N,O-chelated organoboron based dyes.

Exploring the relationship between the “ON-OFF” mechanism of fluorescent probes and intramolecular charge transfer properties

In this study, the excited state charge distribution characteristics and fluorescence mechanism of HClO detection probes HN-ClO (weak fluorescence) and HN-ClO-F (strong fluorescence) probes were investigated based on density functional theory (DFT) and time-dependent density functional theory (TDDFT). The results of electrostatic potential (ESP) map and hole-electron analysis show that the HN-ClO and HN-ClO-F probes have obvious charge separation characteristics in the excited state. The excited state energy decomposition and Merz-Kollman charge analysis demonstrate the existence of distinct planar intramolecular charge transfer (PICT) features in HN-ClO and HN-ClO-F. Due to the strong charge coupling caused by the planar structure, the fluorescence of HN-ClO-F could occur. Furthermore, the weak fluorescence of HN-ClO is caused by inter-system crossing (ISC) between S1 and T1 state. Our result proves that the ICT process could exist in HN-ClO-F, but the PICT process does not cause fluorescence quenching, which have provided an excellent supplement to the mechanism of fluorescent probes. The conclusion is consistent with the fluorescence phenomenon observed in the experiment.

Tuning the Fluorescence and the Intramolecular Charge Transfer of Phenothiazine Dipolar and Quadrupolar Derivatives by Oxygen Functionalization.

A series of new naphthalimide and phenothiazine-based push–pull systems (NPI-PTZ1–5), in which we structurally modulate the oxidation state of the sulfur atom in the thiazine ring, i.e., S(II), S(IV), and S(VI), was designed and synthesized by the Pd-catalyzed Sonogashira cross-coupling reaction. The effect of the sulfur oxidation state on the spectral, photophysical, and electrochemical properties was investigated. The steady-state absorption and emission results show that oxygen functionalization greatly improves the optical (absorption coefficient and fluorescence efficiency) and nonlinear optical (hyperpolarizability) features. The cyclic voltammetry experiments and the quantum mechanical calculations suggest that phenothiazine is a stronger electron donor unit relative to phenothiazine-5-oxide and phenothiazine-5,5-dioxide, while the naphthalimide is a strong electron acceptor in all cases. The advanced ultrafast spectroscopic measurements, transient absorption, and broadband fluorescence up conversion give insight into the mechanism of photoinduced intramolecular charge transfer. A planar intramolecular charge transfer (PICT) and highly fluorescent excited state are populated for the oxygen-functionalized molecules NPI-PTZ2,3 and NPI-PTZ5; on the other hand, a twisted intramolecular charge transfer (TICT) state is produced upon photoexcitation of the oxygen-free derivatives NPI-PTZ1 and NPI-PTZ4, with the fluorescence being thus significantly quenched. These results prove oxygen functionalization as a new effective synthetic strategy to tailor the photophysics of phenothiazine-based organic materials for different optoelectronic applications. While oxygen-functionalized compounds are highly fluorescent and promising active materials for current-to-light conversion in organic light-emitting diode devices, oxygen-free systems show very efficient photoinduced ICT and may be employed for light-to-current conversion in organic photovoltaics.

Donor-acceptor type π-conjugated diphenylsulfone derivatives showing diverse fluorescence response to mechanical force

Two donor-acceptor type π-conjugated fluorophores DPS-OCZ and DPS-OPTZ consisting of diphenylsulfone unit as the electron-accepting and carbazole or phenothiazine moiety as the electron-donating were synthesized and characterized. They showed evident ICT characters endowed both fluorophores with a prominent solvatochromic effect, with emitting color tuning by changing the solvent polarity. Spectral studies and theoretical calculations proved the presence of D-A type conjugated structures of DPS-OCZ and DPS-OPTZ. Interestingly, both fluorophores showed diverse fluorescence response to mechanical force (bathochromic shift for DPS-OPTZ or hypsochromic shift for DPS-OCZ). Based on the results of XRD patterns and X-ray single crystals analyses, the reason is as follows: the planarization of the molecular conformation and the resulting planar ICT upon grinding were responsible for the bathochromic shift in the PL spectra of DPS-OPTZ. In contrast, the highly ordered structure was destroyed, partly changed the packing mode, resulting in the hypsochromic shift in PL spectra of DPS-OCZ. The current work presented a strategy of imposing molecular feature screening for realizing diverse fluorescence response to mechanical force.

Substituent Effect on the Photophysics and ESIPT Mechanism of N,N'-Bis(salicylidene)-p-phenylenediamine: A DFT/TD-DFT Analysis.

Excited-state intramolecular proton transfer (ESIPT) and intramolecular charge transfer (ICT) processes are widely exploited in the designing of organic materials for multifarious applications. This work explores the aftereffects of combining both ESIPT and ICT events in a single molecule, namely, N,N'-bis(salicylidene)-p-phenylenediamine (BSP) exploiting DFT and TD-DFT formalisms. The PBE0 functional employed in the present study is found to yield results with better accuracy for excited-state calculations. The results reveal that introduction of electron donor (-NH2) and electron acceptor (-NO2) substituents on BSP produces a strikingly red-shifted emission with respect to the corresponding emission from the unsubstituted analogue in polar solvents. This red-shifted emission originated due to the coupled effect of ESIPT and planar-ICT (PICT) processes from the coplanar geometry adopted by the substituted molecule (s-BSP). Based on the computed potential energy curves, the ground-state intramolecular proton transfer (GSIPT) was found to take place more favorably in s-BSP than in BSP under all solvent conditions. In the case of ESIPT, the barrier and relative energies of the phototautomers of s-BSP were slightly higher than BSP, which shows that simultaneous substitution of -NH2 and -NO2 groups causes slight perturbation to the ESIPT process. Overall, the computed results show that simultaneous substitution of suitable electron donor and acceptor substituents provides profitable changes in the photophysical properties of ESIPT molecules like BSP. These molecular-level insights will pave way for designing better materials for diverse applications.

New insights into the detection mechanism of β-galactosidase in living cells with fluorescent probes

The mechanism of β-galactosidase detection probes ESIPT-GAL and ESIPT-OH have been investigated. The torsional potential energy curve proves the planar characteristics of the system. Combined with frontier molecular orbital and “hole-electron” analysis, there are apparent planar intramolecular charge transfer (PICT) characteristics in ESIPT-GAL and ESIPT-OH. The barrier-free intramolecular proton transfer explained the single fluorescence. Furthermore, the mechanism of ESIPT-OH fluorescent probe is the cooperation of excited-state intramolecular proton transfer (ESIPT) and PICT. The relationship between the singlet and triplet states combined with the spin coupling constant reveals that the weak fluorescence characteristic of ESIPT-GAL is caused by the intersystem crossing.

Dansyl Emits from a PICT Excited State.

Two derivatives of dansyl (1-dimethylamino-5-naphthalenesulfonyl) in which the amino group is constrained in a ring are prepared as neopentyl esters. Their photophysical behavior is compared with that of the dansyl ester. The solvatochromism and quantum yields are similar for all three. Since the two constrained derivatives cannot twist about the amino group, they must emit from a planar intramolecular charge-transfer excited state. The similar photophysical behavior suggests that dansyl also emits from a PICT excited state instead of a twisted intramolecular charge transfer state.

Phenothiazine and diphenylsulfone-based donor–acceptor π-systems exhibiting remarkable mechanofluorochromism

Two donor–acceptor luminophores PTZ-DPS and BPTZ-DPS, which are constructed by electron-donor phenothiazine and electron-acceptor diphenylsulfone moiety, are designed and synthesized. Through the effective combination of phenothiazine and diphenylsulfone unit, PTZ-DPS and BPTZ-DPS molecules adopt twisted molecular conformation, unique intramolecular charge transfer (ICT) behavior and intense fluorescence in both solution and solid state. The solid-state luminescence efficiencies of the two luminophores reach 32.84% and 39.69%, respectively. It is particularly noteworthy that both PTZ-DPS and BPTZ-DPS exhibit switchable mechanofluorochromism with high contrast. Upon external force stimulation, the fluorescence colors of the PTZ-DPS and BPTZ-DPS solid powders that change dramatically from initial bright green and yellow to orange, respectively, are observed. Meanwhile, the emission bands red shift from 499 nm to 564 nm–589 nm and 592 nm, respectively, suggesting the large spectral red shifts of 90 nm and 28 nm. Moreover, the mechanofluorochromic (MFC) behavior of PTZ-DPS and BPTZ-DPS also show good reversibility when fume the ground powders by DCM vapor. PXRD and spectral data suggest that the phase transition between crystalline to amorphous states is responsible for MFC behavior of PTZ-DPS and BPTZ-DPS, and the red-shift in the PL spectrum upon grinding originates from the planarization of the molecular conformation and subsequent planar intramolecular charge transfer (PICT).

Excitation wavelength dependent ICT character and ISC efficiency in a photocleavage agent of 1-aminoanthraquinone.

Anthraquinone derivatives have been widely used as photocleavage agents and dyes. Here, photoinduced excited state intramolecular charge transfer (ICT) dynamics of 1-aminoanthraquinone in ethanol are studied by femtosecond transient absorption (fs-TA) spectroscopy and quantum chemical (QC) calculations. Four decay associated difference spectra and corresponding lifetime components were obtained by singular value decomposition and global fitting analysis from the fs-TA spectra. The QC calculations prove that the S1 state has obvious ICT character. Planar ICT (PICT) and twisted ICT (TICT) reaction coordinates are observed. On the PICT coordinate, vibrational relaxation (VR) and intersystem crossing (ISC) processes are observed. The ISC efficiency is dependent on excitation wavelength, which elucidates that T'2 state participates in the ISC process on a higher-level than the S'1 state. On the TICT coordinate, the TICT process is dependent on excitation wavelength, which elucidates a direct experimental evidence of an energy barrier in excited state TICT potential energy.

Multi-stimuli responsive fluorescence switching of D-A tetraphenylethylene functionalized cyanopyridine isomers

Three new positional isomers (Z)-2-(pyridin-2-yl)-3-(4′-(1,2,2-triphenylvinyl)-[1,1′-biphenyl]-4-yl)acrylonitrile (o-PyTBA), (Z)-2-(pyridin-3-yl)-3-(4′-(1,2,2-triphenylvinyl)-[1,1′-biphenyl]-4-yl)acrylonitrile (m-PyTBA) and (Z)-2-(pyridin-4-yl)-3-(4′-(1,2,2-triphenylvinyl)-[1,1′-biphenyl]-4-yl)acrylonitrile (p-PyTBA) incorporating tetraphenylethylene (TPE) as donor and pyridine unit and cyano-group as acceptor were designed and synthesized. The positional change exploited in these isomers influenced the acceptor strength and molecular packing. These three isomers displayed distinct solvatochromic behaviour and different aggregation induced emission (AIE) properties. The density functional theory (DFT) studies revealed good separation of the highest occupied molecular orbitals (HOMOs) and the lowest unoccupied molecular orbitals (LUMOs) supporting the distinct intramolecular-charge transfer (ICT) transition. Moreover, the three isomers exhibited reversible mechanochromism in the order o-PyTBA (56 nm) > m-PyTBA (49 nm) > p-PyTBA (43 nm). The powder X-ray diffraction (PXRD), differential scanning calorimetry (DSC) and field emission scanning electron microscopy (FESEM) studies indicated that the morphological conversion from crystalline to amorphous state should be responsible for the mechanochromism. Meanwhile, the extension in molecular conjugation caused by planarization of molecular conformation and subsequent planar intramolecular charge transfer (PICT) process by external force were responsible for the red-shifts in the fluorescence spectra. Interestingly, protonation-deprotonation of the pyridine moiety in o-PyTBA, m-PyTBA and p-PyTBA had a significant effect on the frontier molecular orbitals as well as very distinctive emission characteristics upon trifluoroacetic acid (TFA) and trietylamine (TEA) stimuli. The film of o-PyTBA, m-PyTBA and p-PyTBA achieved a low detection limit for trifluoroacetic acid which was estimated to be 13.2 ppm, 17 ppm and 7.2 ppm, respectively.