Room-Temperature Orange-Red Phosphorescence by Way of

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Room-Temperature Orange-Red Phosphorescence by Way of Intermolecular Charge-Transfer in Single-Component Phenoxazine-Quinoline Conjugates and Chemical Sensing Indranil Bhattacharjee, Nirmalya Acharya, Saheli Karmakar, and Debdas Ray J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b06171 • Publication Date (Web): 29 Aug 2018 Downloaded from http://pubs.acs.org on August 30, 2018

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The Journal of Physical Chemistry

Room-Temperature Orange-Red Phosphorescence by Way of Intermolecular ChargeTransfer in Single-Component Phenoxazine-Quinoline Conjugates and Chemical Sensing Indranil Bhattacharjee, Nirmalya Acharya, Saheli Karmakar, Debdas Ray* *Department of Chemistry, Shiv Nadar University, NH-91, Tehsil Dadri, District Gautam Buddha Nagar, Uttar Pradesh, 201 314. Email: [email protected].

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Abstract. Achieving red phosphorescence from purely organic system is a challenging feat due to the predominant thermal non-radiative decay pathways of the excited electrons. Here we design single-component charge transfer (CT) complexes based on phenoxazine-quinoline conjugates (PQ1-PQ3), in which the phenoxazine ring is covalently attached to the quinolinyl fragment via a C−N bond. These conjugates in concentration-dependent absorption studies show a new lowenergy CT absorption band along with the parent π-π* band with binding constants of up to 102 M−1 due to self-association via intermolecular CT (I2CT). Steady-state emission and phosphorescence decay transient measurements of all the conjugates in solutions, thin films and crystals reveal the signature of I2CT that leads to orange-red phosphorescence (ORP) at ambient conditions. Theoretical calculations show the existence of dimer with stronger I2CT characteristics, and reduced energy gap between the lowest singlet (S1) and triplet (T1) states (∆EST = 0.05-0.14 eV), which is in line with emission measurements. These conjugates are used for solid-state dichloromethane vapor sensors, and PQ3 can be transformed into oxygen sensor. These studies give an insight into the ORP properties and provide a rational strategy for the design of single-component self-CT complexes with ORP feature at ambient conditions.


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1. Introduction Organic charge-transfer complexes1-6 (CTCs) which rely on association of donor (D) and acceptor (A), have been shown to be good candidates mainly for high conductivity7-8, ferroelectricity9-10, because of their possible long-range orientation of their CT dipoles existing in highly ordered networks. Besides, CTCs formed by the co-crystallization of two components have been observed to be highly emissive with red-shifted emission via a low energy singlet1112

, and triplet CT13-16 state at RT and/or 77 K (Figure 1a). In view of these interesting

photophysical properties, particularly the existence of a low-energy transition band, CTCs might have important role in room-temperature red phosphorescence (RTRP) via 3CT emission. However, most of the reported CTCs are based on multi-component systems that strongly favor self-aggregation into D or A assemblies alone, rather than complexation between them.1-16 Single-component CTCs (SCCTCs) potentially offer advantages of low cost and easy fabrication over multi-component systems. Therefore, the search and use of new SCCTCs are of great interest. Recently, few SCCTCs that rely on self-association of two parts of the same molecule were developed by utilizing intermolecular CT (I2CT) interaction which could generate white-light emission17-19, second order non-linear effect20, and semiconductive behavior.21-22 To the best of our knowledge, such SCCTCs with efficient RTRP emission have not been reported. On the other hand, purely organic room-temperature phosphorescence (RTP) materials23-25 have attracted increasing attention due to their utilization of the triplet energy and long lifetime. However, RTP materials are typically based on noble metals26-27 that allow efficient phosphorescence with high decay rates (>104 s-1) from the triplet state through strong spin-orbit coupling (SOC)28, which have been used in light-emitting diodes (LEDs)29-30, advanced security



imaging31, and bio-imaging32. The alternative metal-free organic materials, which were seldom reported in RTP studies, become economically viable because of their low costs and ease of chemical modification. It has been observed that most of the reported organic RTP23-25 systems are based on long-lived green and yellow emission. Achieving intrinsic red emission from pure organic molecules has become a tremendous challenge in photophysics due to difficulty in tuning the energy levels of the molecules, and the energy gap law that describes the increase in non-radiative decay of the excited states with decreasing the energy difference between the excited and ground states. In this context, observation of measurable RTRP from organic compound in solutions is another challenging task, due to predominant non-radiative pathways of triplet states under this condition. Unfortunately, only a handful of pure organic systems are known to exhibit the RTRP in solutions31, 33-36 and solids37-39. Currently, RTRP research has been overwhelmingly limited to metal-based systems.40 Therefore, a key design strategy (Figure 1b) aiming to develop purely organic single-component RTRP (SCRTRP) emitters based on selfassociation is of prime


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Figure 1. Jablonski diagram for light-harvesting of (a) singlet state from multi-component systems, (b) triplet state from single component system via intermolecular charge transfer. Proposed molecular structures (bottom). importance. If such systems could be obtained, the materials would create a new avenue in SCRTRP studies underlying 3CT that emits in the red part of the spectrum. Previously we have reported25 how intrinsic dual emission via thermally activated delayed fluorescence (TADF) and RTP along with thermal enhancement in a carbazole-quinoline conjugate can be achieved. Inspired by this unique strategy, we designed new D−A conjugates (Scheme 1) with the aim of self-association of a single molecule due to I2CT. Herein, we demonstrate a general approach to develop SCCTCs with efficient room-temperature orange-red phosphorescence (RTORP) feature by modifying series of aryl substituents onto acceptor part of the phenoxazine appended quinoline (D-A) conjugates without changing the donor. Photophysical studies revealed that all the conjugates in solutions as well as in films undergo self-association via I2CT that leads to RTORP due to radiative decay of the T1 (CT) state. X-ray structure analysis showed that tilted and co-facial arrangement between the two chromophores of the respective conjugates benefit the construction of self-association. Theoretical calculations using four exchange correlation functions (BLYP, B3LYP, M06-2X, M06-L)41-43 with 6-31G(d) basis set revealed that all the dimer possess: a) intermolecular CT characters, b) the highest occupied molecular orbital (HOMO) on the phenoxazine moiety of one layer, c) LUMO on the quinolinyl moiety and its substituents at 2,4-positions of other layer, d) low energy difference between the lowest singlet and triplet states (∆EST; 0.05, 0.14, and 0.09 eV), thus causing RTORP from T1(3CT). Given the solvent-dependent property, solid-state dichloromethane vapor sensing applications has been achieved for all the conjugates. Interestingly, one of the conjugate,



PQ3 exhibited naked-eye distinguishable orange and red emission in aqueous buffer solution under an atmosphere of ambient and oxygen-free condition. 2. EXPERIMENTAL AND THEORETICAL PROCDURE 2.1. Synthesis. The synthesis of the D-A conjugates, PQ1, PQ2 and PQ3 were achieved in three steps (Scheme 1). First, phenoxazine is treated with 2-fluoro-nitrobenzene to obtain 2. Reduction of 2 (SnCl2) in water followed by multicomponent condensation reaction25 with respective aryl aldehydes, aryl acetylenes, and FeCl3•6H2O at 105 °C under open atmosphere, and purification by column chromatography gives PQ1-PQ3 in 56-70% isolated yield. All the conjugates were characterized by NMR spectroscopy, high-resolution mass spectrometry, and X-ray analysis (see Supplementary Information).

Scheme 1. Synthesis of PQ1-PQ3 2.2. Sample Characterization X-ray Diffraction Technique. Single crystal X-ray diffraction data were collected using a D8 Venture IµS microfocus dual source Bruker APEX3 diffractometer equipped with a PHOTON 100 CMOS detector. Single crystals were mounted at room temperature on the ends of glass


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fibers and all the data were collected at room-temperature. Data collection: APEX2 (Bruker, 2014)44 cell refinement: SAINT (Bruker, 2014)44 data reduction: SAINT; program(s) used to solve structure: SHELXT (Sheldrick, 2008)45 program(s) used to refine structure: SHELXL2014 (Sheldrick, 2008); molecular graphics: Ortep-3 for Windows46. The crystal structure of PQ3 comprised of one toluene molecule as solvent. In case of PQ3 respective solvent molecule was heavily disordered and were squeezed out using PLATON47. Photoluminescence

measurements25:

Steady-state

emission

(Fluorescence

and

phosphorescence) was recorded on HORIBA Fluorolog-3 spectrofluorometer (Model: FL3-2IHR). Phosphorescence emission were collected at 41 ms detector delay (for all of the sample). 2.3. COMPUTATIONAL INFORMATION All molecular calculations were performed via DFT and TD-DFT48 with BLYP, B3LYP, M062X and M06-L41-43 hybrid functional implanted in the Gaussian 0949 program, keeping the basis set same (6-31g(d)) for all the cases. We have incorporated Grimme’s dispersion correction50 in B3LYP and BLYP hybrid function. 3. RESULT AND DISCUSSION 3.1. Single crystal X-ray diffraction (SCXRD) analysis. SCXRD analyses (Figure 2) of all the conjugates reveal that the phenoxazine ring and aryl substituents attached to C8 and C4 atoms of the quinolinyl moiety deviate from planarity (PQ1: -76.85(0)°, 86.07 (0)°; PQ2: -75.76 (3)°, 97.94(3); PQ3: 81.03(3)°, 47.32 (0)°), when viewed along the C(11)−N(2)−C(8)−C(9) and C(18)−C(17)−C(4)−C(3) atoms, respectively (Figures S6-S8, Tables S1,S2). We also found in PQ1 and PQ2 that aryl substitution at 2 position of the quinolinyl moiety causes the aryl substituents to be out of plan with torsions of 20.24(3)° and -14.36(4)° respectively, when viewed



Figure 2. Oak Ridge thermal ellipsoid plots (50% probability ellipsoids) of (a) PQ1, (b) PQ2, (c) PQ3 and (d-f) intermolecular interaction between two neighbors and. The protons are placed in calculated positions. along the C(24)−C(23)−C(2)−N(1) atoms. For PQ3, this torsion was measured to be -0.48(1)°, indicating that the substituted quinolinyl ring at 2-position of the parent quinolinyl moiety is almost planar with the former. Furthermore, multiple noncovalent C−H•••C interactions (PQ1: C6−H6•••C12, 2.792 Å; C6−H6•••C13, 2.769 Å; C3−H3•••C12, 2.829 Å. PQ2: C3−H3•••C32, 2.886 Å, C32−H32•••C18, 2.890 Å, C18−H18•••C10, 2.809 Å, C18−H18•••C19, 2.669 Å, C12A−H12A•••C5, 2.882 Å.) are also seen (Figures S6, S7). In addition, intermolecular hydrogen bonds (H−B) (O1•••H25, 2.593 Å; O1•••H15, 2.71 Å) between two neighboring molecules was also found for PQ1 and PQ2 respectively. These intermolecular forces allow the molecules to form a tilted packing arrangement. However, in PQ3, a co-facial arrangement


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between two molecules with long π•••π interactions (3.989 Å) were found (Figure S8). Similar large number of C−H•••C interactions (PQ3: C13−H13•••C26, 2.863 Å, C14−H14•••C29, 2.870 Å, C7−H7•••C15B, 2.859 Å, C7−H7•••C16B, 2.826 Å) and hydrogen bonding interaction (O1•••H7B, 2.680 Å) were also observed (Figure S8). We believe these intermolecular noncovalent forces served as the increased rigidity that might help to re-inforce I2CT transition between the phenoxazine ring of one layer and quinolinyl fragment of the second layer. 3.2. Optical Absorption Characteristics. The UV-visible(vis) properties of all the conjugates were measured in solvents. The transparent toluene solutions of all the conjugates (1.01 × 10-5 M) show an absorption maximum at ~280−360 nm which can be assigned as π−π* transition (Figures 3a-3c, Table 1). To gain knowledge of self-association behavior of all the conjugates, concentration-dependent absorption studies were explored in toluene solutions. Absorption studies with increasing concentration from 10 µM to 1.0 mM resulted in the emergence of a new low energy broad band at ~370-500 nm, while the parent peak at ~280-360 nm remains almost unchanged except increased intensity (Figures 3a-3c). This new broad band can be assigned to the I2CT absorption band due to self-association. Further details of related studies in other solvents (1,4-dioxane (DOX), tetrahydrofuran (THF), cyclohexane (CH), chloroform(CHCl3)) showed similar behavior (Figures S9-S12). Furthermore, hypsochromic shift of these new low energy bands were recorded on going from CH to THF, which is in accordance with the nπ* (CT) nature of the transition (Figures S13-S15).25 Surprisingly, we observed in dichloromethane (DCM, 1.0 µM-1.0 mM ) solutions that the low energy band is further red shifted to 539 nm, 540 nm and 602 nm for PQ1, PQ2 and PQ3, respectively (Figures 3d-f). In order to verify these bands, trifluoroacetic acid (1.0 equiv.) was added to the DCM solutions of all the conjugates. Measurement of these solutions revealed that all the bands were further bathochromically shifted



(Figure S16) and these new red shifted bands revert back to original positions when triethylamine was added to these solutions. This inverse polarity effect of all the conjugates suggest that DCM unusually favors I2CT. To understand the thermal effect on this I2CT band, temperature-dependent UV-vis studies of PQ1, PQ2 and PQ3 in toluene were undertaken. We found in thermal effect that intensity of the broad I2CT band at ~370-500 nm increases with decreasing temperature (Figures S17-S19). The corresponding association constants (Ka) for all the CTCs at low temperature (250 K) were calculated to be 3.99 × 102, 4.29 × 102 and 4.25 × 102 M-1 respectively, which are relatively higher values (Figures S20-S22) compared to Ka51 (2.74 × 102, 2.52 × 102, 2.91 × 102 M-1) in toluene at RT. To calculate the thermodynamic parameters using Van’t Hoff plot, concentration-dependent experiments at different temperatures in toluene were performed. The enthalpy change

Figure 3. Concentration-dependent absorptions of PQ1, PQ2 and PQ3 in (a-c) toluene and (d-f) DCM. Inset of (a-c) and (d-f) showing the Van’t Hoff plot and naked eye color in DCM.


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were found to be -0.45, -0.637, and -0.476 kcal/mol, respectively (Figures 3a-3c, inset). These observations indicate that CT complexation via I2CT is favored at high concentration. 3.3. PL Characteristics. Steady-state emission of all the conjugates were measured in solvents. The toluene solutions (1µM) of all the conjugates show only one emission peak at ~402-404 nm which is independent of solvent polarity effect (Figures 4a-c, Figure S23). Moreover, a short average lifetime of 2-4 ns confirms a prompt fluorescence feature of the parent bands observed in the conjugates. In addition, a high intense low energy broad emission band at ~580-590 nm was recorded when concentration was increased from 0.1 µΜ to 1.0 mΜ for PQ1 and PQ2 (Figures 4a-c). Interestingly, for PQ3, negligible intensity of this new band was recorded at ambient conditions (Figure 3f). In absence of molecular oxygen similar red-shifted emission band was observed at RT, indicating a triplet state that is formed via I2CT transition. Moreover, positive solvatochromism25,52 effects and broad emission feature (Figure S23) suggest that the conjugates show RTORP via CT triplet state. Furthermore, the observation of single well-defined isosbestic point at 505, 503, and 516 nm for PQ1, PQ2, and PQ3, respectively, indicates the presence of a thermodynamic equilibrium between two species that can be clearly ascribed to the respective monomer and dimer. Comparison of the emission bands of PQ1 and PQ2 with that of PQ3 suggest that red-shifted band of the later is probably due to the extended conjugation of the quinolinyl backbone. The longer decay (28, 6.9, and 51 µs) of the lowest emission bands of all the conjugates in deoxygenated toluene at RT (PQ1: λem = 584; PQ2: λem = 577; PQ3: λem = 600 nm) confirms typical phosphorescence that originated via triplet I2CT state13 (Inset, Figures 4ac). In addition, a prompt component (4.2, 5.1, 1.9 ns) of the later bands for all the conjugates were also recorded, which suggest that low energy gap between the singlet and triplet states is contributing to the dual-state emission feature of the later bands53. Similar emission features of



the conjugates were measured in other solvents (CH, THF, DOX, CHCl3) (Figure S24-S27). The phosphorescence decay transient analysis (Table S3) of the conjugates in such solvents further ensure previous line of argument for prompt and RTP component. However, emission measurements of all the conjugates in DCM solutions at ambient conditions show weekly emitting red-shifted emission bands at 610, 615 and 650 nm, respectively (Figure S28). The intensity of these new bands were substantially increased with increasing concentration from 0.1 µΜ to 1.0 mM (Figures 4d-f) when deoxygenated solutions were measured. To understand the nature of these emission bands, phosphorescence decay transient analysis were undertaken. The lifetimes were measured to be 10.9, 20.3 and 72.2 µs for PQ1, PQ2 and PQ3, respectively (Inset, Figures 4d-f). It should be noted that no prompt component, which was present in other solvents, was recorded. These results clearly suggest that all the conjugates exhibit efficient RTRP via I2CT at high concentration. To elaborate upon our understanding of I2CT transitions of all the conjugates, concentration dependent absorption and emission studies were performed in poly(methyl methacrylate) (PMMA) matrix. For all the conjugates (PQ1-PQ3), a similar trend in concentration-dependent UV-vis absorption spectroscopy was detected during increasing concentration of dopants (Figures S29), thereby assigning the observed features to the possibility of I2CT at RT. Likewise, emission studies of PQ1 and PQ2 show the similar I2CT dependent emission at 530 and 532 nm along with parent LE band at ~380-480 nm when 1% dopant was used, while further redshifted emission at 570 nm was detected for the PQ3 for the same concentration (Figure S30). In addition, ~32-38 nm redshift of the low-energy emission band of all the conjugates is measured when concentration of dopant was increased from 1 to 50%. No further appreciable change in the peak position was detected


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Figure 4. Emission spectra of PQ1, PQ2 and PQ3 in (a-e) toluene at ambient conditions, and (df) DCM at deoxygenated conditions. above 50% dopant concentration (Figure 5a). It should be noted that only one low-energy emission band was observed with increasing concentration (≥90%) of the respective conjugates. The phosphorescence decay transient measurements show a substantial increase in lifetimes of the RTP component from 3.2 to 14.6 ms (PQ1), 7.92 to 25.7 ms (PQ2), and 10.37 to 32.8 ms (PQ3) at RT (Figures 5b, Figures S31, Table S4), when dopant concentration was increased from 10 to 100%. On the other hand, emission measurements were performed in crystals of PQ1, PQ2 and PQ3. We observed single RTP peak at 556 nm (22.9 ms), 558 nm (31.2 ms) and 598 nm (39.7 ms) for PQ1, PQ2 and PQ3, respectively (Figure S32). The absolute quantum yields (QY) of the conjugates were measured to be 19.6%, 38.2%, and 6.1% (Table 1), which were relatively higher to that of literature value54, while radiative rate constants of phosphorescence (krP) are calculated to be 1.32 ×101, 1.49 ×101, and 0.18 ×101, respectively (Table 1). These observations further substantiate our previous discussion of emission enhancement with increasing concentration of respective conjugates in



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Table 1. Photophysical parameters of PQ1, PQ2, and PQ3. Toluene λabs (nm)a

Thin film λem (nm)a

λabs (nm)b

λem (nm)b

࢖

τ (ms)

QY (%)c

࢑࢘ (s-1)

PQ1

315

445

390

584

344

434

410

575

14.6

19.6

1.32 ×101

PQ2

323

446

392

577

346

432

415

574

25.7

38.2

1.49 ×101

PQ3

320

455

388

600

346

461

426

600

32.8

6.1

0.18 ×101

a

[0.1 mM]; b[100% dopant]; ccrystals; λex = 320 nm.

Figure 5. (a) Stead state emission and phosphorescence of PQ1, PQ2, and PQ3 in film (100%). (b) Lifetimes of the conjugates. solutions. Taking all together, we believe that low-energy absorption as well as emission bands originated due to the formation of CTC via I2CT. This is in good agreement with the X-ray data


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of all the conjugates which show tilted (PQ1, PQ2) and co-facial (PQ3) arrangement via noncovalent interactions that might enable I2CT between two neighbors. 3.4. Theoretical Analysis. To elaborate upon our understanding of self-association of all the conjugates, we have calculate the enthalpy change (∆H, kcal/mol) for dimerization, The ∆H values were found to be -17.45, -10.34, and -14.515 kcal/mol (Table S5-S7) for PQ12, PQ22, and PQ32, respectively, which suggest all the conjugates form dimer via self-association. To understand the CT character of the dimers, we have undertaken natural population analysis (NPA).55 The calculation showed that the charge of the phenoxazine rings in all the conjugates are -0.223 (Figure S33a), -0.23 (Figure S34a), and -0.258 (Figure 6a), while corresponding charges in the dimers were found to be -0.198, -0.133, and -0.237, respectively (Figure 6b, Figures S33b, S34b). These results indicate that I2CT is originating from phenoxazine ring of one molecule to quinolinyl fragment of other molecule of the dimer. Therefore, all the dimer possess I2CT characters. The calculated isosurfaces of S1 and T1 states in all the monomers are delocalized on quonolinyl fragment and its substituents at 2,4-positions (Figure S35). Interestingly, the highest occupied molecular orbital (HOMO) of PQ12 and PQ22 are localized on phenoxazine ring of one layer, while HOMO of PQ32 is distributed over the two phenoxazine rings from both layers (Figures 6c, Figure S36). The lowest unoccupied molecular orbital (LUMO) of all the dimers is delocalized over the quonolinyl fragment and its substituents at 2,4-positions from both layers (Figures 6c, Figure S36). To calculate the CT character for HOMO to LUMO transition in the dimers, natural transition orbital (NTO) analysis were performed. We have found that I2CT for all cases were



Figure 6. NPA of (a) monomer and (b) dimer of PQ32. (c) Isosurfaces of S0 (lower left image), S1 (upper left image), and T1 (lower right image) of PQ32 calculated at the M06-2X/6-31G(d) level. found to be >70%, while intramolecular CT is

Room-Temperature Orange-Red Phosphorescence by Way of

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