Muhammad Khalid*a,
Akbar Alib,
Rifat Jawariaa,
Muhammad Adnan Asgharc,
Sumreen Asima,
Muhammad Usman Khand,
Riaz Hussaind,
Muhammad Fayyaz ur Rehmanb,
Christopher J. Ennise and
Muhammad Safwan Akram*ef
aDepartment of Chemistry, Khawaja Fareed University of Engineering & Information Technology, Rahim Yar Khan, 64200, Pakistan. E-mail: khalid@iq.usp.br
bDepartment of Chemistry, University of Sargodha, Sargodha 40100, Pakistan
cA Division of Science and Technology, University of Education Lahore, Pakistan
dDepartment of Chemistry, University of Okara, Okara-56300, Pakistan
eSchool of Health and Life Sciences, Teesside University, Middlesbrough, TS1 3BA, UK. E-mail: safwan.akram@tees.ac.uk
fNational Horizons Centre, Teesside University, Darlington, DL1 1HG, UK
First published on 10th June 2020
Materials with nonlinear optical (NLO) properties have significant applications in different fields, including nuclear science, biophysics, medicine, chemical dynamics, solid physics, materials science and surface interface applications. Quinoline and carbazole, owing to their electron-deficient and electron-rich character respectively, play a role in charge transfer applications in optoelectronics. Therefore, an attempt has been made herein to explore quinoline–carbazole based novel materials with highly nonlinear optical properties. Structural tailoring has been made at the donor and acceptor units of two recently synthesized quinoline–carbazole molecules (Q1, Q2) and acceptor–donor–π–acceptor (A–D–π–A) and donor–acceptor–donor–π–acceptor (D–A–D–π–A) type novel molecules Q1D1–Q1D3 and Q2D2–Q2D3 have been quantum chemically designed, respectively. Density functional theory (DFT) and time-dependent density functional theory (TDDFT) computations are performed to process the impact of acceptor and donor units on photophysical, electronic and NLO properties of selected molecules. The λmax values (321 and 319 nm) for Q1 and Q2 in DSMO were in good agreement with the experimental values (326 and 323 nm). The largest shift in absorption maximum is displayed by Q1D2 (436 nm). The designed compounds (Q1D3–Q2D3) express absorption spectra with an increased border and with a reduced band gap compared to the parent compounds (Q1 and Q2). Natural bond orbital (NBO) investigations showed that the extended hyper conjugation and strong intramolecular interaction play significant roles in stabilising these systems. All molecules expressed significant NLO responses. A large value of βtot was elevated in Q1D2 (23885.90 a.u.). This theoretical framework reveals the NLO response properties of novel quinoline–carbazole derivatives that can be significant for their use in advanced applications.
Several framework types have been reported in the literature, including D–A, D–π–A, D–π–π–A, D–A–π–A, D–D–π–A, D–π–A–π–D and A–π–D–π–A.22–25 The nature of both D and A moieties plays a critical role in developing a good NLO response and the literature is replete with a huge variety of π-linkers. Through assimilation of suitable D, π-conjugation spacers and A, a push–pull architecture has been built for designing A–D–π–π–A and D–A–D–π–π–A organic compounds. These push–pull arrangements decrease the recombination of charges, influence charge detachment, deepen the penetration range within higher wavelengths, magnify the asymmetric electronic distribution and decrease the HOMO–LUMO band gap, hence elevating the NLO response.26–32
Quinoline and its derivatives have been widely studied in various areas, exhibiting a variety of practical applications: e.g. such compounds are used in optoelectronics due to their chemical and thermal stability, fluorescence and electron transporting capabilities and the possibility of their chemical modification.33 Quinoline has the ability to attract electrons and is a good candidate for an organic acceptor moiety. Carbazole derivatives are electron-rich systems (D) making them suitable for organic donor moieties and are frequently used in optoelectronics, especially in solar cell technologies, and as hole-transporter materials in organic light emitting diode (OLED) devices.34 In addition, the carbazole moiety can easily be modified by substitution at the 9-position, modifying the overall solubility and photophysical properties of the NLO molecule.35 In view of the acceptor and donor properties of these moieties, we have taken frameworks from two quinoline–carbazole based compounds (Q1 and Q2) recently synthesized by Slodek et al.36 and have built upon them to develop new compounds (Q1D1, Q1D2, Q1D3, Q2D1, Q2D2 and Q2D3) to study and explore the NLO response properties of quinoline–carbazole systems. We hope that this study will serve as a springboard for experimental chemists and physicists for the synthesis of compounds with excellent NLO response properties.
〈α〉 = 1/3(αxx + αyy + αzz) | (1) |
βtot = [(βxxx + βxyy + βxzz)2 + (βyyy + βxxy + βyzz)2 + (βzzz + βxxz + βyyz)2]1/2 | (2) |
Fig. 1 A sketch map of two families of compounds. The first is designed from Q1 (donor–acceptor) and the second is designed from Q2 (donor–acceptor–donor) configurations. |
Hence, the three compounds with A–D–π–A architecture designed from Q1 are Q1D1 (2-((5′-(2-(4-bromoquinolin-2-yl)-9H-carbazol-9-yl)-[2,2′-bithiophen]-5-yl)methylene)-malonic acid); Q1D2 (2-((5′-(2-(4-bromoquinolin-2-yl)-9H-carbazol-9-yl)-[2,2′-bithiophen]-5-yl)methylene)malononitrile); Q1D3 ((Z)-3-(5′-(2-(4-bromoquinolin-2-yl)-9H-carbazol-9-yl)-[2,2′-bithiophen]-5-yl)-2-cyanoacrylic acid).
Similarly, three compounds with D–A–D–π–A architecture were designed from Q2: Q2D1 (2-((5′-(2-(2-(9H-carbazol-2-yl)quinolin-4-yl)-9H-carbazol-9-yl)-[2,2′-bithiophen]-5-yl)methylene)malonic acid); Q2D2 (2-((5′-(2-(2-(9H-carbazol-2-yl)quinolin-4-yl)-9H-carbazol-9-yl)-[2,2′-bithiophen]-5-yl)methylene)malononitrile); Q2D3 ((E)-3-(5′-(2-(2-(9H-carbazol-2-yl)quinolin-4-yl)-9H-carbazol-9-yl)-[2,2′-bithiophen]-5-yl)-2-cyanoacrylic acid).
With the exception of the terminal acceptor shown in Fig. 2 and which is modulated in all compounds designed from Q1 and Q2, the D–A part is preserved throughout the designs. The structures of the investigated compounds (Q1, Q1D1–Q1D3, Q2, Q2D1–Q2D3) are portrayed in Fig. 3. DFT and TDDFT calculations were performed to investigate how π-conjugated spacers and various acceptors established with quinoline and carbazole units influence the NLO properties: (i) hyperpolarizability (β), (ii) polarizability (α) and (iii) absorption wavelength.
Compounds | E(HOMO) | E(LUMO) | Band gap |
---|---|---|---|
Q1 | −7.232 | −1.072 | 6.160 |
Q1D1 | −7.226 | −1.793 | 5.433 |
Q1D2 | −7.189 | −2.142 | 5.047 |
Q1D3 | −7.362 | −2.030 | 5.332 |
Q2 | −7.196 | −0.969 | 6.227 |
Q2D1 | −7.171 | −1.699 | 5.472 |
Q2D2 | −7.203 | −2.151 | 5.052 |
Q2D3 | −7.137 | −1.934 | 5.203 |
The data in Table 1 show that of the parent molecules, Q1 has the lower HOMO–LUMO energy gap (6.160 eV). Furthermore, the band gap is observed to be smaller in the designed molecules. For the Q1 family, Q1 has the highest Egap value (6.160 eV) and the HOMO–LUMO energy gap decreases in the order Q1 > Q1D1 > Q1D3 > Q1D2 to 5.047 eV. A similar effect on Egap is seen in the Q2 family, with the energy gap being highest in Q2 (6.227 eV) and falling in the order Q2 > Q2D1 > Q2D3 > Q2D2 to 5.052 eV. Overall, the value of the Egap of all the investigated molecules increases in the order: Q1D2 < Q2D2 < Q2D3 < Q1D3 < Q1D1 < Q2D1 < Q1 < Q2. Thus, the introduction of a planar spacer linker to join different suitable acceptor units is an effective way to lower Egap and thus to enhance and influence the NLO properties. Of the terminal acceptors studied, 2-ethylidenemalononitrile (EMN) exhibits the greatest capacity to reduce the Egap of quinoline–carbazole based compounds. Overall, the band gap is found to be low in Q1 and its derivatives relative to the Q2 family.
In addition to the magnitude of the HOMO–LUMO energy gap, the ability of compounds to undergo intramolecular charge transfer from donor to acceptor moieties, mediated by π-linkers, is an important aspect of NLO structure–function relationships. The contour surfaces for the distribution of the frontier molecular orbitals (FMOs) are used to explain the exchange of charges, as illustrated in Fig. 4.
Fig. 4 HOMOs and LUMOs of quinoline–carbazole based Q1 (A–D–π–A) and Q2 (D–A–D–π–A) families of molecules. |
In Q1, the HOMO is predominantly located on the donor carbazole unit while LUMO density is concentrated on the acceptor quinoline moiety. In Q1D1, Q1D2 and Q1D3, the HOMO is predominantly on the donor carbazole unit and partially on the π spacer while the LUMO is concentrated partially on the π spacer and largely on the terminal acceptor. Similarly, the HOMO in Q2 is on the donor carbazole segment and the LUMO is on the acceptor quinoline part. The HOMO charge density in Q2D1, Q2D2 and Q2D3 is positioned on one of the two donor carbazole units, while the LUMO charge density is positioned partially on the bridge unit and largely on the terminal acceptor parts. Thus, significant charge transfer in the studied compounds was directed from the donor HOMO to an acceptor via by the π-spacers. This facilitation of charge transfer indicates that all the designed compounds should be excellent NLO materials.
IP = −EHOMO | (3) |
EA = −ELUMO | (4) |
The chemical hardness (η), chemical potential (μ) and electronegativity (X) have been calculated by utilizing Koopmans's theorem and the following eqn (5)–(7):55,56
(5) |
(6) |
(7) |
The global softness (σ) is described by eqn (8):
(8) |
An electrophilicity index (ω) was introduced by Parr et al., as in eqn (9):
(9) |
The results obtained from eqn (3)–(9) are collected in Table 2.
Comp | IP | EA | X | η | μ | ω | σ |
---|---|---|---|---|---|---|---|
a IP = ionization potential, EA = electron affinity, X = electronegativity, μ = chemical potential, η = global hardness, σ = global softness and ω = global electrophilicity. Units are eV. | |||||||
Q1 | 7.23 | 1.07 | 4.15 | 3.080 | −4.152 | 2.798 | 0.16 |
Q1D1 | 7.22 | 1.79 | 4.51 | 2.716 | −4.509 | 3.742 | 0.18 |
Q1D2 | 7.18 | 2.14 | 4.66 | 2.523 | −4.665 | 4.312 | 0.19 |
Q1D3 | 7.36 | 2.03 | 4.70 | 2.666 | −4.696 | 4.135 | 0.18 |
Q2 | 7.19 | 0.96 | 4.08 | 3.113 | −4.082 | 2.676 | 0.16 |
Q2D1 | 7.17 | 1.70 | 4.43 | 2.736 | −4.435 | 3.594 | 0.18 |
Q2D2 | 7.20 | 2.15 | 4.68 | 2.526 | −4.677 | 4.329 | 0.19 |
Q2D3 | 7.13 | 1.93 | 4.15 | 2.601 | −4.535 | 3.953 | 0.19 |
The global reactivity parameters relate to the reactivity of the molecules studied. For example, ionization potential relates to electron-donating ability and electronegativity relates to the ability of a compound to attract electrons. The negative values of chemical potential (μ) shown in Table 2 indicate that all the studied molecules are chemically stable. Hardness, chemical potential and electronegativity depend on the value of Egap. The values of these parameters in Table 2 correlate, therefore, with the size of the band gap across the two families of molecules. Hence, the calculated hardness values across the two families lie in the order Q2 > Q1 > Q2D1 > Q1D1 > Q1D3 > Q2D3 > Q2D2 > Q1D2, exactly the same as the order of increase of Egap. The order of increasing softness is the reverse of this. Ionization potential, electron affinity, electronegativity and global electrophilicity correlate with Egap in the same manner.
(10) |
Compounds | Donor (i) | Type | Acceptor (j) | Type | E(2)a | E(j)E(i)b (a.u.) | F(i,j)c (a.u.) |
---|---|---|---|---|---|---|---|
a E(2) is the energy of hyper conjugative interaction (stabilization energy in kcal mol−1).b Energy difference between donor and acceptor i and j NBO orbitals.c F(i;j) is the Fock matrix element between i and j NBO orbitals. | |||||||
Q1 | C27–C32 | π | C28–C29 | π* | 372.21 | 0.01 | 0.095 |
C1–C2 | π | C3–C4 | π* | 23.89 | 0.36 | 0.087 | |
C2–H8 | σ | C3–C4 | σ* | 5.16 | 1.16 | 0.070 | |
C3–C4 | σ | C9–Br24 | σ* | 6.93 | 0.88 | 0.070 | |
N25 | LP(1) | C3–C4 | σ* | 11.98 | 0.98 | 0.097 | |
N26 | LP(1) | C27–C32 | π* | 46.95 | 0.36 | 0.121 | |
Q1D1 | C27–C28 | π | C29–C30 | π* | 34.05 | 0.37 | 0.100 |
C12–N25 | π | C9–C13 | π* | 15.43 | 0.42 | 0.073 | |
N26–C27 | ∂ | C37–C38 | ∂* | 0.55 | 1.50 | 0.026 | |
C28–C29 | ∂ | N26–C27 | ∂* | 6.94 | 1.25 | 0.084 | |
S46 | LP(2) | C47–C49 | π* | 30.57 | 0.33 | 0.090 | |
C3 | LP(1) | C9–C13 | π* | 72.78 | 0.18 | 0.126 | |
Q1D2 | C12–N25 | π | C9–C13 | π* | 15.44 | 0.42 | 0.073 |
C47–C49 | π | C51–C53 | π* | 34.36 | 0.38 | 0.102 | |
C2–C3 | ∂ | C3–C4 | ∂* | 4.65 | 1.34 | 0.071 | |
C9–C13 | ∂ | C12–N25 | ∂* | 29.21 | 0.39 | 0.098 | |
S46 | LP(2) | C47–C49 | π* | 28.96 | 0.34 | 0.088 | |
N26 | LP(1) | C27–C32 | π* | 40.09 | 0.37 | 0.113 | |
Q1D3 | C47–C49 | π | C51–C53 | π* | 32.17 | 0.38 | 0.100 |
C12–N25 | π | C9–C13 | π* | 15.44 | 0.42 | 0.073 | |
C1–H7 | ∂ | C1–C6 | ∂* | 0.61 | 1.20 | 0.024 | |
C38–C40 | ∂ | N26–C37 | ∂* | 7.16 | 1.24 | 0.084 | |
C22 | LP(1) | C16–C18 | π* | 78.49 | 0.19 | 0.133 | |
S39 | LP(2) | C37–C38 | π* | 30.71 | 0.35 | 0.094 | |
Q2 | C3–C4 | π | C11–N23 | π* | 20.03 | 0.34 | 0.076 |
C45–C46 | π | C48–C51 | π* | 391.20 | 0.01 | 0.095 | |
C1–H7 | σ | C1–C6 | σ* | 0.61 | 1.20 | 0.024 | |
C26–C27 | σ | N24–C25 | σ* | 6.53 | 1.28 | 0.082 | |
N23 | LP(1) | C3–C4 | σ* | 11.40 | 0.99 | 0.096 | |
N55 | LP(1) | C39–C43 | π* | 47.19 | 0.36 | 0.121 | |
Q2D1 | C3–C4 | π | C5–C6 | π* | 20.02 | 0.37 | 0.091 |
C8–C12 | π | C11–N23 | π* | 34.85 | 0.37 | 0.102 | |
C73–O75 | ∂ | C73–O77 | ∂* | 0.50 | 1.66 | 0.026 | |
C58–C60 | ∂ | N55–C57 | ∂* | 6.99 | 1.23 | 0.083 | |
O78 | LP(2) | C74–O80 | π* | 58.35 | 0.44 | 0.146 | |
S66 | LP(2) | C67–C69 | π* | 30.83 | 0.33 | 0.091 | |
Q2D2 | C3–C4 | π | C5–C6 | π* | 20.10 | 0.37 | 0.081 |
C8–C12 | π | C11–N23 | π* | 34.49 | 0.37 | 0.102 | |
C60–C62 | ∂ | N55–C57 | ∂* | 0.57 | 1.30 | 0.024 | |
C73–C75 | ∂ | C75–N76 | ∂* | 9.51 | 1.78 | 0.117 | |
N24 | LP(1) | C25–C30 | π* | 46.86 | 0.36 | 0.121 | |
S66 | LP(2) | C67–C69 | π* | 29.10 | 0.34 | 0.089 | |
Q2D3 | C73–N79 | π | C71–C72 | π* | 8.00 | 0.48 | 0.058 |
C67–C69 | π | C71–C72 | π* | 34.54 | 0.39 | 0.105 | |
C1–H7 | ∂ | C1–C6 | ∂* | 0.60 | 1.20 | 0.024 | |
C72–C73 | ∂ | C73–N79 | ∂* | 10.18 | 1.79 | 0.121 | |
N79 | LP(1) | C72–C73 | ∂* | 10.77 | 1.16 | 0.100 | |
O75 | LP(2) | C74–O77 | π* | 57.92 | 0.45 | 0.146 |
Four types of electronic transitions were commonly observed; σ → σ*, π → π*, LP → σ* and LP → π*. Among these π → π* were most prominent while σ → σ* were least dominant and LP → σ* and LP → π* were slightly dominant transitions. The presence of conjugation and charge transfer in these compounds can be examined by studying the π → π* transitions. The most significant π → π* electronic interactions are π(C27–C32) → π*(C28–C29), π(C27–C28) → π*(C29–C30), π(C47–C49) → π*(C51–C53) and π(C47–C49) → π*(C51–C53) with stabilization energies 372.21, 34.05, 34.36 and 32.17 kcal mol−1 found in Q1, Q1D1, Q1D2 and Q1D3, respectively. These excitations are the highest such transitions in the Q1 family.
Similarly the most feasible π → π* interactions among the Q2 family of molecules were π(C45–C46) → π*(C48–C51), π(C8–C12) → π*(C11–N23), π(C8–C12) → π*(C11–N23) and π(C67–C69) → π*(C71–C72) with stabilization energies of 391.20, 34.85, 34.49 and 34.54 kcal mol−1 in Q2, Q2D1, Q2D2 and Q2D3, respectively.
Significant resonance interactions were observed. In the Q1 family these are LP1(N26) → π*(C27–C32), LP2(C3) → π*(C9–C13), LP1(N26) → π*(C27–C32), and LP1(C22) → π*(C16–C18) with stabilization energies of 46.95, 72.78, 40.09, and 78.49 kcal mol−1 in Q1, Q1D1, Q1D2, and Q1D3, respectively. For the Q2 family the significant transitions from lone pair orbitals are LP1(N55) → π*(C39–C43), LP2(O78) → π*(C74–O80), LP1(N24) → π*(C25–C30), and LP2(O75) → π*(C74–O77) with stabilization energies of 47.19, 58.35, 46.86 and 57.92 kcal mol−1 in Q2, Q2D1, Q2D2, and Q2D3, respectively.
Hence NBO investigation of these molecules indicates that the extended hyper conjugation and strong intramolecular transfer of charge plays a significant role in stabilizing these systems and provides evidence of charge transfer characteristics that are crucial for potential NLO properties.
The linear and nonlinear responses of NLO materials arise from the electrical characteristics of the entire molecule. The NLO response properties of the Q1 and Q2 families of compounds have been estimated and the results for values of 〈α〉 and β are presented in Tables 4 and 5, respectively.
System | αxx | αyy | αzz | αtotal |
---|---|---|---|---|
Q1 | 644.744 | 396.922 | 137.529 | 393.065 |
Q1D1 | 955.102 | 788.767 | 359.700 | 701.1897 |
Q1D2 | 938.708 | 836.458 | 368.775 | 714.647 |
Q1D3 | 930.100 | 770.333 | 425.275 | 708.5693 |
Q2 | 772.065 | 681.095 | 275.425 | 576.195 |
Q2D1 | 1255.002 | 913.365 | 423.010 | 863.7923 |
Q2D2 | 1060.912 | 849.048 | 755.935 | 888.6317 |
Q2D3 | 1246.859 | 922.663 | 420.967 | 863.4963 |
System | βxxx | βxxy | βxyy | βyyy | βxzz | βyzz | βzzz | βtotal |
---|---|---|---|---|---|---|---|---|
Q1 | 5702.17 | 884.16 | −229.03 | 183.80 | −10.97 | 11.08 | −2.17 | 5568.09 |
Q1D1 | 9834.81 | −6526.60 | 397.07 | −3776.22 | −16.26 | 128.33 | 55.11 | 14420.20 |
Q1D2 | −12728.50 | −11190.89 | −4179.96 | −5623.95 | 19.92 | 70.96 | −65.46 | 23885.90 |
Q1D3 | −11016.63 | −7465.43 | −806.15 | −3690.90 | −193.28 | 46.25 | 92.98 | 16392.40 |
Q2 | −1653.41 | 2627.86 | −673.04 | 1288.99 | 27.04 | 22.09 | 32.91 | 4570.66 |
Q2D1 | −6660.35 | 473.68 | −493.84 | 2352.61 | 193.75 | −65.34 | −31.74 | 7498.18 |
Q2D2 | −3633.41 | 5694.79 | −7459.25 | 4279.34 | −573.46 | −298.39 | 510.96 | 16748.70 |
Q2D3 | −10000.88 | −405.63 | −498.86 | 2153.84 | 186.47 | −79.76 | −1.95 | 10452.70 |
Table 4 shows that the values of average polarizability for parent compounds Q1 and Q2 are 393.065 and 576.195 a.u., respectively. The most prominent transitions in all analysed molecules are along the x-axis (αxx) and modification with different acceptor moieties affects the polarization values. In both Q1 and Q2 families, the parent molecule has the lowest average polarizability and the presence of the strong acceptor unit EMN gives rise to the highest value of average polarizability. That is, the family maximum average polarizability is shown by Q1D2 and Q2D2, with the latter having the largest value for the molecules studied. Average polarizability decreases across the studied molecules in the order Q2D2 > Q2D1 > Q2D3 > Q1D2 > Q1D3 > Q1D1 > Q2 > Q1.
Dipole polarizability values (along the x-axis) are computed using eqn (11):
(11) |
In this equation MgmX indicates the ground and mth excited state transition moment and Egm is the transition energy. In general, a molecule with large value of MgmX and a smaller value of Egm will exhibit a high hyperpolarizability value. Hence, dipole polarizability is a quantitative measure of the NLO response properties of compounds. Second-order polarizability – first hyperpolarizability (βtot) – values for the Q1 and Q2 families of compounds are shown in Table 5.
Table 5 shows that for the parent molecules the hyperpolarizability value of Q1 (5568.09 a.u.) is 1.2 times greater than that of Q2 (4570.66 a.u.). The values of βtot for the set of molecules increase in the order Q2 < Q1 < Q2D1 < Q2D3 < Q1D1 < Q1D3 < Q2D2 < Q1D2. That is, the hyperpolarizability increases in a manner compatible with the decrease in Egap. The highest βtot values are shown by molecules modified with the strong acceptor moiety EMN.
In these molecules, the highest and lowest NLO responses can be attributed to the efficiency of charge transfer from donor to acceptor through their corresponding π-associated connection. Briefly, increases in hyperpolarizability values in the molecules studied here arise in association with the delocalization of π-electrons. This delocalization decreases the HOMO–LUMO energy difference. Calculated values of average polarizability and second-order polarizability of the investigated molecules are significantly greater than for urea (βtot(urea) = 43 a.u.), a standard molecule for the analysis of NLO response.59 Hyperpolarizability values of Q1, Q1D1–Q1D3, Q2, Q2D1–Q2D3 are determined to be 129 times, 335 times, 555 times, 381 times, 106 times, 174 times, 389 times and 243 times higher than the second-order polarizability value of urea, respectively.
To examine the electronic excitations in the optimized geometries, TDDFT calculations were performed. TDDFT generally overestimates the excitation energies in case of charge transfer transitions and extended conjugated systems. It was therefore necessary to adopt a robust and high level of theory to model the UV-visible absorption spectrum of the studied compounds. For this purpose, TDDFT calculations were conducted using the CAM-B3LPY level with the 6-311G(d,p) basis set in DMSO solvent. In performing TDDFT calculations, the six most reduced singlet–singlet transitions were examined in Q1, Q2 and their derivatives. The results for the calculated absorption wavelength (λmax), transition energy, nature of transitions (Ege) and oscillator strength (fos) are listed in Tables S17–S23 (ESI)† while some important values of these compounds are shown in Table 6.
Compounds | Ege (eV) | λmax (nm) | fos | LHE | MgmX (a.u.) | MO transitiona |
---|---|---|---|---|---|---|
a H = HOMO, L = LUMO, H − 1 = HOMO − 1, L + 1 = LUMO + 1 etc.b Experimental values in parentheses are from ref. 36. | ||||||
Q1 | 3.860 | 321 [326]b | 1.447 | 0.964 | 1.989 | H − 1 → L (83%) |
Q1D1 | 3.878 | 320 | 1.438 | 0.964 | 2.101 | H − 1 → L + 1 (85%) |
Q1D2 | 2.844 | 436 | 1.450 | 0.965 | 3.366 | H → L (65%) |
Q1D3 | 2.993 | 414 | 1.502 | 0.969 | 2.533 | H − 1 → L (60%) |
Q2 | 3.885 | 319 [323]a | 1.459 | 0.965 | 0.830 | H − 1 → L (44%), H → L (32%) |
Q2D1 | 3.912 | 317 | 1.283 | 0.948 | 0.140 | H − 1 → L + 1 (69%) |
Q2D2 | 3.874 | 320 | 1.455 | 0.965 | 2.442 | H − 1 → L + 1 (55%) |
Q2D3 | 3.954 | 314 | 1.184 | 0.934 | 2.046 | H − 1 → L + 1 (46%) |
Fig. 5 Simulated absorption spectra of quinoline–carbazole based compounds (Q1, Q1D1–Q1D3, Q2, Q2D1–Q2D3). |
Structural tailoring of compounds Q1 and Q2 modifies the contribution of different orbitals to the electronic transitions. In Q1, Q1D1–Q1D3, Q2, Q2D1–Q2D3, the crucial excited states are produced due to the promotion of electrons mainly from HOMO and HOMO − 1 to LUMO and LUMO + 1. This sort of charge transfer is required for NLO active compounds and forms the basis for the NLO responses of the compounds investigated here.
A crucial factor that determines the optical productivity of the molecules studied here is the light harvesting efficiency (LHE), which is determined using the fos values given by the TDDFT calculations. It is commonly observed that molecules with a large value of LHE exhibit high photocurrent properties. The value of LHE of the analyzed compounds is calculated using eqn (12) and the results are shown in Table 6.
LHE = 1 − 10−f | (12) |
The LHE values of all studied compounds are similar to each other. Structure–property relationships help us to gain an understanding of the reason for an improved NLO in response to changing the nature of the acceptor units. Oudar and Chemla60 formulated a two-state model, eqn (13), which is frequently used in the literature for NLO estimation.
(13) |
In this model the product of oscillation strength (fgm) and transition moment (Δμgm) is directly proportional and the transition energy cube Egm3 is inversely proportional to β. Thus, compounds exhibiting a large fgm and Δμgm and small Egm3 exhibit good NLO response properties. These parameters were estimated and their values are shown in Table 5. The relationship between βtot values and the corresponding two-level model (Δμgmfgm/Egm3) values for the studied molecules are displayed in Fig. 6, in which the βtot and two-level model values can be seen to be in very good agreement with each other.
Fig. 6 Relationship between the βtot (red star) values and the corresponding Δμgmfgm/Egm3 (blue triangles) values for the investigated compounds. |
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0ra02857f |
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