L1 and A1 compounds have the same fragments in the molecular structure: the electron-acceptor dibenzothiophene-S,S-dioxide and two electron-donating diphenylamine fragments connected to the central fragment via meta- and para-positions with respect to the SO2 group. In the A2 compound, in comparison with A1, the tretbutyl groups are replaced by methoxy groups. Methoxy substitution is followed by a small (~ 6 nm) blue shift of the main absorption bands. In the L2, the central acceptor fragment consists of phenylphenanthridine.
Since the mutual arrangement of the diphenylamine fragments and the central fragment leads to a noticeable change in the spectral properties, we shall conditionally designate L1 and L2 as compounds of linear structure, and A1 and A2 as those of angular structure. Thus, linear compounds have a band in the absorption spectra in the region of ~ 440 nm (Fig. 2), while angular molecules do not have one. At the same time, an abnormally large Stokes shift of 7000–9000 cm− 1 is observed in all the studied media for the A1 and A2 compounds of angular structure (Table 1). This fact is clarified by quantum chemical calculations [12]. The long-wave absorption band of these compounds appears to be formed by the S0-S3 transition, whereas the S0-S1 transition has very low oscillator strength (~ 0.08) due to its charge-transfer nature and it is located on the red wing of the absorption band.
It should be noted that all compounds are characterized by a weak solvatochromic dependence, i.e. the change of the ambient medium (ethanol, chloroform and TVD film) has almost no effect on the position of the bands in the absorption spectra (Table 1, 2 and Fig. S1).
Table 1
Spectral-luminescent properties of compounds in CHCl3, ethanol and TVD-films.
Compound | L1 | A1 | L2 | A2 |
Media | CHCl3 | Ethanol | TVD | CHCl3 | Ethanol | TVD | CHCl3 | Ethanol | TVD | CHCl3 | Ethanol | TVD |
λabs, nm | 382, 440 | 376, 432 | 380, 432 | 362 | 360 | 360 | 377, 433 | 370, 440 | 370, 440 | 358 | 350 | 356 |
λPF296(λPF77)*, nm | 525 | 538 (483/513) | 517 (518/539) | 512 | 548 (467) | 485 (488) | 518 | 526 (504) | 502 (501) | 553 | 605 (501) | 535 (530) |
ΔνStokes, cm− 1 | 3680 | 4560 | 3810 | 8090 | 9530 | 7160 | 3790 | 3720 | 2810 | 10330 | 12040 | 9040 |
PLQY296K | 0.94 | 0.46 | 0.2 | 0.2 | 0.2 | 0.06 | 0.2 | 0.15 | 0.03 | 0.4 | 0.02 | 0.13 |
λDL296(λDL77), nm | - | 511 | 527 (539) | - | 465 | 488 | - | 530 | - | - | 605 | - |
λPhos77, nm | - | 596 | 632/687 | - | 523 | 535 | - | 566 | 570 | - | 534/565 | 560/591 |
τphos1/τphos2** ms | - | 2.4/ 11.5 | - | - | 200 | 16.8/ 68.9 | - | 770 | 12.9/ 158 | - | 113 | 16.6/ 66.2 |
Δ(ES1-ET1), ev | | 0.49 | 0.43 | | 0.28 | 0.22 | | 0.27 | 0.30 | | 0.15 | 0.13 |
Δ(ES1-ET1)calc, ev [12] | 0.34 | 0.13 | - | 0.13 |
* the position of the most intense vibronic transition is indicated via a slash ** phosphorescence lifetimes by two-exponential decay |
The fluorescence spectra are more sensitive to the polarity of the solvent. Thus, the A1 in hexane emits with a maximum at 442 nm, and in acetonitrile, it emits with a maximum at 558 nm. This means that changing non-polar to strongly polar solvent, the fluorescence band shifts to the red region by 116 nm. The second A2 compound of angular structure undergoes an even greater shift – 174 nm. The red shift of linear compounds is smaller (74 and 40 nm for the L1 and L2 respectively). The dependences of the maxima position of the absorption and fluorescence bands on the solvent are shown in Table 2 and Fig. S1. The higher the polarity of the solvent is, the broader the fluorescence bands are. For angular molecules, the bands in acetonitrile are 50–60% wider than in hexane. For linear molecules, this value is 15–17%. A noticeable difference in the half-widths of the fluorescence bands of linear and angular molecules in polar solvents indicates a greater variety of geometric conformers of the latter in the electron-excited state. The red shift of radiation with increasing polarity of the solvent indicates a strong redistribution of the electron density in the molecule in the S1 electron-excited state. An increase in intermolecular interactions with the solvent in the excited state can be expected to be due to a significant increase in the dipole moment in the S1 state.
From the previously obtained data [12], it is known that the S0→S1 transitions are of low intensity and they are located on the wing of more intense high-energy transitions in the A1 and A2 angular molecules. Therefore, the dipole moments from solvate models cannot be determined for these molecules. However, in the L1 and L2 linear molecules the S0→S1 transitions have sufficiently high intensities, their position can be determined from the absorption spectra and the dipole moments in the S0 and S1 states can be calculated for them.
The Bilot-Kawski model was used to estimate the dipole moments of molecules in the S0 and S1 states [,]. In contrast to the Lippert-Mataga model, this model gives a more accurate agreement with the experiment [14]. The Supplementary Material provides a method for calculating dipole moments using the Bilot-Kawski model and shows the values of the dipole moments of the L1 and L2 molecules (Table S1). The dipole moments of both molecules have been found to be small in the ground state and they amount to 0.61 and 1.05 Debye for the L1 and L2 respectively. During transition to the S1 state, the dipole moments increase (6.96 and 6.08 Debye for the L1 and L2 respectively). This suggests that even in the molecules with an almost linear arrangement of donor substituents, there is a significant redistribution of the electron density. Therefore, even greater values of the dipole moments themselves should be expected both for the molecules with the angular arrangement of substituents and changes during transition to the excited state. These results confirm the data of quantum chemical calculations [12] and the assumption about the charge-transfer nature of the S0→S1 transition in such molecules.
It is characteristic that the position of the fluorescence maxima of all compounds in TVD films is close to those in weakly polar solvents (toluene and chloroform). This suggests that in TVD films, with the molecules in the sample packed more closely than in low-concentration solutions, the compounds do not enter into specific intermolecular interactions and retain their individual properties. The quantum yield of fluorescence in TVD films has also been found to decrease significantly compared to chloroform solutions (Table 1). These compounds are therefore characterized by concentration quenching since the fluorescence intensity has not changed in the argon atmosphere.
Table 2
Maxima of absorption and fluorescence wavelengths of compounds in various solvents
Solvent | ε | n | L1 | A1 | L2 | A2 |
λabs, nm | λfl, nm | λabs*, nm | λfl, nm | λabs, nm | λfl, nm | λabs*, nm | λfl, nm |
n-Hexane | 1.9 | 1.375 | 423 | 463 | 355 | 442 | 420 | 480 | 350 | 466 |
Toluene | 2.38 | 1.494 | 433 | 490 | 367 | 478 | 431 | 492 | 355 | 516 |
Chloroform | 4.78 | 1.443 | 440 | 525 | 362 | 512 | 433 | 518 | 358 | 553 |
Chlorobenzene | 5.63 | 1.525 | 432 | 508 | 363 | 507 | 431 | 505 | 357 | 549 |
Ethyl acetate | 6.00 | 1.370 | 425 | 500 | 359 | 509 | 422 | 499 | 353 | 549 |
Tetrahydrofuran | 7.22 | 1.404 | 424 | 498 | 360 | 509 | 425 | 498 | 354 | 551 |
Acetone | 20.8 | 1.356 | 425 | 516 | 358 | 546 | 429 | 513 | 352 | 609 |
Ethanol | 25.3 | 1.361 | 432 | 538 | 360 | 548 | 440 | 526 | 350 | 605 |
Dimethylformamide | 36.7 | 1.428 | 428 | 536 | 358 | 550 | 428 | 520 | 352 | 624 |
Acetonitrile | 38.8 | 1.342 | 430 | 537 | 357 | 558 | 422 | 526 | 351 | 640 |
Dimethyl sulfoxide | 48.9 | 1.476 | 434 | 540 | 361 | 563 | 428 | 530 | 354 | 632 |
* the absorption maxima of the observed long-wave transitions are given for angular molecules |
Monitoring of luminescence spectra, when decreasing the temperature in steps of 30° from room temperature to 77 K, was carried out both in the Prompt Fluorescence mode and in the Delayed Luminescence mode, i.e. 200 µs after the flash of the excitation lamp with a registration duration of 5 ms. In the Delayed Luminescence mode, both delayed fluorescence and phosphorescence were recorded. The delayed fluorescence was observed at room temperature and disappeared (or almost disappeared) as the temperature decreased. The phosphorescence in the form of a new band of radiation appeared in the red region of the spectrum and, as a rule, its intensity increased strongly with a decrease in temperature. Such studies were carried out both in ethanol solution and in TVD films.
Absorption and fluorescence spectra in ethanol. A decrease in the temperature of ethanol solutions of compounds leads to a bathochromic shift of the absorption bands (a change in the absorption position was recorded in the fluorescence excitation spectra. Fig. S3). Similar effects were observed in the following works [,,]. It is known that when the temperature decreases, both macroscopic parameters increase – the refractive index of the medium (nD) and the dielectric constant (εD) which affect the shape and position of the absorption and fluorescence bands. As mentioned above, the absorption spectra of the compounds practically did not shift (Fig. S1) when changing the solvent from εD = 1.9 (hexane) to εD = 48.9 (dimethylsulfoxide). Consequently, this parameter did not significantly affect the red shift of the absorption spectra as the temperature decreased.
The increase of nD boosts the power of dispersion interactions, which are stronger in the excited state than in the ground state due to higher polarizability. For this reason, the energy level of the Frank-Condon excited state of the S1FC decreases more strongly than the S0 state. As a consequence, the bathochromic shift of the dye absorption bands during the cooling stage can be explained by increasing nD, E(S0→S1FC )77 <E(S0→S1FC)293, where E is the electron transition energy.
Figure 3 shows the fluorescence spectra of compounds in ethanol solution at a decreasing temperature from room temperature to 77K. With a decrease in temperature, all ethanol solutions of the studied compounds are characterized by a hypsochromic shift of the fluorescence band. This behaviour is associated with a change in the solvent relaxation time which increases with an increase in the number of hydrogen bonds formed between alcohol molecules [,] as the temperature decreases up to the solid-phase state at T ≈ 158K. With an increase in the number of hydrogen bonds in the solvate shell, resolvation of the dissolved molecules slows down during the lifetime of the excited state, which is reflected in the blue shift of the fluorescence bands and a decrease in their half-width by an average of 25%.
Interestingly, the fluorescence intensity of molecules with angular substituents (A1, A2) passes through the maximum as the temperature decreases (Fig. 3, S13, S15). The intensity increases approximately up to the glass transition temperature (the quantum yield of fluorescence increases), but a further decrease in the temperature to 77K is followed by a decrease in the fluorescence intensity. The reason for this phenomenon is still unclear.
The fluorescence spectra in TVD films. The fluorescence behaviour of compounds in TVD films is very different from that in ethanol solution. When the temperature decreases, the position of the bands in the spectra practically does not change (Fig. S16-S19), but the radiation intensity of the L1, A1 and L2 compounds at temperatures below 150K drops sharply (Fig. S8a-S11a). In the A2 compound in the temperature range from 296 to 77K, the intensity changes are less significant (within 20%). The reason for the different behaviour of fluorescence of compounds in TVD films at a decreasing temperature is still difficult to comment on.
Delayed fluorescence and phosphorescence. The radiation spectra recorded with a delay of 200 µs after the lamp flash at room temperature in ethanol solutions coincide in position and shape with the prompt fluorescence spectra (Fig. S4-S7). The coincidence of the radiation bands provides grounds to believe that the recorded radiation belongs to delayed fluorescence.
The delayed emission at room temperature of TVD films of compounds, except the L1, differs from prompt fluorescence (Fig. S8-S11). The maxima of the delayed luminescence bands of the A1, L2 and A2 films are shifted to the red region with respect to fluorescence (Fig. 4).
Weak delayed luminescence is observed at room temperature for the L1 compound. Unlike other compounds, delayed fluorescence of L1 is observed over the entire temperature range (Fig. S8). However, a phosphorescence band appears in the long-wave region at temperatures below 150K. Delayed fluorescence of L1 may be caused by the process of the triplet-triplet annihilation (TTA) as was observed for this molecule in [7]. The long-lived radiation for all other compounds lies in the longer wavelength region relative to fluorescence already at room temperature (Fig. 4) and can be attributed to mix of delayed fluorescence and phosphorescence. With a decrease in temperature, the intensity of phosphorescence increases significantly. The greatest increase in intensity (180 times) is observed for the A1 film. A clear separation according to the position in the spectrum into fluorescence and phosphorescence at low temperatures for the L1, L2 and A1 allows for estimating the energy gap ΔST between the S1 and T1 states. Delayed luminescence of the A2 compound at room temperature lies in almost the same region as fluorescence, but the radiation band is broadened on the red wing side. With a decreasing temperature, the intensity of this band increases and a narrowing appears on the side of the blue wing (Fig. S11).
Table 1 shows ΔST values for both ethanol solutions and films. It can be seen that these values are close to each other in solutions and films and they correlate well with the data of quantum chemical calculations [12]. The lowest value of 0.13–0.15 eV is observed for the A2 compound. Such a small value of ΔST leads to a significant overlap of the fluorescence and phosphorescence bands, increases the rate constant of reverse intersystem crossing and the intensity of thermally activated delayed fluorescence. In addition, it is known [12] that the presence of auxiliary methoxyl groups in the donor fragments of the A2 compound leads to an increase in the matrix element of the spin-orbit interaction by an order compared to A1, which also increases the yield of TADF. Thus, we can expect that the electroluminescence intensity of the OLED structure with the A2 should be sufficiently high due to the borrowing of triplet excitons by the mechanism of thermally activated delayed fluorescence.
The phosphorescence kinetics can be described by a two-exponential decay in TVD films (Table 1). The molecules might be in more than one molecular conformation with a denser packing of molecules in the process of thermal vacuum deposition in the film.