Density functional theory (DFT) and time-dependent DFT (TDDFT) were employed to study ground-state properties, HOMO-LUMO gaps
In the last decade, conjugated polymers have been found to be interesting materials for electronic and optoelectronic devices, such as flat panel display (FPD) [
In general, the optical band gap of conjugated polymers can be controlled via the modification of their chemical structures. In order to achieve the better properties, the chemical structure can be improved through the following methodologies: first [
Recently, theoretical quantum calculations have been the famous tools because they can be used to rationalize the properties of known polymers and also predict those of unknown ones to guidance observed experimental synthesis. These methods include (i) density functional theory (DFT) [
In this work, we study the electronic and optical properties of PFV-
Conjugated polymers for theoretical analysis.
The ground-state geometries of the studied molecules,
The selected optimized interring bond lengths and dihedral angles of these oligomers are listed in Table
Selected interring distances and dihedral angles of PFV-
Oligomer | Interring distances ( | Dihedral angles (deg) | ||||
F-V | V-N | V-P | F-V | V-N | V-P | |
(PFV- | ||||||
1.4655 | 1.4693 | 9.362 | 33.820 | |||
1.4656 | 1.4700 | 12.021 | 35.066 | |||
1.4655 | 1.4699 | 11.839 | 34.987 | |||
1.4656 | 1.4698 | 11.986 | 34.786 | |||
(PFV- | ||||||
1.4626 | 1.4604 | 3.162 | 10.678 | |||
1.4621 | 1.4603 | 5.603 | 14.374 | |||
1.4621 | 1.4585 | 4.806 | 8.928 | |||
1.4622 | 1.4588 | 6.487 | 12.652 |
F is fluorene ring, V is vinylene unit, N is naphthalene ring, and P is phenylene ring.
For PFV-
For PFV-
The highest occupied molecular orbitals (HOMO) and the lowest unoccupied molecular orbitals (LUMO) are useful for understanding more details on excited-state properties. They can provide a reasonable qualitative indication of the subsequent excitation properties and the ability on electron or hole transport in feature of electron density contour. The contour plots of HOMO and LUMO orbitals of PFV-
Frontier molecular orbitals of PFV-
For PFV-
For PFV-
To gain insight into the effect of the alternating groups, the HOMO and LUMO energies of PFV-
The HOMO and LUMO energies of PFV-
There are two theoretical approaches used in this study to obtain the energy gaps. First approach is a crudely estimated from the different energies between HOMO and LUMO (
The
The HOMO-LUMO gaps (
Oligomer | TDDFT | ||
Wavelength (nm) | |||
(PFV- | |||
3.53 | 3.19 | 388.85 | |
3.16 | 2.79 | 445.16 | |
3.04 | 2.67 | 464.88 | |
2.94 | 2.59 | 478.62 | |
2.77 | 2.40 | 516.88 | |
Exp. [ | 2.56 | 484.57 | |
(PFV- | |||
3.30 | 3.08 | 402.78 | |
2.79 | 2.49 | 498.24 | |
2.62 | 2.29 | 540.35 | |
2.56 | 2.23 | 556.89 | |
2.30 | 1.92 | 646.09 | |
Exp. [ | 2.10 | 592.00 |
The HOMO-LUMO gaps (
Moreover, we found that the vertical excitation energies (
Ionization potentials (IPs) and electron affinities (EAs) were employed to estimate the energy barrier for the injection of hole and electron of PFV-
Ionization potentials (IPs) and electron affinities (EAs) for PFV-
Oligomer | IP(v) | EA(v) |
---|---|---|
(PFV- | ||
6.28 | 0.47 | |
5.75 | 1.02 | |
5.48 | 1.24 | |
5.12 | 1.61 | |
(PFV- | ||
6.13 | 0.46 | |
5.50 | 1.11 | |
5.40 | 1.37 | |
4.98 | 1.80 |
The TDDFT/B3LYP/6-31G(d) was employed to obtain the energy of the singlet-singlet electronic transitions as well as transition energies, oscillator strengths, and main configurations for five singlet-excited states of PFV-
Electronic transition data obtained by TDDFT method for PFV-
Electronic transitions | Wavelengths (nm) | Main configurations | |
---|---|---|---|
PFV- | |||
388.85 | 1.0066 | ||
339.28 | 0.0099 | ||
320.33 | 0.2709 | ||
307.31 | 0.0336 | ||
294.86 | 0.0045 | ||
(PFV- | |||
445.16 | 2.4685 | ||
398.95 | 0.0087 | ||
397.57 | 0.0474 | ||
368.08 | 0.0258 | ||
363.98 | 0.6254 | ||
(PFV- | |||
464.88 | 3.8927 | ||
434.31 | 0.0639 | ||
421.81 | 0.2199 | ||
409.94 | 0.4292 | ||
399.71 | 0.0004 | ||
(PFV- | |||
478.62 | 4.9660 | ||
454.05 | 0.2804 | ||
436.32 | 0.5958 | ||
432.48 | 0.2016 | ||
422.51 | 0.0040 |
Electronic transition data obtained by TDDFT method for PFV-
Electronic transitions | Wavelengths (nm) | Main configurations | |
---|---|---|---|
PFV- | |||
402.78 | 1.3722 | ||
341.10 | 0.2476 | ||
313.14 | 0.0576 | ||
301.95 | 0.0012 | ||
282.01 | 0.0228 | ||
(PFV- | |||
498.24 | 3.3091 | ||
430.94 | 0.0006 | ||
408.82 | 0.0044 | ||
376.04 | 0.5267 | ||
369.20 | 0.0039 | ||
(PFV- | |||
540.35 | 4.9913 | ||
470.79 | 0.0004 | ||
466.44 | 0.0077 | ||
433.73 | 0.0671 | ||
431.29 | 0.9033 | ||
(PFV- | |||
556.89 | 6.7096 | ||
502.80 | 0.0003 | ||
486.97 | 0.0003 | ||
463.66 | 1.2450 | ||
459.86 | 0.0007 |
The maximal absorption wavelengths of PFV-
The properties on the excited state were carried out by using configuration interaction singles (CIS). CIS is the cheapest method with reasonable accuracy for studying the excited-state properties. However, the prediction on CIS is not accuracy enough due to the neglecting of electron correlations [
The prediction of different bond lengths between the ground (
Comparison of the excited structures (
In opposite, the HOMO has bonding across
In addition, the fluorescence energies of all oligomers were estimated using TDDFT calculation on B3LYP/6-31G(d), and the results are summarized in Table
Fluorescence energies and radiative lifetime of PFV-
Oligomer | Electronic transition | Fluorescence energies (eV) | Lifetime (ns) | Main configurations | |
(PFV- | |||||
1.2498 | 2.71 | 2.51 | |||
2.5731 | 2.56 | 1.37 | |||
3.1934 | 2.54 | 1.12 | |||
2.45 | 0.36 | ||||
Exp. [ | 2.37 | ||||
(PFV- | |||||
1.5925 | 2.75 | 1.91 | |||
3.4649 | 2.28 | 1.28 | |||
4.7141 | 2.17 | 1.04 | |||
1.86 | 0.61 | ||||
Exp. [ | 1.85 |
Finally, the fluorescence energies and oscillator strengths were used to calculate the radiative lifetime by using the Einstein transition probabilities in the following formula (in au.) [
The predicted radiative lifetimes are collected in Table
Theoretical studies on the electronic structure and optical properties of PFV-
Finally, our results reveal that improving the electronic and optical properties of polymer can be controlled by the appropriate alternating group such as dialkoxyl phenylenevinylene unit which is an interesting molecule for enhancing the performance of LEDs based on easily injection of hole and electron and also long-time emission. In addition, the good agreement tendency between theoretical calculation and experiment indicates that these methods are possible and reliable for precise predicting new compounds as LEDs materials.
The authors would like to express grateful acknowledgement to Department of Chemistry, Faculty of Science, Chiang Mai University. Financial support from the Center for Innovation in Chemistry (PERCH-CIC), Commission on Higher Education, Ministry of Education is gratefully acknowledged. This work is also financially supported by The Thailand Research Fund (TRF) senior research scholar (RTA5080005). And the Graduate School of Chiang Mai University is also acknowledged. V. Sanghiran Lee, P. Nimmanpipug, R. Deang-ngern, C. Sattayanon, T. Piansawan, J. Yana, P. Tue-ngeun, W. Sangprasert, and C. Ngaojampa are acknowledged for their helps.