Abstract:
Organic π-conjugated materials with Nonlinear Optical (NLO) properties have
attracted interest because of their applications in optoelectronic devices. They are
preferred to their inorganic/organometallic analogues owing to their lower cost of
production. In previous works, the electronic and NLO properties of organic
molecules based on phenothiazine and methylidene units have been reported.
However, there is dearth of information on the structural and solvent effects on these
properties. Therefore, this study was designed to investigate the structural and
solvent effects on the electronic and NLO properties of literature experimental
synthesised organic molecules and their modelled analogues.
Quantum mechanical calculations were employed to investigate the electronic band
gap (Eg) and molecular first hyperpolarisability (β) of synthesised phenothiazine and
methylidene derivatives and their modelled analogues using the density functional
theory. Pure Becke Lee Yang Parr (BLYP) and hybrid Becke Three Lee Yang Parr
(B3LYP) correlations were used for optimisation with 6-31G (d) basis set in vacuum
and tetrahydrofuran. Both correlations were chosen in order to validate available
literature experimental results. Time-dependent density functional theory was
employed to calculate the maximum absorption wavelength (λmax). Their values
were compared with that of urea, a standard for organic NLO materials.
The calculated Eg of the synthesised phenothiazine, methylidene, modelled
phenothiazine and methylidene analogues in vacuum were 3.90, 3.85, 2.57–3.84 and
3.30-3.58 eV, respectively with B3LYP while those for BLYP were 2.46, 2.47, 1.38-
2.36 and 2.03-2.28 eV. The available literature experimental Eg for synthesised
methylidene derivative was 2.27 eV. The BLYP (σ = 0.20 eV) correlation predicted
Eg more accurately than B3LYP (σ = 1.58 eV). All values were lower than that of
urea (literature experimental = 6.21 eV; calculated = 8.20 and 5.76 eV for B3LYP
and BLYP, respectively) and decreased in tetrahydrofuran. The calculated β in
vacuum were 1.49 x10-30, 2.44 x10-30, 1.71-5.13 x10-30 and 3.84-9.67 x10-30 esu,
respectively with B3LYP while they were 1.51 x10-30, 2.61 x10-30
, 1.74-5.76 x10-30
and 4.03-10.09 x10-30 esu with BLYP. The SHG efficiency of synthesised
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methylidene derivative was 4.13 times that of urea. All modelled analogues had
higher β than urea’s (0.65 x 10-30 esu). The BLYP (4.02 times urea’s, 97.3 %)
correlation predicted SHG efficiency more accurately than B3LYP (3.75 times
urea’s, 90.8 %) and increased in tetrahydrofuran. The calculated λmax in vacuum
were 293, 344, 276-378 and 369-399 nm, respectively with B3LYP while those for
BLYP were 362, 412, 346-470 and 496-517 nm. Literature synthesised
phenothiazine derivative absorbed at 294 nm while the modelled phenothiazine
absorbed at 294 nm and 362 nm for B3LYP and BLYP, respectively. The B3LYP
predicted the λmax accurately. All λmax values were higher than urea’s (< 200 nm)
and increased in tetrahydrofuran.
The electronic and nonlinear optical properties of modelled phenothiazine as well as
methylidene analogues were sterically enhanced by substituents groups and were
altered by the inclusion of tetrahydrofuran solvent.
