Single wavelength colour tuning of spiropyran and dithienylethene based photochromic coatings

Abstract

Controlling the transmission of thin films with external stimuli is an important goal in functional optical materials and devices. Tuning is especially challenging where both broad band (neutral density filtering) and spectrally varied (colour) transmission are required. The external control provided by photochemically driven switching, between transmission levels and colours, is functionally simple from a device perspective. The limits due to the spectral ranges of individual photochromic compounds can be overcome by combining several photochromes within one material or device. Here we show that a combination of photochromic molecular switches immobilised in a PMMA polymer matrix enables tuning of colour and transparency. We show that only a single excitation wavelength is required through the use of the primary inner filter effect and the layered construction of the films in which the photochromes nitrospiropyran (NSP), and nitrothiospiropyran (TSP) or 1,2-bis-terthienyl-hexafluorocyclopentene (DTE) are separated spatially. The approach taken circumvents the need to match photochemical quantum yields and thermal reactivity of the component photochromes. The photochemical switching of the films was characterised by UV/vis absorption spectroscopy and shows that switching rates and photostationary states are limited by inner filter effects rather than the intrinsic properties of photochromes, such as photochemical quantum yields and thermal stability. The photochemical behaviour and stability of the photochromes in solution and in the PMMA films were compared and the concentration range over which self-inhibition of photochemical switching occurs was established. The rate of photochemical switching and the difference in transmission between the spiropyran and merocyanine forms in solution enable prediction of the performance in the films and enable rational design of colour tuning ranges and responsivity in thin film filters.

Scheme 1. Colouration obtained by selective irradiation of photochrome-containing multilayers deposited on the glass. (a) Short irradiation of one side of the filter yields green, (b) irradiation of both sides yields grey, and (c) irradiation of the other side yields purple. Ring-opening of the (d) nitrothiospiropyran (TSP) and (e) nitrospiropyran (NSP) photochromes to their merocyanine forms TMC and NMC, respectively.

Fig. 1. (a) UV/Vis absorption spectra of NMC (purple) and TMC (green) in PMMA films on glass slides. Photochemical switching to their PSS_365nm of (b) NSP (black to red) and (c) TSP (black to brown) at room temperature. The films were spun from a 2 wt% solution of PMMA in dichloromethane containing 20 mM NSP or TSP.

Fig. 2. Average absorbance between 575 and 585 nm after 10 min irradiation at 365 nm with respect to the average initial absorbance between 340 and 345 nm for films of PMMA of various thicknesses on the glass containing various concentrations of NSP (slope = 2.19, R² = 0.95). An independent data set, prepared from new stock solutions, is shown in red.

Fig. 3. (a) Trajectory towards the PSS_365nm for NSP containing films of PMMA during irradiation. The initial absorbance at 365 nm was held the same for all three films by compensating for film thickness with a change in concentration of NSP. (b–d) UV/vis absorption spectra of each slide before (black), during (grey) and after (colour corresponds to line colour in panel (a)) irradiation.

Fig. 4. (a) Deposition of multiple layers by alternate spin-coating of PMMA and HEC containing solutions and (b) FTIR absorption spectra of a glass slide coated alternately with 10 layers of PMMA (8 wt%) from ethyl acetate and HEC (0.25 wt%) from water. The spectra after deposition of each PMMA layer are shown in black and of each HEC layer in red. The O–H stretching band of HEC at 3400 cm⁻¹ and C–H stretching bands of both HEC and PMMA at 2800–3000 cm⁻¹ are observed in this region.

Fig. 5. UV/vis transmittance spectra of glass slides cated with 6 layers of PMMA first with three layers of TSP and second with three layers of NSP present with intermediate layers of HEC. Irradiation from (a) the NSP side (from black to red) and (b) the TSP side of the slide (from black to blue (initially only to NMC) and then to green (loss of NMC and appearance of TMC)). The full UV-vis transmittance spectra and the trajectory towards the PSS_365nm at 580 nm (purple) and 700 nm (green) are shown in each case.

Fig. 6. Structure of DTE (1,2-bis-terthienyl-hexafluorocyclopentene) and glass slides coated with several layers of PMMA containing first NSP and then DTE. Irradiation at 365 nm on the side with (a) NSP and (b) DTE provides for differently coloured slides.

Fig. 7. Glass slides coated with several layers of PMMA with first TSP and then NSP after irradiation at 365 nm on the side with (left) NSP and (right) TSP. In addition to showing the generally protective inner filter effect, the patterning shows how an insufficient volume of HEC containing solution for the spin coating of intermediate layers leads to imperfect coverage and allows for sublayers of PMMA to be removed by subsequent washing steps (Fig. S7, ESI†).

Scheme 2. Photobleaching upon continuous irradiation of the side of the slide with PMMA containing TSP is manifested in a colour change over time, as the TMC is degraded allowing for the NSP to switch to the NMC form (a to c).