Spacer Directed Comparable and Contrasting Optical Properties in D-A Molecules Enabling Fabrication of Efficient Novel Thin Film Phototransistors

Abstract

Intuitive and insightful reasoning based on experimental evidences can be employed for devising a hypothesis toward development of functional molecules equipped with desired optical properties. The present work relays a detailed study on novel Donor-Acceptor type molecules using a bimodal strategy toward creating contrasting and comparable optical properties. The study relies on architectural distinction in curcumin-BF2 based D⎯A molecules by varying the spacers from rigid (conjugated; D⎯π⎯A,) to flexible (conjugation forbidden, D⎯σ⎯A) as well as the strength of donor from anthracene to carbazole. Four molecules AnBr/CbBr (D⎯π⎯A) and AnBf/CbBf (D⎯σ⎯A) have been designed and synthesized. The combined synthetic, steady state and time resolved photophysical and theoretical approach have established the contrasting emissive behavior of D⎯π⎯A and D⎯σ⎯A, with former being more emissive in solution (ACQ) and later in solid state (AIE). The comparative optical behvaiour was engendered in each category through varying the strength of donor leading to strong dual emission in AnBf contrary to CbBf. However, the higher emission quantum yield of CbBr relative to AnBr proclaims the strong electron donating ability of carbazole. Owing to their specific absorption in solid state, AnBf and CbBf have further been employed for fabrication of blue sensitive photodetector. These photodetectors have been made in phototransistor geometry which shows large variation of threshold voltage and Off current of the devices. Devices with CbBf and AnBf molecules show excellent photoresponsivity of 3 A/W and 1 A/W respectively for the blue light intensity of 1 W/m2.