M. P. Joshi Proc. of ASID’06, 8-12 Oct, New Delhi 228 Broad-band visible emission from UV-exposed TPD solution M. P. Joshi 1* , S Raj Mohan 1 , S. K. Tiwari 1 , T. S. Dhami 1 , T. Shripathi 2 , U.P. Deshpande 2 , M. K. Singh 3 , H. Ghosh 1 1 Laser Physics Application Section, Raja Ramanna Centre for Advanced Technolgoy, Indore, MP - 452 013, India. 2 UGC-DAE Consortium for Scientific Research, Khandwa Road, Indore 452017, India 3 Laser Materials and Device Development Division, Raja Ramanna Centre for Advanced Technolgoy, Indore, MP - 452 013, India. *mukesh@cat.ernet.in Abstract: Strong broad band visible emission from N-N’- diphenyl-N-N’-bis(3-methylphenyl)-1-1- biphenyl-4,4’- diamine (TPD) is reported when solution of TPD (in halomethane solvents like chloroform or dichloromethane) is exposed to UV light. Emission color changes from originally blue to green with exposure. Uv-visible absorption, FTIR and XPS studies of photoproduct suggest photodegradation of TPD. With UV exposure the photolysis of chloroform first oxidizes TPD and with further exposure a permanent transformation/degradation of TPD to possibly its chloride salt and other low molecular weight compounds. Strong blue- green dual emission is observed from the photoproduct dissolved in different solvents (solvent dependent) and also from vacuum sublimed thin film. Compared to pure TPD an improved electrical conductivity has been observed in photodegraded TPD dispersed polymers films. Keywords: TPD; photodegradation, visible emission; exciplex Introduction N,N-diphenyl-N,N-bis(3-methylphenyl)-1, 1-biphenyl-4.4 diamine (TPD) a well-known aromatic diamine dye with emission in blue (λ em ~ 410nm) and used extensively in developing organic light emitting diodes (OLEDs)[1] and solid-state dye laser[2]. Due to lone pair of electrons at nitrogen site the TPD molecule is chemically very reactive and easily undergoes charge transfer reactions with electron acceptors. This property also makes TPD structure unstable under certain environmental conditions of certain gases or solvents[3]. Stability of TPD has been addressed but mostly in doped polymers films and with respect to lasing parameters like pump intensity, repetition rate and polymer matrix[4]. Photodegradation of TPD in solutions and resulting changes in optoelectronic properties has not been reported so far. We report observation broad band visible emission covering the entire blue–green range from TPD when solutions of TPD in halomethane solvents are exposed with UV light. A systematic shift of emission from blue to green is observed with exposure time. We emphasize exclusive use of halomethanes like chloroform and dichloromethane solvents which initiates photochemical reactions and degrade TPD. Resultant solid photoproduct show solvent dependent blue- green broad band emission and is thermally stable as sublimed films also show broad band visible emission. Compared to pure TPD the photoproduct dispersed in polystyrene shows drastic improvement in electrical conductivity. Photoelectron spectroscopy, FTIR and absorption and photoluminescence measurements suggest the origin of emission in green is presumably the characteristics of TPD based chloride salt or an exciplex emission from TPD and its photoproducts. Experiment Solution of TPD (Aldrich, 99.8% pure) in spectroscopic grade chloroform was prepared (~ 10 -4 M concentration) and part of this solution was transferred to a 1 mm path-length quartz cuvette and kept under UV lamp for exposure (we used 4W germicidal lamp, long wavelength UV light (λ peak ~ 365 nm), fluence ~ 8nJ/m 2 s). With exposure, visible inspection showed gradual change of TPD emission from blue to white to finally green with darkening of solution color. Figure 1 shows exposure time dependent changes in photoluminescence spectrum of TPD-chloroform solution. With increasing exposure, the emission maxima ~ 410 nm of TPD decreases and growth of new Figure 1: Changes in photoluminescence spectrum of TPD-chloroform solution with UV exposure taken at different exposure durations (a) 0 s, (b) 10s, (c) 30s, (d) 1.5m, (e) 3.5 m, (f) 8.5m, (g) 18.5m, (h) 38.5m, (i) 68.5m, (j) 113.5m. All the data recorded with excitation wavelength at 365 nm. Inset show structure of TPD. broad band emission centered around 520nm take place. Optimum time for appearance of green emission was ~ 2 hrs for given TPD concentration. Saturation of emission intensity and peak position was observed with further UV exposure. Even for prolonged UV exposure for more than few days there was no change or reduction in green emission intensity. Exclusive use of halomethanes was proved when similar observation were made in dichloromethane but not in other solvents like toluene, dioxane etc. Exclusive use of halomethane solvent seems to be playing an important role in 400 450 500 550 600 650 700 0 1x10 7 2x10 7 3x10 7 4x10 7 5x10 7 (j) (i) (h) (g) (f) (e) (c) PL Signal (a.u.) Wavelength (nm) (a) (b) (d) 410nm 520 nm