Charge Transfer in 1,8-Naphthalimide: A Combined Theoretical and Experimental Approach Anamika Manna and Sankar Chakravorti* Department of Spectroscopy, Indian Association for the Cultivation of Science, Jadavpur, Kolkata, India Received 21 May 2009, accepted 24 July 2009, DOI: 10.1111 ⁄ j.1751-1097.2009.00625.x ABSTRACT Photophysics of 1,8-naphthalimide (NAPMD) in different sol- vents has been delineated in this paper. Theoretically calculated bond distance of N–H and C=O groups rule out any intramo- lecular proton transfer in the excited state. Concomitant increase in negative charge on O atom compared to N atom and dipole moment hints at possible intramolecular charge transfer. Pro- gressive redshift with polarity of solvents in emission and absorption spectra also confirms the theoretical prediction. Weakening of N–H bond helps hydrogen abstraction and anion formation in water with decay time of 2.54 ns through intermo- lecular proton transfer. This was corroborated from the ground state photoexcitation of laboratory synthesized anion of NAP- MD. Amide hydrolysis in higher pH and excess proton avail- ability at low pH are responsible for anion emission quenching. A possible electron transfer diminishes phosphorescence at 77 K with changing pH. INTRODUCTION Naphthalimides comprise an environment-sensitive special class of chromophores (1) whose spectroscopic behavior depends on the properties of the surrounding medium. 4-amino-1,8-naphthalimide (NAPMD) shows sensitivity to the polarity of the local environment, which is very useful as solvatochromic fluorophore (2,3). Due to their strong yellow- green fluorescence and good photostability, the NAPMD derivatives have got considerable application in many areas such as coloration of polymers (4), laser active media (5), potential photosensitive biological units (6), fluorescent mark- ers in biology (7), analgesics in medicine (8), light emitting diodes (9), photoinduced electron transfer sensors (10), fluo- rescence switchers (11), electroluminescent materials (12), liquid crystal displays (13) and ion probes (14). As NAPMD derivatives are very sensitive to the environ- ment, many studies have been performed with these molecules as solvatochromic probes (15). Some spectral analysis for different substitutions at the C4 position of NAPMD (16–18) have been performed but a detailed solvatochromic investiga- tion into NAPMD could not be found in the literature and very little work has been done on the mechanistic properties of imides (19). The thought-provoking feature in NAPMD is the acidic proton is symmetrically placed between two C=O groups. These goaded us to investigate the geometry in the ground and excited states vis-a`-vis the investigation into the photophysics of NAPMD in respect of intramolecular charge transfer (ICT) both experimentally and also theoretically to establish mechanistic details of this probable charge transfer in NAPMD. In this paper the spectroscopic behavior of NAPMD was studied in different organic solvents, which modulates the ICT process in the probe by the probe–solvent interaction at room temperature and at low temperature (77 K) along with effect of pH on the emission of NAPMD. Theoretical calculations hint at the asymmetric nature of two charge acceptor sites in the excited state and a possible ICT state. Steady-state and picosecond time-resolved emission spectroscopy were employed to get the mechanistic details. MATERIALS AND METHODS NAPMD was purchased from Sigma Aldrich Chemical Company and was purified by recrystallization before use. Methylcyclohexane (MCH), acetonitrile (ACN), trimethylamine (TEA) and sulfuric acid (H 2 SO 4 ) were obtained from Merck Specialties Pvt. Ltd. (India) and before using them their emission was checked in the region of interest. Preparation of anion of NAPMD. Sodium hydride (0.05 g, 1.2 mmol, 60% suspension in mineral oil) was washed with 5 mL of tetra hydro furan (THF) under N 2 atmosphere. To this, dry THF (20 mL) was then added, followed by slow addition of 0.197 g (1 mmol) of NAPMD, taken in the same solvent (10 mL). The reaction mixture was then allowed to stir for a period of 30 min. The resultant solution was treated with 20 mL of ice cold water and stirred for another 20 min. The organic phase was extracted with chloroform (3 · 30 mL) and the combined organic phase was dried over anhy- drous Na 2 SO 4 . Solvent volume was reduced by rotary evaporation and allowed to stand at 4°C overnight. The crystalline compound deposited at this stage was collected by filtration, washed with diethyl ether (3 · 10 mL) and dried in vacuum over P 4 O 10 and the yield was 64%. Experimental and theoretical details. The absorption spectra were taken with a Shimadzu UV–VIS absorption spectrophotometer (model UV-2401PC). Hitachi F-4500 fluorescence spectrophotometer was used to obtain the steady-state emission spectra. For the spectroscopic measurements, the sample concentration was maintained at 10 )5 M in each case in order to avoid aggregation. The quantum yield was determined by using a secondary standard method with recrystallized NAPMD. The quantum yields of fluorescence were calculated using 4-nitro-1-hydroxy-2-naphthoic acid (u ref = 0.08) in ethanol (20) as a standard according to following relation where OD¢, A¢, n¢ and OD, A, n represent the optical density at the excited wavelength, the integrated emission band area and the solvent refractive index of the standard and the sample, respectively (21). U fl ¼ u fl 0 A f OD 0 n 2 E 0 A 0 f ODn 0 2 E ð1Þ *Corresponding author email: spsc@iacs.res.in (Sankar Chakravorti) Ó 2009 The Authors. Journal Compilation. The American Society of Photobiology 0031-8655/10 Photochemistry and Photobiology, 2010, 86: 47–54 47