\documentclass{article}

\usepackage{url}
\usepackage{tikz}
\usetikzlibrary{arrows}
\usetikzlibrary{positioning}

\begin{document}

\tikzset{
treenode/.style = {
    align=center,
    inner sep=0pt,
    text centered,
    font=\sffamily},
  arn_n/.style = {
    anchor=center,
    thick,
    treenode,
    circle,
    font=\sffamily\bfseries,
    draw=black,
    fill=white,
    minimum size=1.5em
    }
}

A second try.

\begin{center}
\begin{tikzpicture}[-,
  >=stealth',
  level/.style={
    sibling distance = 3cm, % This parameter can be used to place the nodes
                            % further apart horizontally.
    level distance = 1.2cm % This parameter can be used to place the nodes
                             % further apart vertically.
   },
  scale=0.7] 
\node [arn_n] {$\neg$} % We start with the root (operator)
    % The arn_n is specified to use a specific set that is defined above in
    % \tikzset.
    % This root operator is unary so it has one child.
    child { [sibling distance=6cm] node [arn_n] {$\leftrightarrow$} 
            child { node [arn_n] {$\leftrightarrow$} 
                    child { node [arn_n] {$p$} 
                          }
                    child { node [arn_n] {$\land$} 
                            child { node [arn_n] {$q$} 
                                  }
                            child { node [arn_n] {$r$} 
                                  }
                          }
                  }
            child { node [arn_n] {$\lor$} 
                    child { node [arn_n] {$s$} 
                          }
                    child { node [arn_n] {$t$} 
                          }
                  }
          }
;
\end{tikzpicture}
\end{center}

\end{document}
